NUCLEAR PHYSICS at GANIL - A COMPILATION

1983-1988

C. Detraz, S. Harar

To cite this version:

C. Detraz, S. Harar. NUCLEAR PHYSICS at GANIL - A COMPILATION 1983-1988. 1989,pp.1-251. <in2p3-00383979>

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NUCLEARPHYSICS

GANIL1983 -1988

a COMPILATION

NUCLEAR PHYSICS at GANIL

A COMPILATION

January 1983 - December 1988

GANILBP 5027 - 14021 CAEN Cedex (France)Tel. 31 45 46 47 - Telex : 170 553 F

Fax 31 45 46 65

FALL 1989

CONTENTS

I - COMPILATION

SUMMARY

A - NUCLEAR STRUCTUREA1 - NUCLEAR SPECTROSCOPYA2 - EXOTIC NUCLEI AND DECAY MODES

B - NUCLEAR REACTIONS

B1 - PERIPHERAL COLLISIONS, PROJECTILE-LIKE FRAGMENTSB2 - DISSIPATIVE COLLISIONS, HOT NUCLEIB3 - MULTI-FRAGMENT EMISSIONB4 - PIONS AND PHOTONS

C - DEVELOPMENTS

AUTHOR INDEX

LABORATORY INDEX

II - PUBLICATION LIST

III - SCIENTIFIC ACTIVITIES AT GANIL

PREFACE

About 130 experiments have been carried out at GANIL since the beginning of theexperimental program in January 1983. During these years, the accelerator ran conti-nuously until december 1988, when it was decided to shut it down for a few months, inorder to increase the energy of heavy ion beams. This was considered a good opportu-nity to collect and present the experimental results obtained so far. Since CANIL is anational facility, mainly used by outside physicists, this information is scattered amongvarious laboratory annual reports.

The spokespersons of the GANIL experiments as well as theoreticians who contri-buted to the scientific life at GANIL through regular workshops were asked to presentcontributions of their results. In order to make this document as complete and homoge-neous as possible, we suggested to the authors that they did not write a contribution perexperiment but rather focus their presentation on the main goals and results even if theywere achieved through several experiments. Furthermore, data for which the analysis isnot completed are not presented here, but are rather mentioned in complement of amore complete work.

In summary this document is not a traditional progress report but rather a compila-tion of milestones concerning the topics studied at GANIL in the past few years. Oversuch a long period one can expect a strong evolution in the aim and complexity of bothexperimental approaches and theoretical studies. Indeed work performed at GANILcovers a wide variety of domains such as nuclear structure, reaction mechanisms, hotnuclear matter, production and properties of exotic nuclei, collective motions in nuclei,etc... Contributions have been tentatively ordered under several scientific topics. We areindebted to the authors for the quality of their contributions.

Claude DETRAZ Samuel HARARDirector Vice-Director

ICOMPILATION

A. NUCLEAR STRUCTUREAl NUCLEAR SPECTROSCOPY

ELASTIC SCATTERING AT INTERMEDIATE ENERGYALAMANOS N., AUGER F., BARRETTS J., BERTHIER B., FERNANDEZ B., GASTEBO1S J., GOZJBERT A., PAPINEAU L.,ROUSSEL-CHOMAZ P. - DPhN-CEN SACLAY, GIF SUR YVETTEDOUBREH., uiTTia w. - GANIL, CAENDISDIER D., LOTTB., RAUCH V., SCHElBLIffG F. - CRN, STRASBOURGSTEPHAN C., TASSAN-GOTL. - IPN, ORSAY 1

EXCITATION OF GIANT RESONANCES IN 90Zr AND 308Pb BY INELASTIC SCATTERING OF HEAVYIONS AT GANIL ENERGIESBEAUMELD.. BLUMENFELD Y., CHOMAZ Ph., FRASOARIA N., GARRONJ.P., ROYNETTE J.O., SCARPACIJ.A.,

SUOMUARVIT. - IPN, ORSAYBARRETTS J., FERNANDEZ B., GASTEBOIS J., ROVSSEL-CHOMAZ P. - DPhN-CEN SACLAY, GIF SUR YVETTEMITTUS W. - GANIL, CAEN 3

INVESTIGATION OF THE SPIN-ISOSPIN-FLIP STRENGTH WITH THE (12C, 12AT)-REACTIONVON OERTZEN W. • HMI, BERLIN 6

CHARGE EXCHANGE REACTIONS TO PROBE THE ISOVECTOR ELECTRIC NUCLEAR RESPONSEBERATC., BVENERDM., MARTIN P., HOSTACHY J.Y., CHAWINJ., LEBRVND. • ISN, GRENOBLEBARRETTS J., BERTHIER B., FERNANDEZ B., HICZAKA A. - DPhN-CEN SACLAY, GIF SUR YVETTEMITJIG W., VILLARI A.C.C. - GANIL, CAENBOHLEN G., STOJARIS S., VON OERTZEN W. - HMI, BERLIN 8

ISOSCALAR GIANT RESONANCES AT FINITE TEMPERATURESURAVDE. • ISN, GRENOBLEBARRANCO M. - DEPT. de ESTRUCTURA I CONST. MATERIA, BARCELONATREINER J.- IPN, ORSAY 10

HIGH EXCITATION ENERGY STRUCTURES IN HEAVY ION COLLISIONS AT INTERMEDIATE EN-ERGYBEAVMEL D., BLUMENFELD Y., CHOMAZ Ph., fRASCARIA N., GARRONJ.P.. JACMARTJ.C., ROYNETTEJ.C., SCARPACIJ.A.,

SUOMUARYI T. - IPN, ORSAYBARRETTS J., BERTHIER B., FERNANDEZ B., GASTEBOIS J., ROUSSEL-CHOMAZ P. - DPhN-CEN SACLAY, GIF SUR YVETTEMITTIG W. . GANIL, CAEN 12

MULTIPHONON EXCITATIONS IN HEAVY ION GRAZING COLLISIONSCHOMAZ Ph., VAVTHERIND. - IPN, ORSAY 16

COLLECTIVE EXCITATIONS OF CLOSED SHELL NUCLEI IN HEAVY ION GRAZING COLLISIONSCHOMAZ Ph.. NGUYEN VAN GIAI, VAUTHER1ND. - IPN, ORSAY 17

EXCITATIONS IN GRAZING HEAVY ION REACTIONSDIETRICHK., WERNER K.- TUM, GARCHING b. MUNCHEN 18

A2 EXOTIC NUCLEI AND DECAY MODES

PRODUCTION AND IDENTIFICATION OF NEW ISOTOPES AT THE PROTON AND THE NEUTRONDRIP-LINESANNER., BAZIND., DETRAZ C, OUERREAVD., GUILLEMAUD-MUELLER D., JIANG D.X., MUELLER A.O., ROECKL E,SAINT-LAURENT M.G. - GANIL, CAENARTUKHA.G., GVOZDEVB.A., KALININ A.M., KAMANIN V.V., KUTNER V.B., LEWITOWICZM., LVKYANOV S.M.,NGUYEN HOAICHAU, PEHIONZHKEVICH Yu.E. - JINR, DUBNABERNAS M., BORREL V., GAUN J., HOATH S.D., JACMART J.C., LANGEVBiM., NAULINF., QUOflOUE, RICHARD A.

- IPN, ORSAYSCHMWT-OTT W.D. - INST. F. PHYSIK, GOTTINGEN 19

MASS MEASUREMENTS OF NEUTRON RICH FRAGMENTATION PRODUCTSAUDIO- C3NSM, ORSAYBIANCHIL., FERNANDEZ B., GASTEBOISJ., GILUBERTA. - DPhN-CEN SACLAY, GIF SUR YVETTECUNSOLOA., FOTIA. - INFN, CATANIAGREGOIRE G, MIT7IG W., SOHUTZ Y. - GANIL, CAENSTEPHAN C, TASSAN-GOTL. - IPN, ORSAYMORJEANM., PRANAL Y. - CEN, BRUYERES LE CHATELVILLAR1A.C.C. - I.F.U.S.P., SAO PAULOWEN LONG Z. - I.M.P., LANZHOU 23

STUDY OF ^-DELAYED NEUTRON EMISSION OF VERY NEUTRON-RICH LIGHT NUCLEIANNER., BAZIND., DETRAZ C, GUERREAUD., GUILLEMAUDMVELLER D., MUELLER A.C., SAINT-LAURENTM.G.

- GANIL, CAENARTUKHA.G., KALININ A.M., KAMANIN V.V., LEWITOWICZ M., LVKYANOV SM., NGUYEN IIOAI CHAU,

PENIONZHKEVICH YILB. - JINR, DUBNABORREL V., JACMARTJ.C, POUGHEON F., RICHARD A. - IPN, ORSAYSCHMWT-OTT U.D. - INST. F. PHYSIK, GOTTINGEN 25

0 - DELAYED MULTI-NEUTRON RADIOACTIVITY OF "B, " C AND "B«DUFOUR J.P., JEL MORAL R., FLEURY A., HUBERTF., JEAND., PRAVBXOFFM.S. - CEN BORDEAUX, GRADIGNANBEAVM., FREfaWTJ., GIRAUDETG. - CEN, BRUYERES LE CHATEL

HANELTE. - TEC><. HOCHSCHULE, DARMSTADTMUELLER A.C. - GANIL, CAENSCHMWTK.H., SUMMERER K. - GSI, DARMSTADT 27

BETA - DELAYED GAMMA SPECTROSCOPY ON LIGHT NEUTRON RICH NUCLEIDUFOUR JJ>., DEL MORAL R., FLEURY A., HUBERTF., JEAND., PRAVKOFFM.S. - CEN BORDEAUX, GRADIGNANDELAGRANGEH. - GANIL, CAENGEISSEL H., SCHMIDT KM. - GSI, DARMSTADT

HANELTE. • TECH. HOCHSCHULE, DARMSTADT 28

BETA-DELAYED CHARGED PARTICLES SPECTROSCOPY OF LIGHT NEUTRON DEFICIENTNUCLEIBORREL V., JACMARTJ.C., POUGHEON F, RICHARD A. - IPN, ORSAY

ANNER., BAZIND., DELAGRANGEH., DETRAZC, GUILLEMAUD-MUELLERD., MUELLER A.C., ROECKLE.,

SAINT-LAURENTM.G. - GANIL, CAENDUFOUR JJ>., HUBERTF, PRAVBCOFFM.S. - CEN BORDEAUX, GRADIGNAN 29

SOME PROPERTIES OF B-DELAYED CHARGED PARTICLE EMISSION OF aoTi and *°Ti.ANNER., BRICAULTP., DETRAZ C, LEWITOWICZM., ZHANG Y.H. - GANIL, CAENBAZIN D., DUFOUR J.P., FLEURY A., HUBERT F, PRAVBCOFF M.S. - CEN BORDEAUX, GRADIGNANGUILLEMAUD-MUELLER D., JACMAST J.C., MUELLER A.C., POUGHEONF., RICHARD A. - IPN, ORSAY 33

BETA DECAY OF a aOHUBERTF., DUFOUR J.P., DEL MORAL R., FLEURYA., JEAND., PRAVJKOFFM.S. - CEN BORDEAUX, GRADIGNANDELAGRANGEH. - GANIL, CAENGE1SSELH., SCHMIDT KM - G SI, DARMSTADTHANELTE. - TECH. HOCHSCHULE, DARMSTADT 35

SPIN ALIGNMENT IN PROJECTILE FRAGMENTATION AT INTERMEDIATE ENERGIESASAHIK., 1SHMARA M., 1CMHARA T., FVKUDA M., KUBO T., GONO Y. - RIKEN, SAITAMAMUELLER A.C., ANNE R., BAZIN D., GUILLEMAUD-MUELLER D. - GANIL, CAENBIMBOTR. - IPN, ORSAYSCmUDT-OTT W.D. - 1NST. F. PHYSIK, GOTTINGENKASAGIJ.- TOKYO INST. OF TECHNOLOGY, TOKYO 36

TOTAL REACTION CROSS SECTIONS AND LIGHT EXOTIC NUCLEUS RADIISAINT-LAURENT M.G., ANNER., BAZIN D., GUILLEMAUD-MUELLER D., JAHNKE U., JIN GEN MING, MUELLER A.O.- GANIL, CAENBRUANDET J.F., GLASSER F, KOX S., IMTARD E, TSAN VNG CHAN- I3N, GRENOBLECOSTA G.J., HEITZ C. • CRN, STRASBOURGEL MASRI Y. • FNRS-UCL, LOUVAIN LA NEUVEHANAPPE F. - FNRS-ULB, BRUXELLESBIMBOTR. • IPN, ORSAYARNOLD E, NEUGARTR. - INST. F. PHYSIK, MAINZ 38

SEARCH FOR FAR FROM STABILITY NUCLEI WITH AN ON-LINE MASS-SPECTROMETERDE SAINT-SIMON M., COCA., GUIMBAL P., THIBAULT C, TOUCHARD F. - CSNSM, ORSAYLANGEVINM. - IPN, ORSAYDETRAZO., GUILLEMAUD-MUELLER D, MUELLER AC.-GANIL, CAEN 40

B. NUCLEAR REACTIONSBl PERIPHERAL COLLISIONS - PROJECTILE-LIKE FRAGMENTS

PROJECTILE-LIKE FRAGMENT PRODUCTION IN Ar INDUCED REACTIONS AROUND THE FERMIENERGYBORREL V., GATTYB., TARRAGO X. - IPN, ORSAYGALBfJ., GVERREAUD.-GJiNlL.CAEH 42

ANGULAR EVOLUTION OF THE PRODUCTION OF PROJECTILE LIKE FRAGMENTS IN INTERME-DIATE ENERGY HEAVY ION COLLISIONSBEAUMEL D., BLVMENFELD Y., CHOMAZPK, FRASCARIA N., JACMARTJ.C, GARRONJ.P., ROYNETTE J.C., SCARPACI J.A.,SVOMUASn T. • IPN, ORSAYBARRETTE J., BERTŒRB., FERNANDEZ B., GASTEBOISJ. - DPhN-CEN SACLAY, GIF SUR YVETTEARDOtmtD., MITT1G W. - GANIL, CAEN 44

PERIPHERAL COLLISIONS IN THE *°Ar + e8Zn REACTION BETWEEN 15 AND 35 MeV/ NUCLEONCOFFIN J.P., FAMJA., FINTZP., GUILLAUME G., HEUSCHB., RAW F., WAGNER P. - CRN, STRASBOURGSCHVTZ Y. - GANIL, CAENMERMAZU.C. - DPhN-CEN SACLAY, GIF SUR YVETTE 46

EXCLUSIVE STUDY OF PERIPHERAL INTERACTIONSBIZARD G., BROVR., LAVOLE J.L., NATOWITZJ., PATRYJJ>., STECKMEYER J.C., TAMAINB. - LPC, CAENDOVBREH., PEGHA1RE A., PETER J., ROSATO B. - GANIL, CAENGVUBAULTF., LEBRUN C. - LSN, NANTESHANAPPEF. - FNRS-ULB, BRUXELLESADLOFFJ.C, RUDOLF G., SCHEIBLING F. -CRN, STRASBOURG 48

STUDYING PERIPHERAL COLLISIONS IN THE REACTION Ar + An AT 27.2, 35 AND 44 MeV/u BYTHE ASSOCIATED NEUTRON MULTIPLICITIESMORJEANM., FREHAUTJ., CHARVETJ.L., DUOHENE G., LOTTB., MAGNAGO C, PATIN Y., PRANAL Y., UZUREAU J.L.- CEN, BRUYERES LE CHATELGUERREAUD., DOUBRE H., GALINJ., JAHNKE U., JIANG D.X., POUTHAS J. • GANIL, CAENINGOLD G. - HMI, BERLINJACQUET D. - IPN, ORSAY 50

DISSIPATIVE FRAGMENTATION IN MEDIUM ENERGY HEAVY ION COLLISIONSADORNO A., DITOROM. - DIPARTIMENTO DI FISICA AND INFN, CATANIABONASERA A. - TUM, GARCHING b. MUNCHENGREGOIRE C. - GANIL, CAENGULWNELUF.. DIPARTIMENTO DI FISICA AND INFN, MILANO 51

FORMATION OF HIGHLY EXCITED QUASI-PROJECTILE FRAGMENTS IN THE **Kr + "Nb REAC-TION AT 35 MeV/uCHARVETJJu, DUCHENE G., JOLYS., MAGNAGO C, MORJEANM., PATIN Y., PRANAL Y., SBiOPOUL., UZUREAU JX.• CEN, BRUYERES LE CHATELBULEREYR., CHAMBONB., CHBIHIA., CHEVARŒR A., CHEVARVER N, DRAIN D., PASTOR C, STERN». - IPN, LYONPEGHAIRE A. -GANIL, CAEN 53

PROJECTILE FRAGMENTATION IN HEAVY-ION REACTIONS AT INTERMEDIATE ENERGIESROYER G., REMAUD B., LEBRUN C, OUBAHADOU A., SEBILLEF. - LSN, NANTESGREGOIRE C.- GANIL, CAEN 55

MEASUREMENTS OF TIME DELAYS FOR PROJECTILE-LIKE FRAGMENTS IN THE REACTION *°Ar+ Ge AT 44 MeV/NUCLEONGOME! DEL CAMPO J. - ORNL, OAK RIDGEDAYRASR., WIELECZKO J.P., POLLACCOE.O. - DPhN-CEN SACLAY, GIF SUR YVETTEBARRETTEJ. - FOSTER RADIATION LABORATORY, MONTREALSAINT-LAMENTF. - GANIL, CAENTOULEMONDEM. - CIRIL, CAENNESKOVIC At, OSTOJIC R. - BKI, BELGRADE 56

NUCLEAR CALEFACTIONBERLANGER U., DALOJ D., LUCAS R., NGO C., LERAY S., MAZUR C., RWRAG U., SUOUUARVI T. - DPhN-CEN SACLAY,GIF SUR YVETTECmODELLIS., DEUEYER A., GVWETD. - IPN, LYONCERRU7J C.-LABORATOIRE NATIONAL SATURNE, GIF SUR YVETTB 58

PROJECTILE FRAGMENTATION NEAR THE FERMI ENERGYDAYRAS R., BARRETTS J., BERTHIER B., DE CASTRO-RIZZO DM., CHAVEt E., CISSE O., CONIGUONE R., GADIF.,LEGRAINR., MERMAZ M.C., POLLACCOE.C. - DPhN-CEN SACLAY, GIF SUR YVETTEDELAGRANGEH., MITTIG W. - GANIL, CAENLANZANO G., PAGANO A., PALMER! A. - INFN, CATANIAHEVSCH B. - CRN, STRASBOURG 60

B2 DISSIPATIVE COLLISIONS - HOT NUCLEI

DYNAMICAL ASPECTS OF VIOLENT COLLISIONS IN AT + Ag REACTIONS AT E/A = 27 MeV. PER-SISTENCE OF DEEPLY INELASli: » ACCESSESBORDERS; B., CABOT a, FVCHSH., GARDES D., GAWINH., JAOQVETD., JOUAND., HONNETF., UONTOYA u.,RIVET M.F.. IPN, ORSAYHANAPPE F. - FNRS-ULB, BRUXELLES 62

CHARACTERISTICS OF THE MANY-BODY EXIT-CHANNELS IN HIGHLY DISSIPATIVE COLLISIONSOLMIA., CASINI G., MAVRENZIG P.R., STEFANINI A.A. - INFN, FLORENCECHARITY RJ., FREIFELDER R., GOBBI A., HERRMANN N., HUDENBRAND K.D., RAMIF., STELZER K, WESSELSJ.- GSI, DARMSTADTPETROV1CIH. - INPE, BUCURESTI-MAGURELEGATJNJ., GVERREAVD., JAHNKE U. - GANIL, CAENGNIRSM., PELTED., RAMOLDD. - INST. F. PHYSIK, HEIDELBERGADLOFF J.C..BILWES B., BO.WES R., RUDOLF G.- CRN, STRASBOURG 64

COEXISTENCE OF COLLECTIVE AND PARTICIPANT-SPECTATOR MECHANISMS IN THE INTER-ACTION BETWEEN KRYPTON AND GOLD NUCLEI AT 44 MeV/uADLOFF J.C., BO.WESB., BO.WES R, GLASER M, RUDOLF G., SCHEIBUNG F., STUTTGEL. - CRN, STRASBOURGBOUGAULTR., BROUR., DELAUNAYF., GENOUX-LUBABi A., LE BRUN O., LECOLLEY Jf., LEFEBVRESF., LOUVEL tl.,BIZARD G., STECKMEYER J.O. • LPC, CAENFERREROJ.L. - IFIC, VALENCIACASSAGNOU Y., LEGRAINR. - DPhN-CEN SACLAY, GIF SUR YVETTEGUILBAULTF., LEBRUNC., RASTEGARB. - LSN, NANTESJIN GEN MING, PEGHAIRE A., PETER J., ROSATO R - GANIL, CAEN 67

INCOMPLETE LINEAR MOMENTUM TRANSFERCERRUTIC., CHIODELIJ S., DEMEYER A., GUINETD., VAGNERONL. - IPN, LYONCHARVETJ.L., LOCHARD J.P., MORJEANU., PAT1N Y., PEGHAIRE A,, SINOPOU L., VZUREAU J.L. - CEN, BRUYERES LECHATELGRANIER O.. GREGOIRE C., LA RANA G., LERAYS., LHENORETP., LUCAS R., MAZUR C., NEBBIA G., NGO C., RWRAGM.,SVOMJARVI r., TOHASIE. - DPhN-CEN SACLAY, GIF SUR YVETTELEBRUN C. - LPC, CAEN 69

FORMATION AND DECAY OF HOT NUCLEI IN THE Ar, Ni AND Kr + Th SYSTEMSCASSAGNOV Y., CONJEAVD U., DAYRAS R., HARAR S., LEGRAOf R,, tfOSTEFAI M., POLLACCO E.C., SAWESTRE J.E.,VOLANT C. - DPhN-CEN SACLAY, GIF SUR YVETTEKLOTZ-ENGHANN G., LIPS V., OESCHLER H. - TECH. HOCHSCHULE, DARMSTADT 71

SATURATION OF THE THERMAL ENERGY DEPOSITED IN Th NUCLEI BY Ar PROJECTILES BE-TWEEN 35 AND 77 MeV/uJIANG D.X., DOUBREH,, GAL1N J., GVERREAUD., PIASECKIE., PCUTHAS J., SOKOLOV A. - GANIL, CAENCRAMER B., 1NGOLD G., JAHNKE V., SOHWINNE. - HMI, BERLINCHARVETJ.L,, FREHAUTJ., LOTTB., MAGNAGO C., MORJEANM., PATW Y., PRANAL Y., VZUREAU J.L. - CEN, BRUYERESLE CHATELGATTYB., JACQVETD. - IPN, OHSAY 73

MOMENTUM TRANSFER LIMITATION IN 30Jfe AND *°Ar INDUCED REACTIONS ON 1345nLLERESA., GIZONA. - ISN, GRENOBLEBLAOHOTJ., CRANCONJ., NIFENEOKER H. • CEN-DRF/SPh, GRENOBLE 75

FUSION RESIDUES BETWEEN 35 AND 60 MeV/ABIZARD G., BROUR., LAYILLE J.L'., NATOW1TZJ., PATRYJ.P., STBCKMEYER J.C., TAMAINB. • LPC. CAENDOUBREH., PEGHAIREA., PETER J., ROSATOE. - GANIL, CAENGUILBAULTF., LEBRUN C. - LSN, NANTESHANAPPEF. - ULB, BRUXELLESADLOFF J.C., RUDOLF G., SCHEIBLING F. - CRN, STRASBOURG 77

PRODUCTION AND DEEXCITATION OF HOT NUCLEI IN 27 MeV/U *°Ar + ™8U COLLISIONSJACQUETD.. BORDERIEB., GARDES D., LEFORTM., RIVETM.F., TARRAGOX. - IPN, ORSAYPEASLEE G., ALEXANDER J.M., DUEKE. - SUNY, STONY BROOKGALINJ., GUERREAUD., GREGOIRE C. - GANIL, CAENFUCaS H.- HMI, BERLIN 79

DYNAMICS OF HEAVY - ION COLLISIONS AT GANIL ENERGIESGREGOIRE a, VINETL. - GANIL, CAENREMAUD B., SEBILLE F., RAFFRAY Y. - LSN, NANTES 81

A NEW THEORY OF COLLISIONSGIRAUD B.G. - DPhN-CEN SACLAY, GIF SUR YVETTENAGARAJANM.A. - DARESBURY LABORATORY, WARRINGTON 83

MEDIUM CORRECTION OF TRANSPORT EQUATIONCUGNONJ,, LBJEVNE A. • INST. DE PHYS. AU SART TILMAN, LIEGEGRANGE P. - CRN, STRASBOURG 85

NON LOCALITY EFFECTS IN NUCLEAR DYNAMICSSEBILLE F., ROYER G., DELAMOTA V., RAFFRAY Y. - LSN, NANTESGREGOIRE O. - GANIL, CAENREMAUD B. - IRESTE, NANTESSCHUCK P. - ISN, GRENOBLE 87

THE PSEUDO PARTICLE METHOD FOR THE SOLUTION OF THE LANDAU-VLASOV EQUATIONGKEGOntE C., VINSTL. • GANIL, CAENPIU. - DEPT. d« ESTRUCTURA I CONST. MATERIA, BARCELONAREHAUD B., SEB&LE F. - LSN, NANTESSURAUD B., SCHUCKP. - ISN, GRENOBLE ................................................................... 89

FORMATION AND DEEXCITATION OF HOT MEDIUM MASS NUCLEI AROUND 30 A. MeVBOBDEKJEB., CABOT C., GARDES D., GAUVINH., RIVBTM.F. • 1PN, ORSAYPETER J. - GANIL, CAEN

, BRUXELLES ..................................................................... 91

PRODUCTION AND DECAY OF HIGHLY EXCITED NUCLEI BETWEEN 26 AND 45 MeV/ NUCLEONAUGER F., BERTUER B., FAVREB., WOSLECXKO J.P. - DPhN-CEN SACLAY, GIF SUR YVETTECUNSOLO A., FOTIA. - INFN, CATANIAM1THG W. - GANIL, CAENPASOAVD J.M., QUEBERTJ. - CEN BORDEAUX, GRADIGNANPLAGNOL E. - IPN, ORSAY ................................................................................ 93

HEAVY RESIDUE SPECTRA AND LINEAR MOMENTUM TRANSFER IN INTERMEDIATE ENERGYNUCLEUS - GOLD COLLISIONSALEKLETTK., SOWER L. - UNIV. OF UPPSALA, UPPSALA

LOJEffZmj.O. - UNIV. OF OSLO, OSLOLOVELAND W. - OREGON STATE UNIV., CORVALLISDE SAINT-SIMON M. - LABORATOIRE RENE BERNAS, ORSAYSEABORG G. T. - LBL, BERKELEY .......................................................................... 95

REACTION MECHANISM EVOLUTION FROM 37 TO SO MeV/ NUCLEON FOR THE SYSTEM ™Nc +

ANDREOZZIF., BRONDI A., D'ONOFRIO A., LA RANA G., MORO R., PERILLO E, ROMANO if., TERRASIF. - INFN, NAPOLIDAYRASR.. DELAVNAYB., DELAUNAYJ., DUMONTH., GADYF. - DPhN-CEN SACLAY, GIF SUR YVETTE

GOMEZ DEL CAMPO 1. - ORNL, OAK RIDGESAINT-LAURENT M.G. - GANIL, CAENCAVALLERO S., SPERDUTO L. - INFN, CATANIABRUANDET J.F. - ISN, GRENOBLE ......................................................................... 97

PREEQUILIBRIUM EMISSION AND INCOMPLETE FUSION IN THE *°Ar + 13C, 3*Mg and 4SSc REAC-TIONS AT 27.5 MeV/ NUCLEONCOFFINJ.P., FAHLIA., FINTZP., GONINM., GUILLAUME G., HEVSCHB., JVNDTF., MALKI A., WAGNER P. - CRN, STRAS-BOURGKOX S., MEROHEZF., MISTRETTA J. - ISN, GRENOBLE ....................................................... 99

TARGET RESIDUE CROSS SECTIONS AND RECOIL ENERGIES FROM THE REACTION OF 1760 MeV*°Ar WITH MEDIUM AND HEAVY TARGETS.HUBERT F., DEL MORAL R,, DUFOUR J.P., EMMERMANN H., FLEURYA., PODiOT C., PRAVBCOFF M.S. - CEN BORDEAUX,GRADIGNANDELAGRANGEH. - GANIL, CAENLLERES A. - ISN, GRENOBLE ............................................................................. 101

LIGHT PARTICLES AND PROJECTILE-LIKE FRAGMENTS FROM THE *°Ar + 13C REACTION AT 44MeV/NUCLEONHEUER D., BERTHOLETR., GUET C., MAUREL M., NIFENECKER H., R1STORI CO., SCHUSSLER F. - CEH-DRF/SPh, GRENO-

BLEBONDORF J.P., NIELSEN O.B. - NBI, COPENHAGEN ......................................................... 102

STUDY OF PREEQUILIBRIUM IN INTERMEDIATE ENERGY HEAVY ION COLLISIONSCERRUTIO., BO1SGARD R., NGO O. - DPhN-CEN SACLAY, GIF SUR YVETTEDESBOISJ. - IPN, ORSAYNATOWITgJ. - CYCLOTRON INST., COLLEGE STATIONNEMETH J. - INST. OP THEORETICAL PHYSICS. BUDAPEST 103

STUDY OF LIGHT CHARGED PARTICLES EMITTED IN *°Ar INDUCED REACTIONS AT INTERME-DIATE ENERGIESLANZANO G., PAGANO A. - INFN, CATANIADAVRAS R., BARRETTS J., BERTIBER B., DE CASTRO-RIZZO DM, CISSB O., CONIGLIONE R., GADIF., LBGRAOfR,,HERMAZM.C., POLLACCO E.C. - DPhN-CEN SACLAY, GIF SUR YVETTEDELAGRANGE H., HITTIG W. - GANIL, CAENHEUSCHB. - CRN, STRASBOURG 105

LIGHT PARTICLES CORRELATIONS : (I) SMALL RELATIVE MOMENTA CORRELATIONSARDOUIN D., DELAGRANGE H., DOUBRE H., GREGOOtE C., KYANOWSKt A., MTTJG W., PEGHAKE A., PETER J.,SAINT-LAURENT V., VIYOGI Y.P., ZW1BGUNSKI B. - GANIL, CAENBIZARD G., LEFEBVRESF., TAMAOfB. - LPC, CAENGELBKE C.K., LYNCH W.G., MA1ER M., POCHODZALLA J. - NSCL, EAST LANSINGQUEBERTJ. - CEN BORDEAUX, GRADIGNAN 107

LIGHT PARTICLE CORRELATIONS : (II) LARGE RELATIVE MOMENTA CORRELATIONSARDOUIN D., DELAGRANGE H., DOUBRE H., GREGOIRE C., KYANOWSKJ A., MTTiG W., PEGHAOtE A.. PETER J.,SAINT-LAURENT F., VIYOGI Y.P., ZW1EGLINSKI B. - GANIL, CAENBASRAKZ., GELBKE O.K., LYNCH W.G., MAIER M., POCHODZALLA J. - NSCL, EAST LANSINGSCHUCKP. - ISN, GRENOBLEBIZARD G., LEBFEVRESF., TAHAINB. - LPC, CAENQUEBERTJ. - CEN BORDEAUX, GRADIGNAN 109

THE ROLE OF TIME IN THE DAMPING OF SOME FINAL STATE INTERACTIONS IN TWO PARTICLECORRELATIONSLAUTRIDOUP., BOISGARD R., QUEBERTJ. - CEN BORDEAUX, GRADIGNANLEBRUN a, GUILBAULTF., GOUJDAMD., DURAND D., TAUISIER R., ARDOVIND. - LSN, NANTESPEGHAIREA., SAINT-LAVRENTF. - GANIL, CAEN Ill

40Ar + MZn COLLISIONS : INCOMPLETE FUSION AND LIGHT PARTICLE EMISSIONCOFFIN J.P., FAULT A., FINTZ P., GUILLAVUE G., HEUSCHB., JVNDTF., RAUIF., WAGNER P. - CRN, STRASBOURGCOLEA.J., KOXS. - ISN, GRENOBLESCHUTZ Y. - GANIL, CAEN 113

DENSITY FUNCTIONAL APPROACHES FOR THE DESCRIPTION OF COLD AND EXCITED NUCLEIBARTEL J., BRACK M., STRUMBERGER £. - INST. F. THEOR. PHYS., REGENSBURGGUET C. • GANIL, CAENMEYER J. - IPN, LYONQVENTINP. -LAB. PHY. THEORIQUE, GRADIGNAN 115

STATIC PROPERTIES OF HOT NUCLEISURAUD E - ISN, GRENOBLE 117

PROPERTIES OF HIGHLY EXCITED NUCLEIBONCHEP. - DPhN-CEN SACLAY, GIF SUR YVETTELEVITS. - WEIZMANN INST., REHOVOTVAVTHER1N D. - IPN, ORSAY 119

MEAN-FIELD DESCRIPTION OF NUCLEI AT HIGH TEMPERATUREBONCHEP. - DPhN-CEN SACLAY, GIF SUR YVETTELEVITS.. WEIZMANN INST., REHOVOTVAUTHER1ND.-IPH, ORSAY 120

STATISTICAL PROPERTIES AND STABILITY OF HOT NUCLEIBONCHEP. - DPhN-CEN SACLAY, GIF SUR YVETTELEVITS. - WEIZMANN INST., REHOVOTVAUTHEJUND.-tPti, ORSAY , 121

THOMAS-FERMI CALCULATIONS OF HOT DENSE MATTERSURAVDE., VAUTHEWND. - IPN, ORSAY 122

EVOLUTION OF HOT COMPRESSED NUCLEI IN THE TIME-DEPENDENT HARTREE-FOCKAPPROXIMATIONVAVTHERIND. - MIT, CAMBRIDGETREINERJ., VENERONIM.- IPN, ORSAY 123

NUCLEAR PARTITION FUNCTIONS IN THE RANDOM PHASE APPROXIMATION AND THETEMPERATURE DEPENDENCE OF COLLECTIVE STATESVAVTHEiUND., VttiH MAUN. - IPN, ORSAY 124

TEMPERATURE DEPENDENCE OF COLLECTIVE STATES IN THE RANDOM-PHASE APPROXIMA-TIONVAVTHEPJND., VJNH MAV N. - IPN, ORSAY 125

EFFECT OF CORRELATIONS ON THE RELATION BETWEEN EXCITATION ENERGY AND TEMPER-TATURETROVDETT., VAVTHERIND., VINHMAUN. - IPN, ORSAY 126

THE LEVEL DENSITY PARAMETER OF A NUCLEUS IN THE SCHEMATIC MODELVAVTHERIND. - MIT, CAMBRIDGEV1NH MAU N. - IPN, ORSAY 127

B3 MULTI-FRAGMENT EMISSION

INSTABILITIES IN AN EXPANDING FIREBALLCVGAT0JV/.. INST. DE PHYS. AUSART TOMAN, LIEGE 128

A SCHEMATIC HYDRODYNAMICAL AND PERCOLATION PICTURE FOR MULTIFRAGMENTATIONOF EXCITED NUCLEINFMETHJ.. INST. OF THEOR. PHYS., BUDAPESTBARRANCO H.. DEPT. DE FISICA., PALMA DE MALLORCADESBO1SJ.. IPN, ORSAYBOISGARD R., NGO C. - LABORATOIRE NATIONAL SATUBNE, GIF SUR YVETTE 130

RESTRUCTURED AGGREGATION COUPLED TO LANDAU-VLASOV MODELLERAYS., NGO C., SPINA U.E. - LABORATOIRE NATIONAL SATURNE, GIF SUR YVETTEREMAUDB..SEBILLEF.-LSN, NANTES 132

MULTIFRAGMENTATION AND RESTRUCTURED AGGREGATIONLERAYS.. NGO C., SPINA U.E. - LABORATOIRE NATIONAL SATURNE, GIF SUR YVETTENGO H. - IPN. ORSAY 134

CONTRIBUTIONS TO THE DESCRIPTION AND UNDERSTANDING OF NUCLEAR FRAGMENTATIONRICHERTJ., WAGNER P. - CRN, STRASBOURGSAMADDAR S.K. - SAHA INST. OF NUCLEAR PHYSICS, CALCUTTA 136

SPINODAL BREAK UP AND THE ONSET OF MULTIFRAGMENTATIONCVSSOL D., GREGOISE C. - GANIL, CAENSURE A V B. - ISN, GRENOBLE 138

INTERMEDIATE MASS FRAGMENTS PRODUCED AT LARGE ANGLE IN HEAVY ION REACTIONSPAPADAKIS N.H., VODINASN.P., PANAGIOTOU A.D. - PHYS. DEPT. ATHENSCASSAGNOU Y.. DAYRAS R., LEGRAIN R., POLLACCO E.G., RODRIGUEZ L., SAVN1ER N. - DPhN-CEN SACLAY, GIF SURYVETTEFONTS R., OMB G., RACIT1G. - INFN, CATANIASAINT-LAURENTF., SAINT-LAURENTM.G. - GANIL, CAEN 140

CHARACTERIZATION OF MULTIFRAGMENT CHANNELS IN 45 MeV/ NUCLEON **Kr + 169TbREACTIONMAKJKA Z., SOBOTKA L.G., STRACENER D. W., SARANTITES D.G. - DEPT. OF CHEMISTRY, ST LOUISAUGER G., PLAGNOL E., SCHUTZ Y. - GANIL, CAENDAYRAS R., WIELECZKO J.P. - DPhN-CEN SACLAY, GIF SUR YVETTEBARRETO J. - IPN, ORSAYNORBECKB.- DEPT. OF PHYSICS, IOWA CITY 142

MULTIFRAGMENT PRODUCTION IN KRYPTON INDUCED COLLISIONS AT 43 MEV/nBOUGAVLTR., DELAVNAYF.. GENOUX-LUBAINA., LEBRVNC., LEOOLLEY J.F., LEFEBVRESF., LOUVEL U.,STECKHEYER J.O. - LPC, CAENADLOFF I.C., BILWES B., BILWES R., GLASER M, RUDOLF G., SCHEIBLING F., STVTTGEL. - CRN, STRASBOURGFERRERO J.L. - IFJC, VALENCIA 143

ONSET OF MULTIFRAGMENTATION IN *°Ar ON *7Al CENTRAL COLLISSIONS FROM 25 TO 85 MeV/uHAGELK.. PEGHAJREA., JIN GEN MING, CVSSOL D., DOVBRE H.,PETER J., SAINT-LAVRENTF.,MOTOBAYASSIT.- GANIL,CAENBUARD O., BROVR., LOWEL M., PATRYJ.P., REGOtBARTR.. STECKMEYER J.O., TAMAOiB. - LPC, CAENCASSAGNOV Y., LEGRAINR.. DPhN-CEN SACLAY, GIF SUR YVETTELEBRVNC. - LSN, NANTESROSATO £ - UNIV. DINAPOLI, NAPOLINATOmXgJ. - CYCLOTRON 1NST., COLLEGE STATIONJEONGI.C., ZEE S.U., NAGASHOIA Y., NAKAGAWA T., OGOURA U. - UNIV. OF TSUKUBA, IBARAKIKASAGIJ. - INST. OF TECHNOLOGY, TOKYO 145

EVENT-BY-EVENT STUDIES OF 16O INDUCED REACTIONS AT GANIL ENERGIES - MULTIFRAG-MENTATION STUDIES IN NUCLEAR EMULSIONSJAKOBSSONB., JONSSONG.. KARLSSONL., NORENB., SODERSTROMK. - UNIV. OF LUND, LUNDMONNANDE.,SCHUSSLERF.-ISH, GRENOBLE 147

B4 PIONS AND PHOTONS

EXPERIMENTAL SEARCH FOR SUBTHRESHOLD COHERENT PION PRODUCTIONERAZMUSB., GVETC., GILLWERTA., MITJIG W., SCHUTZ Y., YILLARIA.C.C. - GANIL, CAENKVHN W., RIESS S. - UNIV. OF GIESSEN, GIESSENNIFENECKER H., PWSTONJ.A. - CEN-DRF/SPh, GRENOBLEGROSSES., HOLZMANR. - GSI, DARMSTADTVIVIEN J.P. - CRN, STRASBOURGSOYEVRM.- DPhN-CEN SACLAY, GIF SUR YVETTE 150

PION AND PROTON EMISSION IN NUCLEAR COLLISIONS INDUCED BY 1BO IONS OF 94 MeV/UBADALA A., BARBERA R., AIELLO S., PALMER! A., PAPPALARDO G.S., SCHILLACIA. - BJFN, CATANIABIZARD a., BOUGAVLTR., DVRAND D., GENOVX-LVBAIN A., LAVOLEJ.L., IfVEBVRES F., PATRYJ.P. - LPC, CAENJIN GEN MING, ROSATO E.- GANIL, CAEN 151

LOW ENERGY CHARGED PION PRODUCTION IN 16O + X REACTIONS AT 93 MeV/uLEBRUND.. PEERING., DESAUfTIGNONP. - ISN, GRENOBLELEBRUN C., LECOLLEYJ.F. - LPC, CAENJULtENJ., LEGRAJNR. - DPhN-CEN SACLAY, GIF SUR YVETTE 153

•PION CONDENSATES" IN EXCITED STATES OF FINITE NUCLEIBLVMELR., DIETRICHK.-1VM, GARCHING b. MUNCHEN 155

HARD PHOTONS FROM HEAVY ION COLLISIONS AT 44 MeV/aKVHN W. - UNIV. OF GIESSEN, GIESSEN <; 156

IMPACT PARAMETER DEPENDENCE OF HIGH ENERGY GAMMA-RAY PRODUCTION IN ARGONINDUCED REACTION AT 85 MeV/AKWATO NJOOKM., MAVREL U., MONNANDB., NIFENECKER a., PERROfP., PWSTONJ.A., SCHVSSLER F. - ISN, GRENOBLESCHUTZ Y. • GANIL, CAEN 158

HIGH ENERGY GAMMA-RAY PRODUCTION FROM 44 MeV/A MKr BOMBARDMENT ON NUCLEIBERTHOLETR., KWATO NJOOKM., MAVREL M., MONNAND E., NIFENECKER H., PERRBfP., PBiSTONJ.A., SCHUSSLERF.- CEN-DRF/SPh, GRENOBLEBARNEOUD D. • ISN, GRENOBLEGVET C., SCHUTZ Y. - GANIL, CAEN 160

C. DEVELOPMENTSPROJECTILE FRAGMENTS ISOTOPIC SEPARATION : APPLICATION TO THE LISE SPECTROME-TER AT GANILDVFOUR JJ>., DEL MORAL R., EMMERHANNH., HUBERTF., FLBURYA., JEAND., POtNOT C., PRAVKOFF U.S.- CEN BORDEAUX, GRADIGNANDKLAGRANGEH. - GANIL, CAENSCHMDTKJL - GSI, DARMSTADT 162

PRODUCTION OP RADIOACTIVE SECONDARY BEAMSBJMBOTR., CLAPJER F.. DELLA-NEGRA S., XVBICA B. - IPN, ORSAYANNEiL, DELAGRANGEH, SCOUTS Y. - GANIL, CAENAGUER P., BAS7W G. - CSNSM, ORSAYARDISSON G., HACHEH A. - UNIV. OF NICE, NICECHAPMAN J., LISLE J, - DEPT. OP PHYSICS, MANCHESTEROONO Y., HATANAKA K. - RIKBN, SAITAMAHUBERT F. - CEN BORDEAUX, GRADIGNAN 163

A TELESCOPIC-MODE USE OP THE LISE SPECTROMETER FOR THE STUDY OF VERY FORWARDANGULAR DISTRIBUTIONSBACRI C.O., ROVSSEL P., ANNE R., BERNAS M., BLVMENFELD Y., CLAPIER F., GAWJN B., BERAVLT J.. JACUART J.C.,LATOOER A., LELONG F., POUGHEONF.. SWA J.L., STEPHAN C., SUOtOJARVJ T. - IPN, ORSAY 165

DAT "A TEST OF DETECTORS FOR RECOILING FRAGMENTS"LAVERGNEL., STAB L. - IPN, ORSAYGVSTAFSSON H.A., JAKOBSSONB., KRISTIANSSON A., OSKARSSON A., WESTENWSM. - UNIV. OF LUND, LUNDKORDYASZAJ. - UNIV. OF WARSAW, WARSAWALEKLETTK., WESTERBERG L. - UNIV. OF UPPSALA, UPPSALA 167

INTERACTIONS OF 20-100 MeV/u HEAVY IONS WITH SOLID AND GASEOUS MEDIA : STOPPINGPOWERS, CHARGE DISTRIBUTIONS, ENERGY LOSS STRAGGLING AND ANGULAR STRAGGLINGBMBOTR., GAWINH., HERAULTJ., KUBICA B. - IPN, ORSAYANNER. - GANIL, CAENBASTIN G. - CSNSM, ORSAYHUBERT F. - CEN BORDEAUX, GRADIGNAN 168

SEMI-EMPIRICAL FORMULA FOR HEAVY ION STOPPING POWERS IN SOLID IN THE INTERMEDI-ATE ENERGY RANGEHUBERT F. • CEN BORDEAUX, GRADIGNANB1MBOTR., GAUVINH. - IPN, ORSAY 170

- A -NUCLEAR STRUCTURE

- A 1 -NUCLEAR SPECTROSCOPY

ELASTIC SCATTERING AT INTERMEDIATE ENERGY

N. Alamanosl, F. Auger1, J. Barrette1, B. Berthier1, D. Disdier3, H. Doubre4, B. Fernandezl, J. Gastebois1,A. Gillibert1, D. Lott3, W. Mittig4, L. Papineau1, V. Rauch3, P. Roussel-Chomaz1, F. Scheibling3,

C. Stephan5, L. Tassan-Got5

1) DPhN, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France. 2) Foster Radiation Lab, Me Gill University, MontrealP.Q., Canada H3A 2B2. 3) CRN, 67037 Strasbourg, France. 4) GANIL, BP 5027 Caen Cedex, France. 5) IPN,91406 Orsay, France.

The main purpose of elastic scattering studiesis the determination of the nucleus-nucleusinteraction. At low energy (E < 10 MeV/u), datahave shown that due to strong absorption effects,the interaction potential can be probed only at theedge of the nuclear surface. Before 1984, few dataexisted on this subject at intermediate energies1).In order to study the sensitivity of elastic scatteringto the nuclear potential in this energy range andto compare the energy dependence deduced fromthe data to that predicted by microscopiccalculations2'3'4), we have measured the elasticscattering of ™O on 12C, 28Si, 40Ca, 90Zr and208Po .

The elastic scattering studies reported herecorresponds to two different experiments. Bothused the 16O beam at 94 MeV/u delivered by theGANIL accelerator. During the first one, whichwas performed in CYRANO with a position sensitive

E-E solid state Si telescope, we measured theelastic scattering of 16O on 1 2C, 40Ca, 90Zr and208pD. The energy resolution obtained in thisexperiment (2 MeV) did not allow a proper separationbetween the elastic and inelastic peaks in the caseof 28Si. We therefore performed another experiment,taking advantage of the achievement of SPEC, tomeasure the elastic angular distribution for16O + 28Si. The energy resolution obtained in thissecond experiment was less that 400 keV. In bothexperiments, the angular precision and resolutionwere less than 0.1°.

The angular distributions measured for thedifferent systems are displayed on Fig. 1. For thesystem 16O + 2 0 8Pb, the angular distribution presentsa typical Fresnel type pattern associated with theCoulomb rainbow phenomenon. For lighter systems,strong oscillations appear, which correspond to theinterference of the two scattering amplitudes fromboth sides of the nucleus. The negative angleamplitude increases as the mass of the targetdecreases and, in the case of 1 2C, even dominatesthe angular distribution above 4°. The data havebeen analyzed in the optical model framework, usingstandard VV-S potentials, but also squared W-S anddouble folded potentials^). The solid lines on Fig. 1have been calculated with W-S potentials. Theycorrespond to the best fit of the data, based onminimum 2 .

To study which region of the potential isdetermined by the data, we proceeded in two steps.For each system, we performed first a notch test"'to know the "sensitive region". Then, to evaluatewith which precision the potential is fixed in thesensitive region, we compared the values takenin this region, by the different potentials whichfit the data. The results are summarized on Fig. 2in the case of 16O + 28Si. For both the real andimaginary potentials the sensitive regions are 3 fmwide, that is much wider than at low energy, andthe data seem to be much more sensitive to thereal potential (factor 10 between the 2 curves inthe insert). Indeed, all the real potentials wich fit

Fig. 1 - Elastic angulardistributions for *6O +12C, 28si> 40Ca> 90Zr

and 2 0 8 Pba t1503 MeV.

«bn

Fig. 2 - Radial dependence of real optical potentialswhich describe the 16O + 28Si angular distribution.The full lines are W-S potentials, whereas the dashedand dotted lines are double-folded potentials basedon M3Y and DDM3Y interaction. The insert showsthe results of a notch test on both parts of thepotential.

the data take similar values, within 10%, on thesensitive region. Concerning the imaginary potential,the picture is less clear and this potential is welldetermined only near the strong absorption radius.As a first conclusion, the striking feature ofintermediate energy elastic scattering is that thedata allow a precise determination of the realpotential on a rather wide domain correspondingto a strong overlap of the two colliding nuclei.

Our data at 94 MeV/u completes the numerousdata that exist at low energy for the same systems.Therefore they allow to study the energy dependenceof the interaction potentials from the Coulombbarrier to 100 MeV/u. Fig. 3 displays for differentsystems the evolution of the normalization factor

1.5

LO-

10 100

Fig. 3 - Energy dependence of the normalizationfactor of the M3Y double folded potentials fordifferent systems. The lines are to guide the eye.

N which has to be applied to M3Y double foldedpotentials in order to fit the data. As these potentialshave a very small intrinsic energy dependence, Fig. 3reflects the evolution of the real potential. Theincrease observed for H>o + 208pb n ear the Coulombbarrier is related, through the dispersion relation,to the rapid variation of W at the barrier7). Beyongthis energy, a regular decrease of N is observed,of roughly 40 % from 10 to 100 MeV/u. This decreaseis in good agreement with previous results obtainedwith heavier systems8' (Ar + Ni, Sn, Pb). The variationof the imaginary potential with energy is less wellknown. At every energy it is determined only ina very small region near the strong absorption radius.As this radius decreases with increasing energy,it is difficult to disentangle the variation of thepotential strength from that originating in thevariation of rj/2- However the general trend is alsoa smooth decrease of the imaginary potential inthe region where it is determined. This decreaseof both the real and imaginary potentials is indisagreement with the results of microscopiccalculations2 '3 '4 ' which predict a strong increaseof nuclear strength in the considered energy range.The apparent failure of these models may be dueto collective modes of excitation no'; included inthe calculations and which can modify significantlynot only the imaginary but also the real potentialthrough coupled channel effec 1; Complete inelasticscattering data over a large range of bombardingenergy could be essential to address this questionand to assure a more detailed comparison betweenexperiments and theory.

References1) M. Buenerd et al., Nucl. Phys. A424 (1984) 313.2) A. Faessler et al., Nucl. Phys. A428 (1984) 271c.3) B. Bonin, J. Phys. 48 (1987) 1479.4) R. Sartor and Fl. Stancu, Nucl. Phys. A404,

(1983) 392.5) G.R. Satchler and W.G. Love, Phys. Reports 55C

(1979) 183.6) J.G. Cramer and R.M. DeVries, Phys. Rev.

C22 (1980) 91.7) M.A. Nagarajan et al., Phys. Rev. Lett. 59 (1985)

1186.8) N. Alamanos et al., Phys. Lett. 137B (1984) 37.

References and contributions to Conferences relatedto these experiments- N. Alamanos et al., Phys. Lett. 137B (1984) 37.- P. Roussel et al., Phys. Rev. Lett. 54 (1985) 1779.- P. Roussel et al., Phys. Lett. 185B (1987) 29.- P. Roussel-Chomaz et al., Nucl. Phys. A477

(1988)345.

- N. Alamanos et al., BAPS 30 (1985).- N. Alamanos et al. Proc. XXIH Intern. Winter

Meeting on Nucl. Phys., Bormio (1985) 717.- N. Alamanos et al., Proc. Second Intern. Conf.

on Nucleus-Nucleus Collisions, Visby (1385) 36.- P. Roussel et al., Proc. Intern. Conf. on Heavy

Ion Nucl. Coll. in the Fermi Energy Domain,Caen (1986) 6.

- P. Roussel et al. Proc. Intern. Symp. on MagneticSpactrograph, East Lansing (1989), to be published.

EXCITATION OF GIANT RESONANCES IN wZr AND 208P6 BY INELASTICSCATTERING OF HEAVY IONS AT GANIL ENERGIES

D. Beaumel, Y. Blumenfeld, Ph. Cliomaz* N. Fraacaria, J.P. Garron,J.C. Roynette, J. A. Scarpaci, T. Suomijarvi

Institut de Physique Nuellaire, 91406 Orsay Cedex, FranceJ. Barrette, B. Fernandez, J. Gastebois, P. Roussel-Chomac

DPh-N-BE, GEN Saclay, 91191 Gif-sur-Yvette Cedex, FranceW. Mittig

GANIL, BP 5027, 14021 Caen Cedex, France

Giant resonances were observed for the firsttime about 40 years ago and since then theyhave been intensively investigated by the in-elastic scattering of electrons, protons andalpha-particles. Since the advent of interme-diate energy heavy ion beams the use of in-elastic scattering of heavy projectiles to excitegiant resonances has opened several new per-spectives for these studies. First, the strongforward focusing of the reaction products leadsto an increase of the inelastic scattering differ-ential cross section. Moreover, the increase ofthe strength of the Coulomb interaction en-hances the excitation pf the isovector reso-nances like the GDR. Also high multipole res-onances are expected to be strongly excited inheavy ion collisions.

In order to study the properties of giantresonances excited by inelastic scattering ofheavy ions we have bombarded °°Zr and20SPb targets with 20Ne and *°Ar projectilesat 40 MeV/u [1,2,3,4] and 2 0 8P6 target with86 Kr projectiles at the incident energy of 43MeV/u [5,6j. These experiments were per-formed at Ganil using the magnetic spectrom-eter SPEG whose good energy and angular res-olution has allowed an accurate analysis of thedata.

An inelastic spectrum obtained for 20Ne+20BPb is presented in fig. 1. The gi-ant resonance bump can be seen at about10 MeV of excitation energy with a peak-to-background ratio of 2:1. The main con-tribution to the physical background comes

10E'(MeV)

Figure 1: Inelastic spectrum from20ATe+308P6at 40 MeV/u.

'Division Physique Theorique, laboratoire aasociS auCNRS

IS)

8 10

6 C M (deg.)

Figure 2: Theoretical angular distributions forat 40 MeV/u.

from the knock-out reactions 'which in the caseof heavy projectiles decreases yielding goodpeak-to-background ratios.

The resonance bump generally contains sev-eral different multipolarity components whichcan be separated by their angular distribu-tions. When the projectile size increases themultipole sensitivity of angular distributionsdecreases. Nevertheless, the interference be-tween nuclear and Coulomb interaction yieldsangular distributions characteristic of the mul-tipolarity. The angular distributions of differ-ent multipolarities for 20Nc+208Pb calculatedwith the code ECIS [7] can be seen in fig 2.A striking feature shown on this figure is thevery high differential cross section (about 1barn/sr). Note also the very strong excita-tion of the isovector GDR excited purely bythe Coulomb interaction.

The multipole analysis for the 207Ve and*°Ar data was done by cutting the inelas-tic spectrum into small energy bins and fit-ting the angular distribution of each binby a linear combination of theoretical angu-lar distributions of different multipole transi-tions. The cross section distributions of dif-

10 125 15E* (MeV)

Figure 3: Multipole decomposition of the giantresonance bump in 208.P6.

ferent resonances extracted by this methodfor 2°./V«+208P6 data are presented in fig. 3.Besides the GDR and GQR strength a smallamount (20% of the EWSR) of 1=4 strengthwas also extracted in 2 0 8 P6. Another impor-tant feature to notice in this figure is the GDRcross section which is spread over a large ex-citation energy region. This shape is due tothe Coulomb excitation probability which de-creases exponentially as a function of excita-tion energy. In fig. 4. the cross section distri-butions of the GDR in °°Zr and 208Pfc fromphotoabsorption [8] (Lorentt-curves, dashedlines) are compared to those obtained withinelastic scattering of y°Ne and *°Ar projec-tiles (histograms). When the photoabsorptionstrength is multiplied by the Coulomb excita-tion probability the cross section distributionsobtained with heavy projectiles are well repro-duced (full lines).

In the case of 4(Mr and *GKr data the de-formation effect in the GDR cross section canalso be seen. Furthermore, with all three pro-jectiles the GDR dominates the GQR in 2 0 8 P6.With 20JVe projectiles the GQR cross sectionis still the largest component of the resonancebump in the °"Zr spectrum but with *°Ar theGDR becomes dominating also in zirconium.

The use of intermediate energy heavy ionsyields good peak-to-background ratios andlarge cross sections for giant resonances whichmakes heavy ions an interesting probe for thestudy of the decay properties of these reso-

|5j P. Roussel-Chomar et al., Phys.Lett.B209(1983)187.

[6] P. Roussel-Chomaz et al., XXVI In-ternational Winter Meeting on NuclearPhysics, Bormio (Italy), 25-30 January1988.

|7) J. Raynal, CEN de Saclay, Report Dph-T/7148, 1971.J. Raynal, Phys. Rev. C23 (1981) 2571.

Figure 4: The experimental GDR cross sectiondistributions in g°Zr and 2O8P6 obtained with2 0 Ne and *°Ar projectiles (histograms) com-pared to the photoabsorption data (dashedlines) and to the photoabsorption strengthcorrected for the excitation probability (fulllines).

|8] B.L. Herman and S.C. Fulti, Rev. Mod.Phya. 77 (1975) 713.

nances. Furthermore, we have shown that anaccurate multipole decomposition is possiblewhen very high resolution detection systemssuch as the SPEC spectrometer are used. Inhe-vy ion collisions the Coulomb excitation isimportant favouring isovector resonances andthe very strong excitation energy dependenceof the Coulomb excitation can deform thecross section distribution of the GDR.

References

[1] T. Suomijarvi et al., Nucl.Phys, in press.

[2] Y. Blumenfeld et al., XXVI InternationalWinter Meeting on Nuclear Physics, Bormio(Italy), 25-30 January 1988.

[3] T. Suomijarvi et al., XXVII InternationalWinter Meeting on Nuclear Physics, Bormio(Italy), 23-27 January 1989.

[4] D. Beaumel, Thesis, Univ. Paris XI, Or-say.

INVESTIGATION OF THE SPIN-ISOSPIN-FLIP STRENGTH UITH THE (12C, ' "^-REACTION

W. von Oertzen

Hahn-Meitner-Institut, D-1000 Berlin 39, West Germany

(HMI,Berlin - GANIL.Caen - ISN.Grenoble - Univ. MiJnchen - DPhBE,Saclay - INS,Tokyo - Collab. E85a)

1. Motivation

The charge exchange reaction ( C, N) has two

characteristic features: (i) Isospin-flip AT=1

with T f = T+l in the final nucleus (T isospin

of the target). It corresponds to the (n,p)-

reaction, but a much better resolution is

achieved, (li) Spin-flip AS= 1, induced by the

transition 12C(0*) => 12N(1+) in the projec-

tile. The sensitivity to spin-flip transi-

tions has been already demonstrated [11. The

12C-nucleus is the lowest mass projectile to

induce a charge exchange reaction with the

abov'j characteristics, (except the (d,2p)-re-

action). It is well suited to study the spin-

isospin-flip strength in the T -channel, e.g.

the spin-dipole resonance. The 1 N-ejectile is

particle stable only in the ground state, and

the transition strength in the projectile

12 12,,N (B(GT) = 0.93) is quantitative-qs gs

ly described by the wave function of ref. [2).

2. The reaction mechanism

The charge exchange reaction with a complex

projectile such as 12C can 'proceed via two

different mechanisms: (i) the direct mechanism

via the NN-interaction (meson exchange), and

(ii) the two-step mechanism with the transfer

of a proton from the target to the projectile

and the transfer of a neutron in the opposite

direction. We have studied the energy depen-

dence of these two mechanisms on the re-

target with transitions to resolved states of

B. Angular distributions have been measured

at Ganil/Caen for E/a = 70 MeV/u and at

Vicksi/Berlin for 30 MeV/u. Calculations have

been performed for both mechanisms and a

quantitative description of the data could be

achieved by the coherent sum (fig.1). The NN-

interaction of Anantaraman et al. [3] has been

tween the interactions of [31 and of Franey

and Love [4], Tensor and exchange terms have

been included. The two-step amplitudes are

calculated in the exact-finite-range formalism

with spectroscopic factors of the particle

states (for the neutron) or hole states (for

the proton) in the intermediate channels.

Systematic calculations on this basis show

that the two-step mechanism has a maximum at

about 30 MeV/n and decreases for higher ener-

gies, while the direct mechanism rises further

and dominates at 70 MeV/u [5]. The angular

0 U 8 12 16 200,

used for the direct mechanism for E 10 MeV

and, at higher energies the interpolation be-

CM

Fig. 1:

Angular distributions of the I2C(12C,12N)12B

reaction to the ground state of 12B at diffe-

rent incident energies. The lines represent

calculations of the direct charge exchange

(dashed-dotted lines), the proton-neutron ex-

change (two-step process, dashed lines) and

the coherent sum of both (solid lines).

distributions of the direct process have a

steeper slope at large angles than the two-

step-process. Therefore it is important to

extract the transition strength only at small

angles.

3. The spin-lsospln strength distribution

The (12C,12N)-reaction has been measured at 70

MeV/u in a range of target masses between C

and 208Pb. A typical spectrum is shown in fig.

2 (upper part) for the 40Ca-target. The strong

line at the ground state is due to the

p(12C,12N)n reaction on the hydrogen contami-

nant in the target. The pronounced structure

between E = 7 and 13 MeV corresponds to the

spin-dipole resonance. To obtain its strength

we have assumed that the shape of the back-

ground corresponds to the shape of the spec-

trum at 2.5° (central part of fig.2), where

the dipole angular distribution has a mini-

mum. The difference is shown in the lower part

of fig. 2. Targets with increasing mass number

show a decreasing spin-dipole strength, as

expected from the blocking of the ft -strength

due to the neutron excess [6]. The 'back-

ground' corresponds to the spin-isospin flip

strength of higher multipolarities (L£2). RPA-

calculations are now performed up to E =* 50

MeV for the strength distributions of L = 0 up

to L = 5. They will be folded with DWBA-calcu-

lations and compared with the measured spectra

and, for resolved states, with the angular

distributions.

[1] H. G. Bohlen et al.. Nucl. Phys. A488

(1988) 357c

[2] J. S. Winfleld et al., Phys. Rev. C33

(1986) 1333

S. Cohen and D. Kurath, Nucl. Phys. 101

(1967) 1

[3] N. Anantaraman et al. , Nucl. Phys. A398

(1982) 269

[4] M. A. Franey and W. G. Love, Phys. Rev.

C31 (1985) 488

[5] H. Lenske et al., Phys. Rev. Lett. 62

(1989) 1457

[6] C. Gaarde et al., Nucl. Phys. A369 (1981)

258

toCa[12C,12NJMIKE = 70MeV/u9L=0.0°±0.5°

0 5 1 0 1 5 Z 0 2 3 » 3 S « « »

2.5"±0.50

H-subtracted

0 3 1 0 1 S Z O S 3 » 3 9 0 > < 3 9 0

A=(0.00-2.5°x0.8)

30 '0 SO

Fig. 2:

Spectra of the (12C, t2N)-reaction on *°Ca at

0° (upper part) and 2.5° (central part) and

their difference (lower part). The 2.5°-spec-

trum has been normalized by a factor of 0.8 to

the high excitation energy region of the 0°-

spectrum. The spin-dipoie resonance is seen at

about 10 MeV excitation energy.

CHARGE EXCHANGE REACTIONS TO PROBE THE ISOVECTOR ELECTRICNUCLEAR RESPONSE

C. Berat, M. Buenerd, P. Martin, J.Y. Hostachy, J. Chauvin, D. Lebrun, (ISN GRENOBLE),J. Barretts, B. Berthier, B. Fernandez, A. Miczaika (CEN SACLAY), W. Mittig, A.Vilari (GANIL)

G. Bohlen, S. Stiliaris, and W.V. Oertzen (HMI BERLIN).

I - MotivationsOur present knowledge on the electric

isovector nuclear response is limited mainlybecause of the lack of a suitable probe. Thecharge-exchange reactions, which haveprovided an impressive body of new data onthe isovector spin modes by means of theoutstanding selectivity of the (p,n) reaction,are also potentially good candidates for anexploration of the electric modes, providedone can find a reaction featuring therequired selectivity.

The aim of the present programm wasto test the (13C,13N) reaction at 50 MeV/u asa probe of the electric isovector response ofnuclei, on its ability to excite the analog ofthe well known giant dipole resonance(IGDR), with the further purpose ofinvestigating the isovector monopole mode(IGMR) of nuclei. This particular reactionwas chosen because: 1) - the transition inthe projectile is (p1/2)p -> (P1/2)n, allowingnon spin-flip AS=O transitions in the target.In addition, the ratio of the Fermi to theGamow-Teller matrix elements is about 5,ensuring a dominance of AS=0 over AS=1(spin flip) transitions. 2) - the incident energyis low enough to ensure an acceptablemagnitude of the electric isovector effectiveinteraction which excite the transition.

II - ExperimentThe measurements have been

performed using the magnetic spectrometerSPEG equipped with its standard detectionplus a plastic scintillator providing residualenergy measurement and a fast timingsignal for time of flight measurement. Datacould be taken with the spectrometer setaround zero degree. The beam entering thefull ± 2° horizontal and vertical angularacceptance, was stopped on a faraday cupbetween the two dipoles.

III - Dipole excitationSix targets have been studied in two

runs: 12C,40Ca,58Ni,6C>Ni,90Zr,120Sn,2°8pbover an angular range © = 0-5°. Figure 1shows the angle integrated spectrameasured on the light targets2). Above smallpeaks corresponding to low excitationenergy transitions, the spectra exhibit acommon distinct feature : they are alldominated by a single, strongly excitedpeak, corresponding to the analog state of

the IGDR. The excitation energy and widthobtained for the IGDR are given in the table.

( 1 3 C , 13N) E| . b . 850 U.V

SCO .

Cibte:12C

40 30 20 10 0

Cible:4°Ca

oLSO 10 30 20 10 0

1

Cibte:MNIe = o-3dtg.

50 40 30 20 10 0

figure 1 - spectra on light nuclei

Note that in 4 0K, the results aremarkedly below those measured in the(7i+,7t°) reaction 1). In 58Co and in 60Co thestrength is splitted into two components.The dominance of the IGDR excitationestablishes for this reaction the status ofisovector electric probe of the nuclearresponse, and certainly opens the prospectof a possible future systematic study of thismode.

table 1-measured dipole excitations

Nucleus Ex(MeV) T(MeV)

12B4 0 K

58CO

7.710.11 1 .5+0.310.6+0.31 2.8±0.3

1.9+0.13.1±0.21 .9+0.21 .5+0.3

IV - Monopole excitationIn heavier nuclei, the increasing

nuclear asymmetry is expected to inhibitprogressively the (1 ho) IGDR transition,whereas the (21Tw) IGMR excitation is less

50 40 30 20E> (MeV)

figure 2 - Spectra on medium mass andheavy nuclei

table 2 - monopole transitions (candidates)

Nucleus Ex(MeV) r(MeV)

90yI20|n

208JI

22.1 ±0.818.7±1.614.7±0.916.5±0.6

8.1±14.6±0.84.1+0.83.1 ±0.5

rapidly inhibited. Figure 2 shows the spectrameasured on the heavier nuclei studied3).In 60Co, one observes above the IGDR, awide peak sitting at the same Ex as theIGMR observed in ref.1. A similar situation isobserved for 90Zr and 12°Sn. In 2 0 8TIhowever, the structure observed is verydifferent from that seen in ref. 1.

V - Angular distributionsThe angular distributions measured for

the various transitions observed reveal thatan unexpectedly large component of two-step processes contributes to the cross-section and washes out the diffractivepattern of the single-step contribution2.3).This feature introduces seriouscomplications for the quantitative analysis ofthe data.

VI - Symmetric reactionThe complementary reaction

(13C,13B) which has also been studied doesnot show the same selectivity because thetransition in the projectile (p3/2)p -» (p1/2)n,is dominated by AS=1. The measuredspectra are correspondingly dominated bythe excitation of the spin-isospin strength3)-

figure 3 - Spectrum of the symmetricreaction on 12C.

1) A. Erell et al. Phys. Rev. C34 (1986) 1822

2) C.Berat et al., to appear in Phys.Lett.3) C.Berat, these, Universite de Grenoble,nov.1988. C.Berat et al., in preparation forNucl.Phys. A.

Isoscalar Giant resonances at finite temperature

E. Suraud1, M. Barranco*.2 and J. Treiner**

Institut des Sciences Nucleates, 53 Avenue des Martyrs, F38026 Grenoble cedex,

*) Departament d'Estructura i Constituents de la Materia, Facultat de Fisica, Universitatde Barcelona, Diagonal 64, SP-08028 Barcelona, Spain

) Division de Physique Theorique, Institut de Physique Nucleaire, F91406 Orsaycedex, France

1) Present address : GANIL, BP5027, F14021 Caen cedex, France2) Present address : Departament de Fisica, Universitat de les Illes Balears, E-07071Palma de Mallorca, Spain

The study of nuclear collective motionbuilt on excited states is presently of challenginginterest both experimentally and theoretically[Ij.The proper description of such states is madedelicate by the metastability of hot nuclei.However, at not too high temperatures it remainsreasonable to picture a hot nucleus embedded inan external gas generated during the vibration. Aphysically sounded description has hence toproperly take into account this gas contributionin order to prevent the excitation of spuriouszero hto states, as was recently shown in theframework of Time Dependant Thomas-Fermicalculations [2]. By using a semi-classical version[3] of the Bonche, Levit and Vautherinprescription (BLV,[4]) for describing hot nuclei,we have hence developed a sum rule approachfor isoscalar giant resonances at finitetemperature [5]. The BLV prescription allows tohandle properly with the continuum and sumrules give a nice description of the grossproperties (centroids, widths) of resonances,even in the finite temperature case [6].

By using the semi-classical expressionsfor the moments m - i .m i and m 3 of thestrength function, one can define the two standartestimates of the resonance energy Ei=Vm i/m.iand £3= V m 3/m i. Our calculations have beenperformed with the Skyrme SKM* interaction,together with a phenomenological ExtendedThomas Fermi approximation for the kineticenergy density. The latter functional gives resultsin very good agreement both with zerotemperature RPA sum rules [6] and with theoriginal BLV finite temperature Hartree-Fockresults [4].

The importance of properly subtractingthe continuum contribution is demonstrated inFigure 1 were the monopole E] and £3 energies

20

10 E,90 Zr

Figure 1Values of non-subtracted (dashed line for box size R=14fm and dashed-dotted line for R=12 fm) and subtracted(full line) monopole El and £3 energies (in MeV) as afunction of the temperature T (in MeV) in the case of 9 0

Zr.

are plotted versus the temperature for a 9 0 Zrnucleus. One sees that even at low temperatures(T = 1 MeV) continuum may play a spuriousrole, with, in particular, a strong dependence onthe size of the box in which calculations areperformed. The same spurious effect is alsofound for higher multipolarities and disappearsin the BLV formalism.

In figure 2 are shown the temperatureevolution of the monopole EI and £3 energies ofvarious nuclei. Note the very smooth variation ofthese energies with the temperature. The samecomment holds for Figure 3 in which the £3energy is plotted versus temperature for / =2,2and 4 multipolarities. Note however thattemperature effects are all the less important forlow multipolarities and big nuclei. The firsttrend reflects the fact that it is easy to smooth thenumerous wiggles present in large 1 deformed

10

20

10

•20

10

20 L

10

•Pb

"Sn

" Z r

8

Figure 2Variations of the E I (dashed line) and £3 (full line)energies (in MeV) of the monopole resonance as a functionof the temperature T (in MeV) for the 3 nuclei90 Zr, 120Sn and 2 " 8 Pb. Note that E I and £3 stay very closetogether, which presumably indicates a small spreading ofthe mode.

10

FigureSVariations of the £3 energies (in MeV) of the / = 2,3 and 4giant resonances in 90 Zr (dashed-dotted line), 120 Sn(dashed line) and 208 pD (full line) as a function of thetemperature T (in MeV).

Fermi spheres. The second one is bound to thesensitivity of surface to temperature, surfaceplaying a more important role in light nuclei, ascompared to bigger ones.

In this work we have applied a semi-classical approximation of the ELV subtractionprocedure to describe Giant Resonances at finitetemperature. We have demonstrated thatcontinuum effects play a very important roleeven at moderate temperatures and should hencebe properly taken into account. The temperaturedependence of the isoscalar collective modes wehave studied is very smooth with small variationswith the mass of the nucleus and themultipolarity of the mode.

References

[1] K.A. Snover, Ann. Rev. Nucl. Part. Sci.vol36, 1986 and references therein[2] M. Barranco, J. Nemeth, Ch. Ngo and E.Tomasi, Nucl. Phys. A464 (1987) 29[3] E. Suraud, Nucl. Phys. A462 (1987) 109[4] P. Bonche, S. Levit and D. Vautherin, Nucl.Phys. A427 (1984) 278[5] E. Suraud, M. Barranco and J. Treiner, Nucl.Phys. A 480 (1988) 29[6] M. Barranco, A. Polls and J. Martorell, Nucl.Phys. A444 (1985) 445

11

HIGH EXCITATION ENERGY STRUCTURES IN HEAVY ION COLLISIONS ATINTERMEDIATE ENERGY

D. Beaumel, Y. Blumenfeld, Ph. Chomaz* N. Frascaria, J.P. Garron,J.C. Jacmart, J.C. Roynette, J. A. Scarpaci, T. SuoniijarviInstitut de Physique NucUaire, 91406 Orsay Cedex, France

J. Barrette, B. Berthier, B. Fernandez, J. Gastebois ,P. Roussel-ChomacDPh-N-BE, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France

W. MittigGANIL, BP 5027, 14021 Caen Cedex, France

A few years ago , experimental evidence forthe presence of simple structures at very highexcitation energy in the inelastic and transferchannels of heavy ion reactions at 10 MeV/uwas presented [1]. Since the data suggestedthat the structures were excited through a di-rect process, an interpretation in terms of col-lective modes was tentatively advanced. Theseresults immediately prompted a large amountof theoretical work. A consistent representa-tion of the data was given by the multiphononcalculation [2] which supposes the excitation ofmultiphonon states built mainly with the gi-ant quadrupole resonance. One prediction ofthis model was that heavy projectiles with anincident energy lying around 30 to 50 MeV/uwould be the optimal probes for the excitationof multiphonon states through the nuclear in-teraction.

In the light of these results we undertook atGANIL a new generation of experiments at in-termediate energy. Because of the experimen-tal difficulties connected with the observationof this low cross section phenomenon, a verycareful study of these structures was carriedout. To get a deeper insight into the mecha-nism responsible for the structures, systematicstudies as a function of the target mass andof incident energy were undertaken using theArgon beam delivered by the GANIL facility[SJ.The 40Ar + 20*Pb, 1 2 05n, s°Zr and 4UCaexperiments were performed at 44 MeV/u andthe *°Ar + wZr at 33 MeV/u for differentangles close to the grazing . Special care wastaken to obtain an unambiguous charge andmass identification. A time of flight spectrom-eter using a microchannel plates system with-out any grid on the trajectory of the ions wasspecially built for this purpose (4j. In this

'Division de Physique theorique, laboratoire sssoeitau CNRS

first set of experiments, high excitation en-ergy structures were observed up to 70 MeVexcitation energy in the inelastic channel ofthe four studied reactions. In order to quan-tify the conclusions concerning the existence,energies, widths and angular evolution of thestructures double Fourier transform, autocor-relation and cross correlation analyses werecarried out on the inelastic spectra.The com-parison of the spectra from the four differenttargets shows that the excitation energies ofthe structures depend on target mass. Theremarkable conclusion is that a coherent de-scription of all the excitation energies can beobtained by supposing a target mass depen-dence of the form Ex ~ A~l/3.

In the studied reactions, three body pro-cesses such as the decay of pick up channels("pick up break up reactions") are expected tocontribute strongly to the inelastic cross sec-tion in the region where the structures are ob-served. Thus, a complete knowledge of sucha contribution is necessary. Extensive calcu-lations [5] have been carried out for all thestudied reactions The calculation reproducesvery well the shape of the physical backgroundbut the cross section is difficult to evaluatebecause a precise knowledge of the transfercross section to the continuum would be re-quired. From this work, several conclusionswere drawn:i)at GANIL energies the dominantpick up break up contribution to the inelasticchannel is due to one nucleon transfer reac-tions ii)Tlie contribution to the spectra of thispick up break up process can be very differ-ent for different projectiles. If many levels arepopulated in the ejectile, the pick up breakup contribution will have the shape of a broadplateau. If one level is preferentially populatedin the ejectile the pick up break up contribu-tion from this level can be split into two nar-

12

I7SO

S2SO

3500

40 A r + 90 Z r

E j n c = UMeV/u

17' < 3 < 4.7°

Ar

59Ev=66MeV

37t

!#

67SOO>

80 60 ' 0 20Ex( MeV)

Figure 1: wAr on °"Zr inelastic spectrum.In full line the complete spectrum representedwith 200 keV/channel bins. The high excita-tion energy part of the spectrum is also dis-played with a COO keV/channel binning andan expanded vertical scale.

row components superimposed on the plateaudue to the other excited levels. Such a patternis expected for certain light ejectiles where lev-els are very sparce above the emission thresh-old.In both cases the apparent excitation en-ergy of the centroid of the pick up break upcontribution depends linearly on the bombard-ing energy per nucleon. Consequently a cleanmethod to conclude if the structures observedexperimentally are due to target excitation orto pick up break up processes is to study agiven system at two slightly different incidentenergies .

Since 1986 a new set of experiments hasbeen performed at GANIL using the magneticspectrometer SPEC. The aim of these exper-iments was to compare with high resolutionand high statistics the inelastic spectra fromthe same reaction studied at two different en-ergies and to compare these spectra witli theone obtained on the same target using a differ-ent projectile. Therefore we studied the 4(Mr+ 9"Zr and 4"/ir + 208Pfr reactions at 41MeV/u and 44 MeV/u and the 2°JVe + oaZrand 2"Ne -f 208P6 reactions at 40 MeV/u (seefor example fig.l).

In the first experiment [6], the bombardingenergy was varied by 3 MeV/u which corre-sponds to an expected shift of the pick up

><a

^inF—

;zoo2000-

1500-

1000-

uooo-

13000-

12000-

fi0Ar+208Pb /

^A rM i

Ex=50

E- -UMeV/u

" / *^ /

X

E jnc=41MeV/u

,.

»t

/

/

I

5 si2 *

ifr ji j

T

r ,» *

4 IMeV f IC 1 J ti

1 f I v/' f

V

A

^

V

v^

$ '' '

^

f

iffy

_70 50 30

E x (MeV)

Figure 2: Comparison of the 4 (Mr+208P6 in-elastic spectra at 41 MeV/u and 44 MeV/u.

break up component by 3 MeV which is ap-proximately half of the mean interval betweentwo observed bumps. Fig.2 displays the twoinelastic spectra from the 4{Mr + 2"8/J6 reac-tion at 41 and 44 MeV/u.

Although the centroid of the broad plateauunder the bumps is shifted by approximately3 MeV as expected from a pick up break upcalculation, the narrow structures show up atthe same excitation energy in both spectra. Inthe case of the 4"Ar + °"Zr experiment thebumps are clearly observed at 44 MeV/u butat 41 MeV/u important problems of linearityof the drift chambers were encountered and acorrection function had to be applied to theinelastic spectra. Such a correction induceslarge error bars but nevertheless the resultsare incompatible with a 3 MeV shift of thebumps between 41 and 44 MeV/u.

As dicussed before the choice of the pro-jecti'» in such studies is of great importance.To get more information on the influence ofthe projectile in these experiments the 207Ve

and 2I)8P6 reactions have been stud-

13

ied at GANIL at 40 MeV/u. We have firstcompared the inelastic spectra from these re-actions with those obtained at MSU at lowerincident energies(25 MeV/u and 30 MeV/u)on the same systems [7|. At these lower ener-gies the 208Pb spectra exhibit two clear bumpssuperimposed on the pick up break up plateauwhich are shifted by 5 MeV between the twoincident energies, as predicted from a pick upbreak up calculation for a light ejectile, whilein the zirconium spectra, only very small os-cillations are observed on the pick up break upplateau. The comparison of the 40 MeV/u in-elastic spectra with those obtained at 25 and30 MeV/u in °°Zr and 2 0 8 f 6 shows that theimportance of the pick up break up plateau hasdiminished, thus reflecting the decrease of thetransfer reaction probability with increasingbombarding energy. No clear double humpedstructure is visible in the 2"Ne + 208Pb spec-trum at 40 MeV/u but nevertheless no clearconclusions can be drawn from the comparisonwitJi the 40>4r + 2 0 8P6 spectra. This resultsuggests that in that case the contributions oftarget excitation and pick up break up pro-cess are strongly mixed. Conversely the mNe+ 0<)Zr reaction inelastic spectrum presentsclear bumps which are exactly at the sameposition as the ones observed in the *°Ar +oaZr reaction studied at 33 MeV/u and 44MeV/u, Fig 3 recapitulates all these inelasticspectra .The shape of the background is verydifferent from one experiment to jnother.Thisevolution is well understood by the pick upbreak up calculation.Despite these widely dif-ferent background shapes, structures appearat the same excitation energy in all spectra.This very complete set of data on the zirco-nium target allows to conclude to excitationsof the target around 50 MeV excitation energy.More systematic work is needed qn other tar-gets to precise the position, width and angularevolution of the structures.

The study of inclusive inelastic spectra attwo different energies allows to distinguish be-tween the target excitation and pick up breakup process for the interpretation of the struc-tures but cannot yield the respective contribu-tion of the two mechanisms to the total inelas-tic cross section. Very recently, we have stud-ied the inelastically scattered 4"Ca on *°Ca at50 MeV/u in coincidence with light chargedparticles using Csl detectors at forward an-gles, a plastic wall and E.AE silicon detectorsat backward angles. Preliminary results showthat the pick up break up is clearly seen in theforward detectors and the detection of back-

0)

t/5

Oo15000

12000-

9000-

10500-

9000-

7500-

1000-

1200-

600-

Ex = 59MeV50

44.5 f+

I,,'40MeV/u N^SPEG »V

X

44MeV/u Ar.SPEG

«* !

/ (//»x44Me% Ar.T.QF

fV

^ < /

33MeV/utwArjT.QF

>

*

70Ex(MeV)

50 30

Figure 3: g°Zr inelastic spectra obtained atdifferent incident energies, with different ex-perimental set ups and different projectiles.

14

ward emitted light particles allows to selecttarget excitations.These first results are verypromising and show that light particle heavyion coincidence experiments should provide aunique tool to disentangle the target excita-tion and the pick up break up contributionsand represent a further step towards the un-derstanding of such complex reaction mecha-nisms.

References

|1] N.Frascaria et al. Z. Phys. A294 (1980)167Ph. Chomaz et al. Z. Phys. A318 (1984)167

[2| Ph.Chomaz et al. Z. Phys. A319 (1984)167

[3| N.Frascaria et al. Nucl. Phys. A474(1987) 253Y.Blumenfeld These de Doctoral d'Etat,Orsay 1987

J.C.Roynette et al. HICOFED Caen1986, p9N.Frascaria XXIV Int. Winter Meet.Bormio, Italy 1986, p71

[4| E.G. Pollacco et al. Nucl.Inst. and Meth.225 (1984) 41

(5] Y.Blumenfeld al.(1985) 151

Nucl. Phys. A445

|6] D.Beaumel These Orsay 1988N.Frascaria First topic. Meet, on Gi-ant Res. Excit. in Heavy Ion Collisions,Legnaro, Italy 1987, Nucl. Phys. A482(1988) 245cY.Blumenfeld et al. XXVI Int. WinterMeet. Bormio, Italy 1988, p67N.Frascaria RIKEN IN2P3 Symp.on HeavyIon Collisions, Shimoda Japan 1987

|7) S.Fortier et al. Phys. Rev. C36 (1987)183

15

Volume 139B, number 4 PHYSICS LETTERS 17 May 1984

MULTIPHONON EXCITATIONS IN HEAVY ION GRAZING COLLISIONS

Ph. CHOMAZ

Division de Recherche Experimentale, Institut de Physique ffucleaire, 91406 Orsay Cedex, France

and

D. VAUTHERBMDivision de Physique Theorique *, Institut de Physique Nudeaire, 91406 Orsay Cedex, France

Received 7 November 1983Revised manuscript received 20 February 1984

A semiclassical model is used to study the excitation of giant resonances in heavy ion grazing collisions. The projectile isdescribed as a moving Woods-Saxon potential with a fixed shape, and the evolution of the target state is calculated bytime-dependent perturbation theory. Using random phase approximation wave functions, probabilities to excite various re-sonances arc obtained. Multiple phonon excitations appear as the possible mechanism for the structures observed in heavyion collisions.

CONTRIBUTION AS SENT BY THE AUTHOR

16

Nuclear Physics A476 (1988) 125-158North-Holland, Amsterdam

COLLECTIVE EXCITATIONS OF CLOSED SHELL NUCLEIIN HEAVY ION GRAZING COLLISIONS*

Ph. CHOMAZ and NGUYEN VAN GIAF

Institut de Physique Nucleaire, Division de Physique Theorique**, 91406 Orsay Cedex, France

D. VAUTHERIN

Center for Theoretical Physics, Laboratory for Nuclear Science and Department of Physics, MassachusettsInstitute of Technology, Cambridge, Massachusetts 02139, USA

and Institut de Physique Nucleaire, Division de Physique Theorique**, 91406, Orsay, France

Received 29 April 1987

Abstract: The single and multiple excitations of giant resonances in heavy ion reactions is investigatedin the framework of a microscopic model. The scattering process is treated semi-classically as inthe Copenhagen modei but the target response is constructed from a microscopic RPA calculationusing Skyrme forces. The importance of an accurate description of excitation operators andtransition form factors is stressed. The connection with other approaches (time-dependent Hartree-Fock approximation, boson expansion method, second RPA) is discussed. The model is appliedto reactions involving 208Pb and '"'Ca nuclei. We conclude that, under near-grazing conditions,with medium or heavy projectiles and for incident energies around 30 MeV/nucleon, there is asubstantial probability of observing multiphonon states built on 1ha> giant resonances.

CONTRIBUTION AS SENT BY THE AUTHOR

17

EXCITATIONS IN GRAZING HEAVY ION REACTIONS

K. Dietrich* and K. Werner

Physikdepartment of the TUM, Garcbin«, FRG

* Guest at the GANIL, August 1984 - March 1985

Quite sometime ago, resonance-like .structures were observed in peripheralreactions between composite nuclei').Since then experiments were performedat various beam energies and for seve-ral projectile-target combinations,essentially supporting the originalobservations. It is a challenge tounderstand the origin of these struc-tures, given the fact that simple ex-planations based on subsequent parti-cle emission by the primary reactionproducts^) failed to explain the morecomprehensive data^).

The first question is whether theobserved bumps are due to the excita-tions of coherent states (giant reso-nances) or due to a concentration ofstrength of many incoherent excita-tions in the energy region of majorshells. The majority of authors takesthe first-mentioned stand-point i.e.interprets the observed structures asdue to single^) or multiple^) excita-tion of giant resonances of differentmultipolarity. The 2 n d possibility isinvestigated in refs. 6 and 7. Here itis assumed that independent and inco-herent p-h excitations (" excitons ")are generated during the collisioneither by tranfer of nucleons or byinelastic excitations. The importantparameters of the model are the pro-bability q that Ip-lh excitations aredue to transfer rather than inelasticexcitations, and the mean number z ofexcitons. Although the excitons areproduced independently from each other,a concentration of strength in inter-vals of major shells is obtained due tothe assumption that each exciton, ir-respectively of whether it is producedby transfer of a nucleon or by inelas-tic excitation, corresponds to a meanexcitation energy of one major shelldistance. If one takes into accountthat the levels of a major shell arenot exactly degenerate but distributedover, an energy interval of a typicalwidth F, and if one adopts a semi-classical description of the reaction,one obtains a closed expression forthe cross-section^'^) depending on the2 parameters q and z.

It turns out that a concentration'of strength in certain energy regionsis obtained for q-values close to 1,i.e. if transfer dominates over inelas-tic excitation. This is so since, forgiven net transfer of neutrons and ofprotons, the relative weight of trans-

fer and of inelastic excitations is dif-ferent for different exciton numbers.The data measured for ^°Ca on ^°Ca at abeam energy of 400 HeV^ could be reasona-bly well reproduced by choosing the rela-tive transfer probability q'l and themean exciton number Z=2 .

The distinction between the twomodels is complicated by the fact thatboth yield Poisson distributions for theexcitation probability of k modes andthat the width of a major shell and atypical giant resonance like the quadru-pole resonance are of the same order ofmagnitude ) .

An important difference is that themodel for incoherent exciton productionwould predict less pronounced bumps forreactions between two nuclei which. areboth far from shell closure compared toa scattering between two magic nuclei. Onthe other hand, giant resonances shouldbe excited in both cases with comparableintensity.

1.

D. Hilscher, J.R.A.D. Hoover, W.V.W.WA.CV.EJ.CYEH

Uilcke, J.R.

N. Frascaria, C. Stephan, P. Colombani,J.P. Garron, J.C. Jacmart, M. Riou,L. Tassan-Got, Phys . Rev. Lett. 39(1977) 918

Birkelund,Schroder,Huizenga,

Ilignerey, K.L. Wolf, H.F. Breuer,Viola, Phys. Rev. C20 (1979) 556Roynette, N. Frascaria,

Blumenfeld, J.C. Jacmart,Plagnol, J.P. Garron, A. Gamp,Fuchs, Z. Phys. A299 (1981) 73

J. Decharge, D. Gogny , B. Grammaticos,L. Sips, Phys. Rev. Lett. 4_9_ (1982)982 ; H. Flocard, M.S. Weiss, Phys.Lett. 105B (1981) 14 ; Nguyen Giai ,Phys. Lett. 105B (1981) IIPh. Chomaz, D. Vautherin, Phys. Lett.I39B (1984) 244 ; Ph. Chomaz, These3e Cycle, Universite de Paris Sud ,IPNO-T-84-01, F-91406 Orsay

Dietrich, K. Werner, GANIL P. 85-06Werner, Nucl. Phys. A453 (1986) 486

K.K.N. Frascaria, P. Colombani, A. Gamp,J.P. Garron, II. Riou, J.C. Roynette,C. Stephan, A. Ameaume, C. Bizard,J.L. Laville, 11. Louvel, Z. Phys.A294 (1980) 167

18

- A 2 -EXOTIC NUCLEI AND DECAY MODES

PRODUCTION AND IDENTIFICATION OF NEW ISOTOPES AT THE PROTON AND THE

NEUTRON DRIP-LINES

R. Anne1, A.G. Artukh2, D. Basin1, M. Bernas3, V. Borrel3, C. Dltras1, J. Galin3, D. Guerreau1,B.A. Gvoedev3, D. Guilleniaud-Mueller1, S.D. Hoath3, J.C. Jacmart3, D.X. Jiaag1, A.M. Kalinin3,

V.V. Kamanin2, V.B. Kutner2, M. Langevin3? M. Lewitowicz3, S.M. Lukyanov3, A.C. Mueller1,F. Naulin3, Nguyen Hoai Cliau2, Yu. E. Penionzhkevich2, E. Quiniou3, A. Richard3, E. Roeckl1,

M.G. Saint-Laurent1, W.D. Schmidt-Ott*

1) GAN1L, BP 5027, Caen, France2) Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, VSSR3) Institut de Physique Nucteaire, Orsny, France4) JI Physiknlisches Institut, Universitat Gottingen, F.R.G.

The search for new neutron-rich or proton-richnuclei has much benefited from the effectivenessof heavy-ion projectile fragmentation at relativisticenergies [1], Therefore it was tempting to try simi-lar experiments at Ganil. Establishing the limits ofparticle stability for nuclei has been the goal of a lotof experiments for many years. It is an importanttest of the validity of mass predictions based on thebulk properties of finite-charged nuclear matter oron the microscopic shell model.

The use of the fragmentation of different projec-tile lias allowed us to map the drip-line on bothsides of the valley of stability.

Mapping the neutron drip-line

Using the 40>lr beam at 44 MeV/u on a tanta-lum target, selecting the projectile-like fragmentswith the doubly achromatic spectrometer LISE-atzero degree |2| and identifying the nuclei through&E.E, time-of-flight measurements, it was possibleto produce for the first time the new isotopes 22C*(Tz = +5), 23W (Tz = 9/2) and the two isotopes™Ne, 3"Ne [3,4).

It was also possible to prove the particle unsta-bility of IBB, 2 1C, 2 6O. Using the mass predictionformulae of Garvey Kelson |5] and Uno-Yamada [6]in the case of Boron isotopes, 10B is predicted tobe bound against neutron emission while ltB andmB are unbound against one neutron emission ; allBoron isotopes with A>21 unbound against two-neutron emission. The observed isotopes (fig. 1)

are consistent with these predictions.

H N N.«/•„*•;<•,»*,

•Be

'deecntedon April 11, 1985

200 300 tOO 500

To I Ichmtlsl

Fig. 1 : Two dimensional representation of eventsZ versus time of Sight.

For the Carbon isotopes the same odd-even effecton the stability is predicted, 2 2C being the last par-ticle bound isotope while 21<7 is particle unstable,again in agreement with our results. For the Nitro-gen isotopes, 23JV is observed ; no counts of a*JVare present which is in accordance with its particleunstability prediction.

For Neon isotopes, 29Ne was predicted particleunstable by the mass-formulae with the exceptionof the Uno-Yamada constant-shell term.

The next step of these experiments was madeby fragmentating a 48Ca beam at 55 MeV/u on atantalum target (7). In this experiment the hewisotopes 2 9 F, 36-36M<7, 38-35Al, 4 ( M 1St, «-"4P,

19

4G.40.475i 40.47.48.40C-j a n d 49,60.61^,. ,

served for the first time (fig. 2). The comparisonof these results with the one obtained at Berkeley[8] at relativistic energy shows progress of typicallytwo isotopes.

Uj

01

«s

MgNaNe

Time of flight

Fig. 2 : Two-dimensional representation A/? ver-sus time of Right (~ A/Z). The new isotopes areindicated by their mass number.

Nevertheless one should- note that we are stillaway from the neutron drip-line by about two tofour masses.

Furthermore, from a reaction mechanism pointof view, it is interesting to note the presence ofstrong transfer reaction channel at intermediate en-ergy. Indeed, part of the new fragments containsmore neutrons than the projectile did. The figure3 shows the known isotopes of Nitrogen as well as23O, 34O and the new iuotope 2 S F . The unsta-ble'isotope 2(iO is obviously absent and no countsof 20O are seen. Nevertheless taking into accountthe counting rate of the observed nuclei and usingthe fit of Siimmerer (9) for an estimate of produc-tion cross-section we should have observed only 2or 3 counts. It was therefore impossible to con-clude on the particle stability of 20O (note howeverthat we analyse presently an experiment with muchimproved statistics). 2 9 F is predicted to be boundby the mass formulae and observed. The odd-eveneffect on the stability of nuclei is clearly seen with28F and 30F which are predicted unbound. Exceptfor the constant-shell mass formula of Uno and Ya-mada 2 9 F is predicted to be the la«t stable isotope

I

23,

N

Time of flight

Fig. 3: Two-dimensional representation &E ver-sus absolute time of Sight (~ A/Z). The isotopesare labelled by their symbol. Four counts of thenew isotope 2 0 F are observed.

In the fragmentation of the 80Kr beam |10] it waspossible to produce new isotopes in the 18<Z<27region such as "Ar, 67Ti, *9-e°V, 6 1 - « O , °"-06A/fJ,oo.G7,osJp(.j c».oo.™Co However in the range ofmass, we are still very far away from the neutron*drip-line.

Mapping the proton drip-line

On the proton-rich side, two experiments |11,12|have been performed to produce new exotic nuclei.For the purpose the 4"Ca (77 MeV/u) and 5*Ni (55

20

MeV/u) have been used on a Ni target.

z20-

15-

10-

180 170 160 150 Tof ns

Fig. 4 : Two dimensional representation of eventsZ versus time of flight.

h has been possible for the first time to producethe whole series of light TZ = — 5/2 isotopes namely2 3 5 . , 2 7 S, 3lAr, 3&Ca (fig. 4). The question ofwhether the drip line has been reached up to Z =20 had to Le' handled with some caution due to thesteepness of the valley of /?'stability. Neverthelesscalculating the Ip and 2p separation energies byapplying the charge symmetry formula of Kelsonand Garvey [13] as done by Janecke |5| but using themost recent experimental masses (14] all the nucleibeyond the ones seen in the experiment are largelyunbound with the exception of 22Si. This isotopelias been identified in the fragmentation of an 30Arbeam at 85 MeV/u on a Ni target) 15].

With the 687Vt beam the new isotopes 43V, 44Cr,<°-47A/n, 48Fe, 60-61-MCo, 6I-«JVt, 66-60Cu havebeen identified (fig. 5).

All the new isotopes are predicted to be boundwith the charge symmetry formula of Kelson-Garvey (13). In this region two isotopes predictedbound remain unobserved namely 43CV and 47Fe.The two unobserved isotopes 42V and 64Cu 6f theTZ = —2 series are predicted unbound against oneproton emission by about -400 keV.

The observation of the new proton-rich Vana-dium to Copper isotopes seems to indicate that theproton drip-line has been reached for odd-Z ele-ments Z = 23, 25, 29.

For the Cobalt isotopes Z = 27 a conclusion is

premature at the present level of statistics (Twocounts were seen for the isotope 49C"o, however pre-dicted unbound by -940 keV).

tt O U <5 U IT

Fig. 5 : Mass spectra for the elements V, Cr, Mn,Ft, Co, Ni, Cu. Arrows murk new isotopes identi-fied in this work.

As a conclusion

All these experiments have allowed to map theneutron-drip line up to Z = 7 and for Z = 9, theproton drip-line up to Z = 20 and for Z = 23,25,29.

These experiments have been very important forestablishing the production conditions for subse-quent study of the nuclear properties of exotic nu-clei unknown before (see other contributions). Theywill continue with the availability of heavier beamsof higher energy after the upgrading of the GANILcyclotrons and the LISE spectrometer.

References

|1] T.J.M. Symons et al, Phys. Rev. Lett. 42

21

(1979) 40

|2| R. Anne et ai, NIM A257 (1987) 215

|3 | M. Langevin et al, Pliys. Lett. 150B (1985) 71

(4| F. Pouglieoii et al, Europhys. Lett. 2 (1986)505

[5| 8. Maripuu, special editor, At. Data Nucl.Data Tables 17 (1976) 1

[6| M. Uno et al. INS Report NUMA 40 (1982)

[7| D. Guillemaud-Muelleret al, Zeit. Phys. A332(1989)

|8| G.D. Westfallet al, Phys. Lett. 4391979) 1859

(9) K. Summerer, GSI Scientific Report 87-1(1986) 97

110] D. Guillemaud-Mueller et al, Zeit. Phys. A322(1985) 415

|11| M. Langevin et al, Nucl. Phys. A455 (1186)149

[12| F. Pougheon et al, Zeit. Phys. A327 (1987) 17

(13) I. Kelson et al, Phys. Lett. 23 (1966) 689

(14) A.H. Wapstra et al, Nucl. Phys. A432 (1985) 1

[15| M.G. Saint-Laurent et al, Phys. Rev. Lett. 59(1987) 33

22

10

MASS MEASUREMENTS OF NEUTRON RICH FRAGMENTATION PRODUCTS

G. Audi1, L. Bianchi2, A. Cunsolo3, B. Fernandez2, A. Foti3, J. Gastebois2, A. Gillibert2,

C. Gregoire4, W. Mittig4, M. Morjean4, Y. Pranal4, Y. Schutz4, C. Stephen6, L. Tassan-Got6.

A.C.C. Villari7, Z. Wen Long8.

1. C.S.N.S.M. Orsay, 2. D.Ph.N/BE C.E.N. Saclay, 3. I.N.F.N. Catania, 4. GANIL Caen,

5. C.E.N. Bruyeres-le-Chatel, 6.1.P.N. Orsay.7.1.F.U.S.P. Sao Paulo, 8.1.M.P. Lanzhou.

From the most fundamental models to veryphenomenological ones, the calculations are expectedto reproduce at least some static nuclear properties.Among them, the binding energy is of primeimportance since it is the lowest energy eigenvalueof the nuclear Schrddinger equation. For the stablestnuclei, all calculations reproduce with more orless success the binding energy data. Howeversuch an agreement is ambiguous since a lack ofphysical description may be overcome by moreunphysical free parameters. Thus, new data relativeto 6 unstable nuclei very far from stability arevery useful since they were not previously usedto fit any parameter. The whole calculation maythen be tested with the new initial conditions presentin these nuclei, that is with a very different isospin.An other interest of these measurements is tolock at the behaviour of nuclei very far from 8stability and especially the evolution of magicshell closures.

Projectile fragmentation at relativistic energyturned to be an efficient way to produce very neutronrich nuclei at Berkeley. A naive but rather successfulldescription of the results was obtained using ageometrical overlap model. At lower incident energy- the Ganil energy domain - the relevant mechanismis not yet well understood. Production of nucleiheaving more neutrons than the projectile showsclearly the importance of pick-up or diffusionprocesses. These diffusion processes may be partiallyresponsible for the high production cross-sectionsof exotic nuclei observed at intermediate energies.This, together with the high beam intensity forlight projectile Old11 particles/s) like 40Ar isthe reason why Ganil is so attractive for productionand study of light neutron rich nuclei. The primaryneutron rich projectiles used so far were 40Ar,86Kr and 48Ca. Further, with that kind of reactionmechanism, not only a specified nucleus is produced

and favoured, but all its neighbours in mass andatomic number. What would be a parasitic backgroundfor other studies is very useful here, since manycalibration point - say nuclei with well-knownmass-turned out to be measured (fig. 1).

According to the relativistic formulationbetween the magnetic rigidity Rp, the charge stateq, the mass me2 and the velocity B=v/c,

qBp = mc"B( l -B 2 Hboth the magnetic rigidity and the velocity aremeasured to deduce the mass value. Clearly theexperimental resolutions have to be at least as goodas the final mass resolution required to be in the10~4 range. This goal is achieved with the help ofthe high resolution properties of the magneticspectrometer SPEC and a time of flight measurementover a eighty meter long path. We measured fourtymass values, most of them being unknown so far.The mass resolution A m/m (full width at halfmaximum) is typically equal to 2-3 1(T4. The finalaccuracy of the binding energy is only ruled by thenumber of counts and the systematic error whichwas reduced to •>• 5 10~6. What should be kept inmind is that any measurement locses a large partof its interest when the error bar goes beyond 1 MeV.

As an exemple of results obtained with 40Arprojectile, fig. 2 compares the experimental resultswith theoretical predictions for oxygen isotopes.The shell model calculation only has a good predictivepower, it is a non trivial result that such a calculationis reliable far from stability.

Microscopic Hartree - Fock calculationsperformed by F. Naulin with a Skyrme SIII forceon light even even nuclei reproduce nicely the twoneutron separation energies, at least as well as othercalculations where much more free parametersare included.

The 40Ar beam is well-suited to stud> theshell closure N=20 far from stability. One sees on

23

11

fig. 3 the two neutrons separation energy S%n.

The sodium isotopes at CERN were found earlier

to be much more bound than expected at this major

shell closure. Instead of the classical behaviour

seen with Ca isotopes, the isotopes 31Na and 32Na

do not seem to be much less bound than the N<20

isotopes. This was interpreted in the shell model

frame as the onset of a new deformation due to

the energy lowering of the f7/2 orbital in a prolate

configuration. Poves et al. have extended sd shell

model calculations to .the f.7/2 p3/2 shells but

fail to reproduce the binding energies in this region.

They predict however strong contributions of 2p2h

deformed configurations for N=20 and Z=ll. A

global theoretical description of this region is

therefore still missing.

Data from ^Ca and 8<>Kr projectiles relative

to shell closures N=20 and N=28 are presently

analyzed. For more details, see references.

I

1 1 « '

( 4 1 !

l i t '

4 1 iI I 4

Bf> a 3 88 7m

Fig. 1 : Matrice d' identif ication obtenue avec le pro-ject i le 40Ar a 60 HeV/A et pour la valeur Bp = 2.88 Tm.L'abscisse correspond au rapport A/Z efr1'ordonnee aunutne'ro atomique Z.

Fig. 3 : Sgn curves as a function of the neutronnumber. Open symbols stand for even-Z nuclei,circles for the values of tables and trianglesfor this work. The more neutron-rich sodiumisotopes (N>19) measured at CERN are labelledby a star.

7-

S-HOQ

1 -2-cx

CQ-3-

-i.-

0 isoloprs

k-* -

A UNO YAMADA

. LIRAN ZELDES

o HOLLER NIX

. WILDENTHAL

•'.*'.

8 10 12 If. 16 N

f . 2 :Difference between experimental massexcess and predictions from mass calculationsfor the oxygen isotopes. The error bars cor-respond to the experimental uncertainties.

A. Gillibert et al., Phys. Lett. 176(1986)317 ;

192(1987)39.

Y. Schutz et al,, Preprint Ganil P 8717.

Int. Winter Meetings on Nuclear Physics, Bormio

(1986); (1987) 426.

W. Mittig et al., 5 t h Int. Conf. on Nuclei far

from Stability Roseau Lake (1987) 11 ; Riken,

IN2P3 Symp. on Heavy Ion Collisions (1987) 70.

24

12

Study of B-Delayad Nautron Emission of Very Neutron-Rich Light Nuclei

R. Anna0, A.G. Artukh2', D. Bazin1', V. Borrel35, C. Detraz". D. Guerreau15,D. Guillemaud-Hueller0, J .c. Jactnart3', A.M. Kalinin21. V.v. Kamanin25, M. Lewitowicz2'

S.M. Lukyanov , A.C. Mueller11, Nguyen Hoai Chau2).Yu.E. Fenionirhkevich2'.

A.Richard35, M.6. Saint-Laurent" ,W.D. Schmidt-Git"0F. Pougheon

1} G«NIL. F-H021 CrtEN. 2) Laboratory of Nuclear Reactions, J1NR. PO-Box T9.DUBNA. USSR

IPN F-91-4O6 ORSAV. ^) n . Physical isches tnstltut der Unluersitat. D-3-»OO Gottingen. FRG

Tho experiments on the production and

identification of new nuclei far from stability

which were performed at USE Csee elsewhere in

this books have stimulated us to undertake the

next step in the study of these species. Thoir

increasing distance to the valley of P-stability

translates into increased values of the (J-decay

energy Qp. Eventually, this opens up a. window

for decay into particle-unstable states in the

daughter nuclei. The delayed neutrons which are

observed for the decay of very neutron-rich

nuclei being coincident to the p-rays provide a

very valuable experimental tool for the

measurement of P-decay half—lives T. ._ at very

low counting rate. The Iiquid-scintillator

detector which we have developped for our

experiments fl ] also provides information on the

delayed neutron emission probabilities p .

Systematic measurements of both quantities

are important to test our knowledge of the

nuclear weak interaction. Much theoretical

progress has been made recently Csee below),

partly stimulated by our experiments.

Furthermore, the information on basic decay

properties of the very neutron-rich nuclei is

needed in astrophysical scenarios [23.

So far, we have produced the neutron-rich

nuclei by means of B6Kr and <8Ca beams from

GANIL. at 45 Mevxu and 55 MeV/u, respectively.

Thf* projectile-like fragments from the

interaction with Ta targets were analyzed by

the doubly achromatic spectrometer LISE [3].

They ware stopped, at the exit of LISE, in a

semiconductoi—detector telescope. The implanted

nuclei were identified by their energy-loss and

their time-of-fIight through the spectrometer on

an event-by-event basis. As soon as an isotope

of interest was detected, the GANIL-boam was

switched off for a Period corresponding to the

expected P-half-life in order to allow the

registration of P-neutron coincidences at low

background. For maximum efficiency, the (3-rays

and the neutrons wsre observed by a 411 detector

geometry Csee fig.13: The semiconductor detector

telescope was surrounded by a 3mm tumbler-shaped

NE-1O2A plastic-scintillator for the p-rays, the

whole being inside a NE-213 Iiquid-scintillator

detector for the neutrons. The latter was chosen

for its

properties.

excel lent

LISE SmttiKUr

neutron-T separat ion

\PLH5TIC

SClltTILlflTORHE1D2

LIQUID SCIKTILlfllOR NE213

f n i /

Lj\l

/ i WCUUh CHBII

r|

£1!

^ *

fig.I Schematicact set-vr> of tAa

The qual ity of the observed signals may be

seen in fig.2, which shows the decay of the

isotope 1SB . The half-life of 1O.3 ms is in

Perfect agreement with a previous result £•»]. In

this way we have obtained decay data for the

first time for the following isotopes: 1B'2OC,

Cantsmoo "B : \= 10.3 is

1 I I I I I I I . . 180 is

fig.2 ft-atacaat of t/te isotapa

25

13

Ma have compared, in fig.3. our experimental

results to various theoretical predictions. They

include the "old" gross theory by Takahashi [73,

its recent refinement by Tachibana et al. [8]

and the microscopic model of Klapdor et al. £93.

Furthermore, results are shown from the new

calculation by Staudt and Klapdor [lOj which is

based on a Proton-neutron quasi-particle random

phase approximation.

Fig.3 Ratio of theoreticat to expervnamtat ftnaff-fives versus the mass ntaHber A. The

error bars correspond to (.statistica-fleyferimontat errors. In the ca.tcu.tat ion byStaudt at at. experiment at misses (.Huct. Phys.

*t3Z (.lyes') 1 ) are used for 19N and 34Af; othermasses are taken from v .Groat e at of. <.At. DataNuct. Data Tabtes 17 (.fs>76> -ifa). x derates themean vatue of the togarithm of the ratio betweenthe experimental and the theoreticat hatf-fivesana a the standard, deviation.

10

T thw

JT tip

.

.1

TlUtMlN

"4 \ IT

i ^

X-.O.M a .0.55

TKhltMM •!•).

I

i r'ifir tT ^ ,

*

_X-*0.37 0-0.55

10 20 30 40 50 10 20 30 *0 5

'I,-"10 20 30 40 . 50

IT fi

ttX • * 0.12

Stoudttl*!.

i

#

« *

i

O -O.G9

10 20 30 40 SO

Several features are obvious from f ig.3: The

"old** cross theory substantially overestimates

the experimental half-lives. This rather

systematic deviation Csee the snail value of CT

in fig.33 is clearly corrected for in the

second-generation calculation by Tachibana et

a.1. without deteriorating the scatter of the

individual data points. The microscopic model by

Klapdor et al. yield a rather good average of

x = -O.46 . Here, however the standard deviation

a = 1.9 is indicating several large

discrepancies to the experimental half—lives

which makes the Predictions from this model less

reliable. On the other hand, the new microscopic

model of Staudt and Klapdor seems to exhibit a

remarkable predictive power far off stability. In

the same experiments we could also measure the

delayed neutron emission Probabilities Pn, these

results are also discussed in [5,6].

Concerning the astrophysical relevance of our

data, one may mention, in addition to the global

importance of decay data far off stability for

network calculations of the nucleosynthesis, the

particular case of Tt/>2 of 44S which is a key

parameter for solar isotope abundances [11].

References

1) D. Buin . thes is . • Ca«n university 1987 «ndQANIL-report T.BT-O1O. Bizln. A. c. Mueller and H.D. Schmidt-ottsubmitted to NucX. Inst. Heth. and GANIL-r«port P.8S-O1

Z1 H.u. KlapdorInt. Symp. an Heavy Ion Physics and Nucl-Astrophysical Problems, Tokyo. 1388

3) R. Ann«. O. Bazin, A. c. Muel%«r et ax.Nuct. Inst. Methods AIST C1987) £15

•4) J.P. Dufour. S. Beraud-Sudrcau et al.Z. Phys. A31« C19S*> 23T

5) A. c. lfu«tl«r. D. Bazin «t al.Z. Phys. A33O (19BB) £3

6) n. Leuitouici, vu. E. Pcnionzhkeuitch et Jl.Mucl.Phys.A in press and GANIL-report PB9-O2

7) K. TakahashiProg. Theo- Phys. 47 C197Z> 15OO

8> T. Tachibana. S. ohsugi. n. vamada5th Int. Conf. on Nucl. far from Stability.Ross«au Lake. Canada 1967; to be published

9) H.u. Klapdor. J.Hetiing*r. T. OdaAt. Data NucX- Data Tables 31 C19B4) 81

10) A. Staudt and H.u.Klapdorprivate communication and to be published

11) K.t_. Kratx. P. Holler. W. Hillebrandt et al.5th. Int. Conf. on Nucl. far from Stability.Rosseau Lake Canada 19B7; to be published

14

B-DELAYED MULTI-BEUTSOir RADIOACTIVITY OP 17B. ' aC ABB ' *Be

J.P. Dufour, M. Beau", R. Del Moral. A. Fleury, J. Frehauf,G. Giraudef, E. Hanelt3, F. Hubert, D. Jean, A.C. Mueller',

M.S. Pravikaff, K.-H. Schmidt2, K. Summerer*

C.E.H. Bordeaux-Gradignan, 'GABIL Caen, =GSI Darmstadt3TH Darmstadt, "C.E.A, Centre d1Etudes de Bruyeres-le-Chfttel

Until recently, the experimental possibi-lities to investigate isotopes near theneutron drip line were almost restricted tothe alkaline elements. Therefore, especially1'Li was extensively studied and it hasproven to be the precursor for the emissionof up to three neutrons1' and also tritons2'.

The nuclei " B , 19C and '"Be which havehigh Qs-values up to 24 MeV should also begood candidates for B-delayed multi-neutronradioactivity. In our experiment theseisotopes were produced at GAUL by theprojectile fragmentation of a 60 MeV/u ^He-beam impinging on a 1.2 g/cm2 tantalumtarget. They were separated using thespectrometer LISE3' operated as a momentum-loss achromat and implanted into a 7 mm thickplastic scintillatar which was designed todetect both the implantation signal and thefollowing beta decays. This method ofProjectile-Fragments Isotopic Separation"'has already successfully been applied tospectroscopic studies5' and isotope searchel.Additionally, the nuclei at the exit of LISEcould be electronically identified by a AE-time of flight measurement <fig. 1). This wasdone by means of a silicon transmissiondetector the signal of which was referenced

200

•6,250

•6 300

10 20 30 *0 50

AE / MeV

60

Fig, 1 : AE-TOF-spectrum (logorlthmic inten-sity scale) measured at the exit of U S Ewhich was optimized to separate 17B

t,^/ms

?i .i/K

Da,-./'/.

5

00000

1 7]

.08

.21

.63

.11

.035

.004

}

(5)

(2)

(7)(7)(3)

000

1 «

49

.46

.47

.07

'C

(4)

(3)(3)(3)

4.

0.0.0.

1 1

35

148105

'Be

(17)

( 3)( 4)( 2)

to the accelerator rf. After the signal of adesired nucleus the beam was stopped for100ms and the time between the implantationand the correlated 8-decays during the beampause was recorded. The neutrons occuringpromptly after the g-decay were countedduring a time window of 50us triggered by thedecay signal. They were detected with a meandelay of 11 us and an efficiency of 70% bymeans of a 4ir neutron ball" filled with 5001of liquid scintillatar doped with Gadolinium.Since this detector is also sensitive to K-rays with energies above 0.5 KeV, it wasnecessary to measure the number of backgroundevents. Therefore they were counted in asecond 50ps window following the first oneafter a delay of 50fas.

The off-line analysis had to take intoconsideration three effects changing themeasured neutron-multiplicity spectra : thedaughter activities, the background pile-upand the finite neutron detection efficiency.Figure 2 shows the measured ^-activitiescorresponding to different neutron multipli-cities and their fits as a function of thetime between the nucleus implantation and itsfi-decay.

The results for the normalized neutronmultiplicity branchings p>«r. and the half-lifes ti/2 of the observed nuclei aresummarized in table 1. All nuclei areprecursors for the emission of at least twoneutrons. In the case of 17B a four neutronbranch is reported for the first time. Thus,the B-delayed three neutron emission of ''Liis not an outstanding case any more. Theemission of several neutrons has proven to bea general decay mode of the light andextremely neutron-rich nuclei. This work hasbeen published*3'.

W

Vf

Table 1 : results (the error-bars correspondto one standard rte"l«tion>

o w m so BO s o m H K O » too w-ioot / m s

Ei*.—2. : measured B-activlties of "B andtheir fit (see text)

1 - R.E. Azuma et al., PL, 96B, 31 (1980)2 - M. Langevin et al., PL, 146B, 176 (1984)3 - R. Anne et al., HIM, A257, 215 (1987)4 - J.P. Dufour et al., SIX, A248, 267 (1986)5 - J.P. Dufour et al., Z. Phys. A-Atom.

lucl. 324, 487 (1986)6 - K.G. Saint-Laurent et al., Phys. Rev.

Lett. 59, 33 (1987)7 - J. Frehaut, HIM, 135, 511 (1976)8 - J.P. Dufour et al., PL, B206, 195 (1988)

27

15

BETA-DELAYED GAHKA 5PHCTROSCOPY 0» LIGHT HBUTROH RICH JUCLEI

J.F. Dufour, R. Del Moral, A. Fleury, F. Hubert, D, Jean,K,S. PravikoffC.E.B. Bordeaux, IH2P3, Le Haut Vigneau, F-33170 Gradlgnan

H. Delagrange, GAHIL. B.P. 5037, F-14021 Caen CedexH. Geissel and K.-H. Schmidt, GSI, D-6100 Darmstadt

E. Hanelt, TH, D-6100, Darmstadt

The GAHIL Intermediate energy heavy-ionbeans have proved very efficient atproducing exotic light nuclei. Theapplication to the LISE spectrometer of theProjectile Fragments Isotope Separator",provided beta delayed gonna spectroscopicdata on light neutron-rich nuclei (nassrange 14-40). The production rates and thenature of the measurenents are listed intable 1 for the 22 studied isotopes .

The bulk of information gathered anthese decay is very large. A first list ofgammas, intensities and half-lives hasalready been published2'3'. The half-liveswere all measured with bean pulsationsuncorrelated to the nuclei implantation. Theisotopes listed in table 1 lie in threedifferent shells : p, sd and fp. Thecomparison of the experimental data withshell-model calculations is Host complete inthe sd shell where Vildentbal'spredictions" are detailed and include the (3decay. The accuracy of these predictions isvery impressive.

Huclei in the sd shell : an interestingproperty of the PFIS selection is a verygood rejection rate of the daughter of theseparated isotope. The beta branching of 2 3Fand 3*Si to the ground states of 23He and3*P have thus been obtained through thedaughter's total activity determination. Theexperimental values (30 ± 8% and 65.5 ± 2.3%respectively) compare well with thetheoretical predictions : 33.6% and 56%.Another specificity of the apparatus is itstuning versatility and very short transporttime. Those properties were especiallyuseful in the study of *eBe. When LISE wastuned on 26He very little gamoa activity wasobserved in a 600 us beta-gamma coincidencewindow. The spectrometer was then tuned on

2SBa and it was possible to observe thedirect decay of a 82.5 ± 0.5 keV excitedstate not in coincidence with betas. Amultlspectrum time-analysis starting aftereach implantation of a 26Ia isotope allowedthe determination of the half-life,9 ± 2 us.of this excited state.

Buclei in the fp shell : the extension ofthe shell-model description from the sd tothe fp shell, further away from the 1 S0inert core, has been recently improved byVarburton et al.s> The comparison of theexperimental data with the calculation isvery encouraging. The SDPF interactionproves its predictive power for the I = 21isotones from 3SC1 to 3sSi. Our data on 3*A1are still too scarce for a fruitfulcomparison. The experimental data need to bevery extensive in this transition regionwhere the decay from negative parity ground-states feeds very high energy excited statesin the daughter. A good example is the 12%feeding from 3 eP of the 7271 keV state in3SS not seen in the first study of thisnucleus. Extension of these calculations isin progress for 3'P and 36Si for which gooddata are available.

1 - J.P. Dufour et al. Hud. Inst. Heth.A248 (1986) 267

2 - J.P. Dufour et al. Z. Phys. A324(1986) 487

3 - J.P. Dufour et al 5*-" Int. Conf. onHuclei far from Stability, Canada<1987),344

4 - B.H. Wildenthal et al. Phys. Rev. C28(1983) 1343

5 - E.K. Warburton et al. Phys. Rev. C34(1986) 1031 - Phys. Rev. C35 (1987) 1851

.J ,,C >3N :.„ 22Q

T>/2 o J t 4(s) 0.20(6) 0.30(8) 0,07(4) 2,25(15)

Table 1 : Summary of thespectroscopic data obtain-ed by fragmentation of aAOAr beam at GAHIL. Astar notes a first study,open triangles and circlesrespectively note improve-ments and confirmations ofprevious measurements

I

N/s

" F "He "Re s'Ni 3°Hj* 0 J o &

0.31(8) 0.62(3) 0.23(6) 0.295(20) 0.342118)

i20

I

>40

*

23 >200 41

o

510

»

50

o

>103

"Al "SI 3'A1 "Si 3BSi "Si "Si 3'P

A0.031(6)

t

1700

» J o t * »0.05(2) 0,03(1) 3.S01) 0.78(12) 0,45(6)

o * t *5,35(53)2,3102) 0.61(14) 8.6(21)

t

260

t

40

t

>1600

t

430

t

100

4

22000

t

3300

t

225

t

58

28

16

BETA-DELAYED CHARGED PARTICLES SPECTROSCOPY OF LIGHT NEUTRONDEFICIENT NUCLEI

V. Borrel, J.C. Jacmart, F. Pougheon and A. RichardINSTITUT DE PHYSIQUE NUCLEAIRE, BP n"l, 91406 ORSAY, FRANCE

R. Anne, D. Bazin, H. Delagrange, C. DltrazD. Guillemaud-Mueller, A.C. Mueller, E. Roeckl*

and M.G. Saint-LaurentGANIL, BP 5027, 14021 CAEN CEDEX, FRANCE

J.P. Dufour, F. Hubert and M.S. PravikoffCEN Bordeaux ,Le Haut Vjgneau , 33170 GRADIGNAN, FRANCE

Light neutron-deficient nuclei exhibit several in-teresting decay modes. In this region, the rapidincrease of mass differences leads, through /3+ de-cay, to a wide range of final states in the daughternucleus, including in particular the super-allowedtransition to the isobaric analog state. Due to thesmall proton binding energy, most of these statesdecay by proton emission, or in some cases by al-pha emission. Thus, delayed emission of one, twoor three protons and alphas are to be searched. Inaddition, for a few number of these nuclei, directtwo-proton decay has been also predicted |l] .

In these experiments the decays of the T, = -5/2 nuclei 3lAr (2), 2 7S and 23St and of the T, =-2 nucleus 28S[3] have been studied. The T, = -3nucleus 22Si has been observed for the first time[4Jand its decay has been measured.

1 THE EXPERIMENT

The GANIL facility, equipped with an ECR ionsource operating with enriched 36Ar provides a 85MeV/u 30Ar beam with an intensity of 4.1011 par-ticles per second on the 385 mg/cm2 natural Nitarget. The nuclei under study are produced asprojectile-like fragments and the LISE spectrometeris operated as an isotope separator [5,6]. This Pro-jectile Fragment Isotope Separation method com-bines the magnetic analysis performed by the spec-trometer itself with a momentum-loss selection ofthe ions which fly trough an energy degrader lo-cated in the dispersive intermediate focal plane of

the spectrometer. In this region of light neutron-deficient nuclei, with carefull optimisation of thetwo dipoles magnetic rigidities, the PFIS methodselects a series of isotones and the studied nucleusis collected at a rate going from 0.1 event per sec-ond (for the study of 225t) up to 5 events per second(for 3lAr). The shape of the aluminium degradershas been designed in order to preserve the achroma-tism of the set-up. Thus, the transmitted nuclei areidentified by deltaE and time of flight measurementbetween the target and the silicon-detectors tele-scope (fig.l) located at the final focal point. Theyare implanted in this telescope by slowing down inaluminium foils located in front of the detectors.

Al digrocter 3 m m

B2S^»!8mm•13mm

. .

^

*, '

{

5mm

SSOO;jm

Fig. 1 : The solid-state telescope. The heavy ionsare imp/anted in detectors E4 or E5 and the protonsare detected in detectors A El to E5. The ions areidentified on the two-dimension a/ plot AEJ versustime of flight (measured between the target and theAJ51 detector). EC is used as a veto to reject high-energy protons.

•Permanent address G.S.I., Po.tfach 110552,6100 DARM- T h e radioactive decay of the nuclei of interest isSTADT 11, Federal Republic of Germany studied on line: the charged particles emitted by the

29

17

iniplanteu ioi.a are measured in the same telescope.Since there is no transport of the activity, it is easyto investigate very short half-lives. The detectorsare equipped with two electronic chains which per-mit the detection of low energy protons a few mi-croseconds after the arrival of a very energetic heavyion. Two single channel analysers examine the A £and time of flight signals of each implanted ion andthe beam is stopped each time that an interestingion arrives. The length of the beam-off period canbe chosen. The data-aquisition system consists intwo systems working in flip-flop mode : the firstone records the ions parameters and the second oneworks only during the beam-off periods and recordsthe ligtli particles energy deposits and the elapsedtime between the particle emission and the last ionimplantation.

2 RESULTS

2.1 31Ar

In the 3lAr study, the only contaminant for protondetection is 2 9 S . Its beta-delayed protons, recordedduring the experiment yield a precise proton energycalibration.

ISO-

1 0 0

5 0

0

31Ar1520±«OkeV

\

|

Ml/

2l90±10keV

3 1Ar5«50±100keV

31Ar31

370D±100keV A r

1 6200±100keV

ii JJ|ifl "1* IPI^^Vvllln-Jliy^iL JL . _

f f lJO 2.0 3.0 4.0 5.0 ] 60 7.0 BO 3.0 10.01 E(MeV)

Fig. 2 : Energy spectra of the protons recorded indetector E4 during the 200 ms long beam-off peri-ods. The preceeding 3lAr nucleus is implanted intoE4 and most of the emitted protons are stopped inthis detector. This difference spectrum has beenobtained after subtraction of the 295P contributionestimated on the long-lived part of the spectrum.

In the long-lived part of the energy spectra of theprotons emitted during the beam-off time by nu-clei implanted, the strongest known [7] proton linesfrom the decay of 2 0 5 are clearly seen. In the dif-

ference spectrum (fig. 2), obtained by subtractingthe 3 8 5 contribution, several peaks decay with thesame short half-life and have been attributed to thedecay of 3>Ar.

The half-life of the ai Ar nucleus is found to be15 ± 3 ins , from a two-component fit assuming thelong-lived component to be entirely due to the 187ms 2 0 5 .

From these results a partial 3LAr decay schemeis proposed (fig. 3).

EMeV

20

15

10

3/2*-K~

-2133-1951 '

P+2p

1* 5136

£ _ P / ^ _ 1 T O _

2* 22\y y 2A90 —

_fe

31 ArlS±3m

1/2*

3.C ,

7530

Fig. 3: Partial decay scheme of 31Ar. Forthe3lClnucleus, the levels are deduced from the present ex-periment with the exception of the T=5/2 state, forwhich the energy has been calculated. The energylevels of its mirror nucleus 3lSi are also presented.

For the beta-decay of 3lAr to the iaobaric ana-log state in 31C7, under the assumption of a pureFermi decay, the measured half-life (15 ins) yieldsa branching ratio equal to 5 %. These experimen-tal results have been compared with the shell-modelcalculation of Brown and Wildenthal [8] : the fivemain predicted proton peaks are actually observed,the intensity ratios corresponding roughly to thecalculated ones. The experimental determinationof branching ratios is underway.

30

18

The isobaric analog state in 31C7 ia expected todecay both by one proton and two-proton emission.There is no evidence for any proton peak with en-

" ergy around 12 MeV or 9.8 MeV. This absence maybe due to tiie combined effects of a low branchingratio and of the strong decrease of the detectionefficiency as protons become more energetic. Onthe contrary, a group of peaks, near 7.5 MeV, de-caying with the 3 Mr half-life, appears on the sumspectrum (Ea + E4 + EB). Coincidence rate mea-surements between detectors 3 and 4, 4 and 5 and3, 4, 5 allow to conclude that one of them representsthe summed energies of the two protons emitted bythe T = 5/2 state in 3lCl towards the ground-statein 2 9 P , with a calculated energy of 7.8 MeV [9].

The measured 3iAr half-life of 15 ± 3 ms in goodagreement with shell-model calculations shows that31 Ar does not exhibit an important branching ratiofor direct two-proton decay.

2 .2 2 8 S

With the same method, beta-delayed proton ra-dioactivity has been studied for the 2 8S isotope (T,= -2) [3], This experiment has lead to the locationof the lowest T = 2, 0 + state in the 2*P daugh-ter nucleus, at 5000 ± 2 1 keV. This experimentalvalue allows to complete the isospin quintet of mass28. A comparison of the experimental data to thequadratic form of the isobaric multiple! mass equa-tion shows that the energies of the five T = 2 statesof the A = 28 quintet are well represented by thatrelation.

The measured half-life of 125 ± 10 ms and theproposed partial decay scheme show an excellentagreement with shell model calculations (fig. 4).

2.3 2 7 5 , 2SSi and 225iThe analysis of the data is underway.

El MeV)

10

T = 2

2lp

" SIIZBtlOKns

" S029tiaims

BR V.

lip

THEORY.

Fig. 4 : Partial decay scheme of 3 8 5 . For thenucleus, the levels are talten from the a*S»(p, n)a*Pdata 110} with the exception of the T = 2 statededuced from this work. Only the MP levels above3 MeV have been indicated. Energies are in keV.The theoretical predictions are also presented.

31

References

|1] V.I. Goldansky, Nucl. Phys. 19 (1960) 482;Nucl. Pliye. 27 (1061) 648; Phys. Lett. B212 (1988) 11

(2] V. Bon-el et al, Nucl. Phys. A 473 (1987) 331

[3] P. Pouglieou et al, submitted to Nucl. Phys.A ; IPNO ORE 89-02

[4] M.G. Saint-Laurent et al, Phys. Rev. Lett. 69(1987) 33

|5| R. Anne et al, lust. Meth. A 257 (1987) 215

[6| J.P. Dufour et al, Nucl. lust. Methods A 428(1986) 267 and this book

J7| D.J. Vieiraet al, Phya. Rev. C 19 (1979) 177

[8] B.A. Brown and B.H. Wildenthal, At. Datav and Nucl. Data Tables 83 |l985) 347

|9] V. Borrel et al, to be published

,10) B.D. Anderson et al, Private Communication

Some Properties of -delayed Charged Particle Emission of 39T1 and 4°T1.

R. Anne*. D. Bazln , P. rlcault*, CL Detraz*, J.P. Dufour , A. Fleury,D. Gulllemaud-Mueller , F. Hubert . J.C.. Jacmart , M. Leuitowicz*.

A. Mueller . F. Pougheon, M.S. Pravikoff , A. Richard , Y. H. Zhang*

1- IntroductionThe study of nuclei far from stability has

led to the discovery of new decays modes and newstructures not seen In nuclei near stability.Work on nuclei near the proton drip-line, forexample, has led to the ' discovery of protonradioactivity and beta-delayed one-proton, andtwo-proton decays.

Heavy Ion fragmentation at Intermediateenergy has demonstrated Its effectiveness In theproduction of many proton rich nuclei. In thisregion, the rapid increase of mass differenceleads, though ft -decay, to a wide range of finalstates In the daughter nucleus, Including Inparticular the isobarlc analog state, (IAS),through a super-allowed transition. Because oftheir small binding energy, most of the statesdecay by proton emission. The proton spectra areexpected to give Information on the position ofthe analog state and on the distribution of theGamow-Teller strength.

In the case of Ti, only fl*-delayed protonemission Is energetically possible; but In thecase of Tl, 0 -delayed two-proton or alphaemission and direct two-proton emission may alsooccur. This nucleus, bound against one-protonemission, Is Indeed predicted to be slightlyunbound against two-proton emission, with atwo-proton separation energy, of Sap* -740 keV.Direct two-proton decay has been predicted longago by Coldanskil1,' and discussed by JSneckeand recently by Coldanskil . But the search forthis new radioactivity became possible onlyrecently, with the new techniques used In heavyIon research. The half-life of this radioactivemode depends strongly on the barrierpenetrability, thus on the decay energy and theangular momentum. The detection of a proton pairwith a kinetic energy of few hundred keV Isexperimentally difficult. One may find evidencefor direct two-proton emission by observing ahalf-life much shorter than one expected forbeta decay.

2- The experimental methodVery proton-rich Isotopes are produced

through the fragmentation of heavy ionsimpinging on a target. The separation of theprojectile-like fragments is performed using the0* magnetic spectrometer, LISE4', working Inseparation mode. The Identification of the

B.P. S027, 14021CAKIL,

Prance.

Instltut de Physique

N*l,91406 Orsay, Franco.

C.E.H. Bordeaux, Le Haul

Cradlgnan, France.

Caen, Codex.

Nucltfalre, B.P.

Vlgneau, 33170

nuclei Is done using a AE-time-of-flighttechnique. For this purpose the detectiontelescope consists of several surface-barrierSi-detectors. We used an energy degrader ofvariable thickness placed i_n front of thedetectors tuned in such a way that the isotopeof interest comes at rest in a very thinSi-detector located between two othersdetectors. On line, two single-analyzers examinethe AE and the tlme-of-fllght Information. Eachtime that an isotope of Interest Is inside thecorresponding window the beam is stopped duringabout 200 ms. During this time the heavy-Iondata- acquisition system is set off-line and aclock and a second data-acquisition forregistration of light particle emitted from thisnucleus is started.

3- Experimental results and discussiona) «T1The energy spectrum of the (S-delayed

protons emitted after the implantation of 190nuclei of Ti in the AE3 detector is shown Infig. 1. Total-decay energy spectrum is obtained,since both the proton and the recoil energiesare recorded in the detectors. At least fourpeaks can be identified and assigned to/3-delayed proton emission from 40Ti. The partialdecay scheme Is presented in fig. 2. The timedistribution for all 0-delayed protons of *°T1is presented In fig. 3a. The ji-decay half-lifeis found to be Tiya= 5B-\z ms. The measuredhalf-life Is close to the theoretical predictionof Tachlbana et als), (Ti/2= 66.7 ms ).

20

go

Ti

Channel (80keV/ch.)

Fig. 1. Energy spec trim of /3-dal ayed protonsemitted fro» Tl. Energies are given In MeV.

33

20

I /M.V/

101-

II

Fa * -too IA S

Fig* 2, ProposedThe correspondingand measured BRIn MeV,

part lc.1 decay scheMe of

levels of mirror nuclei,

values are shown. Energies

Fig. 3.

a) Tl

T Irni)

Tine spectra

and b) Tl .

of charged particles from

Error bar* correspond to

one standard deviation.

b) 39T1In the present experiment the existence of

Tl Is shown for the first time. However thecollection of only 75 nuclei of Ti leads to acharged-particle energy spectrum of rather lowstatistics. Only one peak at energy 3.52 ± 0.12MeV Is identified without any doubt. Possiblecontamination by |3-delayed protons coming fromCa can be excluded, since in the energy

spectrum, fig. 4, the main peak of 37Ca( BR = 46.74 •/. ) 6', at 3.063 MeV Is absent.Beta-delayed protons from Ca produced as adaughter nucleus would also be observed in thecase of direct two-proton emission of 39T1. Theabsence of protons from 3?Ca thus give a strongargument against the hypothesis that Tl Is astrong direct two-proton emitter.

From the time spectrum of charged- particledecay, shown in fig. 3b, a half-life of 26^7 msIs deduced for Ti, a value not far fromtheoretical expectation for fl half-life of thisIsotope ( Ti/2=36.3 ms5'), providing a secondargumentemission.

against sizeable direct two-proton

1 4Ti

Fig. Energy

Chann.l (160k«V/ch.)

of 0-delayedspectrum39

particles emitted from TI. Energy are In HeV.charged

4. ConclusionsThe first observation of Ti and Ti

decays are achieved with the very efficientselection of reaction products provided by theU S E spectrometer at GANIL. In the case of Ti,it is the first successful attempt to produceand identify this isotope.

The measured charged particle enerievidence for /3-delayed proton decay ofexperimental half-life (56-la ms)

/ showsTl. Theis in

reasonable agreement with theoretical prediction(66.7 ms31).

In the charged-particle energy spectrum ofTl, no evidence appears for -delayed protons

of Ca. The measured half-life (26-7 ms) isconsistent with a (3-decay mode for TI. Thus noevidence for direct two-proton decay of Ti isfound.

The new improvement of GANIL facility underrealization will give us the opportunity toextend the range of the Investigation to largermasses, with heavier projectiles available witha new ion source. Also, an electrostaticdeflector will be placed Just after the LISEspectrometer. This new device will permit higherbeam intensity on the production target withoutany increase of the background in the detectionarea. This will allow us to search for veryexotic nuclei, in particular exhibiting directtwo-proton decay, without any contamination dueto other Isotopes transmitted by thespectrometer.

1. V.I. Goldanskli, Nucl. Phys. 19 (1960) 482Nucl. Phys. 27 (1961) 6482. J. Janecke, Nucl. Phys. 61 (1965) 3263. V.I. Goldanskii, Phys. Lett. B, 212, N* 1(1988) 11.4. R. Anne, D. Bazln, A.Jacmart, M. Langevin, Nucl(1986) 2675. T. Tachibana, S.Proceedings of the 5-th Int. Conf. on Nuclei farfrom Stability, Sept. 14-19, • 1987, Rosseau Lake.Ontario, Canada6. R. G. Sextro, R. A. Cough,Phys. A234 (1974) 205

C. Mueller, J. C.Inst. Meth. A248

Ohsugi.M. Yamada,

J. Cerny, Nucl.

34

21

BETA DECAY OF " Q

F. Hubert,J.P. Dufour, R. Del Moral, A. Fleury, D. Jean, M.S. Pravikoff,C.E.H Bordeaux, IN2P3, Le Haut-Vigneau, F-33170 Gradignan

H. Delagrange, GANIL, B.P. 5027, F-14021 Caen CedexH. Geissel and K.-H. Schmidt, G.S.I. D-6100 Darmstadt

E.Hanelt, TK, D-6100 Darmstadt

As part of a program investigating the

properties of exotic light nuclei produced at

GANIL, we have studied the beta decay of the

neutron rich nucleus i 2 0 , These nuclei were

produced by a 60 HeV/n*°Ar beam interacting

with a thick Be target, and selected among

all the produced nuclei with the LISB

separator. Both B-f and ji-lf-Y coincidences

could be measured using a Ge detector and a

matched pair of BTal detectors. Five V lines

were attributed to the 0 decay of "Q

according to several criteria'" (table 1).

Coincidence data between the two Mai and the

Ge detectors allow to established the partial

decay scheme of fig. 1.

Values for the excitation energy of " F

levels have been deduced that are consistent

with previously published values-2"*' except

for a ground-state and first excited state

doublet hitherto unobserved. The relative 8

branchings represent only Units on the

feeding of the involved levels.

Table 1

t ray Definite Relative Transition

(keV) coincidence intensity Ex-»Ef

(a) (b)

71.6(2) 637,918,1862 100(32) 72-tO

637.4(2) 72,918,1862 100(7) 710-»72

709.6(3)

918.0(2) 72,637 34(4) 1627-1710

1862.6(3) 72,637 56(7) 2572-1710

1874.2(10) 8(4)

a) Intensities normalized to 100 for the

72keV and 637 keV V lines and corrected

for summing effects.

b) Proposed excited states in 2 2F deduced

from the present measurement.The

uncertainty on the level value is = IkeV.

Regarding the shell-model description of

the B decay of 2 20 the measured and predicted

values for the half-life are in excellent

agreement6". The theory successfully predicts

the observed excited states in -"F61. The

calculation agrees well with the experimental

limits set on the beta branching ratios. But

the experimental v ray results are only

partially understood.

This Work Gate

T,/2=2.06s

;5.S .

>48

<9

2572

51627

s710

72=c22p

Fig. 1 :

Experimental and theoretical = 20 decay

schemes.

1) F. Hubert et al., Beta decay of =-'0,

Accepted for publi. in Z. Phys.

2) R. H. Stokes et al., Phys. Rev. 178 (1969)

1789

3) H. Orr - private communication

4) JF.H. Clarke et al., J. Phys. G : Sucl.

Phys. 14 (1988) 1399

5) B. H. Wildenthal et al., Phys. Rev. C28

(1983) 1343

6) B.A. Brown et al., Private communication

35

22

SPIN ALIGNMENT IN PROJECTILE FRAGMENTATION AT INTERMEDIATE ENERGIES* ##

K. Asahi, M. Ishihara , T. Ichihara, M. Fukuda , T. Kubo, Y. Gono

RIKEN, Wako-shi, Saitania 351-01, Japan

A.C. Mueller'1", R. Anne, D. Bazin++, 0. Guil lemaud-Muel ler+

GANIL, B.P. 5027, 14021 Caen Cedex. France

R. Bimbot

IPN Orsay, B.P. 1, 91406 Orsay, France

W.-D. Schmidt-Ott

II. Physikalisches Institut, Univ. Goettingen, D-3400 Goettingen, FRG

J. Kasagi

Tokyo Institute of Technology, Oh-Okayama, Tokyo 152, Japan

1)1. Introduction

Recent studies of nuclei far from stability

using high- and intermediate-energy heavy-ion

beams have proven that the projectile fragmenta-

tion process provides a powerful tool for pro-

duction of unstable nuclei. Besides the advan-

tage of high production rates available for wide

ranges of unstable nuclei, the strong kinemati-

cal focusing of projectile fragments with re-

spect to both the emission angle and velocity

facilitates isotope separation with high c.llec-

tion efficiency by means of magnetic analysis

during flight. The effectiveness of this method

for isotope production arouses further interest

in whether these product nuclei are spin-

oriented. In fact, the presence of non-zero

spin orientation in projectile fragments would

enable more detailed spectroscopic investiga-

tions such as g-factor measurements on exotic

nuclei. The measurement of the spin orienta-

tion in projectile fragmentation is also inte-

resting as a source of unique information on

reaction mechanisms. For example, a simple

model of projectile fragmentation predicts a

specific momentum. We thus performed a measure-

ment of the dependence of the spin alignment on

fragment spin alignment of projectile fragments.

* Also, Univ. of Tokyo, Tokyo 188, Japan.

** Present address: Osaka Univ., Toyonaka, Osaka

560, Japan.

+ Present address: IPN Orsay, B.P. 1, 91406

Orsay, France.

++ Present address: CENBG Bordeaux, 33170

Gradignan, France.

2. Experimental Procedure

Spin alignment of B nuclei produced in 0

+ 9Be reaction at E(180)/A = 60 MeV/u was mea-

sured. Experimental setup used is shown in Fig.

1. Spectrometer LISE was used for the isotope

separation of the reaction products. A wedge-

shaped energy degrader for the momentum-loss2)

technique ' was placed in the momentum-disper-

sive focal plane between the two dipole magnets

of LISE. Products B were stopped in a stopper

foil (Pt) placed at the achromatic focal point.

Static magnetic field B« = 310 mT parallel to

the two dipole fields of LISE was applied to the

stopper. The 6.09-MeV Y rays emitted after the14g decay of B were detected by using two

Nal(Tl) detectors placed at 0° and 90° to the

direction of BQ. The ratio R = N (0°)/l\l (90°)

of the -y ray yield was measured both with Bg on

(i.e., with the alignment preserved) and off

(with the alignment destroyed).

The spin alignment A produced by reaction in

the beam axis is expressed as A = (2a ,-a ,-2ag

-a_,+2a_p)/2 in terms of the population

probability a for the magnetic sublevel m. As

LISE

1: Experimental setup.

36

23

a result of the spin precession in the dipole

fields of USE and in B,,, a new alignment A' =

-(1/2)A in the axis parallel to Bn is estab-3)

lished after a short period of time characte-

rized by the transverse relaxation time which is

much shorter t.'ian the lifetime of B. Thus the

initial alignment A was deduced from the change

in R between the two conditions, through A =

(-8/3)(RON/ROFF-1) where ON and OFF refer to the

measurements with B~ on and off, respectively.

3. Results and Discusion

In Fig. 2 the result for the spin alignment

is shown together with the measured momentum14 14

distribution of the B particles. The Byield exhibits a broad peak centered at a momen

-10

i : IOOQc

500

18,'0 (60 PleV/u) + • X -

ap(targec)

1.2 1.1 1.6 1,8

p (GeV/c)

Fig. 2:Experimental spin alignment A of ^ B pro-duced in the reaction 180(60 MeV/u) + 9Be;the observed momentum distribution for 1^8is also shown. The plotted value of A isaveraged over the momentum bin indicated byhorizontal bars. The vertical extension ofthe hatched zone corresponds to the uncer-tainty in A (one standard diviatioin).

turn p =4.47 GeV/c which nearly corresponds to

B ions with the projectile velocity. Measure-

ments of A were made for three different momen-

tum windows. Around the peak momentum p , small

values of A were observed both for a narrow

( V p = *! %) and a wide ( V P = ^ 2) momentum

windows. An interesting feature appeared when

the measurement was made for a window in the re-

gion of high-momentum tail: A negative value of

A with a substantial size was observed for the

window around a momentum 6.3 7, higher than p

The negative sign of A in the high-momentum re-

gion conforms with tne prediction from a simple4)projectile fragmentation model , as illustrated

in Fig. 3.

The present result shows thit the observation

of the spin alignment in the projectile fragmen-

tation brings an opportunity to t;?st models for

reaction mechanisms. It has also tlv» important

implication that projectile fragmentation pro-

vides a promising tool to produce spin-oriented

unstable nuclei which must be extremely useful

for detailed spectroscopic studies of nuclei far

from stability and for other applications.

1/2 - Uo/rr

(0) V < Vg (tj) V = Vg (CJ V > Vg

Fig. 3:Predicted behavior of the fragment spinalignment based on a simple projectile frag-mentation consideration. On the bottom,situations for the outgoing velocity (a) v <vg, (b) v ~ VQ, and (c) v > vg are intui-tively illustrated, where VQ denotes theprojectile velocity.

1) D. Guillemaud-Mueller et al., Proc. of the

5th Int. Conf. on Nuclei Far From Stability

(Rossau Lake, Canada, 1987) p. 757.

2) R. Bimbot et al., J. Phys. C4-8, 241 (1986).

3) H. Morinaga and T. Yamazaki, "In-Beam ^-Ray

Spectroscopy" (North-Holland, 1975) Chap. 9.

4) K. Asahi et al., Nucl. Phys. A488, 83c (1988).

37

24

TOTAL REACTION CROSS SECTIONS AND LIGHT EXOTIC NUCLEUS RADII

M.Q. Saint Laurent. R. Anne, D. Bazin, D. Guillemaud-Mueller,U. Jahnke, Jin Gen Ming, A.C. Mueller (GANIL Caen)

J.F. Bruandet, F. Glasser, S. Kox, E. Liatard, Tsan Ung Chan (ISN Grenoble)G.J. Costa et C. Heitz (CRN Strasbourg)

V. El Masri (FNRS-UCL, Belgique), F. Hanappe (FNRS-ULB, Belgique)R. Birabot (IPN Orsay), E, Arnold and R. Neugart (Uni. of Mainz, RFA)

MOTIVATIONThe strong absorption radius is determined

by a measurement of the total cross sections ina nuclear reaction. Its dependence on the numberof protons and neutrons in the interactingnuclei is the subject of two recent works :

- Using the transmission method. He, Li, Beunstable projectiles and Be, C and Al targets,Tanihata et al (l! found remarkably large valuesfor "Li and ''•Be radii which were attributed tostructure effects.

- In a thick Si target experiment in whichthe Y rays from the nuclear reactions weredetected with a high efficiency, Mittig et al(2) measured the reaction cross-sectionintegrated over the beam range, for a variety ofunstable nuclei (Z = 5 - 12) ; their resultsshow a surprising linear dependence of thereduced strong absorption radius on theneutron-excess.

The aim of this experiment is to verificateif exotic nuclei have an anomalous radius.

In the present experiment, we are usingbeams of nuclei which have a neutron-excess toN - Z = 7 in the range Z.<, Zs 10. The resultsthus overlop with those of ref (1) and (2).

EXPERIMENTAL PROCEDUREThe present work is a OR measurement using

the 4 TIY method ; this technique has alreadybeen used with success for stable nuclei (3,6)and is complementary to the transmission methodused by Tanihata et al. Secondary beams ofexotic nuclei are produced through 22Nefragmentation in Be target at 45 MeV/u incidentenergy and selected using the LISE spectrometer.The resulting 95 values of OR have thus beenobtained for 49 isotopes : * '6He, 6~ 9 > ! 1Li,7 , 9 - 1 2 . I ' l g e , 1 0 - 1 5 , 1 7 B 1 1 - 1 9 C ] !3-l%J 1 5 ~ 2 ' 0

I8-21F and 2 0' 2 1Ne.' A convenient ' way ofdiscussing the Op data and comparing them withthe results of other experiments is to expressthem in terms of the reduced strong absorptionradius R0 using the Kox parametrisation (4) :

B,

A j ' 3 + A;'3

with R0 = 1.1 fm, b = 1.85 and in which C is theonly E dependent parameter accounting for thetransparency effect C = 0.14 + 0.015 E/A and Dis the neutron excess termD = 5(A t- 2Zt) Zp/(ApAt). The Ro valuesextracted for each isotopes at differentincident energies are in agreement with them,within uncertainties.

RESULTS AND DISCUSSIONThe R2 values deduced from these

measurements and from Tanihata experiments,using eq. (1) are plotted in fig. 1 versus theneutron number N.

2.Q

_ 1.8

Ji 1-6

U

1.2

1.0

1.8

1.6

1.4

1.2

1.0

1.4

1.2

1.0

D this workH e « Tomhoto

. D thiswork'-' aTomhata

a)

2 3 4 5 6 7 8 9 10 11 12 13N

Figure 1 : The reduced strong absorption radiusR* for Li (a). Be (b) and B (c) isotopes. Fullblack dots are data points obtained in thiswork, open circles are from Tanihata et al. (1)and the triangle from Mittig et al. (2).

The Tanihata R2

agreement with theIncertainties. For 1consistent within the

values are in goodpresent ones within

Be, both results areerror bars and a large

increase in R| for i'B is observed here for thefirst time. The sharp increase in the matterradius for the weakly bound most neutron-richisotopes has been associated with the occurence

38

25

of a spatially extended di-neutron halo (5). ForllLi, we find a smaller value in Ro radius thansuggested by Tanihata. This discrepancy can bedue to the electromagnetic dissociation of " L iinto «Li + 2n, without Demission, in the Coulombfield of the target as measured in ref 7.Neutrons, contrary to y , are not veryefficiently detected by our setup, leading to anunderestimation of OR by our experiment.

Recent shell model calculations (8) of theradii of 6~' ' Li predict a much smootherdependence of < ^2> on the neutron number thansuggested by either experimental data set. Inparticular the increase of < R2> becomes smalleras one approaches the magic neutron number N = 8for M L i . This is in agreement with the generalexpectation about shell effects, but it seems tobe in contradiction to both experimentalresults.

In this context it is interesting to notethat a recent measurement of the spin and themagnetic moment (9) has ruled out the suggestionof a deformed ground state of "Li (10) whichwould alternatively explain the increasein <R2>.

The region of nuclides covered by ourcross-section measurements includes the majorshell closures at N = 8 and Z = S. Here it issurprising to find no pronounced shell effects,i.e. relative minima in the radii, which areprominent in the N dependence of the mean squarecharge radii of heavier systems.

Looking at the more globr L trends, we findnearly constant values of R2 as a function ofN-Z as shown in fig 2. The mean valueR2, = 1.16 fm2 is in agreement with the R| = 1.21fm2 value find by Kox for stable nuclei, withinuncertainties. However, this is in contradictionto the linear increase with N - Z as reported inref 2.

REFERENCES(1) I. Tanihata et al. Phys Rev. Lett 55

(1985) 2676 - Preprint Riken AF-NP-60(1987)

(2) W. Mittig et al. Phys Rev. Lett. 59(1987) 1889

(3) J.F. Bruandet - J. Phys. (Paris) C4(1986) 125

- present report

(4) S. Kox et al. Phys. Rev. C35 (1987) 1678

(5) P.G. Hansen and B. Jonson Europhys. Lett.£ (1987) 409

(6) M.G. Saint Laurent et al. Thirdinternational conference on NucleusNucleus Collisions SAINT-HALO, France,June 6-11, 1988 ISBN2-905461-30-6M.G. Saint Laurent et al. Z Phys. inpress

(7) T.(1

.1 et al. Phys. Rev. Lett. 60

(8) N.A.F.M. Poppelier et al. 5th Int. Conf.on Nuclei far from Stability, RosseauLake, Ontario, Canada, in AIP Conferenceproceedings Vol. 164, Ian S. Towner ed.New York 1988, p. 334

(9) E. Arnold et al. Phys. Lett. B 197 (1987)311

(10) T. Bjornstad et al. Nucl. Phys. A 359(1981) 1

rsi

er"0

t.u

1.8

1.6

1.4

1.2

1.0

0.8

' » 1 1 1 1 1

oHe *Li oBe «B *C °N »0 *F *Ne

I

: I !. * , ' :- J\ ^i | ''' »*: f| (

r r i i I I I T

0 1 2 3 4 5 6 7N-Z

The reduced strong absorption radiusR o as a function of the neutronexcess N - Z for all the isotopesmeasured in this work.

39

SEARCH FOR PAR FROM STABILITY NUCLEIWITH AN ON-LINE MASS-SPECTROMETER.

M. DE SAINT SIMON, A. COC, P. GUIMBAL, C. THIBAULT,F.TOUCHARD,CSNSM Or,ayM. LANGEVIN, IPff OnayC. DETRAZ, D. GUILLEMAUD-MUELLER, A. C. MUELLER, GAN1L Caen

I - AIM OF EXPERIMENT.At the start-up of the new beams of GANIL,

it was tempting to gather most of the techniquesdevoted to collection and identification of nuclei inorder to try to reach the nuclear stability limits,in the neutron rich heavy nuclei region particularly.Among these techniques, we can notice the He jetcollection device, our mass-spectrometer with Z se-lection capability for alkali elements, well suited tocollection at rest of target-like fragments, and af-terwards, the electromagnetic separator LISE de-voted to projectile-like fragment identification andin-flight collection, and the magnetic spectrometerSPEG. Our mass-spectrometer equipment had beensuccessfully used in far from stability nuclei produc-tion and in nuclear structure measurements, and ithad been set on-line to many acceleration facilities:U-300 Cyclotron (JINR - Dubna) [1],PS [2], SC [3],ISOLDE [4] (CERN - Geneva)... Results reportedhere have been measured at GANIL during the 1983-1984 period.

2 - EXPERIMENTAL SET-UP.A conventional mass-spectrometer characteri-

zed by a resolving power ^jff of 300 and a goodtransmission is fitted with a surface ionization typeion source which is composed of 4 components: tar-get, diffuser, ionizing device and focusing device.The target can be located outside or inside the dif-fuser, and in this last opportunity, it is made of 50lmg/cm~ foils mixed with the diffuser foils. The dif-fuser is an assembly of 100 IQmg/cm2 graphite foilslocated inside an oven heated at 1700°C', convenienttemperature to get a fast and selective thermal dif-fusion of alkali elements. The ionizing device is aRe or Ta tube according to the needed work func-tion, heated at 1500°C', convenient temperature toget good ionizing efficiency of alkali elements and tominimize ionizing efficiency for non-alkali elements.Both processes, thermal diffusion and surface ion-ization, are highly selective for alkali elements. Theoutput of the equipment is fitted with a beam line inorder to tranport identified nuclei across a concreteshielding into a low background area where detectorsare located.

3 - EXPERIMENTAL RESULTS.Three designs of the target-diffuser assembly fittedto 3 tvpes of production mechanisms have been per-

formed and a new device designed to get chemicalselectivity with halogen elements has been tested.3-1 Target-like fragments. The isotopic distri-butions of recoils measured with a 93Nb (thickness30rn<7/cm2) are fully independent of the projectilenuclei: 12C, 1SO, wAr and of energy in excess of 25AMeV. The shape of the neutron deficient side ofthe isotopic distribution is governed by the ratio p?in the evaporative chain. As the target thickness islimited by the projectile range in target-diffuser as-sembly, the production rate of neutron deficient nu-clei is determined by the beam intensity only, whichhas been always smaller than 5x 1011ps~1 during themeasurements. As a comparison the yield of 7SRb is100 atoms per second instead of 106 observed withISOLDE and the 600 MeV proton beam of the SC(CERN). With a target exhibiting a different N/Zratio (natMo), the shape of the neutron deficientisotopic distribution remains completely unchanged,as attributed to a low collection efficiency for nucleiwith very weak recoil energy (< 0.1 AMeV) which isexpected for target-like fragments. Then the recoilscannot escape from the target thickness and reachthe diffuser material.

3-2 Projectile-like fragments. Projectile frag-mentation has been under study from the point ofview of nuclear production using 44 AMeV Ar beam,the heaviest projectile accelerated with a large inten-sity at that time, bombarding a lslTa target andlooking at Na fragments. The external target thick-ness has been made adjustable in order to set stop-ping location of recoils in the diffuser assembly; op-tima] target thickness has been determined to be180mg/cm?. Yields are 100 times lower than thoseobtained with a 100/cm2 Ir target bombarded by a10 GeV proton beam of 5 x ID12?*"1-3-3 Secondary reactions. A target-diffuser as-sembly has been designed for a 2 step process, butit could not be successful due to small yields in theprimary step of projectile fragmentation (see 3-2).3-4 Halogen selective ion source. A new ionsource has been designed in order to get Z selectiv-ity for halogen elements based on negative surfaceionization of LaBe with the hope to get diffusiontime as short as for alkali elements (~ 10ms). Effi-ciency measured for Br and I is 10~3 and diffusiontime is as long as 1 minute.

40

27

4- CONCLUSION.

Due to the lack of reaction mechanisms withgood selectivity leading to large production cross-sections as at low energy for fusion-evaporation pro-cess, and due to the peculiar energy range of re-coils, in the energy range of GANIL projectiles thedominant projectile fragmentation process has beenshown less promising than foreseen at least for col-lection of fragments at rest. For projectile-like frag-ments the recoil energy is too large and most ofthe fragments are lost in slowing down process; fortarget-like fragments the recoil energy is too weakand most of the fragments cannot escape from thetarget material. Improvements of the geometry oftarget-diffuser assembly could be undertaken, butthe beam current limitation, at least at that time,was not encouraging to hope to get better results

than the existing facilities such as ISOLDE. Sincethat time, the LISE spectrometer has been shownto be better suited to the kinematics of recoils atGANIL.

References:fl] M. de Saint Simon et al. Pkys. Rev. C14 (1976)S185.fl] M. de Saint Simon et al. Nucl. Instr. Meth. 186(1981) 193.[3] M. de Saint Simon et al. Phys. Rev. CS6 (198S)

141 L. C. Carraz et al. Nucl. Instr. Meth. 158(1979) 69.

41

28

D

NUCLEAR REACTIONS

29

- B 1 -PERIPHERAL COLLISIONS, PROJECTILE - LIKE FRAGMENTS

30

PROJECTILE-LIKE FRAGMENT PRODUCTION IN Ar INDUCED REACTIONS AROUND THE FERMI ENERGY

V. Borrel1', J. Galin2', B. Gatty",D. Guerreau2' and X. Tarrago1}

1) IPN, BP N° 1, 91406 Orsay, Cedex France. 2) GANIL, BP 5027, 14021 CAEN, Cedex France.

The primary aim of these experiments was

to look for the transition from the one-body

to the two-body dissipation through the study

of the isospin degree of freedom (N/Z ratio)

which is known to be, at low energy, the

fastest node to reach its relaxation (~2 10"22

sec). The N/Z ratio of the projectile-like

fragments (PLF) was then supposed to provide

relevant information concerning the

disappearance (or the persistence) of a

collective behaviour of nuclear matter.

The first run was the very first

experiment performed at GANIL in January 1983.

Isotopic distributions and energy spectra of

PLF have been measured in the forward

direction for 26.5 and 44 MeV/u Ar induced

reaction on 5 8 " " Ni, 1 0 3 Rh and 1'7Au (i.e.

targets with significantly different N/Z

ratios, from 1.07 up to 1.47)1"4 . The most

striking feature of these studies is the role

still played by the target in the PLF

production. This is stressed in fig.l which

shows the mean N/Z ratios of the PLF obtained

with 26.5 MeV Ar projectile impinging on the

four different targets. The more n-rich is the

target, the more n-rich the PLF appear to be.

At that time, this was already an indication

for the mean field to play a significant role

in this intermediate energy range. However,

the effect is rather small on the mean values,

and can also be explained as the result of a

rather large energy dissipation leaving the

primary PLF rather excited (30 to 50 MeV

excitation energy according to recent

exclusive experiments 5> ). Furthermore, this

target effect is strongly enhanced when

looking at the production of the most n-rich

fragments, as shown in fig.2 comparing the

production rates for 58Ni and 6 4Ni targets. It

should be stressed that this effect has been

extensively exploited afterwards to produce

isotopes near the n-drip line at these

energies6'.

The second point worth to be stressed

from these "first generation" of inclusive

experiments is connected with the origin of

PLF close to the projectile mass. The results

show the coexistence of pure transfer

reactions and break-up processes. For

instance, 3 8Ar, 39C1 and <11K have been clearly

identified as preferentially produced through

a transfer reaction. The maximum yields in the

energy spectra for stripping reactions are

observed at the expected optimum Q value

whereas they are peaked at an energy close to

the one corresponding to the Qgg for pick-up

products (fig.3). This can be explained as

suggested by Siemens assuming that the

transferred nucleons are at rest in the frame

of the nucleus from which they originate. This

hyprthesis implies that most of the excitation

energy is found in the recipient nucleus (and

not in the donor). The energy spectra have

been analysed successfully in terms of direct

transfer reactions leading to continuum

states. A good agreement is obtained with the

prediction of the diffractionnal model 4 ).

Fig.l. (N)/Z ratios for PLF from a 26.5 MeV/u

Ar bombarding 5 8Ni, 6*Ni, 1 0 3Rh and 1 9 7Au

targets.

42

31

Fig.2. Evolution with PLF atomic number and

mass of the ratio of differential cross

sections obtained with 58Ni and 6*Ni targets

at 26.5 MeV/u.

1) D. Guerreau et al, Phys. Lett. 131B (1983)293.

2) V. Borrel et al., Z. Phys. A314 (1983) 191.

3) V. Borrel et al., Z. Phys. A324 (1986) 205.4) M.C. Mermaz et al., Z. Phys. A324 (1986)

217.5) J.C. Steckmsyer et al., Internal Report

LPCC 89-01.

6) D. Guerreau, J. Physique C4 (1986) 207.

700 BOO 900 1000 1100 1200ENERGY (MeVl

Fig.3. Energy spectra of one and two nucleontransfer reactions measured at the grazingangle. Solid curves are theoreticalpredictions using the diffractional model *'.Arrows correspond to the optimum Q valuefor each channel.

43

32

Angular Evolution of the Production of Projectile like Fragments

in Intermediate Energy Heavy Ion Collisions

D. Beaumel, Y. Blumenfeld, Ph. Chomaz;N. Frascaria, J.C.Jacmart J.P. Garron, J.C. Roynette,

J. A. Scarpaci and T. SuomijarviInstitut de Physique NucKaire, 91406 Orsay Cedex, FVance

J. Barrette, B.Berthier, B. Fernandez and J. GasteboisDPhN/BE, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France

D.Ardouin and W. Mitt igGANIL, BP 5027, 14021 Caen Cedex, France

1) MotivationsThe abundant production of projectile likefragments (PLF) exhibiting velocities close tothe beam velocity is a general feature of all in-termediate energy liea'/y ion reactions. PLF'saccount for a large fraction df the total reac-tion cross section and their study is a nec-essary first step towards the understandingof the underlying reaction mechanisms. Thescattering angles of the PLF's will be deter-mined both by the interplay of the nuclear andcoulomb deflections and by the reaction mech-anism itself (nucleon transfer, abrasion, stick-ing....). The measurement of PLF angular dis-tributions down to angles inside the grazingangle and the study of the properties of thePLF's (isotopic distributions, mean velocities,momentum widths...) as a function of angleshould allow us to enhance our knowledge ofperipheral reaction mechanisms.

2) ExperimentsThe mass, charge, energy, and angular distri-butions of the PLF from the *°Ar + 208P6reactions at 44 |l,2] and 33 MeV/u [2] andthe 40Ar H- *°Ca [1,2] reaction at 44MeV/uwere measured with a time of flight system(3] allowing to obtain simultaneously severalpoints of the angular distribution. For the sakeof comparison two short runs were performedat one angle for the *°Ar + *°Ca system at27MeV/u and the 20Nc + 20aPb reaction at44MeV/u. The elastic scattering counting ratelimits the use of a time of flight system at veryforward angles and hence to obtain data downto 0((t(l=0.50 the *°Ar + 2 0 8Pt reaction at 60MeV/u was studied (4) using the SPEC spec-

'Division de Physique theorique, Laboratoire aseociiau CNRS

trometer.

3) ResultsWe will briefly review the features of the var-ious quantities measured in these experimentsand stress the analogies and differences withthe behaviors observed at lower and higher in-cident energies.Most probable PLF velocities : The PLFspectra are centered at velocities between 90and 100 % of the beam velocity. The generaltrend is a decrease of the velocity with decreas-ing fragment mass. When one moves towardsangles larger than the grazing the velocitiestend to drop. These two last features are anal-ogous to those observed at low bombarding en-ergies when measurements are performed nearthe grazing angle (quasi-elastic reactions).Momentum widths : The energy (or mo-mentum) spectra of the PLF exhibit a gaus-sian shape prolonged by a low energy tail in-dicative of energy dissipation. In the frame-work of the fragmentation model [5] it hasbeen claimed that the reaction mechanisms invarious systems can be compared by fitting thehigh energy part of the spectra and extractingthe reduced momentum width parameter on.We have found that the a,, values for the *°Ar+ *°Ca reactions at 27MeV/u and 2"Ne +208 Pb at 44Mev/u do not agree with the sys-tematics compiled by Murphy and Stokstad|6j. This result seriously questions the rele-vance of the reduced momentum width to fol-low the evolution of reaction mechanisms atintermediate energies.

Isotopic distributions : In general for agiven charge the PLF isotopic distribution isapproximately Gaussian. Neutron rich iso-topes are more strongly populated when neu-

44

33

tron rich targets are used. However when frag-ments close to the projectile are consideredthe isotopic distribution presents two compo-nents, one of them extending over all anglesand the other strongly focused at the grazingangle. This feature shows up spectacularly inthe 40Ar + 208P6 reaction at 33MeV/u wherethe production of 30Cl and 30Ar (the p anda-particle transfer channels) is strongly en-hanced around the grazing angle. This showsthat simple direct transfer reactions subsist atintermediate energies.Angular distributions : The striking evolu-tion of the shape of the angular distributionswith fragment mass is illustrated on fig.l forthe i0Ar + 208P6 reaction at 44MeV/u. ForPLF close to the projectile a bell shaped distri-bution peaked slightly inside the grazing angleis observed while for lighter fragments the dis-tributions decrease monotonously with angle.This pattern resembles the behavior observedat lew incident energies. Classical trajectorycalculations show that the bell shaped angulardistributions are incompatible with an abra-sion mechanism for which the density over-lap required between the two nuclei inducesforward peaked distributions. Thus the frag-ments close to the projectile must be producedby surface transfer reactions. At very forwardangles the strong dependence of the angulardistributions on fragment mass remains. Acalculation supposing fragmentation accompa-nied by a deflection of the PLF by the meanfield was performed. To reproduce the datafor fragments with M < 34 a deflection an-gle between 1.5 and 2 degrees (much smallerthan the grazing angle da) was needed show-ing the importance of the attractive nuclearforce. For heavier fragments the data couldnet be reproduced which points once again tothe importance of surface transfer reactions.

4) ConclusionsWe have shown that the measurement of theangular evolution of PLF production is crucialfor the understanding of the reaction mecha-nisms responsible for their production. A sur-face transfer reaction component dominant atthe grazing angle can be separated from a sec-ond component which cannot be entirely ac-counted for by a simple fragmentation mech-anism. Angular distribution studies yield im-portant information on the deflection of thePLF's by the mean field potential.

E, =«MeV/u

•13 <!» £

CL

3's

5Mg

Figure 1: Angular distributions of PLF emit-ted in the 40Ar+208P6 reaction at 44 MeV/u.

References

[1] Y.Blumenfeld et al. Nucl.Phys. A455(1986) 357

[2] Y.Blumenfeld These d'etat IPNO-T-87-07 (Orsay 1987)

[3] E.C.Pollacco et al. Nucl. Inst.Meth. 225(1984) 51

[4) T Suomijarvi et al. Phys. Rev. C36(1987) 2691

[5] A.S.Goldhaber Phys.Lett. 53B (1974)306

[6] M.J.Murphy and R.G.Stokstad Phys.Rev.C28 (1983) 428

45

34

PERIPHERAL COLLISIONS IN THE 40Ar t 88Zn REACTIONBETWEEN 15 AND 35 MeV/NUCLEON

J.P. COFFIN, A. FAHLI, P. FINTZ, G. GUILLAUME, B. HEUSCH, F. KAMI and P. WAGNERCentre de Recherches Nucleaires and Universite Louis Pasteur, 67037 Strasbourg Cedex (France)

Y. SCHUTZGANIL, B.P. 5027, 14021 Caen Cedex (France)

M. MERMAZC.E.N., Saclay, 91191 Gif sur Yvette Cedex (France)

1. MotivationBelow 10 MeV/nucleon, peripheral heavy

ion collisions are associated with quasi-elasticand deep-inelastic transfer reactions, while athigher energy (a 100 MeV/nucleon), they aregoverned by a fragmentation process. Theinvestigation of the intermediate energy regionis essential to provide information about thenature of the transition from the mean fieldregime, characterizing the low energyreactions, to the nucleon-nucleon interactionregime governing the high energy collisions.

In order to shed some light on thisquestion, we have investigated the competitionbetween few nucleon transfer reactions andprojectile fragmentation process in the40Ar + 68Zn system, as a function of thebombarding energy.

2. Results and discussion

The experiments were conducted at 40Arincident energies of 14.6, 19.6, 27.6 and35 MeV/nucleon,

Bidimenslonal spectra (A,E) and (/ E,E)have been measured at several angles. Bothprojectile-like fragments and productsresulting from low energy dissipative processeswere identified. The momentum widths of theprojectile-like fragments have been deducedfrom the analysis of their velocity spectra.The results indicate the presence of tworeaction processes : transfer reactions andprojectile fragmentation. The competitionbetween these two mechanisms is clearlyillustrated by the observation at 27.6MeV/nucleon (Fig. 1) of two groups of

fragments. For nuclides lighter than 36, thereduced momentum width 0o is constant (o<> =84 MeV/c) and very close to the one observedIn high energy experiments. Fragments close tothe projectile (A a 36) have a much smallerCo value (GO = 49 MeV/c). Other signatures ofthis competition between transfer andfragmentation processes were also obtainedfrom the analysis of the average velocities ofthe projectile-like fragments.

150

100

b 50

—-c0- tO%

ojj^iMeV/c'

10 20 301

Fig. 1 :Reduced momentum widths ao plotted as afunction of the ejectile mass for themAr + ^Zn reaction performed at27.5 MeV/nucleon.

Few nucleon pickup and strippingreactions were analysed (Fig. 2) in terms ofdirect surface transfer reactions leading tocontinuum states with a diffractional model.The data are consistent for stripping reactionswith a direct transfer of nucieons and atarget excitation yielding multlparticlemultihole configurations.

Fig. 3 shows the evolution with thebombarding energy of the reduced momentumwidths of the projectile-like fragments. For

46

35

700 900 1100ENERGY (MeV)

Fig. 2 :Energy spectra of MK, "Cl and *S ejectilesmeasured at 5' laboratory angle. The solidcurves are the result of calculations based onthe diffractional model.

the projectile fragmentation processes the

values of o<> at 27.6 and 35 MeV/nucleon are

represented by solid points. Those

corresponding to • nucleon transfers are

represented by open circles (the value at 35

MeV/nucleon corresponds to an upper limit).

100

tu

— 50

b°

projectile fragmentation .

complex transfer

The solid curve is a diffractional model

calculation for complex transfer of nucleons.

The straight solid line corresponds to

projectile fragmentation as found in the

literature and extrapolated to lower incident

energies. The slanted dashed line, indicating

the tendency of the available experimental o<>

values within the 10 <. EP/A £ 50 MeV/nucleon

energy range, gives a wrong idea of the actual

situation since it suggests that o» increases

with the bombarding energy. In fact, one has

to distinguish the transfer component and the

projectile fragmentation one, as it has been

done In the present work.

In conclusion, two mechanisms have been

identified : few nucleon direct transfer

reactions and projectile fragmentation process

which appears between 20 and 27 MeV/nucleon

and competes strongly with transfer at 35

MeV/nucleon. The momentum width has been

found to be the relevant parameter for

distinguishing between transfer and

fragmentation processes.

3. ReferencesF. RAMI et al., XXIInd Int. Winter Meeting onNuclear Physics, Bormio, Italy (1984)P. RAMI et al., Z. fur Physik A318 (1984) 239M.C. MERMAZ et al., Phvs. Rev. C31 (1985)1972F. «AMI et al.. Nucl. Phys. A444 (1985) 325F. RAMI, Thesis, Universite de Strasbourg,France (1985)F. RAMI et al., Z. fur Physik A327 (1987) 207

50 100 150

Ep (MeV / nucleon!

Fig. 3 :Reduced widths on of the linear momentumdistributions of projectile-like fragments as afunction of projectile energy.

47

36

EXCLUSIVE STUDY OF PERIPHERAL INTERACTIONS

G.Bizard,R.BrouJ.L.Laville,J.B.Natowitz,J.P.Patry,J.C.Steckmeyer,B.Tamain L P C , CAENH.Doubre.A.P6ghaire,J.P6ter,E.Rosato GANIL, CAENF.Guilbault,C.Lebrun L S N , NANTESF.Hanappe U L B , BRUXELLESJ.C.Adloff,G.Rudolf,F.Scheibling C R N , STRASBOURG

l)Motivations and experimental set-up

When the first inclusive experiments were made on peripheral interactions in the GANIL energy range ,the conclusion was that the high energy fragmentation regime, well understood by the Goldhaber theory, wasalready almost achieved with heavy ions beams of a few tens of MeV per nucleon. Later on , it was found thatthings were not so simple and that the so called fragmentation peak was in fact due to the superposition ofdifferent processes : direct break-up, transfer reactions , sequential break-up . The easiest way to disentangle allthese contributions is to try to detect all the particles accompanying the remaining projectile like fragment. Thisis possible using the plastic wall in operation at GANIL. This multidetector covers the forward angular cone(3°< 6 < 30°) with 96 counters, allowing charge identification of the light nuclei up to Z=8. The energy of theparticles can be accessed through! the measurement of their velocity. To complete the experimental set up, a fourelements solid state telescope was located near the grazing angle of the reactions, in order to detect and identifythe projectile like fragments (figure 1). Two systems were studied : Ar+Ag and Ar+Au at four incidentenergies:35,39,50, and 60 MeV per nucleon, in two separate experiments.

2)Transfer reactions

Among the various classes of experimental events , the simplest one is constituted of projectile likefragments which are not accompanied by any charged particle in the wall. Apart from small corrections due torapid particles not detected in the wall (6 < 3° or 6 > 30°) , these events are associated with charge transferreactions mainly from the projectile to the target. In figure 2 the charge transfer probabilities are displayed forAr+Au and Ar+Ag at 60 MeV/A and for Ar+Ag at 35 MeV/A as a function of the projectile like fragment atomicnumber . We can see that these probabilities are independent of the target and that they decrease quite slowlywith increasing energy, indicating that mean field effects are still important at 60 MeV/A.

3)SequentiaI break up

Let us consider now the events for which at least one rapid charged particle is found in coincidence withthe projectile like fragment. If we restrict the detection zone of the wall to the few counters which are locatednearly in the direction of the telescope (those counters which are hatched in figure 1 ) , the correlation plotbetween the energy of the light charged particle (LCP) and that of the projectile like fragment (PLF) exhibits twodistinct regions (fig 3) . This observation is a clear signature of a sequential binary decay mechanism of aprimary excited PLF ,the two regions of the correlation plot corresponding to the forward and backwardemission of the LCP in the center of mass system of the primary PLF. It can be shown that a large number of therapid light particles detected in the whole plastic wall (around 75% of the alpha particles and at least one third ofthe protons ) originate from such a sequential decay of an excited projectile like fragment for a bombardingenergy of 60 MeV/A.

4)Excitation energies

Taking into account the sequentially emitted particles (and correcting for the unseen neutrons and gammarays), the excitation energy of the primary projectile like fragments can be evaluated .The results are displayedin figure 4 for the 60 MeV/A Ar+Ag system and compared with different models : abrasion model, equaltemperature in quasi projectile and quasi target fragments , equipartition of the excitation energy ...They showthat none of these models are able to describe the data.

5)Conclusion

Peripheral interactions at incident energies as high as 60 MeV/A cannot be described by high energymodels (participant spectator) .To account for transfer reactions and sequential decays of the projectile, lowenergy concepts implying inelastic interactions between projectile and target have also to be considered.

6)References

Fragmentation and/or transfer reactions in the 35 MeV/A Ar+Ag systemG. Bizard et alPhys. Lett. B172(1986) 301

Transfer reactions and sequential decay of the quasi projectile in the 60 MeV/A Ar+Ag and Ar+AureactionsProceedings of the 25th winter meeting on nuclear physics -Bormio-(italy) 19-24 January 1987 , P 229

48

Transfer reactions and sequential decays of the projectile like fragments in the 60 MeV/nucleon Ar+Ag,AureactionsJ. C. Steckmeyer et alSubmitted to Nuclear Physics

Etude semi exclusive de la reaction Ar+Ag a 35 MeV/nucleonG. PloyartThese de Seme cycle. Caen 1985

Etude de 1 'emission des fragments quasi projectiles produits dans des reactions induites par ions Ar a 60MeV/nucleonA. ThiphagneThese de I 'universite de Caen 1987

Figure 1

1

V6I

1(3 05

%

1

0

D 35 MeV/A Ar « Ag (e - eg)

• 60 MeV/A Ar«Ag (e -159,)

t- 60 MeV/A A r . A u (e - 8,)

i"T

_o

O -1

" ft4} beami i . . t

14 15 16 17 18

Z PLF

0 100 200 JOO 400 500 SOOLCP energy (MeV)

60 MeV/u Ar + Au

Z(plf)=15 Z(lcp)=21.4 1.8 2.2

PLF energy (GeV)

Figure 2

; ISO

100

Prrinary atomic intmbvr

Figure 3 Figure 4

49

STUDYING PERIPHERAL COLLISIONS IN THE REACTION Ar + Au AT 27.2, 35 AND 44 MeV/uBY THE ASSOCIATED NEUTRON MULTIPLICITIES

M. MORJEAN'-b, J. FREHAUT', D. GUERREAUb, J.L. CHARVET" , G. DUCHENE" •' , H. DOUBREb,J. GALINb, G. INGOLD0, D. JACQUET"1, U. JANHKE6'0', D.X. JIANGb'2, B. LOTT', C. MAGNAGO1 ,

Y. PATIN", J. POUTHASb, Y. PRANAL* and J.L. UZUREAU"

0 CEA Bruyeres-le-Chatel, BP.12, F-91680 Bruyeres-le-Chatel, France.b GANIL, BP.5027, F-14021 Caen Cedex, France.c Hahn-Meitner Institut, Berlin 39, Germany.d IPN, BP. N"l, F-91406 Orsay Cedex, France.

When considering peripheral collisions,the knowledge of the amount of thermalizedenergy shared by the partners is veryimportant to help distinguish between transfera'nd break-up types of reactions. The neutrondetection is well suited for collisionsinvolving heavy targets as they will cool downessentially by neutron evaporation. In thatcase, measuring the multiplicity of evaporatedneutrons provides the information on theexcitation energy deposited in the target-likefragment (TLF).

The system 40Ar + 197Au has been studiedat three bombarding energies 27.2, 35 and 44MeV/u1j2). Peripheral collisions were selectedby detecting the projectile-like fragments(PLF) near the grazing angle and at the sametime, the multiplicity of slow neutrons(originating from the TLF) was deduced bymeans of a 411 neutron detector (500 liters Gdloaded liquid scintillator). The efficiency ofsuch a detector was found to be 74X for Cfneutrons.

Fig. 1 shows, for the three bombardingenergies, the correlation between the mostprobable average neutron multiplicity <Mp> andthe PLF mass. It clearly stresses that, forPLF's with masses between 40 and about 25, thelighter the detected PLF, the more excited theundetected TLF is. This is a clear indicationfor "CM,, > to characterize the degree ofviolence of the collision and, the impactparameter. However, no significant change of<Mn > for a given PLF is observed whenincreasing the incident energy. Thisdefinitely rules out the idea of a dominantmassive transfer process for peripheralcollisions, as it was observed at lowerenergies. These results have been comparedwith a phenomenological model developped byBonasera et al. 3 ) consisting of a two stepprocess (a standard one body dissipationtaking into account the Pauli blocking effectspossibly followed by an abrasion step). Theagreement is quite satisfactory and showsclearly that mean field effects still play asignificant role at these energies.

However, the size of the detectedfragment is no longer characteristic of agiven impact parameter for PLF's with masslower than about 25. This feature can also beseen for very large kinetic energy losses ofthe PLF (fig.2) for which the associated <!!„>becomes rather high and independent of thedetected PLF. This could correspond to aprocess rather similar to -deep inelasticcollisions as observed at lower energies.

Ar + Au

3% 10 s *o ts :o a x x 30 10 35 to ts

:*>:&. * . K . * . If

Fig.l. Evolution with incident energy of thecorrelation between <M n> (not corrected forefficiency) and the mass of the PLF. Solidcurves result from a calculation according toBonasera et al.3'.

Arl27.2MeV/ul

Fig.2. Energy spectra of PLF observed at 8°(solid lines). Dashed lines show the evolutionof <Mn > (not corrected for efficiency) as afunction of the PLF kinetic energy. Arrowsindicate the beam velocity.

1)

2 )

3)

M. Morjean215.D . Guer reau

et al., Phys.Lett. 203B (1988)

collisions,p. 391.Bonasera et al.653.

et al., Proceedings of theSymposium of heavy ionShimoda (Japan), October 1987,

Nucl. Phys. A463 (1987)

50

OISSIPATIVE FRAGMENTATION IN MEDIUM ENERGY HEAVY ION COLLISIONSA.Adorno'J, A.Bonasera'-*', M.Di Toro'}, Ch.Gregoire'J, F.Gulminelli"'') Dipartimento di Fisica and I.N.F.N., Catania, Italy') Sektion Physik der L.M.U., Garching b.Munchen, W.Germany') GANIL, Caen, France-) Dipartimento di Fisica and I.N.F.N., Milano, Italy

Heavy ion reactions at the GANIL energies raise very difficult problems in nuclear dynamics.Pauli blocking is less and less effective and we expect to see a quite strong competition betweenmean field and collisional dynamics. Microscopic treatments require large numerical efforts withsome fundamental problems of difficult solution, e.g. variations in the mean field and effectiveforces, medium effects on the collision term, accurate description of phase space distributions.The aim of this project is to develop a relatively transparent model in phase space, based on safephysical grounds, able to quantitative reproduce a large volume of data and to make reliable pre-dictions (1,2,3). We introduce a two stage mechanism (see Fig.l).In the first stage we have a one-body dissipationprocess (incoherent nucleon exchange through a neck)quite well described within the window formula, suitably corrected due to the partial overlap of momeinturn distributions. We can evaluate energy and ang£lar momentum losses, orbiting and prompt particle £missions. Coherent one-body dissipation is also i£eluded in the approaching phase through the excita-tion of high frequency collective motions, giant re_sonances, expected to play an important role just"on the basis of time matching conditions (fl^n *Across -!'• Tne second stage is an abrasion mechanism with formation of an highly excited intermedi£te source, whose properties (mass, energetics) aresubstantially affected by a still active Pauli bio-

TOUCHING POINT OtSSFATIVE STAGE ABRASION STAGE

Fig.l

ching and one-body corrections. Compression effects can be also included with a possible teston the nuclear equation of state. Three primary bodies are formed in the exit channel! with welldefined properties: excitation and kinetic energies, spins, masses, deflection angles. All possi-ble correlations can be worked out, and sequential statistical emissions of particles can be eva-luated. Since the classical impact parameter analysis of different reactions, the model is parti-culary suitable to study exclusive measurements, using as a trigger fragments of different massbins in forward direction. Applications are discussed on: i) transtion from deep inelastic pro-cesses to peripheral fragmentation (1); ii) exotic isotope production (5); iii) formation anddecay of fragments with high excitation energy and angular momentum (4); iv) energetic particle(pions, hard photons, h.e,protons), emission from a hot intermediate source (2,6); v) dynamicalpolarization of nucleon momentum distributions and effects on fragment momentum widths (7).

Fig.2

Cue to the persistence of mean field effects, alarge amount of energy is dissipated in the "spe-ctator" fragments. This has been observed in theneutron emission from target-like fragments (Fig.2)and it has noticeable effects on isotopic produ-ction. Fig.3 shows the primary projectile fragmentproduction cross section and excitation energy for35 and 100 MeV/A ""Ca beans on " a T h . At 35 MeV/Awe have much less fragmentation, only with PLF ma£ses larger than 30 units and more neutron rich.

10. 20. 40 50A(PLF)

Fig.3

51

100.

1

.01

100.

1.

01

I i i i i | i i i i i i i i rz-e z.e 35MeWA

z-6 Z-B 100MeV/A,

"~-

F i g . 410 15 20

44MeV/A

«> so oo n eo K> noC_,(MeVI

«o SO 00 10 W 90 WO

Cn-(MeV)Fig.5

,-210

10-

10 <

UJ-2 -

Fig.6

4O 60

Ej CMeVJ

However (Fig.3b) we have also much larger excitation energieswhich finally will compensate the different initial conditi-ons with a longer ablation chain. The final production yieldwill be essentially the same (Fig.4): probably for this pro-jectile-target system some intermediate energy {•*• 50 MeV/A)will optimize the production of neutron rich isotopes.Intermediate mass fragments (IMF) can be produced by directbreak up of projectile and/or target. Moreover the primaryPLF and TLF can have a relatively large excitation energyand angular momentum. In this view it would be important tolook carefully at sequential decays before discussingdirect mu Hi fragmentation.The fireball-like source can emit energetic particles. Someinclusive pion spectra are presented in Fig.5. Since we havean impact parameter-PLF correspondence, our results are sui-table for exclusive measurements. An example is shown inFig.6 for hard photon production in inclusive measurementsand in coincidences which select peripheral processes.We reproduce not only the absolute values but also the slopevariation of the spectra.In conclusion we have developped a quite nice model, easy towork with, which reproduces, without free parameters, the

main features of dissipative medium energy collisions and can be used to plan new, more exclusive,experiments.

1. A.Bonasera, M.Di Toro and Ch.Gregoire:Nucl.Phys. A463, 653, 1987

2. A.Bonasera, M.Di Toro and Ch.Gregoire:Nucl.Phys. A483, 738, 1989

3. M.Di Toro:in "Trends in Nuclear Physics", E.Fermi School, Varenna 1987, Ed.

4. A.Adorno, F.Gulminelli, A.Bonasera, M.Di Toro and Ch.Gregoire:Int. Winter Meeting on Nuclear Physics, Borraio 1988, p. 491

5. A.Adorno, A.Bonasera, M.Di Toro, Ch.Gregoire and F.Gulminelli:int. Conf. on Nuclear Reaction Mechanisms, Varenna 1988, p. 231

6. A.Adorno, A.Bonasera, M.Di Toro, Ch.Gregoire and F.Gulrainelli:St. Halo 1988, Nucl.Phys. A488, 451c, 1988

7. M.Di Toro, G.Lanzano and A.Pagano:Phys.Rev. C37, 1485, 1988

P.Kienle

52

FORMATION OF HIGHLY EXCITED QUASI-PROJECTILE FRAGMENTSIN THE 84Kr + 9 3Nb REACTION AT 35 MeV/u

J.L. Charvet1*, R. Billerey2, B. Chambon2, A. Chbihi2, A. Chevarier2, N. Chevarier2,D. Drain2, G. Duchene1, S. Joly1, C. Magnago1", M. Morjean1b, C. Pastor2, Y. Patin1,

A. Peghaire3, Y. Pranal1, L. Sinopoli1, M. Stern2, and J.L. Uzureau1.

(1) CEA, Bruyeres-le-Chatel, Service PTN, BP 12, F-91680 Bruyeres-le-Chatel(2) IPN, Universite Claude Bernard, Lyon 1, F-69622 Villeurbanne Cedex(3) GANIL, BP 5027, F-14021 Caen Cedex.

a) present address: CEA Saclay, DPhN/BE, F-91191 Gif-sur-Yvetteb) present address: GANIL, BP 5027, F-14021 Caen Cedex

1. IntroductionIn heavy ion induced reaction at

intermediate energies, the production offragments having a velocity close to theprojectile one has been generally interpretedin terras of a projectile fragmentation process(1). The abrasion model including dissipationseems to reproduce the essential features ofargon induced reaction data (2), althoughrecent calculations based on a primarytwo-body process, similar to a deep inelasticcollision, give also a good understanding ofthe data (3, 4). Nevertheless, when usingprojectiles heavier than argon, the situationseems much more complex (5, 6) and we havechosen to study the properties of theprojectile-like fragments issued from the8 4 Kr + " Nb reaction at 34.5 MeV/u. Inaddition f> the inclusive measurements wetried to determine the excitation energy ofthe primary fragments by measuring thefragment-fragment and fragment-light particlecorrelations in the forward direction.

2. Inclusive measurementsOn one side of the beam direction the

experimental set-up was constituted of a largegas ionization chamber and located between3.5° and 15.5°, which measured the nuclearcharge (Z-38 down to 10) and the energy of theprojectile-like fragments. The main propertiesobserved for these fragments can be summarizedas follows (7): i) all of the fragment energydistributions exhibit a relative broad peak atan energy corresponding to a velocity about80-90% of the beam velocity; ii) theproduction of light fragments (Z-<20) becomepredominant beyond the grazing angle. Theseresults cannot be understood in a puregeometrical abrasion-ablation model.

3. Fragment-fragment correlation measurementsOn the other side, with respect to the

beam axis, we placed at 7° a three-stagetelescope and measured the fragments (Z2) incoincidence with those detected in the chamber(Zl). The average of the sum Z1+Z2 of allfragments in coincidence is about 28-30, lowerthan the projectile, charge (Z-36). Therelative velocityVl-V2 between two fragmentsin coincidence has been calculated event byevent and a typical distribution is shown infig.l for an angle in the chamber selected at8 °. The raost probable value of about 2cm/nsagrees with the expectation of Coulombrepulsion between two fragments both having acharge of the order Z-15. The broadening of

the distribution can be explained by thenucleon evaporation effect from the fragments(5). We concluded that our data are compatiblewith a binary break-up of a source having avelocity around 90% of the beam velocity and amass close to the projectile one, which arethe characteristics of a quasi-projectilefragment (8).

500.

400.

300.

200.

100.

0.

4Kr * 9^Nb34.5 MeV/u

«e,=83

1

' 'k! i=

0. I. 2. 3. 4. 5.RELATIVE VEtOCnY(cm/ns)

Fig.l. Spectrum of relative velocity of twofragments measured in coincidence between thechamber at 8° and the telescope at -7°.

4. Fragment-light charged particle correlationmeasurements

The back of the ionization chamber wascovered with a thick titanium foil in order toplace six light charged particle detectors,each one constituted of a 1mm thick NE102scintillator followed by a photomultiplier andlocated at 4.3°, 7.6°, 10.5°, 12.1', 13.6° and15.3°. In fig.2 we have reported typicalenergy spectra for Z-l and Z-2 in coincidencewith fragments having a charge 10<Z1<20 andintegrated over the entire angular range ofthe chamber. The correlated fragment energy

53

spectra exhibit approximately the same shapesas measured in the single spectra. The solidcurves shown in fig.2 result from a fit usingthe expression of the moving source model (9).The extracted velocities of the sourcescorresponding to emission of particles withZ-l and Z-2 are almost equal (94X and 93* ofthe beam velocity respectively) and close tothe one obtained from the correlated fragmentmeasurements. The T parameters are indicativeof the source temperature and we deduced thevalues of 5.1 HeV and 6.4 HeV showing a highlyexcited source. It should be noted that thediscrepancy at high energies can be connectedto the presence of direct particle emission.

We clearly showed that the intenseproduction of fragments with charge Z<-20 areoriginated from a source with a highexcitation energy and velocity close to thebeam one (8). Such data could be consistentwith the early stage of deep inelasticcollisions in which the quasi-projectilearised with strong excitation energy anddecayed by break-up associated with both pre-and post- break-up evaporation.

"Kr."Nb 4.5 MeV/u

10'

10'

!? ios

z

I 10'

10'

10'

I01

10*

Fig.2. Energy spectra of Z-l and Z-2 particlesdetected at various angles in coincidence withthe 10<Z<20 fragments measured between 3.5°and 15.5°

References

(1) D. Guerreau, Nucl.Phys. A447 (1985) 37c.(2) R.A. Dayras et al., Nucl.Phys. A460 (1986)

299.(3) R.A. Dayras et al., Phys.Rev.Lett. 62

(1989) 1017.(4) L. Tassan-Got, Ph.D. Thesis, Orsay, 1988.(5) M.J. Murphy et al., Phys.Rev.Lett. 53

(1984) 1543; Phys.Rev. C33 (1986) 165.(6) G. Rudolf et al., Phys.Rev.Lett. 57 (1986)

2905.(7) J.L. Charvet et al., Z.Phys. A321 (1985)

701.(8) J.L. Charvet et al., Phys.Lett. B189

(1987) 388.(9) T.Awes et al., Phys.Rev. C25 (1982) 2361.

54

PROJECTILE FRAGMENTATION IN HEAVY-ION REACTIONS AT INTERMEDIATE ENERGIES

G. ROYER, B. REMAUD, C. GREGOIRE+, C. LEBRUN, A. OUBAHADOU and F. SEBILLE

Laboracoire de Physique Nucleaire (UA CNRS N°57) 44072 NANTES Cedex 03-FRANCE

+GANIL, BP 5027, 14021 CAEN Cedex-FRANCE

1. Motivation

At intermediate energies (10-100 MeV/A) the peripheral collisions lead to three dis-

tinct emitting sources : a target-like source, a projectile source and an intermediate

source. The mere abrasion-ablation model developped to describe these sources at relati-

viseic energies is expected to lose gradually its validity with decreasing energies since

collective effects play a larger role. We study1,2) how far the binary fragmentation of a

•projectile can be describe macroscopically within a rapid fission process induced by thetarget field.

2. Three-body macroscopic model of the projectile fission

The projectile is considered as the combination of two nuclei 1 and 2 while the

third particle is the target. The potential energy is the sum of three contributions : V =V12 + ^13 ^23' ^ n e P o t e n tial Vi, which governs the projectile fission is the liquid-dropmodel energy including the nuclear proximity energy. The shape sequence describes the path

leading from a sphere to two unequal spheres. The potential V j , and V^o are calculated in

the sudden approximation. Friction forces between the part 2 of the projectile and the

target have been added to take into account the dissipation energy.

3. One and two peak energy spectra

Besides the fusion and elastic or inelastic reactions the projectile breaking occurs

naturally. For small impact parameters, the projectile part which is in contact with the

target is strongly slowed down by the nuclear and frictional forces and is trapped by the

target. The remainder part (the quasi-projectile) tends to escape almost freely. The kine-

tic energies are well reproduced (Fxg.l). For medium impact parameters the two fragments

are emitted and the part 2 which has grazed the target has a velocity of about 0.6V beam

(Fig. 2). It might be a contribution to the relaxed fragment events detected at intermedia-

te angles (Fig. 3). The sequential fission of the projectile far from the target is also

reproduced .

1000

4000

+ '"Au

50 100 150 200 250 300 350 400 450

40Ar(35MeV/n)+109Ag

IMPACT PARAMETER (fm) LFig.1iTheoretical value of Fig.2:Relative velocity ofmean energies of the quasi- the two emitted fragments,projectiles and one-peakspectra.

Fig.3:Theoretical value of

the mean energy of the two

emitted fragments in the

40Ar(35 MeV/A) + 197Au reac-

tion and experimental data.

^. Conclusion

In heavy-ion reactions at intermediate energies, the ion-ion potential between the

projectile and the target induces strong stress between the projectile parts and leads to

the fission projectile. The description of this asymmetric fission process within the li-

quid drop model including the proximity energy allows to reproduce semi-quantitatively the

mass distribution of the quasi-projectiles and the position of the maxima in the one and-

two peak energy spectra. The target is supposed unaltered and such a model cannot describe

the frontal collisions and the competition between fission and evaporation.

1) G. ROYER, Y. RAFFRAY, A. OUBAHADOU and B. REMAUD, Nucl. Phys. A466(1987)139

2) C. GREGOIRE and B. REMAUD, Phys. Lett. 1276(1983') 308

55

PE54. MEASUREMENTS OF TIME DELAYS FOR PROJECTILE-LIKE FRAGMENTS IN THE REACTION " ° A r + Ge AT

44 MeV/NUCLEON

J . Gomez del Campo

Oak Ridge National Laboratory,* Oak Ridge, Tennessee 37831

R. A. Dayras, 0. P. Uieleczko, and E. C. Pollacco

Servicie de Physique Nucleaire-Basse Energie, CEN Saclay, 91191 Gif-sur-Yvette, Cedex, France

J . Barrette

McGill University, Foster Radiation Laboratory, 3610 University StreetMontreal PQH3A2B2, Canada

F. Saint-LaurentGANIL, B.P. 5027, 14021 Caen, Cedex, France

M. ToulemondeCIRIL, B.P. 5133, 14040 Caen, Cedex, France

andN. Neskovic and R. Ostojic

Boris Kidric Institute of Nuclear Sciences, Belgrade, Yugoslavia

To achieve a better understanding of thereaction mechanisms in peripheral collisionsbetween heavy ions in the intermediate energydomain of 20-1000 MeV/nucleon, it is importantto know the mass and energy transfer to pri-mary fragments during the collision. Thesequantities can be obtained through detailedexclusive measurements of all particlesemitted in coincidence with the fragments. Inthe work reported here an approach, sensitiveto the primary fragment excitation energies,is taken. The formation times of the secon-dary fragments produced in the reaction aremeasured by the crystal blocking technique.1

In a recent review of the crystal blockingmethod1 preliminary accounts of this workwere reported and now a more detailed descrip-tion of the results will be given. Particularattention is placed in the extraction of thedecay times which for fragmentation reactionsis ambiguous due to the uncertainty on theangular distribution of the primary fragments.Basically, the blocking technique provides anexperimental determination of the perpen-dicular displacement, measured with respect tothe nuclei arranged in a row of one of thecrystal channels, of the reaction products.This displacement is equal to the product ofthe reaction time and the velocity of therecoiling nuclei and therefore, only for caseswhere the velocity is known, like in light ion

resonance reactions, heavy ion fission orfusion reactions, the decay time can be uni-quely extracted.

In the present experiment, models have tobe used to predict the experimental displace-ments, and the resulting time distributionswill depend mainly on the modeling of theangular distributions and excitation energiesof the primary fragments.

The experiment was performed by bombardinga 10 pm crystal with a 1.76 GeV ''"Ar beamextracted from the GANIL cyclotron facility.The reaction products were detected using atwo-dimensional position sensitive gas propor-tional detector system placed at 1.85 m fromthe target and equipped with a solid-statetelescope to identify the nuclear charge (Z)of the fragments. The first stage of thetelescope consisted in a 300 urn thick, 600 mm2

solid state detector followed by another 5000urn thick and 600 mm2. A beam spot of 2 mm wasused and the measured intrinsic position reso-lution of the detector was .8 mm. Thesevalues combined gave an overall position reso-lution a = 0.04°.

The basic ingredient of the crystalblocking experiments consist of the measure-ment of the two-dimensional blocking patterns.In the present experiments these patterns wereobtain at four laboratory angles: 3.5°, 5.3°,8°, and 11.8°.

~nw tubmrntd rmnuBcnpt hM bMnauthored by • contractor of tht U.SGowmmant infer contact No. K-ACO!rUO«21«X>. AceordnoJv. tha US.Government ratama » nonMdmiM.roran>-frat fcana* to pubMi or raproduoarha pubWMd form of tnb ujlililjmluit. or•low othm to do *o. for U.S. GovammantpurpOftM.-

56

Fragments of Z>9 were analyzed and showedthe typical kinematic characteristics of pro-jectile fragmentation products. The resultingtwo dimensional blocking pattern obtained forall reaction products of Z>9 emitted at alaboratory angle of 5.3° and along the <110>axial direction is shown in Figure 1, wherethe typical <110> axial structure can be seen.This pattern was obtained after the <110> axisof Ge was aligned to the beam direction byobserving the typical "star" channeling pat-terns on a zinc sulfide screen placed at zerodegree. Rotating the <110> axis to an angleelab = 5.3°, where the detector was locatedthe two dimensional pattern of Figure 1 wasobtained. To observe the axial and planarpatterns a very small beam divergence < 0.02°was necessary.

Fig. 1: Two-dimensional blocking patternof all the projectile-like fragments of Z>9produced at 9iab = 5.3°.

Integrating the two dimensional patternsin the azimuthal direction of the axis ablocking angular distribution is obtained, andby analyzing these blocking angular distribu-tions the decay time information can beextracted. The reduction of the yield(blocking dip) seen towards zero degree (<110>direction) is directly proportional to the

decay time of the primary fragment. For veryshort decay times, the primary fragments deex-cite on a lattice site; hence, the secondaryfragments are clocked by the lattice atoms,yielding a minimum. For longer decay times,the deexcitation point is displaced towardsthe center of the channel thus increasing theyield along the channel direction.

From the analysis of the blocking angulardistribution we conclude that for allfragments, a significant filling in of theblocking dip is observed, indicating sizabletime delay effects. These delays show thatemission of light particles is an importantprocess to be considered in the description ofprojectile fragments produced in intermediate-energy heavy-ion reactions. The extracteddecay times are consistent either with a broadprimary distribution of fragments with rela-tively low excitation energies (<50 MeV), ofthe type given by the abrasion model, or witha deep-inelastic like process in which theprimary fragments have a narrow mass distribu-tion with high (-150 MeV) excitation energy.

• This ambiguity results from the fact that theblocking technique is sensitive to the productof the recoil velocity and the decay time,rather than to the time alone. Removal ofthese ambiguities can be accomplished byimproving the measurements incorporating Z,and M identification and some angular correla-tion measurements between the PLF and theemitted light particles.

*0ak Ridge National Laboratory (ORNL) isoperated by Martin Marietta Energy Systems,Inc., under Contract No. DE-AC05-840R21400with the U.S. Department of Energy.

1). J. Gomez del Campo, Nucl. Instrum. Meth.Phys. Res. B24, 447 (1987).

57

NUCLEAR CALEFACTION

R. LucasM.Berlonger1. S.Chlodelli2, D.Dalili1, A.Demeyer2, D Guinet2, R. LucC. Ngfl , C.Cerruti , S.Leray , C.Mazur , M.RIbrag , T.Suomijarvi

1 DPhN/MF Saclay, Z IPN Lyon, 3Laboratoire National Saturne

At low bombarding energy the fusion of twovery heavy Ions is impossible due to their toolarge Coulomb repulsion. Instead of that, oneobserves a large deep inelastic cross section.As the bombarding energy goes above about 10MeV/u one start to observe some products comingfrom three body processes (!]. The probabilityof the three body process increases with thebombarding energy [1J. The question is : whathappens at larger bombarding energies with veryheavy projectiles?

An Inclusive experiment performed on theKr+Mo,Ag and Au systems at 22 MeV/u [2] hasshown the existence of events which might comefrom a three body mechanism. Basically, if onedetects the products emitted between 6° and 12°In the laboratory system, one observes acorrelation between their itass and kineticenergy which is displayed in fig. 1: Here onlythe most probable values of th& two dimensionalplot are shown (points). The horizontal barscorrespond to the standard deviation of thekinetic energy distribution. One observes two

OPhN Saclay E 83167~1 1 1 1 / I T"*—1 ' '

components : the first one with a ridgeextending from A=84, E=1800 MeV down to A=50,E=600 MeV. A second one where the ridge ispractically at constant kinetic energy (E^SOOMeV). Similar results have been observed with Xeprojectiles at 23 MeV/u [31.

At very high bombarding energies one knowsthat nucleon nucleon collisions dominate. As aconsequence one has. to a first approximation,the participant spectator of ref.[4J. One getsthree kinds of subsystems which have quitedifferent velocities : the fireball and thespectators of the projectile and target.

If one applies this picture to our data onecannot reproduce the mass-energy correlationshown In fig. 1. This Is not be too surprisingsince we are at a bombarding energy where meanfield effects are still important.

One can reproduce the mass-energycorrelation of fig. 1' if one modifies theparticipant picture. Let us start, in theinteraction region, with the three zones of theparticipant spectator picture. Then, because ofthe mean field, which is still important at thisenergy, there are interactions between thepieces. For projectiles like Ar or smaller,there will be a fusion of the three pieces after

Projectile

Partial fusedsystem

Participants

0.0Spectators

Fig. 1 Fig. 2

58

the hot zone has emitted some preequi 1 ibriumparticles. This would correspond to incompletefusion. Such a process Is not possible when theprojectile and target are too heavy. Indeed, weknow that two very heavy ions cannot fuse.However, nothing prevent that two, among thethree parts, fuse together. Let us for instanceconsider the fusion of the participant zone withthe spectators of the projectile. One gets apartial fused system moving with a velocitywhich depends on the number of participants ofthe target. We could Imagine that, after sometime, this partial fused system reseparates intothe projectile spectators and the participants,before thermalization occurs (fig.2). This couldbe understood in analogy with caiefaetian.phenomenon., an effect observed in macroscopicphysics when one pours a drop of water on anoverheated plate.

The above simple picture can be comparedwith the experimental data. This gives the twolines in fig.1. One observes a surprisingly goodagreement between the assumptions and the data.Why is it so is still an open problem whichdeserves further attention.

[1] A.Gobbl, Topical Meeting on phase spacedynamics, triestrs (1985)S.Gralla et al, Phys. Rev. Lett. 54 (198LO 183312] D.Dalili et al, Mud. Phys. A4S4 (1986J 163C.Ngd et al. Prog. Part. Nucl. Phys. IS (198S)171[3] O.Granler et al, Nucl. Phys A481 (1988) 109[4] J.D.Bowan et al, LBL report n°29098 (1975)G.D.Westfall et al, Phys.Rev. Lett. 37 (1976)1202[5] M.Lefort et al, Nucl. Phys. A216 (1973) 166

59

46

PROJECTILE FRAGMENTATION NEAR THE FERMI ENERGY

R. Dayras1, J. Barrette1, B. Berthier1, D.M. de Castro-Rizzo1, E. Chavez1,

O. Cisse !, R. Coniglione1, H. Delagrange3, F. Gadi1, B. Heusch4, G. Lanzano2,

R. Legrain1, M.C. Mermaz1, W. Mittig3, A. Pagano2, A. Palmeri2 and E. Pollacco1.

1 D.Ph.N/BE, C.E.N. Saclay, 91191 Gif-sur-Yvette. 2 I.N.F.N. Catania, Corso Italia 57, 1-95129 Catania.

3 GANIL Caen, BP. 5027, 14021 Caen Cedex. 4 C.R.N. Strasbourg, 67037 Strasbourg Cedex.

1. Motivations.

In peripheral heavy ion induced reactions,

production of fragments in the beam direction

with velocities close to the projectile velocity

is a very common process at all projectile energies1'.

For projectile energies smaller than •>. 20 MeV/u,

these fragments are clustered around the projectile

mass and are produced either by a transfer of few

nucleons (stripping or pick-up) or by an incomplete

fusion process in which only part of the projectile

fuses with the target. On the other hand, for

projectile energies greater than 200 MeV/u, these

fragments are thought to be produced by a fast

removal (abrasion) of the nucleons in the region

of overlap between projectile and target. Then

the questions arise to when and over which energy

range does the transition between those two

processes occur. In order to answer those questions,

we have undertaken two kinds of experiments:

i) inclusive measurements (isotopic yields, energy

spectra, angular distributions) of the fragments

from an 40Ar projectile bombarding Al and Ti Targets

at 44 MeV/u and Ag targets at 30 and 60 MeV/u.

The purpose of these experiments ivas to determine

the general properties of the projectile-like

fragments (PLF). ii) measurements of the correlations

between projectile-like and target-like (TLF)

fragments in the reactions 4uAr + 27A1 at 44 MeV/u

and 40Ar + na tAg at 30 and 60 MeV/u in order

to characterize the mechanisms responsible for

the fragmentation process.

2. Properties of the projectile-like fragments.

As in high energies (E/A > 200 MeV) collisions,

the mass spectrum of the fragments with

approximately the beam velocity extends from

the projectile mass down to the lightest

elements2""). The production rate is large and

exhausts about two thirds of the reaction cross

section and is well reproduced by the geometrical

abrasion model'''). However some marked differences

can be noted with the high energy data. The presence

of fragments heavier than the projectile clearly

indicates that direct surface transfer reactions

may still play an important role in the fragment

production at least in the vicinity of the

projectile**'9). Further evidence of nucleon exchange

between projectile and target is provided by the

slight dependence of the neutron to proton ratio

of the fragments upon the target5 '1").

Although the widths a^ of the fragment momentum

distributions in the beam direction are well

reproduced by Goldhaber's relation11' with a value

of a 0=85 MeV/c in agreement with high energy

data, these momentum distributions are asymmetrical

with a low momentum tail indicating dissipative

processes. Thus, although the essential features

of the data may be reproduced by abrasion models

including dissipation5'12', the features described

above cast some doubts on the validity of such

models in the intermediate energy domain.

3. Correlations between projectile-like and

target-like fragments.

Fig. 1 shows the PLF-TLF mass correlation

in the reaction 40Ar + 27A1 where the PLF's were

detected at 3.1° relative to the beam direction

whereas the TLF's were detected on the other

side of the beam between 15° and 85" (refs. 8,13,

14). The data points represent the ridge of the

correlation whereas the horizontal (vertical) bars

indicate the full width at half-maximum of the

PLF's (TLF's) mass distributions for a given mass

of the TLF's (PLF's). On average, the mass lost

by the target is equal to the mass lost by the

projectile which is consistent with an abrasion

process as indicated by the full line in fig.l. However,

the dependence of the TLF recoil angle upon the

TLF mass is reminiscent of a more simple two-body

mechanism were only two excited primary fragments

emerge from the collision, which then decay by -

particle emission. Assuming just such a process,

60

47

25

•20

Fig. 130 35 40

Detected PLF Mass

we have use momentum conservation for a full

kinematic reconstruction of the binary events,

which yields primary quantities like fragment

masses, excitation and kinetic energies. The average

primary masses of the PLF's and of the TLF's,

independently reconstructed are shown in fig.2 a-b

as a function of the detected PLF mass. The total

kinetic energy loss EIOSS of the primary fragments,

derived from energy conservation is compared

in fig.2c to the total excitation energy E*tot

deduced from the mass difference between the

primary and detected fragments. As the mass

of the detected PLF's decreases, E*t0t increases

more rapidly than Ejo s s . This behaviour is consistent

with an emission of less than five fast nucleons

prior to the separation of the fragments.

Thus the data are consistent with a two-body

process reminiscent of the early stage of deeply

inelastic collisions in which projectile and target

share an approximately equal amount of excitation

energy. Recent calculations15\ assuming a

stochastic exchange of nucleons between projectile

and target give a good description of the

40Ar + nat^g data at 30 MeV/u. However, in order

to solve the ambiguities between the two processes

just discussed (abrasion vs. two-body) it would

be necessary to accede directly to the excitation

energy of the primary fragments for which the

two approaches give very different predictions.

We have just undertaken such measurements by

looking at the spectra of the light particles emitted

0 -

Fig.2

25 30 35 40

Mass of the detected PLF's

by the fragments.

1) R. Dayras, J. Phys. (Paris) C4(1986)13.

2) J. Barrette et al., Proc. 22nd Int. Meeting on

Nuclear Physics, Bormio (Italy), Jan. 1984.

3) R. Dayras, Proc. 8thOaxtepec Simposium on

Nuclear Physics (Mexico), Jan. 1985.

4) R. Coniglione, Tesi di Laurea, Universita Degli

Studi di Catania, 1984.

5) R. Dayras et al., Nucl. Phys. A460(1986)299.

6) F. Gadi-Dayras, These, Universite de Paris

Sud (Orsay), Juin 1988.

7) J.D. Bowman, W.J. Swiatecki and C.F. Tsang,

LBL report n° LBL-2908, 1973 unpublished.

8) O. Cisse, These, Universite de Paris Sud (Orsay),Sept. 1985.

9) M.C. Mermaz et al., Nucl. Phys. A441(1985)129.

10) D. Guerreau et al., Phys. Lett. 1316(1983)293.

11) A.S. Goldhaber, Phys. Lett. 536(1974)306.

12) A. Bona^era, Nucl. Phys. A463(1987)653.

13) R. Coniglione et al., Nucl. Phys. A447(1986)95c.

14) R. Dayras et al., Phys. Rev. Lett.(1989) in print.

15) L. Tassan-Got, These, Universite de Paris

Sud (Orsay), 1988.

61

48

- B 2 -DISSIPATIVE COLLISIONS, HOT NUCLEI

49

DYNAMICAL ASPECTS DF VIOLENT COLLISIONS IN Ar + Ag REACTIONS AT E/A = 27 MeV.

PERSISTENCE OF DEEPLY INELASTIC PROCESSES

B.BORDERIE, C.CABOT, H.FUCHS, D.BARDES, H.GAUVIN, F.HANAPPE*, D-JACQUET,D.JOUAN, F.MONNET, M.MONTOYA, M.F.RIVET

Institut de Physique Nucleaire, F-914O6 ORSAYFNRS and U.L. Bruxelles, B-1O5O Bruxelles

1. Motivation

observation of intermediate—mass

Sem i — c I ass i ca 1 calculat ions

It was very tempting to quantitativelyThe observation offragment (IMF) emission in heavy-ion compare our experimental observationscollisions at intermediate energies has wjth a semi-classical approach based on

fewappeared, in the last -few years, aspuzzling question giving birth to manytheoretical speculations. Experimentally,using a three-source parametrization, itwas possible to successfully reproducethe measured inclusive energy spectraC13. To better understand the origin ofthe intermediate-velocity source and tohave an overview on violent collisionsinvolving IMF emission,measurements between IMFparticles and heavy residues

performed bombarding a Ag

the Landau— VI asov equation [33. For cen-

coinc idenceor 1igh t(HR) «

target by

109O MeV Ar ions-

2. Exper imental results

The in—plane data were analyzed assu-ming two body kinematics after preequi-librium particle emission C21. The meanrelative velocities V between the IMF

and HR were calculated as well as themean velocities of the center of mass ofthe binary system V (IMF-HR) ; this

' ' c.m. 'last quantity was derived assuming thatthe particles emitted prior to separationof the two partners, which are essen-tially preequi1ibrium particles, do notcarry a sizeable mean transverse momen-tum. The figure summarizes the results.The evolution of V , between 7 and 3O°

relis similar to what was observed in incom-pletely damped deeply inelastic colli-sions at low energies. For &,„.- > 4O° a

IMFcomplete damping seems to be observed ;from an experimental point of view, IMFcan be regarded as emitted from a fusedsystem as well as the light partner of acompletely damped deeply inelastic colli-sion.

The bottom part o-f the figure shows theevolution of V (IMF-HR) which can be

c.m.compared with the c.m. velocity of thetotal system (V )

FMTthe deviations

observed from this value clearly revealthe appearance1 and the increase of pree-quilinrium partinle emission with the*degree* nf dissipation of the collision.

o

6ifiC

EO"^ 4"3

c;

£ ,uZ

E

a:rE0

1

I • I • I ' I ' I ' I • I •Ar + Ag

E/A=27 MeV-. (9)

5— Z — 1 4" t--•\18)1

n- f)

- i{

( b = 5 . 5 £ m )

I I 1 1 . 1 I [ I 1 I 1 i1 1 ' 1 1 1 '

I

v- 1 (b.5.5) F^f8, ,?, •£ { = J _a>-o75f«i

T ' , ,* EXP '* — Landau-Vlasov

. 1 . . l . l . l . l . i .

O

6

4

0 10 20 30 40 50 60 70 80

Fiq. 1_ :Comparison betweencalculations (see text)mental results.

Landau—V1asovand experi-

tral collisions (O to 5 fm) an incompletefusion occurs for which only one HR andnucleons are found in the exit channel(no complex particles or IMF can be eva-porated in such a calculation). At t =

~-2?4.1O s the momentum distribution ofemitted nucleons becomes isotrcipic andone can then define this instant as theend of the preequi1ibrium phase for col-lisions (b = O-B fm). The recoil velocityof the fused system can then be compared

62

50

(IMF-HR) (see figure 1). Expe-

calculation agree

H i t h Vc.m.riment and calculation agree an onepaint : the amount of preequilibriumemission increases with the violence ofthe collision. However, for the morecentral collisions the experimental va-lues are lower than the calculated ones ;experimental results on protons emittedin coincidence with heavy residues aftersubtraction of an isotropic componentattributed to heavy-residue evaporationalso show a disagreement with some of thecharacteristics of the preequi1ibriumprotons given by the calculation [43.This discrepancy may have its origin ineither the approximate character of thenuc lean-nut: lean effective interactionused in the calculations or in someunderestimation of the nucleon mean freepath.

Beyond b - 5 fm two main products arefound in the exit channel Cane HR and oneIMF with average primary Z = 151. Therelative velocity between the two part-ners is found larger than a pure Coulombrepulsion and depends on #TMp which is a

negative angle. The correlated valuesV , and S> „,_ can thus be compared withrel inrexperimental ones (see figure' 1). A goodagreement is observed which is confirmedby the measured cross—sections.

and by incorporating results from Landau-Vlasov calculations for preequi1ibriumemission, it was passible to deduce themissing mass AA which is associated toevaporated particles. Figure 2 shows thestrong difference between incompletelydamped DIC ^S.|l.f. = 15*) and statistical

emission from incomplete fusion process(e.MT- = 67") ; in this latter case, asIMF

expected the missing mass or the calcula-ted excitation energy is found ratherconstant whatever ZTM_ [53.IMF

4. Conclusions

Through experimental and theoreticalstudies dynamical aspects of heavy-ioncollisions around E/A = 3O MeV have beeninvestigated. After a preequilibriumemission depending on the violence of thecollision it was shown that highly dissi-pative processes arts present namelyincompletely damped deeply inelasticcollisions at intermediate impact para-meters and incomplete fusion for centralcollisions.

As a consequence, intermediate—massfragments detected at intermediate angles("intermediate velocity source") are fora large part the final light partners ofdeeply inelastic collisions.

4. Excitation— energies involved

From the knowledge of the two main^ (AjM

et al, Z. Phys.

partners, and ZjM[..-t-O.5> ,

D U

^ ^

D— ' 50

u,2g

f 40Qj3C

1 30ucu

U 20oH

<

< 10

1 ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 .

— -x X x ^ x x X :

if w X V K

— x -*F

* ** $ * a. -* T * * ** ? :

AA E* + +

°IMF~^-'-'

K X &IMF=6 ' ' 'C

. . . . 1 . . . . 1 . . . . 1

D U U

rax500 0H>

400 dO•z,

300 W

PI

200 O

> —

100 K

r»

0 5 10 15

Cl] B.Borderie(19B4) 315.C23 M.F. Rivet et al, Proc. of the TexasA&M Symposium on Hot Nuclei, CollegeStation (USA), p. 4OO World Scientific(19BB).B.Borderie et al, Proc. XXVI Int. WinterMeeting on Nuclear Physics, Bormio(Italy) p. 84 (19BB).B.Borderie et al, Phys. Lett. B2O5 (19BB)26.[33 B.Borderie et al, Third Int. Conf. onNucleus—Nucleus Collisions, Saint Mala(France), contribution p. 125 (198B).M.F.Rivet et al. Phys. Lett. B215 (19BB)55.[43 D.Jauan et al, Proc. XXV Int. WinterMeeting on Nuclear Physics, Bormio(Italy), p. 135 (1987).[S3 B.Borderie et al, to be published.

Fig- Z see text.

63

51

CHARACTERISTICS OF THE MANY-BODY EXIT-CHANNELS INHIGHLY DISSIPATIVE COLLISIONS

A. Olmi*, G. Casini*, P.R. Maurenzig*, A.A. Stefanini*, R.J. Charity*, R. FreSfelder*,A. Gobbi*, N. Herrmann*, K.D. Hildenbrand*, F. Rami', H. Stelzer', J. Wessels*, M. Petrovici*,J. Galin*, D. Guerreau0, U. Jahnke1, M. Gnirs0, D. Pelte0, D. Raimold", J.C. AdlofF,B. Bilwes', R. Bilwes1 and G. Rudolf.'GSI, Darmstadt, W. Germany;*INFN and Univ. of Florence, ItalyjtINPE, Bucharest,Romaniaj'GANIL, Caen, France;°Univ. of Heidelberg, W. Germanyj'CRN, Strasbourg,France.

At incident energies fro^i 12 to 24 MeV/nucleon, the excitation energy of the primaryfragments formed in a deeply inelastic reaction spans a very broad range (0 to ~1 GeV)and leeds to the observation of processes involving three, four or even more heavy frag-ments in the exit channel. These processes have been systematically investigated in thereaction of 100Mo + 100Mo using incident energies of 23.7 MeV/nucleon at GANIL andof 12.0, 14.7 and 18.7 MeV/nucleon at the UNILAC. The heavy fragments (A> 20) weredetected with high efficiency employing a, set-up of 12 parallel plate avalanche countersarranged to cover ~ 80% of the forward hemisphere.

- QCO

Ol_

Q.

(0JDO

001 r

0.001

001 r

0.0010 200 400 600 800 1000 1200 0 1 2 3 4 5 6

TKELr (MeV) E V A (MeV)

Fig.: 1. Ratios (P3 and P4) of 3- and 4-body cross sections to the total cross section for2-, 3- and 4-body events measured at four bombarding energies.

As a result of these measurements, it has been possible to extend the observation ofternary processes1 to a broad range of incident energies and to the observation of 4-bodydecay channels. Figure la summarizes all the data concerning the probability of these pro-cesses obtained between 12 and 24 MeV/nucleon. Detailed studies of the 3-body processusing a refined kinematical coincidence method2 show that these events are associatedwith the fission-like decay of a highly excited deep-inelastic product; P4 seems to scale

64

52

approximately quadratically with P3, as would be expected for independent decay proba-bilities of the two deep-inelastic products. Beside the high values of the fission probabilityand the similarity of P3 at different bombarding energies1, one has to note: (i) the shiftof the curves at 23.7 MeV/nucleon with respect to those at lower bombarding energies,and (ii) the decrease of P3 and P4 with increasing TKEL for fully damped interactions.The second feature seems to be a consequence of an equilibrium particle emission duringthe first step deep-inelastic collision. The strength of such an emission has been estimatedusing as ingredients: the interaction times derived from a phenomenological model andthe particle emission time calculated with the statistical model. One effect of equilibriumemission during the collision is to lower the excitation energy per nucleon E*/A of theemerging primary fragments, thus explaining the decrease of P3 and P* at high TKEL, asshown in Fig. Ib.The equilibrium emission can however not explain the shift seen between the 23.7 andthe 12 MeV/nucleon excitation functions. One possibility is, that the shift arises frompre-equilibrium particle emission (PEPE) during the early collision stage. This effect isexpected to be more important at the higher bombarding energies. The magnitude ofsuch pre-equilibrium effects can be estimated if Pa is assumed to be predominantly depen-dent on the thermalized excitation energy of the primary fragments. P3 is thus used as a"thermometer" to deduce the "temperature" (or excitation energy) of the fragments. Itwas calibrated at 12 MeV/nucleon where no pre-equilibrium particle emission is expected.Having inferred the thermalized excitation energy of the primary fragments, it is possibleby applying the energy balance to estimate the amount of energy lost due to PEPE.This can then be directly compared to calculations of the Qunatum Mechanical PhaseSpace Model4. In- figure 2 the ranges of pre-equilibrium energy losses consistent with thedata are shown. (The impact parameter was derived from the measured differential cross-section dtr/dTKEL).

— 200

(/>(/)O

150

100O)

0}

lS 50

LUCL 0

10 12 U0 2 i 6 8

b (fm)Fig.: 2. The range of pre-equilibrium energy losses consistent with the data are indicated

65

53

by the bands. The dashed lines shows the predictions of the Quantum Mechanical PhaseSpace Model at 19 MeV/nucleon. According to the experiment for the 23.7 MeV/nucleonreaction as much as 100 - 150 MeV are removed by PEPE.

The statistical equlibrium decay model appears to be important in characterizing the heavyfragment exit channel probabilities. The measured angular distribution of the fission-likesecond step process seems however to contradict the equilibrium hypothesis and is indica-tive of the role played by dynamical properties. This conclusion can be drawn from theexperimental finding of a strong alignment of the fission separation axis with the initialdirection of flight of the fissioning nucleus in the 3-body exit channel. Figure 3 showsthe in-plane angular distribution of the heavier fission fragments for two different bins inmass-asymmetry 77 = \(ml — m2)|/(mi + m2). <f> = 0° refers to the emission in the direc-tion of flight of the fissioning nucleus; the window in TKEL was chosen such that it waspossible to clearly identify the fissioning system by gating on the relative velocity betweenthe fragments. It can be seen that for asymmetric fission in the second step, there is ahigh probability for collinear emission of the three fragments suggesting that the decisionfor fission may already be taken during the first step of the reaction.

800

0.5*1)50.6

150

Fig.: 3. In-plane angular distribution for the heavier fragment resulting from the scis-sion of one of the primary products in the collision 100Mo + 100Mo at 18.7 MeV/nucleon(350 < TKEL < 550MeF).

1 A. Olmi et al., Europhys. Lett. 4 (1987) 1221.; 2 G. Casini et al., NIM A227 (1989)445.;3G. Wolschin and W. Norenberg, Z. Phys. A284 (1978) 209.;4W. Gassing et al., Phys.Lett. B181 (1986) 217.

66

54

COEXISTENCE OF COLLECTIVE AND PARTICIPANT-SPECTATOR MECHANISMS IN THE INTERACTION BETUEEN

KRYPTON AND GOLD NUCLEI AT 44 MeV/u.

20 40 60 80 100V,/VP

J.C.Adloff, B.Bilwes, R.Bilwes, M.Glaser, G.Rudolf, F.Scheibling, L.Stuttge, C.R.N.StrasbourgG.Bizard, R.Bougault, R.Brou, F.Delaunay, A.Genoux-Lubain, C.Lebrun, J.F.Lecolley, F.Lefebvres,

M. Louvel, J.C.Steckmeyer, L.P.C. CaenJ.L. Ferrero, IFIC Valencia (Spain)

Y. Cassagnou, R. Legrain, DPhN/BE SaclayF. Guilbault, C. Lebrun, B. Rastegar L.S.N Nantes

G.M.Jin, A. Peghaire, J. Peter, E. Rosato GANIL Caen

1.MotivationAt low bombarding energy, the interaction

between two nuclei is of collective nature,i.e. all nucleons participate to it. At highbombarding energy, only some of them partici-pate, the others being spectators. The tran-sition between one mode and the other isexpected to occur in the Fermi energy domain,i.e. for relative velocities close to thevelocity of the nucleons inside the twonuclei. This domain Is precisely that coveredby Ganil. First experiments, based on inclu-sive measurements, have generally indicatedthat the high energy regime starts veryearly. More recent experiments based on coin-cidence measurements have reached the oppo-site conclusion.

Because in this domain collisions are quiteviolent, many fragments are expected to beproduced. A special set of multidetectors hasbeen developped to allow measurements asexclusive as possible: XYZt and OELF detectfragments at forward angles and around thetarget, respectively, while the HUR and theTONNEAU measure light particles in the sameangular ranges.

The present results have been obtained intwo experiments. They show that both types ofmechanisms coexist in the interaction of Krwith Au at 44 MeV/u.2. Velocity correlations between fragments

In Fig.l we show the correlation betweenthe velocities of two fragments detected inXYZt. All these fragments have intermediatemass (A ~ 16-50). Grossly speaking, we candistinguish three kinds of fragments: i) fastones (V/VpXJ.75}, ii) slow ones (V/Vp<0.25)and iii) intermediate velocity fragments(IVF) with a velocity close to half of that

Fig.lof the beam (Vp). These latter ones are ofparticular interest: they may be producedeither by the decay of a participant zone, bythe fission of the projectile or byevaporation from the target. The origin ofthese fragments is therefore a signature ofthe mechanism.3. Deep-inelastic collisions.

We shall first investigate the origin ofthe fast-intermediate coincidences in Fig.l.Light particle multiplicities show that thiscomponent corresponds to intermediate impactparameters. Kore precisely, we shall selectevents in which one single IVF is detected incoincidence with one fast and two slow frag-ments (4-fold coincidences). These slow ones(measured by DELF) have typical properties offission fragments from a heavy and slowlymoving nucleus. The relationship between thecharge of the fast fragment and that of thesum of the two slow ones (full dots in Fig. 2)seems fairly well reproduced supposing that

67

55

the fast fragment is the spectator of theprojectile and the slow fragments areproduced by the fission of the spectator ofthe target. The calculation is based on amodel ') in which the mechanism has first acollective step and then a participant-spectator one (dotted curve). However, thisagreement is misleading. Indeed, the relativevelocity between the IVF and the center ofmass of the two slow fragments demonstratesthat this IVF has been evaporated by atarget-like nucleus before it fissioned. Thecharge of the IVF must be added to the sum ofthe two slow fragments. Me obtain thus thecharge of the target-like nucleus minus theevaporation of light particles (open dots).The relationship between this charge and thatof the fast fragment agrees well with acalculation based on a deep-inelastic modelin which the fast fragment is the pro-jectile-like nucleus and the excitationenergy is shared equally between the twopartners {full curve).

80

70-

60-

£50-115CO

40-

8 16 24 32

Z of the fast fragment

Fig.24. Fragments emitted bv a participant zone.

Coincidences between two IVF's correspondto the most central collisions observed intins experiment. They are observed in double(Fig.l) and triple coincidences in XYZt: inthis case, the third fragment is sometimes afast one, which excludes that this componentis due to the fission of a very slowed-downprojectile. Shown in Fig.3 is the relation-ship between the difference in velocity andthe relative angle between the two IVF's. The

00 1 2 3 4

IIVJ-IVJI (cm/ns)

reported in Fig.3a, athe decay of a source

and velocity ~ 0.4VP in

Fig.3experimental data aresimulation based onwith mass = 120Fig.3b and another one based on a deep-inelastic reaction followed by a doubleevaporation from the target in Fig.3c. Thecalculation lends definitely support for theexistence of a separate participant zone.

A close inspection of the characteristicsof the fast fragment in the triple coinci-dences shows that the system evolves from themechanism described in section 3. to that ofsection 4. when about 1 GeV kinetic energy isdissipated.1) A.Bonasera et al., Nucl.Phys A463

(1987)653

68

56

INCOMPLETE LINEAR MOMENTUM TRANSFER

S.ChiodelliC.Cerruti1, J.L.Charvet2

re , D.Guinet , G. La Ranaa, C. Le_. _.. , „ , .R.Lucas3 C.Mazur3, M. Morjean , G.Nebbia3, C.Ngo,M.Ribrag , L.Sinopoli2, T. Suomi jarvi3, E.Tomasi , J.Uzureau*, L. Vagneron

l i 1 , A.Demeyer1, O.Granier3,i4, S.Leray3, P.Lhenoret3. J.P.

Y. Pat in , A.Pe;

C.Gregoi-Lochard ,ghaire ,

1 IPN Lyon, 2 CEN Bruyeres le Chatel, 3 DphN/MF Saclay, 4 LPC Caen

At low bombarding energies central

collisions between heavy ions lead to the

fusion of the projectile and target into a

compound nucleus. When the energy increases

fusion becomes incomplete, that is, only part

of the incident nuclei fuses together to form a

quasi compound nucleus. When beams at

intermediate energies became available the

questions were : what is the mecanism for

incomplete fusion ? how does the rate of linear

momentum transfer, LMT, vary with energy and

with the mass of the two ions ? Is there a

limitation to this process and in particular to

the energy deposit into the quasi compound

system ?

In order to answer these questions we have

performed a serie of experiments with Ar

projectile on 137Au and 238U targets at

different bombarding energies. Because we were

dealing with heavy targets the main decay

channel of the compound system was expected to

be fission. Thus, information about the

90 120 150 180 90 120 150 1809fo.d

Fig. J. Folding angle distributions for the

foldlast

percentage of linear momentum transferred to

the quasi compound nucleus was obtained through

the measurement of the folding angle, 9 ,fold

between the two fission fragments, since 8

increases with decreasing LMT. In the

experiment light particles were also detected

in coincidence with the fission fragments.

Fig.1 shows the folding angle distributions

for the system Ar + U that we obtained at

SARA3' (a) and at GANIL1"3' (c,d) as well as

results of Jacquet et al4' (b). At 20 and 27

MeV/u two peaks can be seen : the first one at

low 0 values can be ascribed to incompletefold

fusion while the one with 8 around 170° isfold

due to sequential fission following peripheral

collisions. The arrow,label led FMT indicates towhich value of 8 full momentum transfer

fold

would correspond. The position of the peak is

in good agreement with an empirical law

established on lighter systems (the Violasystematics

10

) as shown by the arrow labelled

6

12

160

oO CO

50 100 150 200 250 300AD (u)

Fig.2. Excitation energy per nuclecn depositedin a quasi compound nucleus vet.-. :s its mass.The filled and half filled circles correspond

Ar(from

+ V system at different bombarding energies to systems for whigh no fusion products wereom ref. ' ' ). observed (from ref. ').

C9

57

on the figure. The situation is quite probability of cluster emission supposed to be

different at 35 and 44 MeV/u since no peak can

be seen where one would expect it. This

suggests that the quasi compound nucleus does

not decay any longer via usual ways or that

incomplete fusion becomes impossible above 35

MeV/u. The same conclusions can be drawn from

results obtained by other authors with

different targets : It appears that one no

longer observes evaporation residues ' or81fisson fragments in central collisions above

35 MeV/u.

One possible explaination is that the

disappearance of these products of fusion could

be due to the reaching of the limit of the

excitation energy which can be deposited in a

compound system. In fig.2 the excitation energy•

per nucleon, c , in the quasi compound nucleus

is plotted versus its mass, A , for different9)systems . Systems for which the usual decay

products of a quasi compound nucleus were not

observed are represented by filled or half

filled circles. It appears that all the systems

for which products of fission were not observed

are above the dashed region in fig.2.

In a simple model we have evaluated the

maximum excitation energy, e , that a nucleusmax

can sustain before boiling off into nucleons

and clusters from the binding energies of

nucleons and clusters in nuclei and the81 1 1 1 1

g 7—U

S 6C .<U

a.> 5

50 100 150MASS (afliu.)

200 250

Fig.3. Maximum excitation energy that can bedeposited in a nucleus versus its masscalculated assuming that clusters of A areformed with a probabjUty A~ for differentvalues of T (from ref. ).

proportional to A" . As it can be seen in fig.3

where e is plotted, for different values ofmaxi, versus the mass of the nucleus this model is

in good agreement with what can be deduced from

the experimental data in fig.2.

We have also developped a theoretical model

for promptly emitted particles based on the

coupling between the intrinsic motion of the

nucleons in a nucleus and the relative motion

of the two ions. This model fits rather well

the experimental data concerning fast protons

detected in coincidence with fission3)fragments and neutrons measured by Holub et

12)al but cannot explain the measured amount of

transferred linear momentum. This indicates

that a large part of the missing mass is

carried away by alphas or even heavier

fragments.

1) S. Leray et al, Nucl. Phys. A425 (1984) 3452) S. Leray et al, Z. Phys. A320 (1985) 5333) Y. Patin et al, Nucl. Phys. A457 (1986) 1464) D. Jacquet et al, Phys. Rev. Lett. 53 (1985)

1405) V. E. Viola Jr et al, Phys. Rev. C26 (1982)

1786) G. Auger et al, Phys. Lett. 169B (1986) 1617) A. Lleres et al, J. Phys. 47 (1986) 365c8) E.G. Pollaco et al, Phys. Lett. 1468(1984)

299) S. Leray, J. Phys. 47 (1986) 275c10)C. Ng6 et al, Z. Phys. A322 (1985) 419IDS. Leray et al, Z. Phys. A320 (1985)38312)E. Holub et al, Phys. Rev. C28 (1983) 252

70

Formation and decay of hot nuclei in the Ar. Ni and Kr + Th systemsY. Cassagnou. M. Conjeaud. R. Dayras. S. Harar. R. Legrain. M. Mostefai'.

E.G. Pollacca. J.E. Sauvestre, C. VolantDPhN/BE. CEN Saclay. 91191 Gif-sur-Yvette Cedex. France

B. Klotz-Engmann. V. Lips. H. OeschlerInstitut fur Kernphysik. Technische Hochschule Darmstadt, Germany

In compressing and heating nuclear matter via

nucleus-nucleus collisions our ultimate objective is

to draw information on the Nuclear Matter Equation

of State [NMES). Our work at GANIL has focussed

on a particular aspect, namely : what are the limits

in thermalised excitation energy E can a nucleus

contain ? An answer is very pertinent since NMES

is very sensitive to this l imit 'J. Thus, experimentally

we have considered different systems with varying

available energy and looked for the conditions whereby

the evaporation residue ceases to exists. To date no

limit can be given however a number of key results

have been obtained which open very interesting

prospects.

The main results are shown in f ig. 1. The

experimental technique employed is the angular

correlation method where the recoil velocity of the

fissioning nucleus as well as the mass of both fragments

are measured^!. Of interest are the peaks at small

Bff (large linear momentum transfer [LMTJ] which

correspond to events of almost complete fusion or

otherwise called, central collisions tCC). By performing

kinematic balance calculations we obtain an estimate

of E which in turn is compared with the final measured

mass. From the ensemble of measurements we draw

a number of salient results :

- the attained energy E is high of the order of 1 GeV

and correspond to BO % of the total binding energy^'1".

For the Kr + Th system, values are even higher and

lead to values v,ell above the expected theoretical

and extrapolated experimental values^!.

- Heavy projectiles yield a higher cross section for

the formation of very hot nuclei.

- By using heavy projectiles we conclude that the fall

in the yield of the CC peak is not as a result of limiting

E* values but rather due to dynamic effects. Therefore.

71

- Dynamic effects are present. From fig. 1. it is clearly

seen that the CC position is independent of incident

energy. In fact a scaling of linear momentum transfer

with mass of the projectile is clearly shown by our

work. This suggests that in heavy ion reactions around

the Fermi energy, one can only deposit a limited amount

of energy.

A problem which we have also addressed is whether

the hot nuclei being formed are indeed thermalised

and how do very hot nuclei desexcite. This work is

in its infancy.however the preliminary results^ are

most encouraging. In f ig. 2 are shown data of angular

correlations with and without coincident intermediate

mass fragments [IMF). The fact that the condition

filters out the most violent collisions is of primary

importance since it allows an additional method to

scrutinize these events. Further, analysis of the spectra

from the detected IMF's show fall off's which are

coherent with the LMT measurements.

It is clear from our work and others that dynamics

play an important role. In particular it is not yet clear

how the heating up of nucleon matter is achieved and

how compression, for example, affects the limits.

The second generation of experiment will seek

to establish a ITT coverage. This will yield a greater

selectivity for the events, and permit to obtain

information on the production mechanisms and hence

allow the establishing of limits in excitation energy

and their interplay with other parameters.

-< 1 1 1 1 r

ao w iffl 11.0 M to

Fig. 2

1] S. Levit and P. Bonche. Nucl. Phys. A"»37 (I985W26.

2] E.G. Pollacco et al.. Phys. Lett. 1168(19BMJ 29.

3) M. Conjeaud et al., Phys. Lett. I59B [19B5] 2MM.

Ml C. Volant et al.. Phys. Lett. 195B [19B71 22.

b] E.C. Pollacco et al.. Nucl. Phys. A18B (I9B8) 319c.

6) C. Volant et al.. Third International Conference

on nucleus-nucleus collisions. Sain-Malo June 6-11.

1988, contributed papers p. 100.

72

SATURATION OF THE THERMAL ENERGY DEPOSITED IN Th NUCLEI BY ArPROJECTILES BETWEEN 35 AND 77 MeV/u

D.X.Jiang'1. H. Doubre1. J. Galin1, D. Guerreau1. E. Piasecki"1. J. Pouthas1. A. Sokolov1. B.Cramer2, G. Ingold+2, U. Jahnke2, E. Schwinn2 J.L. Charvet++3, J. Frehaut3. B. Lott*3. C. Magnago3.

M. Morjean33, Y. Patin3, Y. Pranal3. J.L. Uzureau3 B. Gatty4, D. Jacquet4

1) GA.VIL, BP. 5027. 1-1021 Caen-Cedex, France2) Halm Meitner Institut Berlin -D-lOOOBerlin 39. Germany FRG3) CE Bruyeres le Chatel, BP 12 91680 Bruyeres-le-C'hatel. France.•4) IPX BP 1, 91406 Orsay-Cedex, FrancePresent address : Dept. of Physical Technics, Univ. of Beijing. China* Present address : Inst. Of Exp. Phys. Warsaw University. Hoza 69. Warszawa+ Present address : Phys. Dept.. State Univ. of Now York. Stony-Brook. NY 11794++ Present address : DPhN/BE, CEN Saclay. 91191 Cui-sar-Yvette CedexA Present address : CRN Strasbourg, BP 20 CRO. 67037 Strasbourg cedexC Present address : GANIL, BP 5027, 14021 Caeu-cedex.

1 Motivations 3 Saturation of the thermal energyThe fate of nuclei heated-up to high temperatures (T>5MeV) has been a hotly debated question for severalyears. In order to progress in this domain, one needsto know how to prepare these hot nuclei, and how tocontrol the amount of energy which is eventually storedas randomized energy (heat) in them.

The influence of the projectile velocity on theamount of dissipated energy is still under debate ; satu-ration of the momentum transfer has been documentedfor projectile energies larger than 30 MeV/u. How-ever, these observations rely mainly on the binary fis-sion channel, and one cannot preclude the existence ofother important exit channels at higher bombarding en-ergies. In order to investigate this question, we believeon the one hand, that it is necessary to probe the en-ergy dissipation by way of all exit channels as opposedto just one. On the other hand, all exit channels are nota priori known. Thus, it follows that one has to find asuitable observable which can be expected to introducethe minimum possible bias to the measurements.

We have chosen the multiplicity of evaporation- like light particles (n, H. He) as our minimum biasprobe. This choice was motivated by one simple fact:irrespective of the decay process - standard fission, in-termediate mass fragment emission or multi - fragmen-tation - all massive fragments finally "cool" by emittinglight particles.

2 Experiment

We present data for the reactions Ar + Th for threedifferent projectile energies (35. 44 and 77 MeV/u)1.

The neutron multiplicity data have been collectedusing a 4;r. Gd loaded liquid scintillator detector. Thedetection efficiency of such an apparatus depends pri-marily on the kinetic energy of the neutrons, and on theamount of liquid surrounding the target. Two detectorshave been used in successive experiments : the first onefrom CE Bruyeres le Chatel of 500 liters at 35 and 44MeV/u. the second one from HMI Berlin of 1500 litersat 4-1 and 77 MeV/u. As checked in Monte Carlo de-tection simulations, the growth in detector size resultsin a spectacular gain in the efficiency from about 60'/fto about 809c for evaporative neutrons. However, theefficiency remains small for high energy preequilibriumneutrons.. Light charged particles were detected closeto 160" by standard Si telescopes.

A glance at the inclusive neutron multiplicity distribu-tions fig. 1 for the three bombarding energies gives al-

SCO I ni-j

U*,V/u ' i - e»"'rn

.-•'"-. **

./•-•.. '«

.. -••'''"••-.. '"

-an dKKtcr

35*»y/u W*rcn 113Tn

*'—-•*"""••..,

.-"""\ H»

."""% JH.•" "*.

/""••••_ '", .--•. v..-••

30 aMn

S»l fKinn etlic!*

U M,'//u "ir c/lmn> | 77 MiV/u "tr a>"'TX

JL"-"'~

.-•••-.. Hf"._

...•" '»•"••.'*

.••••-..'H

"• * * » • • *

H*

'H

'" ' . '•''H • • .

to n K a a a joAfrl Mn

Figure 1: Inclusive and exclusive (triggered by lightcharged particles emitted at 160°) neutron multiplicitydistributions as measured with two detectors.

ready an interesting information : no sizeable shift isobserved in the location of the high multiplicity bump,associated -to the most dissipative collisions. The mostspectacular effect is related to a change in detection ef-ficiency when the 44 MeV/u experiment is performed,first with a 500 1 detector and. then with a 1500 1 detec-tor. Apart this experimental effect, no important changecan be seen from 35 MeV/u up to 77 MeV/u. Trig-gering the neutron detection by backward emitted lightcharged particles (l.c.p.) leads also to similar neutronmultiplicity distributions whatever the bombarding en-ergy. Moreover, the evaporation like l.c.p. ha.'e beendetected at 160° with similar multiplicities. Thus, theextra energies of 307 MeV and 1433 MeV brought bythe projectile, in the center of mass Ar + Th system ascompared to the initial 1194 available bombarding en-ergy at 35 MeY/u have hardly any effect on the neutronmultiplicity data as well as on evaporative l.c.p. emittedbackwards.

73

i n tg u n g u u

Fig.i. Calculated spectra for alpha particles

emitted by the composite nucleus (CE: dotted

lines) by the fission fragments (FE:dashed

lines) and their sum (full lines) compared to

the experimental spectra (histograms).

1) D. Jacquet et al. Phys. Rev. Lett. 53

(1984) 2226.

D. Jacquet et al. Phys. Rev. 32 (1985)

1594.

D. Jacquet et al. Submitted for publica-

tion.

2) D.X. Jiang et al. Contribution to the III

Conference on Nucleus-Nucleus Collisions,

St.Malo (1988).

3) D. Hilscher et al. Phys. Rev. Lett. 62

(1989) 1099.

4) D.X. Jiang et al. Submitted for publica-

tion.

data 3 } . It is a rather cold nucleus which

undergoes scission irrespective of its initial

temperature. Due to their origin the light

charged particles constitute a good probe of

the hot nuclei. The slope temperature which is

deduced from the energy spectra indicates that

a-particles are emitted rather early by

strongly deformed and spinning nuclei. These

strong deformations are also found in a

Landau-Ulasov simulation of the collision for

different impact parameters. The shape degree

of freedom are shown to slowly relax in the

course of the cooling of the nuclei. Thus it

is quite consistent to observe a signature of

the deformation in the evaporated particles.

To summarize, light charged particle

evaporation becomes increasingly important

when the temperature of heavy nuclei is

raised, as confirmed by additional data on

similar systems *' . These charged particles

constitute a good probe for investigating the

average properties (temperature, spin,

deformation) of very excited nuclei (T s 5

MeV) formed after violent nucleus-nucleus

collisions.

74

MOMENTUM TRANSFER LIMITATION IN 20Ne AND 40Ar INDUCED REACTIONS ON 124Sn

A. Lleres+, J. Blachot , J. Cranjon , A. Gizon+ and H. Nifenecker

+ Institut des Sciences NucMaires, 53 Avenue des Martyrs, F-38041 Grenoble, France* Centre d'Etudes Nucteaires, DRF/Ph.N, BP 85 X, F-38041 Grenoble Cedex, France

A limitation of the linear momentum transferin central heavy ion collisions has been observed ina lot of various experiments performed atintermediate incident energy on heavy and lighttargets (Ref. 1,2,3). It was shown that, as theincident energy increases, the complete fusionprocess becomes less and less probable and givesplace to increasingly incomplete fusion. To followthe evolution of fusion processes in centralcollisions on medium mass targets, we have focusedour effort on a series of systematic velocitymeasurements of heavy residues recoiling atforward angles in 12C to 40Ar induced reactions on124Sn.

In this paper, we report a set of results on20Ne and 40Ar induced reactions at energy from 20to 60 MeV/nucleon. Simple incomplete fusioncalculations are applied to estimate the amount oflinear momentum transferred to the fusion-likesystems in the most violent collisions. Estimatedmomentum transfers are compared to predictions ofa pre-equilibrium emission model and a simpleFermi jet model.

The experiments were performed withions of 20, 30, 40 and 49 MeV/nucleon and 4OAr of24, 27, 30, 35, 44 and 60 MeV/nucleon. Thehighest energy beams were delivered by theaccelerators GANIL (27 - 60 MeV/nucleon) and SCat CERN (49 MeV/nucleon) and the lowest ones bythe SARA facility (20 - 30 MeV/nucleon).

The experimental technique is based uponoff-line gamma-activity measurements followingtarget irradiations associated with on-line collectionof reaction products (Ref. 4). Heavy productsemitted around the beam axis were collected by astack of thin aluminium foils (1 or 2 urn) set behinda thin target (400 ug/cm2), perpendicularly to thebeam axis. A collimator set between the target andcatchers defined a solid angle with an angularaperture of 10°. Identification and production crosssections of heavy reaction residues (70 < A < 130)were established from the analysis of gamma-activities measured on the catchers. Velocitydistributions were extracted by use of range-energyconversion tables.

Typical residue velocity-mass spectra obtainedfor the 2°Ne + 124Sn and 40Ar + l^Sn reactions areshown in figures 1 and 2, respectively. On thesefigures, the dashed lines represent the evolution ofthe most probable velocity as a function of mass ;the highest values are associated to the most violentcollisions. Vcm is the center of mass velocity.

80 100 120 80 100

Fig. 1: Velocity-mass spectra of forward emittedheavy residues from the 20Ne + 124Sn reaction at20,30,40 and 49 MeV/nucleon (see text).

One observes clearly that, at each bombardingenergy, the most probable velocities of the residues,although relatively high, are lower than the center ofmass velocity. This indicates that, for all incidentenergies, the final products correspond toevaporation residues coming from incomplete fusionmechanisms. Moreover, with increasing incidentenergy, the difference between the measured mostprobable velocities and the center of mass velocityincreases, indicating that the involved momentumtransfer is more and more incomplete.

To estimate the amount of linear momentumtransferred from the projectile to the fusion-likesystem, several assumptions can be done to describethe formation and the de-excitation of die fusion-likenucleus (Ref. 4,5,6). We assume that a part a of theprojectile fuses with the totality of the target, thespectator nucleons being emitted (as nucleons,clusters or a quasi-projectile fragment) along thebeam axis with a part p of their initial velocity. Wesuppose that the composite nucleus de-excites byisotropic nucleon evaporation. Linear momentumtranfers were estimated by fitting a and p toreproduce the measured residual most probablevelocities and masses. Calculated p values show thatnon-absorbed projectile nucleons keep around 75 to100 % of their initial velocity.

75

It is far too early to conclude unambiguously on this point. Nevertheless, Landau-Vlasov calculations stronglysuggest that compression energy could represent a sizeable part of the total excitation energy for bombardingenergies of about 50 MeV/u. Our aim in the future will be to find signatures of these features. One way for thatis to study fusion nuclei of comparable total excitation energies but with different values of the compressionenergy involved in the collisions.

4) ReferencesA Semi-Exclusive Study of Heavy-Residue Production in the ^Ar+Ag Reaction at 35MeV/u Incident EnergyZ. Phys. A - Atomic Nuclei 323,459-464 (1986)Evolution of fusion reaction with AT projectiles from 30 to 60 Mev/uInvited talk Texas A and M Symposium on Hot Nuclei. College Station USA Dec 1987Evolution de la fusion pour les systemes ^ A r + ^ A g et 4°Ar+197Au entre 39 et 60 MeV/AP. Eudes - These de 1'Universite" de Caen, 1988.

E MeV/A

395060

IBV MeV

4.2±0.44.2±0.25.5+0.1

TMvMeV

3.8±0,44.5±0.55.410.2

TnvMeV

4.0±0.25.110.96.9+1.0

60 fWf l flR «•

V 2 0

w

Ii s *

Figure 1

i t 1

1 2Residue velocity (cm/ns)

Figure 3

250 500 750 1000

100

50

Ar * Ag 60 MeV/u

J _

0.5 1. 1.5 2.

Residue velocity (cm/ns)

Figure 2

J50Q 1000 E'(39Hev)

.£•

1 7

e i

? 3C

iS 1

t30

ilj i

500 ' 1000 ' E"(50Hev)

500 1000 ' E*(60MeV)

• 39MeWu A ' A • •- • SOHeV/u

» 60MeV/u ' *A

A • A

• *

* •

9 Ar » Ag system . evaporated protons

• •

* VtM(39HeV/«)

' , , , , I ,

A e

A*» * evaporated at

• A

' , - - , 1 ,

1 2Residue velocity (cm/ns)

Figure 4

76

FUSION RESIDUES BETWEEN 35 AND 60 MeV/A

G.Bizard,R.Brou,J.L.Laville,J.B.Natowitz,J.P.Patry>J.C.Steckmeyer,B.Taniain L P C , CAENH.Doubre,A.Peghaire,J.Pe"ter,E.Rosato GANIL, CAENF.Guilbault,C.Lebrun L S N , NANTESF.Hanappe U L B , BRUXELLESJ.C.Adloff,G.Rudolf,F.Scheibling C R N , STRASBOURG

1) Motivations

The purpose of our experimental program is to study the evolution of the fusion process for Ar projectiles up to60 MeV/A .At low bombarding energies, fusion nuclei may be recognized in detecting either fission fragments orevaporation residues. Our experiment considered the latest ones. Two targets have been used : silver and gold. Aspecificity of our measurements lies in the use of multidetectors. The triggering signal is given by a telescopewhich identify the evaporation residue. In coincidence with this residue, we are looking at light charged particlesemitted either in the forward direction (detected in the plastic wall) or at larger angles (detection in the barrel). Theexperiment performed up to now has been achieved by using the whole plastic wall but only few pieces of thebarrel covering a large polar angular range but a limited azimuthal one. The results may be analysed in twosections : results concerning the inclusive detection of the residue ; and results obtained by analysing thecoincident light charged particles.

2) Inclusive measurements

The fusion nuclei have been identified by the detection of evaporation residues at forward angle in a time of flightsilicon telescope. The energy threshold was set low enough to enable detection of very slow heavy residues(v > 0,3 cm/ns). As shown in Fig.l, which is an example of experimental data of correlations between massand velocity residues from Ar+Au system at 60 Me V/A , the classes of events appear clearly : quasi-projectile,fission and evaporation residue contributions. In the case of Ar+Ag system there is no evidence for fissioncontribution. It appears that the evaporation residue velocity distributions do not exhibit any peak above thevelocity threshold, where as it exists at 27 Me V/A. However evaporation residues are observed with recoilvelocities which can represent a significant fraction of the center of mass velocity (arrows of Fig 2&4), whichdecrease with bombarding energy. The analysis of coincident light charged particles will show that the eventscorrespond really to very hot fusion nuclei formation. The existence of such events for bombarding energiesreaching 60 MeV/A is a first qualitative result of our experiment. In Fig.2 experimental results (horizontal lines)are the most probable evaporation residue masses for each recoil velocity bin. The points have been calculated inthe framework of an incomplete fusion model. The projectile is assumed to partially fuse with the target, itsremaining part flying away at zero degree with the beam velocity. This model is able to nicely reproduce the mostprobable masses of the detected residues. The agreement is a first indication that excitation energies as high as700-1000 MeV could be reached in fusion nuclei. For Ar+Au system at 60 MeV/u there is a sizableevaporation cross section which have a recoil velocity exceeding half the center of mass velocity. The abovesimple incomplete fusion model indicates that in this case, the fusion nucleus would have a mass of at least 220amu and an excitation energy of about 1 GeV . The fact that such a nucleus does not undergo fission with astronger probability cannot be understood in the static statistical model but only in a dynamical statistical modelwhere evaporation takes place before fission.

3) Coincident measurements : semi exclusive analysis

Velocities of light particles detected in coincidence with evaporation residues at 39,50 and 60 Me V/A, havebeen measured. The backward spectra (90-150°) exhibit a maxwell shape and have been fitted by standartevaporation code (Lilita) in which it was assumed that all the backward detected particles were evaporated fromthe fusion nucleus. It has then been possible to calculate the corresponding forward contribution and, to subtractit from ilie forward detected spectra. The corresponding spectra account for particles which are not evaporatedfrom •!.- fusion nucleus. These particles may be due to direct processes or to preequilibrium emission or toseque.a.ui decay of a projectile like fragment. We will call it fast component. By integration of this component ithas been possible to estimate the percentage of the initial beam energy which has not been dissipated in the targetnucleus.

In Fig.3 the corresponding charge multiplicity is compared with the prediction of the massive transfer modelfor various velocities of the fusion nucleus (VR). In the case of small VR values (peripheral collisions) thedisagreement between the curve and the experimental points, is due to the undetected very forward projectile likefragments; for large VR (central collisions) the agreement is very nice which shows that the massive transfermodel is able to predict the excitation energy dissipated in the fusion nucleus. This excitation energy is plotted onthe upper axis of Fig.4. These figures are related to the fusion nucleus decay particles multiplicities. Both p anda multiplicities increase and saturate for an excitation energy of about 750 MeV whatever incident energy.Such result is surprising when considering temperatures extracted from backward proton kinetic energy spectraslopes (silicon telescope at 150°) which are tabulated . The three columns low (LV), medium (MV) and highvelocity (HV) correspond, to more and more violent (or central) collision. The absolute values of T increasewith bombarding energy. How do we can reconcile these results with the multiplicity evolution noted above ?One explanation may be found in the excitation of collective (compression) modes. In such a framework, thefusion nucleus excitation energy would be divided in two parts (thermal+Collective) and the collective part couldaffect significantly both the slopes of the particle kinetic energy spectra and the relative multiplicity of clusters.

77

4 0Ar , e=o°.io°

120 80 100 120

Fig. 2 : Velocity-mass spectra of forward emittedheavy residues from the 40Ar + 124Sn reaction at24,30,44 and 60 MeVlnucleon (see text).

250

> 200

<• 15°CL" 100

2°Ne +

20 40 60 80Energie / nucleon (MeV)

Fig. 3 : Linear momentum transfers per incidentnucleon estimated for the 20Ne + 124Sn and4^Ar + 124Sn reactions. Comparison with pre-equilibrium emission calculations (dashed line) andFermi jet calculations (solid line).

The linear momentum transfers estimated forthe most central collisions are displayed in figure 3.We clearly see a limitation in the linear momentumtransfer which occurs at an incident energy around40 MeV/nucleon for neon induced reactions and20 MeV/nucleon for argon. The maximum value ofthe transfer is 190 MeV/c per incident nucleon forneon (the total transfer is 3.8 GeV/c) and 150 MeV/cper incident nucleon for argon (6.0 GeV/c).Estimated momentum transfers per incident nucleonexhibit a clear dependence upon projectile mass:heavier is the projectile, less efficient is the transferto the target nucleus.

Linear momentum transfers calculated in theframework of a pre-equilibrium emission model anda simple Fermi jet model are plotted in figure 3. Inthe pre-equilibrium model (Ref. 7), one supposesthat all the projectile and target nucleons form acomposite system which equilibrates by internalnucleon-nucleon scatterings and emissions into thecontinuum (two-body process). In the Fermi jetmodel (Ref. 8), which is based on momentum spaceconsiderations, one assumes that nucleons areejected from the projectile due to the target meanfield (one-body process). These two modelsqualitatively reproduce the observed momentumtransfer limitation, indicating that it could be due tofast particle emission. For neon, momentumtransfers extracted from experimental data arequantitatively in good agreement with pre-equilibrium calculations, suggesting that nucleon-nucleon collisions contribute strongly to the incidentenergy dissipation. For argon, the disagreementobserved with pre-equilibrium calculations could becorrelated with (static) limitations of hot nucleistability or (dynamic) compression effects.

References:

1) H. Morgenstern et al., fhys. Rev. Lett. 52(1984) 1104.

2) A. Fahli et al., Phys. Rev. C34 (1986) 161.3) S.Leray, J. Phys. 47 (1986) C4-275.4) A. Lleres, These d'Etat, Universit^ de Grenoble

(1988).5) H. Nifenecker et al., Nucl. Phys. A447 (1985)

533c.6) A. Lleres et al., J. Phys. 47 (1986) C4-365.7) M. Blann, Phys. Rev. C31 (1985) 1245.8) C. Gregoire andF. Scheuter, Phys. Lett. 146B

(1984)21.

78

PRODUCTION AND DEEXCITATION OF HOT NUCLEI IN 27MeV/u *°Ar + S 3 aU COLLISIONS

D. JACQUET'1', G. PEASLEEb), J.-M. ALEXANDER"1, B. BORDERIEa), E. DUEKb), J. GALINC), D. GARDES'",

D. GUERREAUC), G. GREGO1REC), H. FUCHS", M. LEFORTa), M.-F. RIVET"1 and X. TARRAGO"'

a) Institut de Physique Nucleaire, Orsay, France.b) State University of New York, Stony

Brook, USA. c) GANIL, Caen, France, d) Hahn-Meitner Institut, Berlin, Germany.

The aim of the present study was to inferthe properties (temperature deformation and

spin) of hot nuclei formed in the Ar + U

collisions from the characteristics of the

charged particles they evaporate. A selection

of the hottest nuclei which are formed

exploits the recoil properties of the fused

nuclei, assuming that the greater the linear

momentum transfer from the projectile nucleus

to the target nucleus, the larger the energy

deposit must be. With a highly fissile target,

like uranium, the momentum transfer can be

deduced from the folding angle of coincident

fision fragments. Three-fold coincidences

were thus measured between two fision

fragments and light charged particles, the

latter being detected at several angles with

respect to the fission plane or in a plane

containing the beam axis but perpendicular to

the fission plane. Such an arrangement was

chosen in order to increase the sensitivity to

spin effects. Special emphasis has been put on

a precise measurement of all light particles,

particularly those of low velocity which arestopped in the first silicon member of each

telescope. Fission fragments were detected, on

one hand, by small areas standard silicon

counters and, on the other hand, by position

sensitive counters to precisely record the

relative emission angle of the fragments.

The usual folding angle picture (fig.l)

shows that the events are roughly evenly

divided between fission following fusion with

about 80% momentum transfer and sequential

fission of weakly excited target-like nuclei1'

Such results were confirmed lately by

detecting che evaporated neutrons in

coincidence with the fission fragments2'.

200C

1SOO

1000

500

L27 MeV/u Ar on U

• I • '125

Correlated charged particles exhibit

quite different behaviors depending on whether

they are emitted forward or backward (fig.2).

The differential multiplicity of those emitted

forward and thus of both equilibrium and

pre-equilibrium origin does not show a very

strong sensitivity on the folding angle of the

fission fragments, thus indicating that a

forward hodoscope cannot be used as a very

sensitive filter on the violence of the

collision. On the contrary the multiplicity of

backward emitted particles, all of evaporative

origin, is extremely sensitive to the folding

angle and thus can be efficiently used to

select the most dissipative collisions.

Fig.l. Folding angle distribution of the

fission fragments. .

Fig.2. Differential multiplicity for H/He for

various angles 6 H / H € as a function of the

folding angle of the fission fragments.

A kinematical analysis of the charged

particles was then performed to infer their

origin: either the hot nucleus prior to

fission, or the accelerated fission fragments

(fig. 3). Eighty percent of the oc-particles

could be unambiguously attributed to emission

prior to scission, in good agreement with what

has been observed more recently from neutron

79

A crude estimate of the excitation energies atsaturation can be performed by adding up the energiescarried away by all the neutral and charged evaporation-like particles. The measured neutron multiplicity needsto be corrected for detector efficiencies. As for the I.e.p.

obtained with Kr beams around 30 MeY bombardingenergy clearly show that the linear momentum transferis roughly twice as large as the one observed with Arand that the excitation energy, as simply deduced frominclusive neutron multiplicity measurements, exceeds byabout 50'/? the one deduced with the Ar beam.

AEV

UjCD

10 :0 20 30 a SO SO 70 iO

E (MeV/u)

Figure 2: Evolution with bombarding energy of the to-tal number of neutron (0) corrected for detector effi-ciency and evaporated charged particles (V) summedover Z=1.2, released from the most dissipative colli-sions. The excitation energies are givan by the righthand scale.Data at 10 MeV/u and 27 MeV/u are pub-lished elsewhere1

one has to extrapolate from the single recording angle,assuming an isotropic angular distribution in the refer-ence system of the emitter and carrying out the jacobiantransformation. Since the solid angle transformation de-pends only weakly on the recoil velocity, the result is nottoo sensitive to the integration procedure. The recoil ve-locity \'f> has been taken independent of the bombardingenergy as suggested by both the fission data and by thebehaviour of the energy spectra of the particles observedbackwards.

The result of this evaluation does not show anysignificant increase in the excitation energy from 35 upto 77 MeV/u within the experimental uncertainty(fig. 2)The value of 650 ± 100 MeV. or T = 4.5 ± 0.4 MeV (witha ~ j ) . matches very nicely the temperature extractedfrom the slope of the o-particle energy spectra recordedbackward.

4 Conclusions

Clearly, in these Ar induced reactions one witnesses theeffects of the dynamics, but by no means the manifes-tation of a limiting temperature that the composite nu-cleus could sustain. Indeed, recent experimental data

References[1] D.X. Jiang et al. accepted for publication in Nucl.Phys. A.See also J. Galin et al. proceeding of the Symposiumon Nuclear Dynamics and Nuclear Disassembly (DallasApril 1989) published in World Scientific.

80

DYNAMICS OP HEAVY-ION COLLISIONS AT GANIL ENERGIES

C. GREGOIRE

*

B. REMAUD F. SEBILLE L. VINET* Y. RAFFRAY*

GANIL BP 5027, 14021 CAEN Cedex+LPN - UA 57, Universite de Nances, 44072 NANTES Cedex 03"iRESTE, La Chantrerie, Universite de Nantes, 44087 NANTES Cedex 03

1. Motivation

In the energy range of GANIL facili-

ty, pure mean-field theories fail in des-

cribing the stopping power of nuclei, which

prevents them to cross each other with re-

latively small perturbations.The Vlasov

equation completed with the Uehling-

Uhlenbeck collision term provides a semi-

classical framework ' which treats dynami-

cally the balance between the mean-field

effects and the two-body collisions. A se-

rie of studies within this theory has been

undertaken to analyze the regimes of heavy

ion reactions that could be expected in the

energy range of 30 Mev/u to 100 Mev/u.

2. Regimes of heavy-ion reactions

At these energies, the Pauli blocking

is strong enough to forbid most nucleon-

nucleon collisions, the nucleon mean free

path (5-10 fro) is still^of the order of the

nucleus diameter. However, the collision

rates are sufficient to destroy the nuclear

transparency predicted by pure mean field

theories. Below 30/40 Mev/u in symmetric

reactions (and higher for asymmetric ones),

the Landau-Vlasov approach describes reac-

tion regimes inherited from lower energies

: fusion at central impact parameters and

formation of quasi-projectile/quasi-target

fragments for peripheral collisions. Howe-

ver, these processes are incomplete since

preequilibrium emissions (see fig. 1) - in

increasing multiplicities with increasing

energies - indicate that some nucleons do

not participate to the equilibration pro-

cess.The preequilibrium processes last less

— 22

Chan 60 fm/c (2 10 sec ) and are progres-

sively replaced by isotopic emissions; ap-

parent thermalizations of the initial kine-

tic energies may then occur in time ranges

of few 10 sec, for central collisions.

30-

25 -

30-

G-

10-

s-

W1THIH SPHERE IvtTH RADIUS tOtn

PR£-EQUILIBRIUMEMISSION

o X so yj tlOKOitant itoijo TMSllmjc]

o 30 60 so ao iso iso 210 2to so TIME(fm/cf

Characteristics of nucleon emission in theentrance channel of H-I reactions.

In more peripheral collisions, a partici-

pant zone is formed in the region of the

overlapping densities ; this participant

zone evaporates quickly and two fragments

emerge with reduced relative velocities.

Although the correlation between the masses

of quasi-projectile and quasi-target are

close to a pure geometrical abrasion , the

process is similar to deep inelastic reac-

tions if one considers the strong deviation

of the fragments trajectories induced by

the mean-field. When the initial kinetic

energies increase, peripheral collisions

come closer to abrasion limit.However a re-

latively quick transition occurs in central

collisions where the disassembly of the fu-

sed system is observed at 40/50 Mev/u in

symmetric systems.

3. Excitation mechanisms

As a transport equation, the Landau-Vlasov

81

equation can go beyond the description of

bulk observables as linear momenta, angular

distributions, etc...; it allows the micro-

of Che collective resonance (mainly monopo-

le) in the limitation of the excitation

energy immediately available to the system;2 }scopic and local analysis ' of the various it provides a clue for the interpretation

mechanisms for the dispersion of the cohe-

rent initial kinetic energy.

of the experimentally observed limitation

of "nuclear temperature".

MeV/ub ; 0 fin

-5

-10

-20

-30

/Touching point

0 20 40 60 80 100 120 140TIME (fm/c)

Fig. 2

Analysis of the contributions to the totalenergy. Top figure enlarges the upper partof bottom figure.

The lower part of fig.2 shows that the ini-

tial kinetic energy (E c o l l at t = 0) is a

small part of the total kinetic energy; due

to the mutual stopping power of the nuclei,

this collective energy could be progressi-

vely transformed in excitation energy. Ho-

wever after a compression phase at t = 40

fm/c, the system expands very quickly,

which very efficiently removes excitation

energy, while preequilibrium nucleons and

first evaporative ones start leaving the

system.

Such calculations emphasize the role

4. Conclusions and outlooks

The Landau-Vlasov approach provides a na-

tural link between the mean-field models

(as TDHF) at low-energy and pure cascade

models which apply at very high energy. It

meets success in the description of bulk

one-body observables ; however, it presents

limitations both on theoretical and techni-

cal sides.

As a mainly one-body approach, it is not

fitted to systems with large fluctuations.'

In chat sense, ic is'an entrance channel

model which needs to be completed to des-

cribe exit channels, particularly when

fragments are formed.

On the technical side, its numerical solu-

tion with Monte-Carlo methods is very de-

manding to to-day computers. Studies of

phenomenas like induced fission, competi-

tion between fission and particle evapora-

tion, excitation energy share between nu-

clear fragments, etc..., seem at the limit

of actual possibilities for systems with

more than 100 nucleons. However the above

list constitutes the open field of studies

for the Landau-Vlasov equation in the near

future.

5. Main references

1) C. Gregoire et a l . , Nucl. Phys. A465

(1987) 317.

2) B. Remaud et a l . , Nucl. Phys. A488(1988)

423c.

8 2

A NEW THEORY OF COLLISIONSB. G. Giraud

Service de Physique Theorique,IRF,CEA,CEN-Saclay,F-1191-Gif-sur-Yvetteand

M. A. NagarajanDaresbury Laboratory, Warrington, WA4-4AD, UK

Abstract: We introduce a time-independent generalization of the static Hartree-Fockequations, which become inhomogeneous.

There is a large domain of heavy ion physics where the only degrees offreedom to be considered are just nucleons, while pions, exchange currents, nucleonisobars, etc. are neglected. This purely nucleonic domain, however, subdivides into a"high energy" domain (HEND), where all nucleons are active and a "low energy" domain(LEND), where the dynamics reduces to fewer degrees, usually of a collective nature:deformations, rotation velocities, etc. From DWBA to hydrodynamics, many ad hoctheories provide elegant, but restricted descriptions of some aspect of thesedomains.

It would be desirable, however, to use a fully quantal and microscopictheory, as a reasonable approximation of the Schroedinger equation, in order tointerpolate beween all these domains. As a matter of fact, the GANIL energy range,10-100 MeV/nucleon, demands such a theory valid for transitions between LEND andHEND.

For several years, there was a hope that TDHF would provide the answer. Itturned out, unfortunately, that TDHF could not account for large fluctuations in theobservables of heavy ion physics. The reasons for this failure are unclear: maybe themean field approximation is not justified, or inaybe the boundary conditions of TDHFare not correct. We have now a new microscopic theory, which takes care of theboundary conditions.

Our idea is very simple: instead of approximating the time-dependentSchroedinger equation (TDSE) by a mean-field approximation (UFA) such as TDHF, let usapproximate the time-independent Schroedinger equation (TISE) by another MFA. This isnot just an academic trick, for the Fourier tranform which connects TDSE to TISE doesnot commute with a MFA. In other words, there is no Fourier transform equivalencebetween TDHF and a MFA performed on TISE.

We describe the initial channel by a Slater determinant lx>, with boostedorbitals, like TDHF. What is new is that we describe also the final channel by anexplicit, time-independent determinant <x" I, while in TDHF the final channel was ablack box, often time dependent. We know the prior and post potentials V and V ,respectively, and now calculate the (multistep) T-matrix element D=<x' IV(E-H)~'vix>,via a MFA.

The details of the formalism have been published1"25. The amplitude D iscalculated from an EXACT Breit-Wigner formalism, D=rT/(E-E'), with transitions viatwo "doorway" states $', *, and !><*' IVI?<>, P-<x'IV l<fc>, E'-<qV IHI*>. The MFAconsists iii taking Slater determinants for <f> and <f>' .

A variational principle3' defines the orbitals <p,. , <p' of <£, <f>' , respectively,i

via generalizations of the static Hartree-Fock equations,

(TIJ - h, ) l(pj> = I0i>, <a 'I = «p'J (rij - h,- ), (1)i i

where the source terms cr( ,(j' are due to the initial and final channels1'Z>.Like ini

static Hartree-Fock, one finds mean-field Hamiltonians h^ and self energies i\^ .

This theory has been successfully tested on exactly soluble models for

83

two-body problems4•5'and three-body problems6*. The cases of four7 •8 •9Jand five105

bodies are also investigated, with very positive results. At present we areinvestigating a realistic proton-triton collision, with a Gaussian two-body force. Itturns out that the mean-field equations, Eq.(l), are technically more difficult tosolve than static Hartree-Fock equations, but still manageable. We show on the Figurethe momentum density plots of several "transition orbitals" <pj , <p' , taken out of a

whole atlas generated by our computer.

We have thus reached a first, and important conclusion: this new theory ofcollisions, based on non-linear equations with yet poorly known properties, does havesolutions. Moreover, as shown by the Figure, the "transition orbitals" seem to besmooth, like extensions of shell-model states. We are able to calculate quantaltransition amplitudes.

Besides comparison of our transition amplitudes with experimental data, ouratlas of orbitals raises, and probably partially answers, several interestingquestions. Among them, is there a systematic zoology of such orbitals, similar to theclassification provided by the static shell model? VThat are the criteria for theidentification of a modification of reaction mechanisms? Last but not least, theself-consistent mean-field potentials contained in the single particle hamiltonianshj define a new optical theory, whose interpretation and zoology is just beginning.

References:l)B.Giraud, M.Nagarajan andI.Thompson,Ann.Phys.152,475(1984)2)B.Giraud, M.Nagarajan andC.Noble, Phys.Rev.A34,1034(1986)3)B.Giraud and M.Nagarajan,J.Phys.04,1739(1978)4)M.Nagarajan and B.Giraud,Phys.Rev.027,232(1983)5)B.Giraud, Physical9D,112(1986)6)Y.Abe and B.Giraud,Nucl.Phys.A440,311(1985)7)B.Giraud and M.Nagarajan,Phys.Rev.C28,1918(1983)8)B.Giraud, S.Kessal andA.Weiguny,J.Math.Phys.29,2084(1988)9)J.Lemm and A. Weiguny,private communication10)B.Giraud, S.Kessal andL.C.Liu, Phys.Rev.C35,1844(1987)

3 -3 -1.6 -0.6 0.6i

1.6 3

I i I i J I I !_

-1 1 I 1 I 1 I 1 1_

84

MEDIUM CORRECTION OF TRANSPORT EQUATION

J. Cugnon*, P. Grange1** and A. Lejeune*

*Universitd df Liege, Physique Nucl^aire Theonque, Inst i tut de Physique au Sart Tilman, B.5,B-400.0 LIEGE 1, Belgium

**CRN , B.P. 20, CRO, F-67Q37 STRASBOURG Cedex, France

A la rge consensus has emerged on the fact that a t ranspor t equat ion su i t ab le

to descr ibe heavy ion c o l l i s i o n s at in termedia te energy has a Landau-Vlassov (LV)

form :

_ U ) . - $ ) f ( r , p , t )

P

^ _-P-i-Pi. )6(e(p) + e(p,) - e(p,) - e(p.))34 Z 3 4

{W(Pp2 * P3p4)f3f4(1-f)(1-f2)-(0(pp2 H- P3p4)ff2(1-f3)(1-f4)}. (1)

In this equation, f(r,p,t) can be considered as the usual Wigner transform of the

one-body density, U is the average single-particle field, e(p) is the single-par-

ticle energy, <o is the transition probability and f stands symbolically for

f(r,p ,t). The situation is still confuse on knowing whether equation (1) retains1 i

the right physics. For instance, it does not retain retardation effects in the1 ) 2 )

collision term , which could be important . Moreover, even if eq. (1) is con-

sidered as a good starting point, the question arises to know what are the "best"

input data (L),e,u) for this equation. The point is that the LV equation can be

obtained as a truncation of the density matrices (BBGKY) hierarchy. In the sim-

plest case (the weak coupling limit), the LV equation is obtained in second order

in the interacting potential v . Of course, in the nuclear case, the relevant

physics cannot bf described at this order, unless resummation and medium renorma-

Jization of the successive interactions are introduced. This is shown in refs.

' , which establish the connection with the Brueckner theory of nuclear matter,

where the same concepts are used. It is then natural to identify the input data

with the corresponding quantities in Brueckner theory (in the local density ap-

proximation)U = UB(p,T,p) (2)

e(p) = *£• + UB(p,T,p) (3)

where g(p,T) is the Brueckner g-matrix at density p and temperature T, UD is tne_5)single-particle field in the Brueckner-Hartree-Fock approximation. In ref. ,

we give this quantity for some range of density and temperature. The input e and

co can be introduced in a very tractable manner. For e(p), one can replace them

in eq. (1) by kinetic energies, provided the collision term is multiplied by the56) '

effective mass m*, which is given in refs. ' . For the medium correction of

the transition matrix, we presented our results in the following form

ot(p) <pp2|T|p3p4> , (5)

where T is the free transition matrix. The value of the coefficient o is given

in fig. 1, for various temperatures and densities. Convenient pararaetrizations

ran be found in rrF. ' . Further refinements including the angular distribution

85

for scattering inside nuclear medium are in progress.

1.5-

1 2 3

P'P,-

F" i g. 1. The ratiofor various densiti

a eq. (5), as a Funclioh of%the relative momonlum p/pr andles. Full curve : T = 0 MeV ; dashed curves : T = 10 MeV.

1) L.P. Kadarioff arid G. Baym, Quantum Statistical Mechanism (Benjamin, New York,1962).

2) P. Daruelewicz, Arin.Phys. 152 (1984) 305.3) R.F. Snider, J.Chem.Phys. 3_2_ (1960) 1051.4) W. Botermans arid K. Malfliel, Phys.Lett. 171B (1986) 22.5) A. Lejeurie, P. Grange, M. Martzolff and J. Cugnori, Nucl.Phys. A455 (1986) 189.6) J. Cugnon, A. Lejeune and P. Grange, Phys.Rev. C35 (1987) 861.

86

NON LOCALITY EFFECTS IN NUCLEAR DYNAMICSF. Sebille. G. Rover, C. Gregoire', B. Reraaud", P. Schuck ' " . V. De la Mota. Y. Raffray.

Laboratoire de Physique Nucleaire. UA 5?. WI NANTES Cedes 03 France* GANIL BP5U2? F-HU21 CAENCedex France

** IRESTE route de la Chantrene F-MUG3 NANTES Cedex France* * * ISM. F-3&U2S. GRENOBLE Cedex. France

L MotivationCurrent theoretical investigations on

heavy ion collisions in the GANIL energyrange, widely use kinetic approaches based ona self-consistent microscopic framework [1].Generally, in these semiclassical descriptionsof nuclear dynamics, the effective nuclearinteraction is taken as a simplified Skyrmeforce, local and density dependent. Their maindrawback is to neglect the momentumdependence of the nuclear force and theresulting effective mass, important effectswhich stem from the Brueckner theory. Thenworks 121 have been undertaken in order tostud)' signatures of non • locality effects onsensitive observables in phenomena rangingfrom induced fission to collective transversematter flow observed in heavy ions reactionsnear 100 MeV per nucleon

2. Description of nor* locality effects.Description of non locality effects

and nuclear dynamics have been madestarting from a semi classical approximationof the time dependent Hartree Fock equation'.the Vlasov equation), where the residualinteraction? are introduced with the Uehling-Uhlenbeck term, taking into account inmedium etlects. Even if the final aim is toderive the mean field and collision termsfrom a bare interaction, a first attempt to curethe drawbacks of local forces consist inimproving the functional from of the effectiveinteraction. The effective interaction proposedby G->gny allow? to reach this goal, this forcerelies upon hmte range exchange terms andprovide? a successful description ol manvnuclear properties in the phase spacerepresentation the Gogny force generate? amomentum dependence of the mean fieldthrough its finite (range exchange terms infig i the enernv dependence of the potential

depth gives some clear insights in thephysical mechanisms introduced by the nonlocality. A salient feature is the momentumdependence of the attractive potential whichweakens with increasing momenta, leading toa less attractive global potential. Anotherimportant consequence is the influence of themomentum dependence on the nucleonmotion inside the nucleus, the resultingeffective mass implies a faster motion of thenucleons inside the nuclear matter than infree space.

110 150Energy (MeV)

Fig. 1. Real nuclear optical potential i'ui theGogiy Dl-Gl interaction, compared withexpirimental results.

3. Signatures on collective observables.The field of application of our model

implemented with the Gogny force isessentially the low and medium energy rangeof heavy ion collisions. Nevertheless, asunderlined by several recent works, theGogny interaction contains highly interestingproperties which motivate to explore theenergy range around 100-400 Mev pernucleon. In addition, this is supported bypromising experimental and theoretical workswhich aim at extracting informations on thf.Equation of State (E.O.S), especially from theexperimental evidence of a nuclear mattercollective flow near 100 MeV/u

Thus, the non locality effects onnuclear dynamics can be illustrated with anobservable called flow and defined within thetransverse momentum analysi? of the

87

sideward collective matter flow. In Fig. 2.results are displayed for three effectiveforces, namely, the finite range Gogny force,the soft and stiff local Zamick forces. Theiriiicompressibility modulus are respectively228 MeV. 200 MeV and 380 MeV. This figurereveals that the momentum dependence ofthe Gogny force is able to reproduce flowsonly obtained with a high incompressibilitymodulus when local forces are considered.

This can be explained by themomentum dependence of the mean field. Inthe overlap region of two colliding nuclei, anucleon feels not only the field of his initialnucleus, but also the one due to the othernucleus whose nucleons are moving fast withrespect to it Considering Fig. 1, we observethai high relative kinetic energies weakenconsiderably the field experienced by theincoming nucleon It provides a morerepulsive character to the momentumdependent forces compared to the local oneswiih the same compressibility

100

° 50

o Gogny

D Stiff E.O.S

4 Soft E.O.S

0 2 4 6

Impact Parameter (fm)

Fig. 2 Dependence of tlie flow on the impactparameter for various nuclear interactions"-•ith the Nb i! 50 MeV/nucleon.1 - N'b system.

4. Conclusions.We have introduced for the first

time the effective finite range interaction ofGogny in the semi-classical description ofheavy ion reactions based on the LandauVlasov equation. This has been worked out inorder to take properly into account the nonlocal character of the effective nuclear mean-field and to track down the non locality-effects on observables at medium energiesStudies of phenomena Jike induced fissioncompetition between fission and evaporationcollective matter flow are actuallyconcerned. The previously meniionned resultsrelated to the transverse matter flow near100 MeV per nucleon. underlines that themomentum dependence of the nuclear forcehave to be properly described if one wants toextract informations on the Nuclear Equationof State, either at high energ.es or at mediumenergies

[IjC fir ego/re et al.. Nucl. Phys. A465 ' i987>317B. Reinaut! el al.. Nucl. Phys A488 (i93*5423C.

121 F. Sebille et al.. XXVII International wintermeeting on nuclear physics (Bormio) 1989,and to be published.

88

THE PSEUDO PARTICLE METHOD FOR THE SOLUTION OF THE LANDAU-VLASOVEQUATION

P. SCHUCK, Institut des Sciences Nucleaires, 53 Avenue des Martyrs,38026 Grenoble-Cedex, France

GREGOIRE C, VINET L.a), PI Mw, REMAUD M., SEBILLE F 3 , SCHUCK P., SURAUD E.d>

a) GANIL - Caenb) Dept. de Estructura i consliiuentes de la Materia - Barcelonac) LSN - Nantesd) ISN - Grenoble

The solution of the Landau-Vlasov alias Vlasov-Uehling-Uhlenbeck (LVUU)equation represents a formidable numerical challenge. It is a non linear integro-differential equation in seven (!) variables (six phase space and one time variable)and it is clear that such an equation can not be solved "exactly". Nevertheless thepseudoparticle method which we want to explain here has proven to be a verypowerful tool and it has turned out that certain average quantities like e.g. the"nuclear flow" are well converged as a function of a manageable number ofpseudoparticles per nncleon.

In order to keep the discussion as transparent as possible we will neglect thePauli principle which is not essential for the explanation of the pseudoparticlemethod and the assessment of its validity. The two basic equations are the (coupled)quantal equation of motion for the one body density matrix p i r and the correlatedpart of the two body density matrix g(2h2 1'2' :

flf pir(t) - [hH-F- ,p]n '=- 2 (V1234 g(2)341'2 - g(2)1234 V341'2) (1)

g(2>12 1'2'- [hH F ( l ) + hH F(2), g(2)]l2 1'2' = 2 (V1234 P31' P41'~ P13 P24 % * 17') (2)

where hH-F-is the nieanfield hamiltonian and v.... the antisymmetrized matrixelement of the (effective) nucleon-nucleon interaction. It is evident from eq. (2) thatthe rhs has been approximated by its first order Born term what makes (1,2) aclosed system of equations.

In order to arrive from (1,2) at the classical Boltzmann equation, oneapproximates the r h s of (1) by its classical limit, i.e. one transforms it into phasespace and replaces the commutator by the classical Poisson bracket. The r h s of eq.(1) as well as of eq. (2) are evaluated in local density approximation, that is onecalculates everything like in nuclear matter and gives p and g(2) a parametricaldependence on the position R. This then leads to the Boltzmann equation in replacingv2 by the differential cross section do/dE. However even in nuclear matter the r h sof (1) and eq. (2) are still quan ta l expressions. In order to arrive at thepseudoparticle method we have to recognize that the rh sides of (1,2) have acommutator structure. These commutators then have to be replaced by theircorresponding classical Poisson brackets. Solving then (2) for g(2) in terms of p andinserting into (1) yields a completely classical equation for p alone. With thepseudoparticle ansatz

f(R, p, t) = S 8(R - Rc(t)) 8(p - Pic(t))

89

where n is the number of pseudoparticles per nucleon and A the nucleon number,we obtain the following set of classical equations for R|C(t) and pic(0 (in practice weuse Gaussians instead of 5-functions in (3) but this should not be the point here).

Pic = - 1 c

However, in order to arrive at equation (5) an additional approximation hasbeen applied, i.e. when there is a collision it should occur as if the meanfield wereabsent. This condition can only be fullfilled if the meanfield varies little within therange of the two body force. For a step function like two body potential, the forcewould be S-function like and then this condition would be fullfilled exactly. In factfor consistency eq. (5) should also be treated in this spirit. Indeed if the scatteringprocess is almost instantaneous, all what counts is the deflection angle orequivalently the differential cross section. One therefore follows all pseudoparticlespairwise in time and whenever a pair at closest approach is within the nucleon-nucleon cross section reduced by a factor of n we redistribute the momenta of thepair according to the differential cross section. As a matter of fact only s-wavescattering has been considered so far and thus the relative momenta of the pairs areredistributed with equal probability whereas total momentum and individualenergies are conserved in each scattering event.Under the condition that thedifferential cross section is isotropic (this condition could be released) and for thecase that results are converged with the number of pseudoparticles this should thencome quite close to an exact solution of the Boltzmann equation (see below).

So far we have not spoken about the Pauli principle which has to beincorporated when dealing with fermions. This, however, is easily done in evaluatingthe phase space occupancies f (R ,p i ) and f (R,p2) of the two scattered pseudoparticles (R is_ the localisation of the scattering event) and in multiplying the crost:sections with f (R, p i ) . f (R, pa) where 7 = 1 - f . Only if the distance of closestapproach is within this reduced cross section, the scattering event is effective.

Fortunately certain quantities like e.g. the nuclear flow converge relativelyfast with the number of pseudoparticles. With n = 30 the flow of Nb + Nb at b = 4 fmis converged for E/A = 150 and 250 MeV. Also the relaxation of the momentumtensor of two interpenetrating pieces of nuclear mater compares well with n = 3 0with an "exact" numerical integration of the LVUU equation. For the Boltzmannequation exist certain analytically solvable models. Again the exact solution is wellreproduced in the case studied with the pseudoparticle method.

In conclusion the pseudoparticle method seems to work very well at least forinclusive quantities. The main open question remains in the calculation of theeffective medium modified cross section entering the collision integral. In principlethis should be calculated self consistently as the reaction between the heavy ion goeson. This task goes beyond our present facilities and we have to rely on G-matrices inlocal density approximation which are calculated at equilibrium. This impliesuncertainties coming from the initial phase of the reaction where non equilibirumprocesses are important.

90

FORMATION AND DEEXCITATION OF HOT MEDIUM MASS NUCLEI AROUND 3O A.MeV

8. BORDERIE, C.CABOT, D.J^ARDES, H.GAUVIN, F.HANAPPE*,J.PETER , M.F.RIVET

Institut de Physique Nucleaire, F-914O6 Orsay*FNRS and U.L. Bruxelles, B-1O5O Bruxelles

**G.A.N. I.L. , F 14071 Caen

I. Motivation

When increasing the bombarding energy,

incomplete fusion become more and more

domitnant with rather light projectiles

(A < 2O> . The present study was a -first

step to explore the mechanisms involved

in central collisions with Ar .project!les

and medium mass targets <Ar and Ho).

Ar + Ag

lOSOMeV 9-6°

50 IOO 150M(u)

Fig. 1_ : Typical mass-velocity diagram.

was not so evident in view of the large

energy deposit ; such collisions account

for about 15 X of the reaction cross-

section [23. Moreover, the characteris-

tics of invariant energy spectra (width

and variation of their maximum versus et

and angular distributions (Fig. 2) of

heavy residues were found in complete

agreement with what is expected for

evaporation residues.

300 -

200 -

100

Inclusive (heavy residues - Fig. 1) [13

and exclusive (fission fragments) measu-

rements were performed to carefully study

the competition between fission and eva-

poration during the deexcitation of fused

nuclei.

2. Incomplete fusion characteristics

From direct (heavy residues) or deduced

recoil velocities from angular correla-

tions (fission fragments), average linear

momenta transferred to fusion-like nuclei

were derived C23. The measured AP/P

values around 7O 7. indicate a strong

persistence of collective effects which

40 50elob ! d e9'

Fio. 2. r Angular distributions of evapo-ration residues.

3. Deexcitation

Competition fission-evaporation

Evaporation residue and fission charac-

teristics (final masses, out-of-plane

distributions) indicate that one can have

some confidence in the simple reaction

scheme used (part of the projectile which

fuses with the target) to derive

excitation energies. The most probable

excitation energy of fusion-like nuclei

is about 5OO-6OO MeV for both systems and

the distribution extends up to more than

7OO MeV C21. This indicates that nuclei

have been farmed in rather extreme condi-

tions, close to the limit of instability

(static calculation).

The evolution of the ratio °£R/1<*ER +

cvt ) with incident energy is displayed

in Fig. 3. For both systems, a change of

b£0.5

030

0.20

0.10

0.5 I 5 10 50 100(E-VC)/A (MeV)

Fig. 3 : Evolution o-f residue over fusioncross-sections with incident energy.

the slope is observed when the bombarding

energy reaches 8-9 MeV per nucleon, »ndi-

cating inhibition of the fission path in

favor of the evaporation path. Two

reasons can b e found to explain this

evolution of the competition between

fission and evaporation. Firstly, the

onset of incomplete fusion occurs in this

energy region, lowering masses and atomic

numbers of fusion—like nuclei and there-

fore their fission probability. The

second reason can be found in the

deexcitation process itself. We know that

prefission evaporation occurs when

temperatures larger than about 3 MeV are

reached. This evaporation is now well

understood, at least qualitatively, in

terms of particle emission time and

f i 'B i on t i me.

4. Conclusions.

Through measurements of nuclei formed

in 27 MeV/nucleon Ar induced reactions in

Ag and Ho, it was shown that fusion-like

reactions still occur at this interme-

diate energy ; they account for 15 'A of

the reaction cross-section. Fusion—like

nuclei have been characterized through

the properties of their cold remnants,

either evaporation residues or fission

fragments. All these properties show that

fusion-like nuclei were formed with a

very high excitation energy, close to the

limits predicted by different theoretical

models. Cross-section fractionation

between ER and fission shows that fission

barriers still play an important role,

even at high excitation energy, due to

the very short particle emission time as

compared to fission time.

C13 B.Borderie et al, Z. Phys. A316C19B4) 243.M.F.Rivet et al, Proc. of the TsukubaInt. Symposium on Heavy Ion Fusion Reac-tions, Tsukuba (Japan), p. 311 WorldScientific (19B5).C21 B.Borderie et al, Proc. XXIII Int.Winter Meeting on Nuclear Physics, Bormio(Italy), p. 445 (19B5).M.F.Rivet et al, Phys. Rev. C54 (1986)1ZB2.M.F.Rivet et al, HICOFED, Caen (France),Contributed papers p. 39 {1986).

92

PRODUCTION AND DECAY OF HIGHLY EXCITED NUCLEI BETWEEN 26 AND 45 MeV/NUCLEON

F. Auger", B. BerthieH1, A. Cunsolob, B. Faure8, A. Fotib, W. MittigC, J.M. Pascaud*3,E. Plagnole, J. Quebertd, and J.P. Wieleczko8

,aDPhN/BE, CEN Saclay, 91191 Gif-sur-Yvette Cedex,Fisica Nueleare, 1-95129 Catania, Italy. CGANIL,

33170 Gradignan, France. eIPN

MotivationsTwo fundamental questions of Nuclear Physics

are : what is the limiting excitation energy of thenuclei ? How do the deexcitation modes of thesenuclei evolve with increasing excitation energy ?

The Ganil facility allowed to investigate thosequestions. In 1983, it was well known that all centralheavy ions collisions lead to the fusion of the wholesystem for beam energies below 10 MeV/nucleon.It was also known that above this value the incompletefusion mechanism ( composite system doesn't containany more all the mass of the system) plays anincreasing role as the beam energy increases*'.However, one expected to produce hotter and hotternuclei as the available energy would increase.Concerning the underlying mechanism, experimentsusing light beams around 10 MeV/nucleon had shownthat the nucleons of the projectile which don't fusewith the target act as spectators of the reaction(massive transfer)2 \

Experimental choicesWe have chosen to study AT-100 nuclei who

mainly deexcite by light particles evaporation. Themean recoil velocity of the residues (cold nucleiat the end of the deexcitation chain) gives the sourcevelocity. Mass and excitation energy of the sourceare generally deduced from the recoil velocity inthe framework of the massive transfer.

Statistical model predicts that the probabilityfor complex fragments emission (Li..., C, 0, ...)increases with the excitation energy. These fragmentsshould thus sign the hottest nuclei produced in thereactions. If the projectile is heavier than the target,the inclusive spectra of the complex fragmentspresent two branches (forward and backward emissionwith respect to the velocity of the source) whosepositions provide both the source velocity and theemission velocity in the reference frame of thesource.

According to these points, and in order tostudy the influence of the asymmetry of the systemand of the incident energy on the incomplete fusionmechanism, we have bombarded 1 2C, 27A1, 48Titargets with a 84Kr beam of 26, 34 and45 MeV/nucleon.

Inclusive velocity spectra, atomic numberand mass of nuclei from A = 10 to A = 90 have beenmeasured with time of flight telescopes consistingof three solid state detectors. Mass and velocitycorrelations were also measured between a telescopekept at 3° on one side of the beam and another onewhich was moved on the other side of the beam.Integrated cross sections were obtained with a goodprecision (MO 96) for most of the fragment.

Highly excited nuclei productionKinematical simulations of the spectra show

that for a given system :- the major part of the complex fragmentscross section (A < 40) can be attributed tothe asymmetric fission of the same equilibrated

France. bDipartimento di Fisica and Istituto Nazionale di14021 Caen Cedex, France. dCEN Le Haut Vigneau,Orsay, 91406 Orsay Cedex, France.

source S. The mass and excitation energyof the source are consistent with the massivetransfer hypothesis.- A > 60 fragments result both from centraland peripheral collisions. Residues from centralcollisions come from the same mean source S'.The mass distributions of these residues areconsistent with the massive transfer hypothesisexcept for Kr + Ti at 34 MeV/nucleon.- fusion becomes more incomplete as theincident energy increases.In addition the simulations show that for

a given incident energy the fusion is the moreincomplete as the colliding system becomes moresymmetrical.

The excitation energies per nucleon E* ofthe sources S and S' are plotted on figure 1 as afunction of the available energy per nucleon ea v =Ecm/<Aproj. + Atarget)-One observes that :

- although fusion is more and more incomplete,e* increases regularly with e a v .

- the source associated with the complexfragments is always hotter than the oneassociated with residues. As expected comple1.fragments are a good tool to probe the hottestnuclei.- for 8 MeV/nucleon available energy thelargest excitation energy is 5.8 MeV/nucleon.This value agrees with the theoretical limitof ref. 3.

Beyond e a v = 8 MeV/nucleon we encounter-ed difficulties to analyse the data (Kr + Aland Ti at 45 MeV/nucleon). This may indicatethe opening of another reaction channel suchas prompt or sequential multifragmentation.More exclusive experiments are needed toprecise this point.

0 • t

A AAIV T t i

t

*

4 *

T

i ' * ' +

I %

4

Fig. 1 : Evolution of the excitation energy of thesource of the complex fragments (open symbols)and af the source of the residues (full symbols)with the available energy per nucleon.

93

DeexcitationMass distributions corresponding to the

deexcitation of the source of complex fragmentsare presented on figure 2. A qualitative analysisof the distributions in terms of statistical andsequential deexcitation of a nucleus gives excitationenergies compatible with those obtained with thekinematical analysis except for the hottest nucleus(Kr + Ti 34 MeV/nucleon). In this case the statisticalanalysis attributes about 4.5 MeV/nucleon to thesource instead of 5.8 MeV/nucleon. This differencecan be taken as the error in the determination ofthe excitation energy, and we may write £ = (5.± 0.8)MeV/nucleon.

A quantitative analysis of the mass distributionswoulJ require sophisticated entrance channel modelfor the incomplete fusion mechanism.

One observes that the residues bump (A n,60-80)progressively flattens as the excitation energyincreases. Finally the complex fragments cross sectiondominates the mass distribution.

ConclusionNuclei of AT. 100 can be heated up to (5.± 0.8)

MeV/nucleon by an incomplete fusion mechanismwith 8 MeV/nucleon available energy. Beyond thisvalue, the experimental results can no longer beanalysed with the simple models used, and one needsmore exclusive experiments to draw conclusions.

The underlying entrance channel mechanismis not really understood although the massive transferhypothesis agrees reasonably well with most of thedata.

Experimental yields are consistent with astatistical and sequential decay of the compositesystem after incomplete fusion. The discrepanciescan be due both to the uncertainties of the entrancechannel model and of the statistical model in thisenergy domain.

We have shown for the first time that at highexcitation energy the emission of complex fragmentsbecomes and important or even dominating process.References1) V.E. Viola, B. B. Back, K.L. Wolf, T.C. Awes,

C.K. Gelbke and H. Breuer, Phys. Rev. C26 (1982)178.

2) J. Wilczynski, K. Siwek-Wilczynska, J. Van Uriel,S. Gonggrijp, D.C.J.M. Hageman, R.V.F. Janssens,J. Lukasiak, R.H. Siemssen and S.Y. Van derWerf, Nucl. Phys. A373 (1982) 109.

3) S. Levit and P. Bonche, Nucl. Phys. A437 (1985)426.

Publications- Physics Letters 154B (1985) 259.- XXIII International Winter Meeting

Physics (Bormio, Italy - January 1985)- J.M. Pascaud - Thesis - Bordeaux (1985)- Physical Review C35 (1987) 190.- XXV International Winter Meeting on Nuclear

Physics (Bormio, Italy - January 1987).- B. Faure - Thesis - Orsay (1987).- International Workshop on Gross Porperties of

Nuclei and Nuclear Excitations (Hirschegg, Austria,January 1988).

- XXVI Internationa] Winter Meeting on NuclearPhysics (Bormio, Italy - January 1988).

- Winter Workshop on Nuclear Dynamics V (SunValley, Idaho - February 1988).

- Third International conference nucleus-nucleuscollisions, contributed papers (Saint-Malo, France,June 1988).

on Nuclear

qu'

Ti 26.4 MeV/n

C'fMeV/n)

vh« -

I i l 't Ti"T~Tt I i I

20 40 60 80A

Fig. 2 : Final mass distributions for the deexcitationof the source of complex fragments. Normalizationis arbitrary. We have indicated the excitationenergies obtained with the kinematical analysis(figure 1)

9 4

HEAVY RESIDUE SPECTRA AND LINEAR MOMENTUM TRANSFER

IN INTERMEDIATE ENERGY NUCLEUS - GOLD COLLISIONS

K. Aleklett3, J.O. Liljenzinb, w. Loveland0 ,M. de Saint-Simon*1, G.T. Seaborg6, and L. Sihvera.

[aj University of Uppsala, Studsvik, S-611 82 Nykoping, Sweden[b] University of Oslo, Oslo, Norway

[c] Oregon State University, Corvallis, OR 97331, USA[d] Laboratoire Rene Bernas, Orsay, France

[e] Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA

I. Motivation

The production of heavy residues isan important reaction channel in interme-diate energy nucleus-nucleus collisions[1,2J. The heavy residue production crosssection exceeds the fission cross sectionfor Ar-Au reactions at projectile ener-gies exceeding -25 MeV/nucleon and isimportant at all intermediate energies(Figure 1). Thus an understanding of themechanism(s) involved in heavy residueproduction is essential for understandingthe majority of intermediate energy nu-cleus-nucleus collisions. Accordingly, weare engaged in a broad survey program ofsingle-particle inclusive measurements ofheavy residue production using radiochem-ical techniques.

The use of radiochemical techniquesto study heavy residues is dictated bythe extremely low residue energies. For

a(mb)

4000

3000

2000

1000

I I IHeavy residues

Fission

10 20 30 40 50

Projectile energy (AMeV)

60

Figure 1. The dependence of the fissioncross section (open circles) and theheavy residue formation cross section(open squares) on projectile energy forthe interaction of Ar with Au[2].

800

700

600

tefl 5 0 0

o>

I

400

300

200

100 —

= 43 A MEV 86Kr + 197Au= 85 A MEV12C +197Au

1 , 1 , 110 20 30 40

A"

50 60 70 80 90 100110— Atarget "-fragment

Figure 2. Mean heavy residue energies asa function of the difference between theresidue mass number and the target massnumber. The 85 A MeV C + Au data pointsare from reference 1.

example, in the reaction of 43 MeV/nucl-ftfi 1 Q*7

eon Kr with Au, the mean residueenergies ranged from 15 keV/nucleon(A-189) to 300 keV/nucleon (A-135) (Fig.2). This finding has important conse-quences for measurements of heavy residueproperties by non-radiochemical techniques.A time-of-flight spectrometer with atypical low velocity cutoff of 0.5 cm/nswill miss 40% of residues with A-155 and

95

100% of the residues with A>180, A demon-stration of this effect is in the meas-urements of the heavy residue yields inthe reaction of 35 MeV/nucleon Ar with197

Au where a radiochemical measurement[21 gave a residue production cross sec-tion of 2790 mb while a time-of-flightmeasurement gave a lower limit of 315 mb13).

II. Results and Discussion

( -40 Mev/c per projectile nucleon) andwell below that expe> tad from one-bodydissipation (Figure 4). The strikingsimilarity between the dependence oflongitudinal momenta upon AA (and theirmagnitudes) for the two disparate reac-tions may be a consequence of the kine-matics of momentum transfer in nucleon-nucleon collisions and the occurrenceof incomplete fusion/massive transfermechanisms.

We have measured the heavy residuedifferential range distribution for theinteraction of 43 HeV/nucleon 86Kr with197

Au as a function of residue Z and A inan attempt to characterize the linear mo-mentum transfer. The range distributionswere converted to energy spectra usingknown range-energy relationships. The de-duced heavy residue longitudinal momenta(the linear momentum transfers) are shownin Figuu'3 3 along with similar data forthe interaction of 85 MeV/nucleon 1 2C

197with Au. The maximum fractionallinear momentum transfer (FLMT) for thecarbon induced reaction agrees withprevious measurements, but the FLMT forthe krypton induced reaction is very low

(X

4OUU

4000

3500

3000

2500

2000

1500

1000

500

n

: ' 1 ' 1 ' 1 ' 1 ' 1 ' 1'-

~r .

li*fflt. $**m

VFy~ ' • = 43 AMeVL • = 85 AMeV

= , ! , 1 , 1 , 1 , 1 , Ij

1 ' 1 ' 1 ' 1 ' :

j \

jL |T Ij-j-_——_

86Kr+197Au --.12C +197Au J

1 , 1 , 1 , 1 , ^

oT

70

60

50

40

30

20

10

' = 43 AMeV 8 6Kr + l97Aui = 85 AMeV 12C + l97Au

• •M • » • • • •

I , I . I , I . I . I

0 10 20 30 40 50 60 70 80 90100110A — A

target fragment

Figure 4. Deduced heavy residue fractio-nal linear momentum transfer as a func-tion of the mass loss of the target nucle-us (the excitation energy of the targetlike fragments).

References.

1. K. Aleklett, M. Johansson, L. Sihver,W. Loveland, H. Groening, P.L. McGaughey,and G.T. Seaborg; Nucl.Phys.A (accepted)

2. K. Aleklett, W. Loveland, L. Sihver,Z. Xu, C. Casey, D.J. Morrissey, J.O.Liljenzin, H. de Saint-Simon and G.T.Seaborg; Phys.Rev.C (submitted).

3. G. Bizard, et al;Nucl.Phys. A456(1985)173.

0 10 20 30 40 50 60 70 80 90 100110A — A

target fragment

Figure 3. Deduced absolute values of thelongitudina^ momentum transfer as a func-tion of the mass loss of the target nucle-us (the excitation energy of the targetlike fragments).

96

REACTION MECHANISM EVOLUTION FROM 37 TO SOMeV/NUCLEONFOR THE_SYSTEM ;°Ne + 6°Ni

F.Andreozzi", A.Brondi', A.D'Onofrio', G.LaRana1, R.Moro', E.Perillo*, M.Romano",F.Terrasi', R.Dayras+, B.Delaunay+, J.Delaunay+, H.Dumont+, F.Gady+,

J.Gomez del Campo", M.G.StLaurent+ + , S.Cavallaro'", L.Sperduto*", J.F.Bruandet++

'Dipartimento di Scienze Fisiche and INFN-Sezione di Napoli- 80125 Napoli Italy;+DPHN/BE CEN Saday-91191 Gif/Yvette Cedex France;

"ORNL Oak Ridge.TN 37830 USA; ++GANIL BP 5027 Caen Cedex France;••-Dipartimento di Fisica and INFN Sezione di Catania-95129 Catania Italy;

+++Institut des Sciences Nucleaires-38044 Grenoble Cedex France

1.MOTIVATION

The evolution of reaction mechanisms in heavy-ioncollisions for intermediate mass systems as a func-tion of relevant parameters of the entrance channel(mass asymmetry, incident energy etc.) has beenthe object of extensive experimental and theoreti-cal investigations over the past decade. In partic-ular, limitations to the complete fusion have beenexperimentally proved down to 4MeV/nucleon anddifferent limitation regimes found, with increasingincident energy. These limitations have been inter-preted as due to the nuclear potential acting betweenprojectile and target or to the location of a statisticalyrast line of the intermediate system, besides otherdynamical properties of the interaction, like promptor preequilibrium particle emission. The influenceof the mass asymmetry in the entrance channel onthe onset of different regimes of limitations has alsobeen explored1'.

With the aim of pursuing this kind of investiga-tions we have undertaken the study of the system20Ne+GONi at Ganil energies, where a transition fromthe above mean field description to an interpretationin terms of interactions between two sets of quasi-free nucIcons is expected.

2.EXPERIMENTAL_RESULTS

Measurements have been performed at three inci-dent energies, just above the Fermi energy: 37.l',44and SOMeV/nucleon. Beam intensities ranging from4 to lOpna and enriched selfsupporting 60Ni tar-gets of thicknesses from .5 to 5 mg/cm2 were used.Charged particles of Z from 1 to 10 and T-rays weredetected. For the former ones telescopes made-upof Si solid state detectors and Nal scintillators wereemployed. For the latter ones Hyper-Pure Ge de-tectors of average efficiency of 24% and energy res-olution of l.SkeV at 1.33MeV were used. In the ex-periment at SOMeV/nucleon the detection of 7-rayswas accomplished by a special detector consistingof three Ge crystals arrrnged in the same cryostat

JThe run for this energy was carried out using the SARA facility inGrenoble

in a three-stage telescope-like mount, the first stagebeing a planar detector 13mm thick. Coincidencesbetween charged particles detected at different an-gles and discrete 7-rays were also recorded. Off-line-y-ray radioactivity spectra were collected by meansof stack foil methods. More details about the exper-imental arrangements can be found in z'3'4'Discrete lin»s in singles 7-ray spectra were mainlyproduced by the deexcitation of low-lying levels oftarget-like nuclei with masses below the target mass.The intensities of ground-state transitions were usedto deduce mass distributions of target-like nuclei.As an example the mass distribution measured atSOMeV/nucleon b shown in fig.l (full dots with er-ror bars).

60

Fig.l-Measured mass distribution of target-like re-sidues (full dots) compared to a modified geometri-cal model calculation (full line)

Radioactivity spectra from targets of different thick-nesses and from Aluminum foils placed in front andon the back of the targets (stack foil arrangement)were used to measure production cross sections ofradioactive residues. The percentage of cross sec-tion in Ni targets with respect to the total (tar-get + absorbers) depends upon the component par-allel to the beam axis of the residue velocity: the

97

greater the percentage, the lower the velocity. Forthe SOMeV/nucleon run we used a target 1 mg/cm2

thick and Aluminum foils of 15 /an and 99.99% pu-rity; the corresponding results are summarized infig.2 (full dots with error bars). As it ran be seen,residues with lighter masses are faster compared toresidues with heavier masses.

90 i

80 I

;o '

60 :

? 50 i

J 40 '.

-So:20 (

KH

40 45 50 5S:' 6 0A...

Fig.H-Ratios of cross sections of radioactive residuesstopped in the target to corresponding total cross sec-tions

3.MODIFIED GEOMETRICAL MODELCOMPARISON WITH DATA

First attempts to reproduce data at intermediate en-ergies were done using pure geometrical models ofa participant-spectator type, which had been suc-cessful! in describing data at relativistic energies 6 '.Then more refined models, in the same geometricalframework, appeared to take into account mean fieldeffects still present at intermediate energies. Thesemore refined models smoothed the rigid scheme ofthe relativistic participant-spectator picture, depict-ing the exit channels as two cold projectile- andtarget-like spectators and a very hot "fireball" made-up of participant nucleons in the whole range of im-pact parameters, allowing,different landscapes formore central collisions. The model of ref.4 eval-uates one- and two-body energy dissipation in theframework of a Fermi-gas description of the collid-ing nuclei. One of the main features of the model isits capability to predict the value of the impact pa-rameter for which the transition between massivetransfer and three-body processes takes place, aswell as the dependence of the latter upon the in-cident energy for a given system. Due to the ge-ometrical basis of the model, one can establish a

correlation between impact parameters and target-like residue masses5', so that the ability of the modelto reproduce target-like mass distributions is a sig-nificant test of its reliability. As a matter of factfig. I shows the kind of agreement between calcu-lated and experimental mass distributions found atSOMeV/nucleon. Good agreements were also foundat 37.1 and 44MeV/nucleon2'3'. For the latter en-ergy, the dependence of charged particle multiplic-ity versus the heavy residue mass, deduced fromparticle-7 coincidences, shows a decrease with in-creasing residue mass, coherently with the energydissipation processes included in the model *'.An observable more closely related to the dynam-ic; of the interaction (compared to mass distribu-tions) is the linear momentum transfer and sharingamong reaction products. The percentages reportedin fig.2 reflect the linear momentum distributionsof the target-like fragments. Since the model is ofa semiclassical nature, it does not predict distribu-tions for the velocities of target-like fragments, butonly average values for each mass. Using these val-ues and taking into account the finite thickness ofthe target, one gets the dashed curve of fig.2, thatdoes not reproduce the data, especially for lowermasses. To improve the agreement, the model hasto be upgraded in such a way that it can predictdispersions in the calculated quantities. For exam-ple, assuming gaussian distributions for the dissi-pated energy having standard deviations a=25% ofthe average values, a substantially better agreementis obtained, full line in fig.2 (see ref.6' for more de-tails).

4.CONCLUSION

Inclusive <vnd exclusive measurements have been per-formed for the system 20Ne+60Ni at three incidentenergies, 37.1, 44 and SOMeV/nucleon. A modelfor the reaction mechanisms predicting an evolu-tion depending on the impact parameter and inci-dent energy was successfully employed in reproduc-ing target-like mass distributions. The importanceof including dispersions in the semiclassical modelhas been pointed out, in order to reproduce linearmomentum distributions of target-like residues.

IJH.Dumont et a!., NPA435(1985)3472)A.D'Onofrio et al., Lett. Nuovo Cim.42(1985)3473)A.D'Onofrio et al., Phys.Rev. 035(1987)11674)A.D'Onofrio et al., in press in Phys.Rev. C5)S.Nagamiya et al.,Ann.Rev.N.P.S.34(1984)l556)F.Andreozzi et al.,to be published

98

85

PREEQUILIBRIUM EMISSION AND INCOMPLETE FUSION IN THE13C, 24Mg and «Sc REACTIONS AT 27.5 MeV/NUCIM

J.P. COFFIN^, A. FAHLI», P. FINTZ«, M. GOWN", G. GUILLAUME=, B. HEUSCH". F. JUNDT". A. MALKI*.

P. WAGNER8, S. KOX", F. MERCHEZb and J. MISTRETTA"

a) Centre de Recherches Nucleaires and Universite Louis Pasteur, 67037 Strasbourg Cedex (France)b> Instltut des Sciences Nucleaires, 383026 Grenoble Cedex (France)

1. MotivationBoth preequillbrlum emission and

incomplete fusion in heavy ion collisions carryinformation about the early stage of thecollision and the manner in which the systemevolves towards thermalizatSon and theformation of a highly excited compoundnucleus. Such studies present, however,several experimental difficulties that can bepartly overcome by studing fairly light systemsfor which the fusion-fission component isrelatively small compared to that of fusion -evaporation and by using inverse kinematicreactions to separate, at backward angles,preequilibrium emission from that subsequentto the decay of the thermalized system.

In order to obtain more informationabout incomplete fusion and preequilibriumemission, we have studied at 27.5 MeV/nucleonthe two Inverse kinematic «°Ar + "C and*°Ar + 24Mg reactions and the nearlysymmetrical system 40Ar + «Sc.

2. Results and discussionThe experiments were performed by

using a beam of wAr ions of 1100 MeV.Evaporation residues (ER) were detected bymeans of a time of flight system. ER lighterthan the projectile could be separated fromthe heavy fragments resulting from reactionmechanisms other than fusion.

The process of incomplete fusion wasstudied by analyzing velocity distributions. InFig. 1 are reported the ratios R of thevelocity of the compound nucleus formed inincomplete fusion (VER/cose) and incomplete fusion (VFMT) as a function of themass AER of the ER, for the three systems.

The figure shows also the evolution of the fullwidth at half maximum of the velocitydistributions. The ratios R and the widthsexhibit a linear dependence with AER. R < 1for <0Ar + "Sc and R > 1 for the inversekinematic reaction 40Ar + "Mg : this implies

i oAr Ep=1100MeV

1.3

1.2

1.1

1.0 -

1.6

1.2

0.8

•i T— i | r | i "" I r

o— -

^ ° - ^

•"~~""*-~.

•

i . i .

i —

---.

I38 1.0 42 U

AE R(u)

46

A5Sc E=iiooMev

10

09

08

0.7

0.6

3

2

1

1 -

35 45

AE R(u)

50

Fig. 1 :Ratios R of the velocity of the CN formed inincomplete and complete fusion as a functionof the mass of the ER and full width at halfmaximum of the ER invariant velocitydistributions plotted versus AEX.

99

86

that incomplete fusion is present in the tworeactions, lighter is the ER more incomplete isthe fusion and that the emission prior tofusion comes mostly from the lighter reactant.

We have analyzed the ratios R with amodel based on linear momentum conservationto examine the pre-equiHbrium emission (PE).Fig. 2 shows the ratio f> of the PE velocity tothat of the projectile and the multiplicity ofPE with respect to AER. The dashed lines areto guide the eye. The histograms represent therelative experimental ER yields. The error barsindicate the variation on p within which theexperimental AER are reproduced. Two PEregimes may exist, one cun be described withthe exclton model and a second is related tothe Fermi-jet concept and correspondsessentially to the heavier ER. Coincidencemeasurements between particles with VPE > Vrand ER have been performed and analyzed Inthe context mentioned above.

Other Interesting informations concerningexcitation energies s and temperatures T wereobtained. For the «°Ar + «Sc reaction, e isnearly constant and close to 6.1 MeV(T = 7 MeV). The «°Ar + S4Mg system leadsto lower values : t = 3.5 MeV and T = 5 MeV.

In conclusion, the process of incompletefusion has been studied : Detailed analysis ofthe velocity distributions has permitted toconfirm the relevance of the relative velocityof the light reactant in the center of masssystem as a parameter governing incompletefusion. Two regimes of pre-equilibriumemission have been determined. The«°Ar + 13C reaction appears to be a specialcase as concerns the pre-equilibrium emission.

3. References

M. GONIN et al., XXVnd Int. Winter Meeting onNuclear Physics, Bormio, Italy (1987)J.P. COFFIN, VInd Adriatic Int. Conf. onNuclear Physics, Dubrovnik, Yugoslavia (1987)J.P. COFFIN, Workshop I N2 P3 - RI!<~N.Shimoda, Japan (1987)M. GONIN, Thesis, Unlversite de Strasbourg,France (1987)M. GONIN et al.. Phys. Rev. C38 (1988) 135

24Mg + "Ar

X .

UJ

>°-

198 173 15

Is"o-.-e 11 .«l» 9

.»:S 75 -

1 '

38 42 44

1.1

10

0.9

0.8

07

as

-E

il .....

Fig. 2 :The ratios P of the PE velocity (Vn) to thatof the projectile (Vr) and the number of PEnucleons (nrs) are plotted as a function ofAt*. Note that, for the inverse kinematic case,the analysis has been done after conversion todirect kinematics.

100

TARGET RESIDUE CROSS SECTIONS AND RECOIL ENERGIES FROM THE REACTION

OF 1760 HeV »°Ar WITH MEDIUM ASD HEAVY TARGETS

F. Hubert, R. Del Moral, J.P. Dufour, H. Emmet-maim, A. Fleury, C. Poinat, M.S. PravikoffCEN Bordeaux, IN2P3, Le Haut Vigneau, F-33170 Gradignan

H. Delagrange, GANIL, B.P. 5027, F-14021 Caen CedexA. Lleres, ISS-IB2P3, USMG, 53, Avenue des Hartyres, F-38026 Grenoble Cedex

In the search for significant, experi-mental facts revealing the true nature ofthe transition between the low and the highenergy domain in heavy ion inducedreactions, measured properties of heavytarget residues are of prime interest. Sincethese residues lie relatively close to thetarget in the chart of nuclides, theirdegree of survival can be a clue to gainsome knowledge of the violence of thereaction.

Deexcitation calculations of highlyexcited medium and heavy nuclei have shownthat the final residues and whatever the Aand Z of the initial excited nuclei are,always end-up in the same region (the so-called "residue corridor"), This region,remote from the stability valley is parallelto the line of 3 stability near the r^/P^lline (r,, and FV being the particle decaywidths for neutron and proton respectively).Then most of the independent yields fortarget residues are to be found for neutrondeficient species with short half-lives.Rapid detection of these nuclei is thenrequired. The method of electrostaticcollection of nuclides we used makespossible on-line collection (in fewmilliseconds) of target residues on thesurface of an alpha detector1'. For rareearth nuclei an island of radioactive alphadecay exists for the N=84-85 neutrondeficient isotones located in the residuecorridor. The present investigation isdevoted to the study of the formation ofthese isotones in several targets. Thus, incontrast to the usual experiments wheredifferent residues are observed for a giventarget, our results are obtained bydetecting specific residues from varioustargets-7'.

We have measured production crosssections (fig. 1) and mean recoil energies(fig. 2) of heavy residues (the rare-earthisotones N=84-85 which are a-emitters)formed in "°Ar induced reactions at 1760 MeV

Fig. 1 : Deduced distribution of the residuecross sections in the (AN, AZ) plane. Thedashed line gives the ridge line, i.e. theline for maximum yield. The open squaresrepresent the location of the experimentalmeasurements. The curves are the loci of theresidues for a given value of the crosssections.

with medium and heavy mass targets. The bellshape of the isotone yields is an indicationof the origin of these isotones in the decayof highly excited precursors. The similarityof these data with those obtained in '--Cinduced reactions (at 1 GeV) in comparableconditions confirms this interpretation.Significant production yields o'f trans-target isotopes have been measured. Ve areable, in the framwork of our analysis, toestimate the pattern of heavy residueproduction from an "average target".

A semi-quantitative analysis based on anincomplete fusion picture was used to try toclassify our data. The experimental masslosses are reproduced, to first order,within this framework. For large masslossed, the behavior of the deduced energyremoved (per nucleon) during the decayprocess seems to indicate the evaporation ofsome heavy particles ( He and heavier). Ourdata concern reactions where the meanfractional momentum transfer is rather small( 30%). Such reactions can then be consideredas peripherical reactions. Even if it isquite paradoxical at first sight, there canbe a large amount of excitation energy inthe primary fragments (E* = 200-600 HeV).

It is far beyond the scope of thisexperimental study to go through exhaustivetheoretical investigations. We have chosento compare our data with, on one hand, theextended abrasion-ablation model of Dayrasand, on the other hand, intranuclearcalculations. As far as our results areconcerned, there is no evidence of atransition from an abrasion-ablationbehavior to an incomplete fusion picture.Comparison with intranuclear "cascadecalculations pinpoints the fact that n-ncollisions only cannot lead to enoughexcitation energy in the precursors. So farthese theoretical investigations have onlybeen able to predict some qualitativeaspects of the studied interactions ; butthese data really stand as a meaningful testfor more elaborate models.

30

25

I 2 0

J n

10

1 1 1 1 1

~-4

i X ;- {\• I.TGGeV 4 0 A r *Y>. ;

-5O -40 -30 -20 -10

Fig, 2 : Experimental mean forward projectedrecoil energy ER as a function of "the meannucleon loss <AA>. The dashed line is drawnto guide the eye.

et inci-

101

LIGHT PARTICLES AND PROJECTILE-LIKE FRAGMEHTS FROM THE 40Ar + 12C REACTION AT 44 MeV/HUCLEON

D. Heuer, R. Bertholet, C. Guet, M. Maurel, H. Nifenecker, Ch.Ristori, F.Schussler

CEN-G,DRF/SPh, F - Grenoble

J.P. Bondorf, O.B. Nielsen, N.B.I., DK-Copenhagen

Doubly differential spectra of light particles and projectile-like fragments were measured at

laboratory angles in the range 3°-23° for the Ar + C reaction at 44 MeV/nucleon. The

kinematical conditions of this reaction lead to a strong focussing of the reaction products in the

forward direction and therefore to a good detection efficiency.

Production rates for heavy fragments (of charge around Z = 14), velocity distributions4

characterizing these fragments and also that characterizing the He emission were consistently

interpreted as resulting from the decay of a thermally equilibrated nucleus, initially made up by

the transfer of an a -particle from C to Ar. Although depending upon a desexcitation model, an44

average excitation of about 3.6 MeV/nucleon of the compound system ( Ca) is consistent with data.

More details are found in ref. . This experiment was among the first to identify the formation of

highly excited nuclei and since then hot nuclei with higher temperature have been currently

produced and observed.

1' 0. Heuer et al., Phys.Lett.161B, 269 (1985)

102

STUDY OF PREEQUILIBRIUM IN INTERMEDIATE ENERGY HEAVY ION COLLISIONS

C. Cerruti*, J. Desbois**, R. Boisgard*, C. Hgo*, J. tlatowitz*** and J. Nemeth****

*Laboratoire National SaCurne, CEN Saclay, 91191 Glf sur Yvette Cedex, France**Division de Physique Theorique"1", Institut de Physique Nucleaire, 91406 Orsay Cedex, France

***Cyclotron Institute, Texas A&M University, College Station, TX 77843, USA****Institute for Theoretical Physics, Eotvos University, Budapest, Hungary

To describe the first step of an heavyion collision, we propose to use apreequilibrium modelD. This approach willbe, in principle, valid for very asymmetricalnuclear systems. Our aim is to calculatethe number of emitted nucleons during thefirst stage of the collision, the linearmomentum transferred from the projectileto the target and also the target excitationenergy when the statistical equilibrium isreached. We proceed as follows.

At t=o, the volume of the target isfilled with the total number of nucleons,namely Aj + Ap (A-J- and Ap are, respectively,the target and projectile masses). The addednucleons (Ap) are put on the energy statesof the system corresponding to £ , theincident energy per nucleon (this correspondsto a "sudden hypothesis"). Following Harpand Miller*', the nuclear system is describedas a two components (protons and neutrons)Fermi gas which is entirely characterizedby the occupation numbers of its singleparticle states. The evolution of theseoccupation numbers is governed by a set ofBoltzmann-like master equations, the relaxationof the system being produced by nucleon-nucleon collisions and nucleon emission-*).These equations are solved numerically andwe stop when the initial peak correspondingto the projectile energy has completelyvanished. This occurs at time tjflf 10~^2 s.Therefore, it is reasonable to consider that,at time t^, the preequilibrium phase isfinished. The results concerning Ne(tj) (numberof emitted nucleons) and £ *(tj^) (excitationenergy per nucleon of the residual nuclearsystem) can be fitted by the following

) )

where V* is the reduced mass of the systemand £ the incident energy per nucleon. Thequality of the fits can be seen in Figs.1-2 where Ne(t!) and t*(t!) are displayed forthree different systems (C, Ne , Ar+Au) as functionsof £ and also of Eg f[ (total center ofmass energy ) .

At the end of the preequilibrium phase,the nuclear system is excited (presumablyhot and compressed). We can study itssubsequent evolution, using an hydrodynamicalapproach coupled to a percolation model (fordetails, see refs.2~3) and, also, the

expressions^ ' :

tle(ti) = 0,96 . |J(.5/4 ( 1 -ex|

6 * ( t l ) - 6 , B . A T ' 1 / 3 p 2 / 3

,( £ )1

[ 1 , f^ 1 GXp ^

Tt;n u.-1/J

contribution of J. Nemeth et al. in thisbook). In this approach, the nucleus willexhibit monopolar oscillations ("breathingmode"). The percolation parameters, p andq, are, at each time, connected with thedensity and excitation energy of the nuclearsystem. Break-up will occur if themultifragmentation region (in the p-q plane)is reached. In Fig.3 we present the results for

Fig. 1

Fig. 2

a central Ar+Au collision at variousbombarding energies (time t2 correspondsto the end of the first half-period ofoscillation). We see that the criticalincident energy for multifragmentation isabout 50 MeV/u for that system. In Fig.4,we show the critical regime foi varioussystems (C, Ne, Ar+Au). Comparisons withthe existing experimental data are ratherencouraging.

In conclusion, we can say, in somesense, that we have built a complete scenariofor a central heavy ion collision. So, weare able to study the influence of differentparameters on the appearance ofmultifragmentation (essentially, theprojectile and target sizes and the bombarding

+ Unite de Recherche des Universites Paris 11 et Paris 6 Associee au C.N.R.S.

103

Ar+Au

o. as

Fig. 3 Fig.4

energy). We must say that, owing to thesmall size of a nuclear system, the transitionfrom incomplete fusion to multifragmentationis rather smooth when the energy increases*'.Recently, we have extended our model tonon central collisions^' and also to reactionsinvolving very heavy ions-*). However, weare aware that this approach is stillschematic and sometimes very crude. He thinkthat a lot of work remains to be done(concerning, in particular, preequilibriumand also the definition of the percolationparameters) in order to obtain a realisticdescription of an heavy ion collision.

1) G.D. Harp et al., Phys. Rev, C3 (1971)18472) C. Cerruti et al., Hucl. Phys. A476 (1988)74.3) J. Nemeth et al., Z. Phys. A325 (1986)347 ;

J. Desbois et al., Z. Phys. A328 (1987)101.4) C. Cerruti et al., to appear in Hucl.Phys. A.5) 0. Granier et al., Contribution to theXXVII Int. Winter Meeting on Nuclear Physics,Bormio (1989).

104

Study of light charged particle* emitted in mArinduced reaction* at intermediate energie*

G.Lanian6, A. Pagano - INFN, CataniaR. Dayras, J. Barrette, B. Berthier, D.M.De Castro Riiio, O. Cisse, R. Coniglione,

F. Gadi, R. Legrain, M.C. Mermas, B.C. Pollacco - DPhN/BE, CEN SaclayH. Delagrange, W. MiUig - GANIL, Caen

B. Heusch - CRN, Strasbourg

1. Motivation

The study of the evolution, with the incident projectileenergy, of the reaction mechanism involved in peripheral in-teractions between heavy ioni, has recently found a greatopportunity with the delivery, at Garni, of a variety of inter-mediate energy beams. In this aim we have accomplished aseries of inclusive and coincidence experiments in which wehave studied both the heavy products and the light chargedparticles (LCP) emerging from the reaction.

As the gross features deduced for the heavy fragmentsart tieated elsewhere in this compilation1', in the followingwe shall present some results on the LCP produced in re-actions induced by an *°Ar beam on an "Al target, at 44and 60 MeV/n.

2. Experimental apparatu*

The experiments were performed using 44 and 60MeV/n *°Ar beams delivered by the Ganil accelerator atCaen.

LCP were detected into telescopes, consisting each oneof 2 AE ( 100 urn and 4000 Mm thick) silicon detectors anda E ( 5 cm in diameter, 10 cm long) INa or BaF2 detector,in an angular range between 15° and 112.5° .insteps of 7.5".

In a coincident experiment two of these telescopes wereplaced at -10° and + 25° with respect to the beam direction,while the coincident projectile-like fragments (PLF) weredetected ( mass and charged identified ) in a five elementsilicon telescope, placed at +3.1° with respect to the beamdirection.

S. Resulta and discussion

The LCP energy spectra, especially in the forward di-rection, are complex, suggesting that more than one emit-ting source is contributing. Fig 1 shows a typical energyspectrum for alpha-particles detected at 15° in the reac-tion 40Ar +2 T Al at 60 MeV/njwe notice in it a relativelylow threshold (wSMev/nucleon) and a dip near 80 Mev,due to a dead layer in the second thick silicon detector ofthe telescope. We have analysed these spectra in terms ofan abrasion-ablation model, that has already been success-full in explaining the gross features of the heavy remnantsof the reaction31. In this model, LCP are produced during

QU 10.H

•jnT| r r r y n - r i s . i p n , | > i > | > . rj , , , | , , , | ,-jr+'f'M CO McV/u

, ^ . —-i , j . Energy &|>eclrum

"'•>''•-.

the ablation stage subsequent to the abrasion, so that threesources can be distinguished :

i) A high velocity source ( HVS ) associated with theprimary PLF, characterised by a velocity near to thatof the beam and a relatively low temperature

ii) An intermediate velocity source (IVS), associated withthe participant lone and characterised by a velocityintermediate between that one of the beam and thatof the target,and by a high temperature

iii) A low velocity source (LVS), associated with the pri-mary target-like fragments(TLF) and characterised bya low velocity and temperature.We have described the LCP emission from each source

by a Maxwell-Boltimann distribution, including the effectof the Coulomb repulsion3).

As an example, table I shows the best set of parameterscharacterising the three sources, found by a fit procedure,for alphas and protons produced in the reaction *°Ar +vt

Al at 60 MeV/n. With this set of parameters we wereable to reproduce remarkably well the experimental spectrathroughout the explored'angular range, from 15° to 112.5°.Fig 1. gives an example of the obtained results and, inparticular, the relative contribution of the three sources tothe spectrum. An interesting result of these calculationsis shown in Fig. 2, where the relative contribution to theenergy spectrum for each source is given as a function ofthe lab. angle for alpha particles. It is evident from thisplot that an experimental signature of the participant sonecan be obtained only in a limited angular range, dependingon the type of particle. For instance, for alpha-particles

' the most suited angular range is located between 23° and33°, where the ratio between the IBS intensity and the totalintensity reaches a maximum value of w 0.70.

0. 40. 80. 120. 160. ZOO. 240. 280. 320. 360. 400.

ErJERCVC MEU >

160

dldeorees)

Figl Fig. 2

105

TABLE I

SOURCE v/c T(Mev) Ec(Mev) Irel(mb)

ALPHAS

HESIESLES

0.310.180.06

6.912. 07.5

4.05.03.0

2306.1538.1378.

PROTONS

HESIESLES

O.330.16O.05

4.914.54.7

3.O4.02.0

22O6.2302.1497.

This result is confirmed by the coincidence experiment40'Ar +" Al at 44 MeV/n, even if the correlations betweenPLF and LCP were performed at only two angles. We wantto stress here only one aspect of the obtained results, refer-ring the reader to ref.4) for a more-detailed presentation ofthe resists.

Fig. 3 shows the dramatic change in the shape of theenergy spectrum for alpha-particles detected respectivelyat -10° and + 25°, in coincidence with PLF detected at+3.1°. As compared to the spectrum at -10°, at +25° thehigh energy component has almost completely disappearedand the spectrum is now peaked at an energy correspond-ing to a velocity equal approximately to 60% of the beamvelocity. Furthermore, the slope of the high energy side ofthe spectrum is much less steep than at -10°, indicating ahigher temperature of the emitting source. This compari-son suggests that the bulk of the emitted alpha- particles at-10° and +28° has its origin in two different sources, con-firming the scenario of an abrasion ablation mechanism, assuggested by the inclusive experiments. The full line in Fig.

A further confirmation that 2S° is a suited angle tostudy the products originating from the participant lone,comes from the inspection of fig. 4, where the differen-tial alpha-particle multiplicity, as measured at +25°, isreported as a function of the mass of the PLF detectedad +3.1°. If we make the reasonable assumption that the.number of alphas emitted by the participant 'one is pro-portional to its mass, then the differential alpha-particlemultiplicity shown in Fig. 4 is nothing but the correlationbetween the PLF's mass and the mass of the participantsone. The full line in fig. 4 represents just this correlation, as predicted by the abrasion model and calculated usingthe simple formula of ref.&'

50-

40

30

20

20

LUO2LUQO

6(J

10'

fig. 4

80 160 240 320 Ea(MeV)

Fig. 3

3 represents just the result of a two sources calculation (the LES has been neglected ), in which velocities and tem-peratures, as predicted by an extended Abrasion model3',have been used as source parameters, see ref. *'.

4. Conclusion

In conclusion, we have presented some features of lightcharged particles at intermediate energies. We have shownthat the data can be successfully explained in terms of threethermalised sources, reflecting a scenario of a fast abrasion-ablation mechanism, in which two spectators (PLF andTLF) and a participant sone are created. We have alsostressed the fact that only in a narrow angular range theintensity of the alpha-particles having the characteristics ofan intermediate velocity source is predominant on the inten-sities relatives to the spectators.At the moment, however,due to the limited angular range explored in the coincidentexperiment,we cannot exclude that the IVS could resultfrom the emission of fast, preequilibre particles, withoutformation of a participant sone.To better elucidate the re-action mechanism responsible of the observed LCP energyspectra and angular distribution, we have just undertaken amore complete coincident experiment ,by looking,»t severalangles,at the spectra of LCP emitted in coincidence with agiven fragment.

References1) R.Dayras et al, this compilation2) R.Dayras et ai, NucI.Phys.A460(1986)299-323 and

R.Dayras et al, Phys.Rev.lett., in press3) T.C. Awes et al,Phys.rev.C 25 (1982) 23614) G.Laniano et al. in Proc. of the XXIV Int. Winter

Meeting on Nuclear Physics, Bormio (1986), pag. 183' - 1915) G.Lansand et al, Nuovo Cimento Vol. 99A (1988) 839-

856

106

EXPERIMENT N°50

LIGHT-PARTICLE CORRELATIONS : ( I ) SMALL RELATIVE MOMENTA CORRELATIONS

D. ARDOUiN1', G BIZARO, H. DELAGRANGE. H. DOUBRE, c.K. GELBKE, c. GREGOIRE. A. KYANOWSKI. F. LEFEBVRES, w. LYNCH, M.MAIER,W MITTIG. A PEGHAIRE, J PETER. J POCHODZAUA2, I. OUEBERT, F. SAINT-LAURENT, B. TAMAIN, Y. P. VIYOGI3.B. ZWIEGLINSKI.

Laboratoire National GANH, BP. 5027, Caen Cedex, France.National Superconducting Cyclotron Laboratory, Michigan State University, East-Lansing, Ml 48824, USA.Laboratoire de Physique Corpusculaire, University de Caen 14032 Caen Cedex, France.Centre d'Etudes NucUaires de Bordeaux-Gradignan 33170 Gradignan, France

Most atcempcs to obtain experimental infor-

mation about the temperature of highly ex-

cited systems in heavy-ion collisions were

based on l igh t -par t ic le kinetic spectra

analyses. Within a thermal model, the rela-

tive populations of particle-unstable sta-

tes can be used to determine the temperatu-

re at the point where the particle leave

the equilibrated system. We have measured

these populations for the Ar + Au and 0 +

Au reactions at E/A = 60 and 94 MeV respec-

tively. Light-particles (Z < 4) were detec-

ted by a close-packed hexagonal array of &E

- E telescopes positioned at laboratory an-

gles of 30° and 45 . Energy calibrations of

the Nal E detectors were established for

all particles using momentum selected reac-

tions products . In addition, coincident

charged par t ic les , detected in the

NAUTILUS Plastic Wall (6=3-30°), were used

as a multiplicity filter in the case of the

Ar + Au reaction. The correlation function

R(q)is defined by a12 (p1,p2)=o*1(p1) .a2(P2^

[l+R(q)l where q is the two-particle rela-

tive momentum.

The population of different highly excitedfl 7 ft

particle-unstable states in Be, Li, Li,

Li and He has indicated -^^ an emission

temperature T=4-5 MeV (Fig.1) whereas lower

temperatures were deduced from low-lying

states. A quantitative comparison must ad-

dress the question of sequential feeding.

The inclusion of sequential feeding within

the Quantum statistical model made it pos-

sible to describe the measured populations

by a thermal distribution with a common

temperature T=5.5 MeV. These results are

much lower than the kinetic slopes of in-

clusive spectra indicating possible distor-

tions by collective or expansion motion of

the system. Several interesting and puzz-

ling features have been revealed by the

very little sensitivity ' of these measured

temperatures to the charged particle multi-

plicity at forward angles or the kinetic

energy of the emitted clusters (between 10

and 50 MeV/A). A dynamical limitation to

the deposited thermal energy has been pro-

posed '. This is corroborated by Landau-

Vlasov calculations where a limitation of

excitation energy is found to result from a

competition between preequilibrium emission

following expansion and monopole mode exci-

tation on one hand and thermalisation on

the other hand.

Because of their sensitivity to final-state

interactions, the correlation-function is

known Co contain information about the spa-

ce-time extension of the emitting system.

Another goal of the present experiment

was based on this determination using dif-

ferent coincidences pairs : (p,p), (d,d),

(t,t), (p,tt) and (d.tt). The space-time ex-

tent of the source was found to depend on

the particle species and to decrease with

the kinetic energies of the particle pairs.

This is an indication of a sequential free-

ze-out of different species at different

stages of the reaction. The energy depen-

dence could be caused by a collective ra-

dial expansion of the interaction zone or

an earlier stage of emission for the more

energetic particles. The comparison of the

source space-extension as a function of the

107

multiplicity shows an increase by a factor

of 2 over the investigated range : higher

multiplicities, associated with lower im-

pact parameters producing larger extension

and smaller correlations. It is the first

time that such an observation is made in

the intermediate energy domain. This points

to a large sensit ivity of the collision

time and the emitting zone spatial exten-

sion with the impact parameter.

* Spokesman at the "Com!Its d'Experiances GANIL"

1} On leave from University of Nantes (France).

2) Present address : lost. Kernphysik, Frankfurt (FRG).

3) On leave from VECC, Calcutta (India).

Publications relative to this work :

1) W. Mittig et al. Nouvelles de Ganil N* 7(1984) 22.

2) J. Pochodzalla et al,Phys. Rev. Lett. 55 (1985) 190 ;Phys. Lett. 161B (1985) 256 ;Phys. Lett. 161B (1985) 275 ;Phys. Rev. C 35 (1987) 1695 ;D. Ardouin et al, Nucl. Physics A 447(1985) 585 c.

3) Z. Chen et al, Phys. Rev. C 36 (1987)2297.Phys. Lett. B 199 (1987) 171.

4) A. Kyanowski et al, Phys. Lett. B 181(1986) 43.

5) F. Saint-Laurent et a l , Phys. Lett. B202 (1988) 190.

40Ar-*-197Au, E /A=60MeV, 0 B V =30 '

. - QSM: T=5.5 MeV. p / p 0 = 0 . 0 4

5Li1 6 .6 6 /5Li,. , .

«20.1/a«.s.

0 5 10 15APPARENT TEMPERATURE (MeV)

FIG. Apparent emission temperatures for *°Ar inducedreactions or. "7Au at E/A=60 MeV. The histogram shows theresults of a quantum statistical calculation which includes thefeeding by sequential decay; an initial temperature of T= 5.5MeV and a dcnsitv of p/po=0.04 were assumed.

108

EXPERIMENT N°50

LIGHT-PARTICLE CORRELATIONS : ( I I ) LARGE RELATIVE MOMENTA CORRELATIONSD. ARDOUIN' , 2 BASRAK, o. BIT.ARD, H. DELAGRANGE, H. OOUBRE, c.K. GELBKE, c. GREGOIRE, A. KYANOWSKI, F. LEFEBVRES,

W G. LYNCH M. MAIER, W. MITTIG, A. PEGHAIRE, 1. PETER, 1. POCHODZALLA2, J. O.UEBERT, F. SAINT-LAURENT, P. SCHUCK, B. TAMAIN,Y P. VIYOGI3. B. ZWIEGLINSKI

GANIL, BP 5027, 14032 Caen Cedex, France.

LPN Nanles, I rue <f* la Hausslniere, 44072 Nantes Cedex 03, France.

ISN GRENOBLE, 53 avenue des Martyrs, 38026 Grenoble Cedex. France

LPC, Un ivers i l i de Caen, 14032 Caen Cedex, France.

CEN Bordeaux, 33170 Gradignan, France.NSCL, Michigan State University, East-Lansing, MI 48824, USA.

A comparison of experiment and theory which

can reveal the expected interplay between

mean-field effects and two-body collisions

is of gretic importance for Che understan-

ding of heavy-ion dynamics at energies

around the Fermi energy. Light particle

correlations at large relative momenta of-

fer an opportunity to reveal mean-field ef-

fects via azimuthal and polar effects on

the correlation-function.

We have measured such two-particle correla-

tions for p,d,t,a species for the Ar + Au

and Ar + Ti reactions at E/A = 60 MeV. Ele-

ven AE - E telescopes were mounted at 8 po-

lar angles of 22°, 30°, 37*. 70*, 110",

135° and 160' with possible variable rela-

tive azimuthal angle (A$) 0, 49°, 90°, 135°

and 180° between them. For the Ar + Au

reaction, the NAUTILUS Plastic-Wall cove-

ring A9 = 3° - 30° was also used as a char-

ged particle multiplicity filter.

The most salient feature is the strong

energy dependence (Fig. 1) of the azimuthal

distribution of the correla-

An almost azi-

muthal isotropy has been observed at 60

MeV/A while a marked same-plane enhancement

was established in previous studies at

E/A = 25 MeV. (Fig. 1). We very satisfac-

torily can explain this behaviour by convo-

lucing two singles emission probabilities

P(0, $) = ff(8,$)/ <7(9) with respect to the

azimuthal angle $ ; the distribution P(8,f)

has been calculated from the Landau-Vlasov

A$ = $2 ~

tion function ffj_ / <7. x (T-.

equation. According to the comparison of

our data with these calculations, the in-

creasing importance of the two-body colli-

sion rate with increasing energy, as oppo-

sed Co mean-field effects is responsible

for the above observations.

The importance of mean-field effects is

found in the observation of collective mo-

tion felt by the participating nucleons.

Such a signature was visible in the in-

plane enhancement at E/A = 25 MeV (Fig.l).

At E/A = 60 MeV, we have shown that the po-

lar structure of the correlation function

(Fig. 2) contains a fingerprint of the

mean-field effect too. Indeed, the maxima

observed around 8 = 60° result from the fa-

vored deflection of participant nucleons by

the mean-field of the two colliding nuclei

towards negative angles. This effect is ex-

pected to offer also a target mass depen-

dence : heavier targets giving less centri-

fugal ejecting forces as observed. Fig. 2

shows the observed dependence which is

again well accounted for by Landau-Vlasov

calculations including both mean-field and

collisional effects. The spread of the

maxima itself depends highly on the rate of

two-body collisions. The position of the

maxima, via mean-field dependence, is also

expected to reflect the attractive or re-

pulsive part of this mean-field. Our multi-

plicity selected data do show such a beha-

viour as smaller deflections seem to be as-

sociated with higher multiplicities or

smaller impact parameters.

109

In summary the large angle two-particle

correlations have been shown Co contain one

of the rare known fingerprint of the compe-

titive role between nuclear mean-field and

two-body collisions in nucleus-nucleus in-

teractions.

* Spokesman at the "Comite's o"Experiences GANH"

1} On leave from University of Nantes (France).

2) Present address : Inst. Kernphysik, Frankfurt (fRfi).

3) On leave from VECC, Calcutta (India).

Publications relative to this work :

D. ARDOUIN el al, Z Phys A 3J9 (1988) 505 and to be pu-blished

0

<oTof 1.0o:

0.5

1 E/A=60MeV

• E/A=25MeV

90°

Fig 1

180°A 0

bT—

C

0 ™

o 1.0©<J_ 0.8

<MO> S

b

. . 1 ^T"

® 0.8oc

. . : , j . . , . « . , . j , . .

EXPERIMENT LANOAU-VLASOVCALCULATIONS

a) b)Ar+ Au

- ^- A0=O

,

- ~S*•ff^t*

' ,,,!,

f" r-A0*n A0=0

Ar+Ti

/y A** ^^1 1 1 1 a i iii

X\w •

AO-n^

r\ :NV -\

1 1 1

90° 0° 90° " 90° 0° 90°

I vzBEAM

Fig 2

110

THE ROLE OF TIME IN THE DAMPING OF SOME FIHAL STATE INTERACTIONS INTVO-PAKTICLE CORRELATIOSS, P. Lautridou, K. Boisgard, J. Quebert

Centre d1Etudes Nucleaires de Bordeaux-Gradignan, Le Haut-Vigneau 33170Gradignan, France ; C. Lebrun, F. Guilbault, D. Goujdaml, D. Durand,R. Tamisier, D. Ardouin, Laboratolre de Physique Bucleaire de Nantes,

2. Hue de la Houssiniere, 440V2 Nantes, France ; A, Peghaire,F. Saint-Laurent, Laboratolre Ganil, 14021 Caen Cedex, France

The purpose of this study is to probe

the specific range of tine ao-a"-10-=2s>

which governs particle emission from a

highly excited (and possibly compressed)

nucleus. We study in fact the emission of

two particles for which a few resonances of

given lifetimes can be used as probes in

the final state. Thu=>, relative tine emis-

sions of the two particles can be studied

since a prompt emission should tune in the

resonance whereas induced time correlations

land relative time delay) should play a

damping effect.

The method of two-particle correlations

at small relative momenta is particularly

well suited to study these modulation

effects (for which other origins have been

considered '-•="). On the other hand, when

the correlated pair concerns two like-

particles, another effect due to Bose-

Einstein or Fermi-Dirac statistics provides

an interference pattern at small relative

momenta which is also dependant on the

lifetime of the source. These latter

effects are in fact "nambiguous in ir-it

correlations *' ; To date, they have not

been evidenced with heavier particles

because of the prominant effects of final

state interactions which often smear out

the expected pattern.

In a recent study oi such correlation

functions in the backward hemisphere of

"--0 + '•-''Au at 94 HeV/A, we observed for

the first tine a progressive transition

from the well known 6> final state interac-

tion of two protons (zHe> to a typical

shape that we assign to statistical inter-

ferometry. This is shown in Fig. 1, where a

progressive damping (57 degrees) is obser-

ved to give rise to a typical new shape of

the correlation function with complete

cdisappearance of -He (135').

197Au('60.pp)X. E/A=94MeV6.v=45°

• < 120 MeV° > 120 MeV

0.5 -

0 i • ' • i • i • i • i • i • i •' •' •'—0 SO 100

. z.Chen etal5)

PUESENT

WORK

® a v="°

=115"

= 135°

Fig.1 :

p-p correlation functions measured at large

scattering angles in l eO + ls""'Au at

94 MeV/A, we notice a disappearance of the

final state interaction in the backward

hemisphere.

This is also observed in another way in

Fig. 2a> through the coincidence plot due

to the only counters (Csl) which are set

at 5 degrees from each other : whereas a

clear B B e resonance is observed with ot-a

coincidences in Fig. 2 b ) , a lack of data

along the expected locus of the p-p

coincidences in Fig. 2a> shows that the

final state resonance is no longer formed.

111

In tiie sane time, the singles proton

spectra display a characteristic evapora-

tion shape at backward angles which allows

to conclude that we deal with a real

stochastic emission of protons (evaporation

and no final state interaction).

a)

1 • • '

c»=«20MeV/(Residual] X

1330 MeV s>

Ep(KcV)

20 4 0 .

Fig. 2 :

Comparison between p-p and a-a coincidences

at 6,1 =•• 5 degrees. On the right are shown

the expected *inal state interactions and

the limits due to a residual system with a

given excitation energy e*,

The new shape of the correlation func-

tion is then assigned to statistical inter-

ferometry because many data concerning

other correlations like p-d or d-d show no

influence of Coulomb distortions at such

large angles.

For a uniform spherical source of radius

Eu, decaying with lifetime T, we get the

following expression B' :

1)Q Eu 1 +

With T * 0, the usual correlation func-

tion versus q - Q/2 has to be considered as

a projection of a two-dimensional plot

<0.q.,J. It can be noticed that the limits

of q., to be used in the integration are

dependant on q as shown Fig. 3a>.

The relevant projection of the plot

(Fig. 3b> is then fitted with the following

values :

T * 1.1 10-21S ; Ru = 6.5 fm

The value of T characterizes the average

coherence time of emission in a stochastic

process like proton evaporation.

iO-

10 20 30 40 50 60,k7_k-g/2(MeV/c)

Fig. 3 :

The characteristic correlation function

fitted after integration of expression 1)

over the range of q..-, values. A lifetime of

the order of 10"-" s is needed to account

for the data with a reasonable value of Ru

(6.5 fm).

Further studies an tine effects are In

progress in other kinds of collisions.

1) S. Koonin, Phys. Lett. 70B 43 (197V)2) W.G. Lynch et al, Phys. Rev. Lett. 51,1850 (1983)3) D.H. Boa! et al, Phys. Rev. Lett. 2i,2901 (1986)4) T. Humanic et al, Z, Fur Physik 3fi, 79(1988)5) Z. Chen et al, Phys. Rev. C3S., 2297(1987)6) G.I. Kopilov, H.I. Podgoretski, Sov. J.Hucl. Fhys. 15_, 219 (1972)

112

« A r + " Z n COLLISIONS: INCOMPLETE FUSION AND LIGHT PARTICLE EMISSION

J.P. COFFIN'. A. FAHLI", P. FINTZ", G. GUILLAUME". B. HEUSCH'. F. JUNDT". F. KAMI". P. WAGNER3.

A.J. COLE", S. KOX" and Y. SCHUTZC

a) Centre de Recherches Nucleates and Universite Louis Pasteur, 67037 Strasbourg Cedex (France)

b) Institut des Sciences Nucleaires, 38026 Grenoble Cedex (France)

c) GANIL, B.P. 5027, 14021 Caen Cedex (France)

1. Motivation

With increasing projectile energy (- 10

to 40 MeV/nucleon) In central collisions,

incomplete fusion becomes preponderant with

respect to complete fusion and the total fusion

cross section is expected to decrease strongly.

The aim of the present work was to gain some

information about the incomplete fusion

process and to contribute to a better

understanding of the fall off of the fusion

cross section.

The <°Ar •*• 68Zn reaction has been

investigated in a serie of experiments

performed at 14.6, 19.6, 27.6 and 35

MeV/nucleon.

2. Results and discussion

The fusion-evaporation and fusion-

fission components resulting from complete and

incomplete fusion were identified and cross

sections were measured for both components at

all the energies. The velocity distributions of

the evaporation residues were analyzed and

the linear momentum transferred to the

compound system deduced. The momentum

transfer appears to depend on the asymmetry

of the system and on the projectile mass when

they are compared to other results. Fig. 1

shows the velocity of the evaporation

residues, divided by the velocity of the

compound nucleus formed by full momentum

transfer, plotted against the relative velocity.

The long-dashed curve represents theoretical

predictions for the "°Ar + MZn reaction and

the short-dashed line corresponds to the Viola

systematics. The mass, excitation energy and

/ \CD

Suh-

z

UJ

v

1.0

0.8

0.6

0.4

Q2

1 i i i 1 1 1 1 1 1 1

f*--iJiVjy .

I N + MQ T i i " * -- * " A l T P ^ K - -

~i ? 6 o + 27At 1 X X N " " - "

- j W A r + 6 8 2 n c e t r a V Q i l 1

__40Ar + S8Zn calcuUNGO et al- systematiques

i i i i i i i i i i i0 1 2 3 4 5 6 7 8 9 10 11

[2(E c m -V c ) /y . ] V 2(cm/ns)

Fig. 1 :Overall average velocity VEK of theevaporation residues divided by VFMT coseplotted against the relative velocity.

08

wC? 0.4

b

02

0

" 1 1 1 1

} J2S*'6Ge -

- I'V.^Zn-

" N. -X- "

/, i i i i "T - I

Fig. 2 :Ratio of fusion to reaction cross sectionsplotted against the^temperature of the reducedcompound nuclei CN,

113

temperature of the compound nuclei formedhave been determined. Fig. 2 shows the ratioof fusion to reaction cross sections plottedagainst the temperature of the reducedcompound nuclei. Fusion disappears slightlyabove 35 MeV/nucleon which corresponds to acompound nucleus excitation energy of500 MeV (5.9 MeV/nucleon) and to atemperature of 6.8 MeV according to the simpleparameterization T = (E,«c/AcN/8)1'2.

The nature of the falloff of the fusioncross section at higher energies remains anopen question : although the reduced compoundnucleus is highly excited, and may havereached the limit of stability, the largenumber of fast particles lost before fusionoccurs may be responsible for this limitation.

The light charged particles emitted inthe <0Ar + MZn reaction have been measured

i0Ar+68Zn= 19.6MeV/'nui:teon E = 35 MeV/nuc leon

LJT>

10010

10010

100101

01

10010

101

10010

10°

20°

50°

90°

120°

150°

1000100101

10010

120°

10 30 50 70

10 30 50 70

E (MeV )

Fig. 3 :Proton energy spectra measured at variousangles and two incident energies. The solidcurves correspond to the prediction of themoving source model.

inclusively. The energy spectra were analysedIn terms of preequilibrium emission, movingsource and coalescence models. Moving sourceparameterization used in these calculations(Fig. 3) is in complete agreement with thosededuced from heavy fragment analysis. At 14.6and 19.6 MeV/nucleon, the data can bereproduced by assuming particle emission ofstatistical origin and from a sequential decayof the quasi projectile. At 35 MeV/nucleon athird source of intermediate rapidity isrequired. It can be correlated to theappearance of projectile fragmentation between20 and 35 MeV/nucleon already observed inheavy fragment measurements. The emission ofcomposite particles (d.t.a) raises the questionof their production mechanism. Instead ofbeing a preformed subset of nucleons they mayresult from the coalescence of nucieons. Thecoalescence model supposes that if a number Aof nucleons has a relative momentum less thana given value p<> the nucleons form acomposite particle. The coalescence radii ofthe thermalized volume related to p<> andextracted at 35 MeV/nucleon (- 3 fm) maysuggest the formation of a hot spot.

A complete coherence between the lightparticle data and those obtained separatelyfrom heavy fragment studies is obtained.

3. ReferencesG. GUILLAUME et al.. Int. Conf. on Nucleus-Nucleus Collisions, Visby, Sweden (1985)J.P. COFFIN et al., Symposium "Many Facets ofHeavy-Ion Fusion Reactions", Argonne NationalLaboratory, U.S.A. (1986)

A.J. COLE et al., Workshop on hot Nuclei andNuclear Disassembly, I.S.N. Grenoble, France(1986)A. FAHLI et al., Phys. Rev. C34 (1986) 161A. FAHLI, Thesis, Universlte de Strasbourg,France (1986)A. FAHLI et al., Z. fur Physik A326 (1987) 169

114

DENSITY FUNCTIONAL APPROACHES FOR THE DESCRIPTIONOF COLD AND EXCITED NUCLEI *>

J. Bartel1'"), M. Brack1), C. Guet2'6>, J. Meyer3>, P. Quentin4*, E. Strumberger1>c)

J) Inst. f. Theor. Physik, Universitdt, D-8400 Regensburg

V GANIL, B.P. 5027, F-14021 Caen

3> IPN, Univ. Lyon-I (and IN2P3), 43 bd du 11-11-1918, F-69622 Villeurbanne

4) Lab. Phys. Theor., Univ. Bordeaux-I (and CNRS), r. du Solarium, F-33170

Gradignan

°> permanent address: Phys. Theor., CRN, B.P. SO, F-670S7 Strasbourg

6> permanent address: Service de Physique, DRF, CENG, F-38041 Grenoble

c) present address: Physik Department, TU Munchen, D-8046 Garching

*) Work partially supported by the French-German exchange program 'PROCOPE'

A systematic predictive description of nuclear properties either for cold systems (as

exotic nuclei at low excitation energy) or for excited nuclei (as resulting from heavy ion

collisions) is contingent upon the availability of microscopic calculations which should be

both reliable and tractable. We report here on two contributions fitting in this broad

framework which have been performed for a substantial part at GANIL. They use an

effective interaction of the Skyrme type and the Hartree-Fock (HF) approximation, which

together allow to express the total nuclear energy as a functional of the density matrix p.

One may furthermore simplify this functional, either by a suitable ansatz for the non-

local part of p (cf. Sect. 1 below) or by performing a semiclassical approximation - which

turns out to be exact at sufficiently high temperatures - in the extended Thomas-Fermi

(ETF) approach (cf. Sect. 2 below).

1. Extended gaussian approximation for the density matrix [1]

A very simple parametrization for the nonlocality of p has been proposed as />(R, s) =

p(R)(l — s2/ot2)exp[—s2/(2<72)], where R and s are the center of mass and relative coordi-

nates. In the above expression, a, a are functional of p(R) which are chosen to yield the

115

correct local semiclassical kinetic energy density as well as the projector character of p in

an integrated form.

The validity of this ansatz has been assessed in various nuclei for physical quantities

sensitive to the non-local nature of the density matrix, such as the momentum distribution

in the nucleus, the exchange energies for the Gogny and Coulomb forces and the two-body

center of mass energy correction. In all cases, a very good overall agreement with exact

(i.e. HF) results has been found. It had in most cases a better quality than previous

approaches, such as the Slater or DME approximations. Obviously, the full relevance of

the proposed phase-space distributions should be tested in dynamical calculations.

2. Liquid drop parameters for hot nuclei [2]

Highly excited nuclei at temperatures T ^ 3 MeV exhibit no shelf structure and can

thus be ideally described by semiclassical methods, such as the ETF density variational

method which has been widely employed for the selfconsistent description of average nu-

clear properties. For many practical applications, a droplet model type expansion of the

nuclear free energy has been found useful and sufficient. The coefficients of this expan-

sion have been determined selfconsistently from one of the most successful Skyrme forces

(SkM*) as functions of the temperature T. They were further approximated by a low-

temperature expansion quadratic in T and valid up to T ~ 4 MeV.

The application of these parameters in a very simple evaluation of the temperature

dependent fission barriers of heavy nuclei has been shown to rather well reproduce the

results of the more time-consuming variational ETF calculations.

The dependence of the infinite-nuclear-matter incompressibility on the temperature,

which plays an important role in the equation of state, relevant e.g. in astrophysical

supernovae calculations, was also discussed.

[1] J. Meyer, J. Bartel, M. Brack, P.Quentin, S. Aicher, Phys. Lett. B 172 (1986) 122

[2] C. Guet, E. Strumberger, M. Brack, Phys. Lett. B 205 (1988) 427

116

Static Properties of Hot Nuclei*

E. Suraud1

Institut des Sciences Nucleaires53 Avenue des Martyrs, F-38026 Grenoble Cedex , France

1) Present address : GANIL, BP5027, F14021 Caen cedex, France

Heavy-ion collisions at intermediateenergy (10-100 MeV/A) offer a very useful toolfor investigating the properties of nuclei at finitetemperature. Experimental evidences for theformation of such excited systems (E*/A < 2.5-3MeV, T < 5 MeV) are now well established (seeRef [1] for a recent review). From a theoreticalpoint of view the formation of hot nuclei hasbeen studied within the framework of dynamicalapproaches [1,2], but their properties, onceformed, may be described in a static picture, atleast for a restricted amount of time [1,3]. Onecan then picture out the system as a hot nucleusin equilibrium with its own evaporated nucleons.The major difficulty is then to properly take intoaccount the effects of evaporated nuclei.

An elegant solution to this problem wasrecently proposed by Bonche, Levit andVautherin (BLV, [4]) in the framework ofquantal Hartree-Fock (HF) calculations. Theseauthors propose to isolate the "liquid"contribution by performing simultaneously twomean field calculations, one for the nucleus inequilibrium with its evaporated nucleons and onefor the nucleon gas alone. These two "phases" arecoupled by the Coulomb interaction which iscalculated from the "liquid" proton density alone.However, these fully quantal calculations arevery heavy, numerically speaking, in particularat high temperature. As shell effects are knownto disappear at around T= 2-3 MeV [1] it is veryinteresting to develop semi-classicalapproximations [5] of the BLV formalism [6]. Itturns out that these approximations allow toreproduce very accurately average quantal valueswhile remaining much more tractable. A detailedstudy of these methods and a comparison with HFresults is presented in Ref [6]. In the followingwe shall hence only focus on some physicalapplications.

*) Part of this work was done at the IPN, Orsay, France.

so

55

5.0

Cslltl

"1- ___ PrlS.ll

Er17.91

130 KO GO

A160

Figure 1Limiting excitation energies of various nuclei in the A=130-160 mass region and along some Z/A lines. Also areindicated the corresponding limiting temperatures. Thedashed-dotted line is only given for guiding the eye,presented results corresponding to actual microscopiccalculations.

In the framework of BLV calculations hotnuclei exhibit a limiting temperature beyondwhich no solution to the coupled probem can befound anymore [4]. One can show that this"instability" is due to the Coulomb interactionwhich overcome the nuclear stabilizing effect athigh temperatures (T= 8 MeV). We haveperformed a systematic study of this phenomenonin the A= 150 mass region for whichexperimental data were avaiable [7]. Calculationshave been done with the SKM Skyrme interactionand with various semi-classical approximationsfor the energy density functional [5]. Results areshown in Figure 1 were the limitingtemperatures and excitation energies are plottedin this mass region. As suggested by schematiccalculations [8] we find that the charge over massratio is a relevant parameter for the study of thislimiting phenomenon.

The study of the temperature dependanceof the level density parameter is also a subject oflarge interest [1] and we have hence used our

117

o 100 2so 500 iooo E'(MeV)

01 s0 E* 100 250

10500

9

(MeV')Ba

7

°Zr

0 1 S T0 E * 50 100 200

10

Ca

0 1 T(MeV)

Figure 2Temperature dependance of the mean field pan of the leveldensity parameter of 3 nuclei. The plotted quantityessentially reflects the temperature evolution of the singleparticle level density around the Fermi energy. Note thatheavy nuclei are more sensitive to temperature than lighterones with larger Z/A ratios.

semi-classical BLV approach to study its meanfield part. In the lowest order semi-classicalapproximation the level density can be simplyexpressed in terms of integrals involving thesingle particle potentials associated to the twocoexisting phases [9]. As can be seen in Figure 2,it turns out that up to about T= 4 MeV the effectof temperature is negligible so that, as a firstapproximation, zero temperature values (a<=A/8)can still be used, for example in estimatingdensities of states. In order to understand thepossible effects on " a " at around T= 4 MeV onehas however to include corrections beyond the

mean field for which the temperature plays amore important role [10].

In this work we have investigated somestatic properties of hot nuclei in the frameworkof the semi-classical counterpart of the BLVsubtraction procedure. We have found a verygood agreement between the semi-classical valuesand the average quantal results. We haveperformed a detailed study of the limitingtemperature of hot nuclei. The experimentalstudy of this kind of situation remains an excitingsubject, with the forthcoming possibility ofaccelerating heavy beams at moderate energies.Our study of the level density parameter has alsoshown the almost temperature independance ofthe mean field contribution in this quantity.

References

[1] E. Suraud, Ch, Gregoire and B. Tamain, toappear in Progress of Nucl. Part. Sci (1989)[2] G.F. Bertsch and S. Das Gupta, Phys. Rep160 (1988) 189[3] J. Treiner, Nucl. Phys. A488 (1988) 279c[4] P. Bonche, S. Levit and D. Vautherin, Nucl.Phys. A427 (1984) 278 and Nucl. Phys. A436(1985) 265[5] M. Brack, C. Guet and H.K. Hakansson, Phys.Rep. 163 (1985) 276[6] E. Suraud, Nucl. Phys. A462 (1987) 109[7] B. Borderie and M.F. Rivet, Zeit PhysA321 (1985) 703 '[8] P. Bonche and S. Levit, Nucl. Phys. A 43 7(1985) 426[9] E. Suraud, P. Schuck and R.W. Hasse, Phys.Lett. B164 (1985) 212[10] R.W. Hasse and P. Schuck, Phys. Lett. B179(1986)313

118

PROPERTIES OF HIGHLY EXCITED NUCLEI

P. BONCHE

Service de Physique Theorique, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France

S. LEVIT

Department of Nuclear Physics, Weizmann Institute of Science, 76100 Rehovol, Israel

and

D. VAUTHER1N

Division de Physique Theorique, Institut de Physique Nucleaire\ 91406 Orsay Cedex, France

Received 24 October 1983(Revised 20 March 1984)

Abstract: A prescription is proposed for calculating the contribution of unbound states in nuclearHartree-Fock calculations at finite temperature. The method is based on the remark that a staticHartree-Fock calculation at finite temperature describes a hot nucleus in equilibrium with anexternal nucleon vapor. Properties of the hot nucleus including continuum effects are obtained byextracting the contribution or the external gas, which we calculate from a second Hartree-Fockcalculation. We show that for a one-body potential this subtraction procedure yields standardformulae for partition functions in terms of phase shifts. Numerical calculations are performed iri56Fe and 208Pb. The resuls indicate that continuum contributions are large beyond temperaturesof the order of 4 MeV. We also find the existence of a critical temperature, of the order of 10 MeV,beyond which solutions of the equations can no longer be found.

CONTRIBUTION AS SENT BY THE AUTHOR

119

Nuclear Physics A428 (1984) 95c-100cNorth-Holland, Amsterdam

MEAN-FIELD DESCRIPTION OF NUCLEI AT HIGH TEMPERATURE

Paul BONCHEService de Physique Theorique, CEN SACLAY, 91191 Gif-sur-Yvette Cedex,France

Shimon LEVITDepartment of Nuclear Physics, Weizmann Institute of Sciences, 76100 Rehovot,Israel

Dominique VAUTHERINInstitut de Physique Nucleaire, Division de Physique Theorique*, 91406 OrsayCedex, France

We propose and discuss a prescription suitable to include the contributionof continuum states in mean-field calculations at high temperature.

CONTRIBUTION AS SENT BY THE AUTHOR

120

Nuclear Physics A436 (1985) 265-293© North-Holland Publishing Company

STATISTICAL PROPERTIES AND STABILITY OF HOT NUCLEI

P. BONCHE

Service tie Physique Theorique, CEN, Saclay, 91191 Gif-sur-Yveile, Cedex, France

S. LEV1T

Department of Nuclear Physics, H'ei:mann Institute of Science, Rehovot 76100, Israel

and

D. VAUTHERIN

Inslinn tie Physique Nucleaire *, 91405 Orsay Cedex, France

Received 6 June 1984(Revised 6 September 1984)

Abstract: Results of temperature-dependent Hartrcc-Fock calculations for equilibrated hot nuclei are.presented, extending to the highest temperatures at which the nuclei remain stable. A subtractionprocedure developed earlier for isolating the properties of the nucleus from the nucleus-)-vaporsystem is applied. The temperature dependence of various quantities characterizing hot nuclei isinvestigated. The influence of different effective interactions in the Hartree-Fock equations isexamined. Special attention is devoted to the study of the high-temperature stability limit of hotnuclei. This limit in nuclei with the Coulomb interaction artificially switched off (i.e. unchargednuclei) is shown to correspond to the critical temperature of the liquid-gas phase transitionexpected on the basis of hot nuclear matter calculations. In realistic charged nuclei theCoulomb repulsion causes a nucleus to become electrostatically unstable and to fall apart atmuch lower temperatures than its uncharged partner. The approach to and the temperature ofthis Coulomb instability are very sensitive to the choice of the nuclear interaction. Studying thisinstability in compound nuclei with different charge-to-mass ratio provides a sensitive measure ofthe temperature dependence of the nuclear surface properties as well as of certain features of thenuclear equation of stale.

CONTRIBUTION AS SENT BY THE AUTHOR

121

Volume 138B, number 5,6 PHYSICS LETTERS 26 April 1984

THOMAS-FERMI CALCULATIONS OF HOT DENSE MATTER

E. SURAUD and D. VAUTHERINDivision de Physique Theorique *, Institut de Physique Nudeaire, 91406 Orsay Cedex, France

Received 5 December 1983

We use the imaginary time-step method to perform fully variational Thomas-Fermi calculations at finite temperature.Results are given for hot nuclei and for the equation of state of the hot dense matter present in the late stages of stellarevolution. Transitions between various phases are calculated and a comparison with the results of Hartree-Fock calcula-tions is given.

CONTRIBUTION AS SENT BY THE AUTHOR

122

Volume 191, number 1.2 PHYSICS LETTERS B 4 June 1987

EVOLUTION OF HOT COMPRESSED NUCLEIIN THE TIME-DEPENDENT HARTREE-FOCK APPROXIMATION *

D. VAUTHERIN 'Center for Theoretical Physics, Laboratory for Nuclear Science and Department of Physics. Massachusetts Institute of Technology,Cambridge, MA 02139, USA

J. TREINER and M. VEN^RONIDivision de Physique Theorique *. Institut de Physique Nudeaire. F-91406 Orsay Cedex, France

Received 29 July 1986; revised manuscript received 12 March 1987

Spherical solutions of the TDHF equations are computed for initial conditions corresponding to hot nuclei under differentcompressions. Special care is devoted to the emission of particles and to the resulting separation between liquid and vapor phases.

CONTRIBUTION AS SENT BY THE AUTHOR

123

Nuclear Physics A422 (1984) 140-156© North-Holland Publishing Company

TEMPERATURE DEPENDENCE OF COLLECTIVE STATESIN THE RANDOM-PHASE APPROXIMATION

D. VAUTHERIN and N. V1NH MAU

Division de Physique Theorique*, Instilut de Physique Niicleaire, F-91406 Orsay Cedex, France

Received 25 November 1983

Abstract: We investigate the temperature dependence of collective states in the framework of therandom-phase approximation at finite temperature. We show that sum rules can be exicndcd 10collective energies at finite temperature. Numerical methods are developed to solve the RPAequations at finite temperature. Results are presented and discussed in the case of 40Ca for isovectordipole and isoscalar octupote vibrations, using oscillator wave functions and a zero-range force.We show that the broadening of giant dipole resonances observed experimentally, appears as anatural consequence of the structure of the RPA equations. Comparison is made u»»vh vhcschematic model for which the temperature dependence of collective states can be worked outanalytically.

CONTRIBUTION AS SENT BY THE AUTHOR

124

Volume 120B, number 4,5,6 PHYSICS LETTERS 13 January 1983

NUCLEAR PARTITION FUNCTIONS IN THE RANDOM PHASE APPROXIMATION

AND THE TEMPERATURE DEPENDENCE OF COLLECTIVE STATES

D. VAUTHERIN and N. VINH MAUDivision de Physique Theoriqite1, Inaitut de Physique Nucleate.F-9J406 Orsay Cedex. France

Received 16 September 1982

Green functions techniques at finite temperature are used to calculate nuclear partition functions in the random phaseapproximation. The theory is shown to yield corrections to the results of functional methods neglecting exchange terms.We discuss the special case of a schematic model for which the level density and the temperature dependence of collectivestates can be worked out explicitly.

CONTRIBUTION AS SENT BY THE AUTHOR

125

Nuclear Physics A458 (1986) 460-474North-Holland, Amsterdam

EFFECT OF CORRELATIONS ON THE RELATION BETWEENEXCITATION ENERGY AND TEMPERATURE

T. TROUDET. D. VAUTHERIN and N. VINH MAU

Division de Physique Theorique*, Insiiiui de Physique Nucleaire, F-91406 Orsay Cedex, France

Received 18 November 1985(Revised 15 May 1986)

Abstract: Formulae for the excitation energy of a nucleus as a function of its temperature are derivedin the framework of the random phase approximation (RPA). Two methods are investigated. Wefirst calculate the energy as the derivative of the grand potential obtained in random phaseapproximation. We also calculate the energy as the thermal average of the Hamiltonian operatorusing RPA Green functions at finite temperature. The two methods are compared by analyzingtheir content in terms of diagrams. We also discuss the effect of correlations on the relation betweenchemical potential and particle number.

CONTRIBUTION AS SENT BY THE AUTHOR

126

Windsurfing the Fermi Sea, Volume IIT.T.S. Kuo andJ. Speth (editors),© Elsevier Science Publishers B. V.. 1987

THE LEVEL DENSITY PARAMETER OF A NUCLEUS IN THE SCHEMATIC MODEL*

D. VAUTHERINT

Center for Theoretical Physics, Laboratory for Nuclear Science, and Department of Physics,Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, UJS.A.

and

N. VINH MAU

Division de Physique The'orique* Institute de Physique Nucle"aire, F-91406, Orsay, Cedex,France

The level density parameter of a nucleus is calculated in the random phase approximationusing the schematic model of Brown and Bolsterli. Using the known collective states inlead-208, we find that RPA correlations increase the level density parameter by 30% at lowtemperature. This increase disappears near T = 4 — 5 MeV in qualitative agreement with therecent measurements by Nebbia et a/.

CONTRIBUTION AS SENT BY THE AUTHOR

127

INSTABILITIES IN AN EXPANDING FIREBALL

J. CugrionUniversity de Liege, Physique Nucle'aire The'onque, Instilut de Physique au

Sart Tilman, B.5, B-4000 LIEGE 1, Belgium

The idea that the dynamical origin of the multifragmentation could be due to

spontaneously growing instabilities, linked with the entrance of the system in

the so-called spinodal zone was first proposed in ref. , and later discussed in2 )

ref. . We recall here the basic ideas arid list the remaining probltms.

A piece of matter, heated and compressed, expands, due to the work of its

bulk pressure. There are good reasons to believe that the system evolves along

an isoentrope. If the initial entropy per baryon is lower than ~ 2 units., the

system will be unstable under any density fluctuation when it enters the iscentro-

pic spinodal zone (see ref. ). This is substantiated by the change in free,-g -*

energy induced by a density oscillation of amplitude a (6p = a e ' ) in nuclearmatter. Fig. 1 illustrates this point.

-Wr-UJ -

Fig. 11 Full curves : variation of the free energy ne-

cessary to create a fluctuation of amplitude a

outside (a), inside (c) or at the boarder (b)

of the isoentropic spinodal. Dotted curves :

probability of observing a fluctuation in a ca-

nonical calculation.

(c)

X7

Even though there is a large consensus on the relevancy of this scenario

for the dynamical origin of the fragmentation, many points are still to be clari-

fied :

1. What are the importance of the finite size and surface effects ? Evapo-

ration, i.e. the appearance of a gazeous phase at the surface more or less stabi-

lizes the inner core

2. Nuclt-ar matter is a Fermi liquid. Perturbations do not propagate as in

an ordinary fluid. Does this matter ? This question has been investigated in2 )ref. , where it is shown that the stability of the matter is increased by this

aspect.

3. How to describe the evolution of the instabilities ? How do they lead to

the birth of clusters. Several lines of approach have been studied in condensed

matter physics, especially the phase separation occuring after the quenching of4)a ferromagnet above the Curie point . Transport theories, still at a phenomeno-

logical stage, have been devised , which describe the separation of ferromagne-

tic and diamagrietic domains. The nuclear case is more difficult because of sur-

face tension and because of the expansion of the system.

128

4. It may be questioned whether the phase separation may take the form of

bubble formation and growth. The strong nuclear surface tension as well as the

surface thickness cooperate in such a way that only bubbles with large sizes can

survive. This is shown by a study of bubble dynamics , The critical radius may

be as large as 4-5 fm. A similar conclusion is arrived at in a recent microsco-

pic study of static bubble energy .

5. Is there some scaling law as in critical phenomena ? Experimentally,p \

such laws seem to hold . Apparently, the granular nature of nuclear matter do

not play an important role.

In conclusion, according to this scenario, the onset of multifragmentation

(energy, size,...) could depend upon the characteristics of the spinodal and of

transport properties, but the mass yield may just depend upon a few critical ex-

ponents, which presumably do not depend much upon the equation of state.

1) J. Cugnon, Phys.Lett. 135B (1984) 374.2) C.J. Pethick and D.G. Ravenhall, Nucl.Phys. A471 (1987) 19c.3) N. Nemeth et al., ?.Phys. A323 (1986) 419.4) J.W. Cahn and J.E. Hillard, J.Chem.Phys. 31 (1959) 688.5) J.D. Gunton et al., Phase Transitions and Critical Phenomena, vol. 8, (1983) 1.6) J. Cugnon, to be published.7) H. Flocard et al., to be published.8) X. Campi et al., Phys.Lett. 142B (1984) 8.

129

- B 3 -MULTI-FRAGMENT EMISSION

A SCHEMATIC HYDKODYHAMICAL AND PEiqOUTION PICTURE FO1

MULTIFRAGMENTATIOU OF EXCITED NUCLEI

J. Nemeth*, M. Barranco**, J. Desbois***, C. Ngo**** and R. Boisgard****

*Institute for Theoretical Physics, Eotvos University, Budapest, Hungary** Departament de Fisica, Universitat de les Illes Balears, E-07071 Palma de Hallorca, Spain*** Division de Physique Theorique"*", Institut de Physique Hucleaire, 91406 Orsay Cedex, France

**** Laboratoire National Saturne, CEtJ Saclay, 91191 Gi£ sur Yvette Cedex, France.

Multifragmentation can occur in violentheavy ion collisions. In order to understandwhat conditions are required to reach thiskind of break-up we propose the followingsimple model^'.

We start with an initial hot andcompressed nucleus and assume that it remainsspherical during its evolution (a ratherdrastic constraint, of course). The hot andcompressed nucleus expands and this processis described by a time dependent Thomas Fermimodel (TDTF)2'. This expansion is assumedto be isentropic, which is probably a verygood approximation in regards to cascadecalculations performed for heavy ion collisionsat higher bombarding energies3^. The resultof such a calculation is presented in Fig.lwhere a 208Pb nucleus is considered (Full line :initial density, dashed curve : density profile

are, respectively,at t =and thermalthe initial compressional

excitation energies).At each stage of the expansion, the

magnitude of the fluctuations of the meanfield is evaluated using a 3-dimensionalsite-bond percolation model on a cubic lattice'1' .We recall that such a percolation is governedby two parameters :

- p, the fraction of occupied st tes ;- q, the probability that two neighboursoccupied sites are linked.

The connection betweenTDTF is made as follows :

percolation and

p(t) andB(t=o)

where <P(t)> « f 0 » are the average densitiesat time t (t=o), B (t=o) is the binding energyper nucleon at time t=o and C (t) is the ther-mal excitation energy per nucleon at time t. So,

1.0

0.5

0.0

1.410~22s'

Multi fragmentation

0.0 0.5 1.0

Fig. 2

0.3-

V.02

0.1

0.0

- 275 MeV

= 501 MeV

r(frn)10

1'ig.l

we can follow tbe dynamical evolutionof the nucleus in p - q plane. Thecase of a 208Pb nucleus is depictedin Fig.2 with two different sets ofinitial conditions :

A EJ* = 102 MeV, E£ » 799 MeVB V* = 275 MeV, E ^ = 501 MeV

As long as the fluctuations of theriean field remain small, the meanfield approach used to describe theexpansion process is perfectly suited.However, it can happen that this isno longer the case. Then, the nucleusbreaks up into several pieces. Sucha phenomenon is expected to occurfor both cases in Fig. 2 : thenultifragmentation region is reachedafter the time indicated in the figure.

The most significant result ofthis model is that compressional energyturns out to be more efficient to;->reak up nuclei than thermal energy.This is illustrated in Figs.3-4.

Unite de Recherche des Universites Paris 11 et Paris 6 Associee au C.N.R.S.

130

fc

1.0

0.8

0.4

0.2-

0.0

i,

C c = 0 MeV

C * r 0.5 MeV

0 50 100 150 200 250A(u)

Fig.3

Multifragmentation

Normal- de.excitation

In Fig.3, we have plotted the quantity £ */B( £ : critical excitation energy per nucleonto undergo multifragmentation, B : bindingenergy per nucleon) as a function of the massA of the initial nucleus. The two curvescorrespond to two different values £ * of theinitial compressional energy per nucleon. InFig. 4, we show the phase diagram of a 208pbnucleus in the plane E *- £ ~ . We can see thata small amount of compressional energy greatlylowers the total excitation energy neededfor the break-up.

This effect can be easily understoodif one remembers that thermal excitationcorresponds to desorganized energy whilecompression corresponds to coherent energyin the mode of instability.

To conclude, concerning our model, wewant to say that a dynamical calculation,without any condition of spherical symmetry,is highly desirable. So, we expect thatmultifragmentation could occur dynamically,without recourse to percolation. Work in thatdirection is currently in progress.

1) J. Nemeth et al., Z. Phys. A325 (1986) 347 ;J. Desbois et al., Z. Phys. A328 (1987) 101.

2) J. Nemeth et al., Z. Phys. A323 (1986) 419.3) G. Bertsch et al., Phys. Rev. C24 (1981) 2514.4) J. Desbois, Nucl. Phys. A466 (1987) 724 ;

X. Campi et al., Contribution to the XXIIIInt. Winter Meeting on Nuclear Physics, Bormio(1985) p.497.

£* IMeV)

Fig.4

131

RESTRUCTURED AGGREGATION COUPLED TO LANDAU-VLASOV MODEL

S. Leray1, C. Ngd1, B. Remaud2, F. Sebille2 and M.E. Spina1

Laboratoire National Saturne2 University de Nantes

A central collision between two nuclei

leads to the formation of a highly excited and

compressed system. Before thermal equilibrium

is reached, fast particles remove a large part

of the available energy. When thermalization

has been achieved the system is still hot and

compressed, so it expands and cools down. If

the expansion phase leads to a very low density

situation the formation of clusters can be

expected and thus multifragmentation can occur.

Indeed, while a nuclear medium at normal

density can be described by the mean field

created by the whole set of the nucleons, at

low density the distance between neighbouring

nucleons can become larger than the range of

nuclear forces. Thus, density fluctuations are

important and some nucleons are close enough to

each other to interact and therefore form

clusters. Due to repulsive Coulomb force

between the created clusters, the system disas-

sembles. It is clear that in this case the mean

field approximation is no longer valid and that

calculations based on this approximation have

Au + Au 200 MeV/u b = 0

1000-

100-

10-

1 -

1E-Q1 -

1E-02-

1E-03-

iF-rid.-

t = 40 fm/c

1 A~ .

1000-]

100-

10-

-o 1-0*f 'E-OI-

1C-02-

IE-03-

1E 04-

t - 50 fm/c

1

\ . / ^

• — . .JT :

100 200 300 400 0 100 200 300 400

Mass Mass

lOOO-i

100-1 = 60 fm/c

10-

' " l1E-01- \

1E-02- \ ^

1E-03- ' " — •

1 E - 04 - i i i < i •• • t i i i i i i ' | i i0 100 200 300

Mass

lOOO-i

100-

10-

s '•^ 1E-01-

tE-02-

1E-03-

1 E-04 -

t = 70 fri/c

1

i•

400 100 200 300 400

Mass

Fig.l. Mass distribution of the fragmentsobtained at different time steps during thecollision for the system Au + Au at 200 MeV/u

to be stopped at the point where the fluctua-

tions of the mean field become too large.

In the intermediate energy domain the model

of Gregoire et al which uses the Landau-

Vlasov (LV) equation has proved to be very

successful in describing experimental data

associated with one body observables. In order

to study the possible disassembly of the system

we couple the LV calculation, which describes

the dynamical evolution of the system, to the2)restructured-aggregation model described

3)elsewhere in this compilation , which

evaluates the fluctuations of the mean field.

With this approach we know when the

fluctuations of the mean field are too

important to carry on the LV calculation. When

this happens, we stop the LV calculation. The

mass distribution of the fragments is obtained

with the restructured-aggregation model and can

be compared to the experimental one if we

assume that the clusters are no longer modified

as they separate.

This model was applied to the system197Au+197Au at 200 MeV/u. In fig. 1 we present

the yield of clusters as a function of their

mass at different time steps and at zero impact

parameter. At 40 and 50 fm/c most of the

nucleons aggregate into a big fragment. This

000^

-

•100-z

:-.

10-:

:•-

1-

A U + A U 200 MeV/u+ t

* f * J $ j * b = 0 f m• * • * „ b = 3fm

» + , » « b = 6fm* * + • x b = 9fm

" * AA b =13 fm* - B I

mA

MM A

W * 1 f

10 15 20 25

Multiplicity

Fig.2. Fragment multiplicity, distribution as afunction of impact parameter for the Au + Ausystem calculated at 60 fm/c.

132

means that a mean field approximation is still

valid and that we can carry on the LV

calculation. On the other hand at 60 and 70

fm/c only small mass fragments are left indica-

ting that the system has undergone multifrag-

mentation. Several impact parameters were

studied : for Osbs6 fn the transition occurs

around 55 fm/c in less than 10 fm/c, while at

b=9 fm we do not reach a probability equal to

one and the mass distribution shows that there

is always a fragment of mass around 100 left,

which would correspond to spectators in a

participant-spectator picture. At b=12 fm the

probability remains zero since the two nuclei

reseparate without loosing there identity. In

fact, multifragmentation arises when the

density of the system has decreased down to

around 0.4 times the initial value, the same as

when studying the onset of multifragmentation3)in a single nucleus with variable density .

The system 197Au+197Au at 200 MeV/u was4)studied by Doss et al . In this experiment

intermediate mass fragments with 3sZslO were

detected in the forward center of mass

hemisphere and the centrality of the collision

was evaluated through the total charged

particle multiplicity. We can compare the

experimental multiplicity distributions (fig.44)of ref. ) to our calculation. In our case the

intermediate mass fragments have mass between 6

and 25. The distributions are taken at 60 fm/c

when the system has undergone

multifragmentation and are shown in Fig.2. The

general trends of the data are rather well

Ar+AI000 -=

100-

1 0 -

.

j _

c

• lBsi«:So. + * * * o E

+ " • ? « E

D ° _ B * E* o c ^ ^

o * » 8 D

0 + S

IK + DO +

1— T— 1 — Q -i 1 , A * i

. , , . , . , , . f . . l + f .) 5 10 15

Multiplicity

w

= 25 MeV/u= 35 MeV/u= 45 MeV/u= 55 MeV/u= 65 MeV/u

20 25

reproduced : for central collisions (Osbs6) the

distributions are similar while their maximum

is shifted towards lower multiplicity values

when the collision becomes more and more

peripheral. The mean value of the multiplicity

is also rather well reproduced by our model.

Some experiments were performed at Ganil on

the Ar+Al system at different bombarding

energies ' and it has been claimed that

multifragmentation begins to appear between 25

and 36 MeV/u. We have applied our model to this

system at 25, 35, 45, 55 and 65 MeV/u for zero

impact parameter. We found that multifrag-

mentation begins to be present at 35 MeV/u as

observed experimentaly, but represents a

significant part of the cross section only

above 45 MeV/u. What is different from the

Au+Au system is that the probability of multi-

fragmentation is never 100 '/. in this range of

energies (even at 65 MeV/u it reaches only 98 '/.

at maximum). If we stop the LV calculation when

the probability of multifragmentation is

maximum we can construct multiplicity distri-

butions for particles with mass between 2 and

20 as Fig. 3 shows. It can be observed that

there is a substantial difference between the

25, 35 MeV/u and the three other energies. As

in fig.2 for the multiplicity distributions

obtained for different impact parameters it

seems that the shape of multiplicity distribu-

tion can be a probe for the occurrence of

multifragmentation. Our results are in quan-go

tative agreement with the ones of ref. which

show a difference in the multiplicity spectra

between 25 and 36 MeV/u. This difference was

attributed to the appearrance of multifrag-

mentation. However, in our case, the difference

is more significative between 35 and 45 MeV/u.

1) C.Ng6 et al, Nucl. Phys. in press.2} C.Gregoire et al, Nucl. Phys. A436 (1985)365.3) S.Leray et al, this compilation.4) K.G.R.Doss et al, Nucl. Phys. A471 (1987)241c.5) G.Auger et al, Phys. Lett. 169B (1986) 161.6) G.M.Jin et al, 3 Int. Conf. on NucleusNucleus Collisions, Saint-Halo (1988)

Fig.3. Fragment multiplicity distributionobtained at different bombarding energies.

133

MULTIFRAGMENTATION AND RESTRUCTURED AGGREGATION

S. Leray1. C.Ngd1, H.Ngo2 and M.E. Spina1

1 Laboratoire National Saturne, z Physique theorique, IPN Orsay

We have developed a restructured aggreg-ation model to study nuclear multifragmentation[1]. The aim of this approach is to try toevaluate the importance of the fluctuations ofthe mean field and thereby to find when thesystem becomes unstable with respect tomultifragmentation. In this model each nucleonis supposed to be represented by a sharp sphereof radius i = 1.03 fm. This value corresponds toabout the range of the nuclear forces. For agiven configuration of a system of mass A, onedraws at random the position of the A nucleonsand look if they overlap. Some of them do. Inthis case the overlapping nucleons are supposedto restructure in a single sphere of radiusR = 1.15 A1'3, where A is the mass of the

c ccluster. After a first iteration one is leftwith a distribution of clusters and nucleons.Some of these particles may still overlap due tothe restructuration. If it is the case, oneiterates again the aggregation process and therestructuration until no overlap exists anymore.In the end one is faced with two situations :either there is a percolation cluster or not. Inthe first case the fluctuations of the meanfield are small while they are large in thesecond case which ends by multifragmentation.This process, done for a particular sampling ofthe nucleons, is repeated several times in orderto generate an ensemble average and studyaverage properties.

In order to illustrate the kind of resultthat one can obtain with a restructured aggreg-ation model, let us present some results ofref.l. Fig.1 shows the mass distribution of theproducts obtained for a 208Pb nucleus at dif-ferent densities p :

There are two components in the massdistribution for p=0.07 fm"3: one, peaked aroundmass 200, corresponds to the percolationcluster. The other component, at smaller massvalues, contains the nucleons and the light massclusters. This mass distribution is typical of asituation in which the fluctuations of the meanfield are small.- For p = 0.04 fm"3 the shape of the massdistribution is completely different. Thepercolation cluster has disappeared and itremains a component containing light and mediummass fragments only. In this case, thefluctuations of the mean field are large and thesystem disassembles.

p = 0.06 fm"3 andtransition region

where one goes from percolation to multi-fragmentat1on.

The two other examplesp = 0.05 fm" , are in the

Due to the small number of particlespresent in the system and to its finite size, agiven event can lead to a percolation cluster orto multifragmentation'with a probability whichvaries smoothly with the density of the system.For an infinite system the size of the percol-ation cluster is infinite. When it disappearsthe mass of the largest cluster drops suddenly.For nuclei, we do not have such an evolution butrather a continuous decrease of the size of thelargest cluster. In order to estimate the inf-luence of the finite size of the system it hasbeen assumed that one has a percolation clusterif the size of the largest cluster is largerthan half the initial mass of the system.Consequently, for a given event, the systemundergoes multifragmentation only if the size ofthe largest cluster is smaller than half the

ir-o?i1E-03,JE-04

208 R b

p-0.07 fm-'

100-

10-

o

a; 1C-01-,> 1E-02

1L-03-J

IE-04

p^O.06 fm-5

0 80 160 240

Mass

fm->

IDOi

1 1

lE-02i1E-OJ-iIE 04

C

,0=0.04 fm-J

\ ,80 160 240

Moss

100-,

80-

60-

20-

O O O O O O O

oooo

O O O 0 O0.000 0.020 0.040 0.060 0.080 0.100

Density

Fig.l Fig. 2

134

Initial mass of the system. This is arbitrarybut one cannot avoid to introduce a convention(which of course could be chosen differently)due to the smoothness of the transition. Themultifragmentatlon probability is deduced byaveraging over many different events for eachdensity. The results for the Pb are shown infig.2. Multlfragmentation appears for densityvalues between 0.06 and 0.07 fm~ and the multi-fragmentation probability is equal to unitybelow p = 0.04 . fm~ . Similar results areobtained for the Ca nucleus but the transitionis smoother due to the smaller number ofparticles.

The smooth evolution due to the smallnumber of particles contained in the system isillustrated also In fig.3 which shows thecorrelation between A , the mass of the

maxlargest cluster, and the density of the system.As we see, there is a continuous evolution ofA as p decreases and the transition is notwaxsharp.

It is interesting to perform a momentanalysis as suggested by Campi [2].For examplethe correlation between Log S and Log S (see

Z 3ref. 12] for a definition of the moments) isrelated to the critical exponent T entering intothe inclusive mass distribution of thefragments. It is shown in fig 4 for an ensembleof multifragmentation events at differentdensities. This correlation is almost linearand, from the slope of the average behaviour ofLog S as a function of Log S , one finds

3 2

T = 2.2 which is very close to the value deducedfrom emulsion experiments (r=2.2±0.1) [3].

pyf

* 208

2

Log S2

Fig. 4

[1] C.Ngd, H.Ng6, S.Leray and M.E.Spina, preprint[2] X.Campi, J. Phys.A : Math. Gen. 19 (1986)1917.

[3] C.J.Waddington and P.S.Freier, Phys. Rev.C31 (1985) 888.

208p b

0.05 0 10

p (fm-3)

Fig. 3

135

CONTRIBUTIONS TO THE DESCRIPTION AND UNDERSTANDING OF NUCLEAR FRAGMENTATION

J. RICHERT*, S.K. SAMADDAR**, P. WAGNER*

*Centre de Recherches Nucleaires et Universite Louis Pasteur, Strasbourg, France**Permanent address : Saha Institute of Nuclear Physics, Calcutta, India

The study of nuclear fragmentation is aimed to describe and explain how and why stronglyexcited nuclei can decay into many pieces. Experimental and theoretical investigations have leadto. several questions nost of which remain unsettled up to now. Among other points one may thinkabout the dynamical time evolution of the multifragmentation which may develop sequentially orcorrespond to a violent quasi instantaneous process. This may depend on the energy regime, thedetails of the nature and range of the interactions among the constituents which are involved andthe conditions under which the excited system is generated. Another point concerns the interestingquestion whether the multifragmentation regime sets in as more or less sharp transition governedby relevant physical quantities, the most evident being the degree of excitation of the system.We have tried to contribute to these problematics through two different investigations.

We considered the sequential decay of a strongly excited nucleus. This mechanism may be atwork in heavy ion induced reactions at low GANIL energies. The model describes an evaporationprocess in which an initial excited nucleus decays by emitting particles and clusters of particleswhich, if they are excited, decay themselves into smaller pieces. As time flows the whole systemexpands in space due to the kinetic energies of particles and clusters and their mutual Coulombrepulsion. The yields of different fragments are obtained through n set of coupled master equa-tions'). Resulting mass (fig. 1) and isotopic distributions for the reaction Ar on C at27.5 MeV/A ~) show quite a nice agreement with experimental results which are obtained under theassumption of preequilibrium particle emission followed by the decay of the remaining heavy sys-tem. In fig. I the bars indicate normalised measured mass yields. Unfortunately there exists nounivoque identification of fragments with mass A<£25. In this calculation the system shows nocritical behaviour with respect to the temperature when a percolation moment analysis-^) is imple-mented. This is due to the too small yields of intermediate mass fragments obtained in the calcu-lation. One would need at least the explicit measurement of the yields of fragments with massA< 25 in order to state whether the present sequential decay picture is ir trouble or not.

Percolation models are able to interpret qualitatively and quantitatively mass distributionsgenerated through violent multifragmentation4,5). This interpretation involves the existence ofa critical behaviour which is governed by pure space occupation characteristics of the system. Itraises the question of the relevance of correlations between the onset of break up and internalfeatures of the system. We investigated these points on a finite size Ising type model"). Thereone is able to show the connection between cluster multiplicities and the correlations generatedby the two-body interaction acting between the spin constituents above, at, and below the criticaltemperature of the corresponding infinite system. The mass distributions are typically thoseexpected in the percolation approach, the temperature being the relevant parameter. The quantita-tive effect of the spin interaction on the mass distribution has been investigated numericallyon two and three dimensional models (fig. 2). Short (indicated by (T)) and long range (indicatedby © ) attractive interactions show marked differences at low excitation (see T<2.5) and lessdiscrepancy at high excitation (see T=5.0). The present analysis and the links of the models withthe percolation approach raises the question of the existence or not of critical threshold formuZtifragmentation. This is definitely a point one would like the settle in the future in as muchas other models seem to exhibit critical features^). In order to clarify the situation it wouldhowever be necessary to overcome the gap which exists between the present spin systems and actualnuclei (kinetic energy, density and Coulomb interaction effects, non equilibrium dynamics).

From the experimental side one faces the necessity to get more detailed information on massand isotopic distributions. One knows now also from experience that these quantities may berather insensitive to the details of the dynamical fragmentation process. Hence one would alsodefinitely like to gather more exclusive information such as for example time and velocity corre-lation measurements between outcoming fragments in order to get closer to the problematics andthe possible critical character of the process.

1) C. Barbagallo, J. Richert, P. Wagner, Z. Phys. A324 (1986) 972) J. Richert, P. Wagner, Z. Phys. A330 (1988) 2833) X. Campi, J. Phys. A : Math. Gen. Jj) (1986) L9I74) T. Biro, J. Knoll, J. Richert, Nucl. Phys. A459 (1986) 6925) X. Campi, Contribution to the International Workshop on Nuclear Dynamics at Medium and High

Energies, Bad Honnef, W. Germany, October 19886) S.K. Samaddar, J. Richert, to be published in Z. Phys. A

136

7) S.K. Samaddar, J. Richert, to be published in Phys. Lett. B8) D.H.E. Gross, Yu-ming Zheng, H. Massmann, Phys. Lett. 200B (1988) 397

0.001 L

Fig. 1

10 20 30 10 20 30 40

1O

6O 0

Fig. 2

2O 40

N.

60

137

Spinodal break up and the onset of multifragmentation

D, Cussol, Ch. Gregoire and E. Suraud*-1

GANIL, BP5027-, F14021 Caen cedex, France

*) Institut des Sciences Nucleates, 53 Avenue des Martyrs, F38026 Grenoble cedex, France

1) Present address : GANIL, BP5027, F14021 Caen cedex, France

Heavy-ion reactions provide a precioustool for investigating the nuclear matterEquation Of State (EOS). The onset ofmultifragmentation seems to be connected to theexploration of low density regions of the EOS bythe excited nuclear system, as predicted byschematic models [1). In order to study thiseffect in a more realistic way we have made ananalysis in the framework of Landau-Vlasovsimulations [2], which provide a dynamicalmicroscopic description of one body observables.The complete description of final products,namely of the numerous produced fragments ishence beyond the scope of such an approach. Themodel however allows a reasonable descriptionof the path towards the low density, unstable,spinodal region.

We have studied in detail two symmetricreactions (40 Ca + 40 Ca and 40 Ar + 50 Ti) atvarious beam energies. While at low energies thesystem fuses, it undergoes multifragmentationbeyond E/A = 40-50 MeV/A and vaporization athigher energies. In order to exhibit the spinodalregion we extract from the total energy thekinetic collective component. The remainingenergy corresponds to an intrinsic energy whichrepresents an analogon of an EOS of the systemwhen plotted as a function of the density (Figure1). One can see on figure 1 that the actualexplosion is connected to the crossing of thespinodal line corresponding to this EOS (between40 and 60 MeV/A beam energies) [3,4].

In fact, as pointed out by Pethick andRavenhall, crossing the spinodal line is anecessary but not sufficient condition for asystem to explode [5]. The crucial quantity, for agiven mode, is given by the integral I of thegrowing rate of instability over the time duringwhich the system stays in the spinodal region. Asthe collective mode associated to the expansion ofthe system towards the spinodal region isdominantly of monopole nature [4] we havemade an hydrodynamical monopole analysis of

WOO

0)

-woo

-.

Co* Co

MeV/u

1./(9f3(fmJ

Figure 1" Equation Of State " of the Ca + Ca reaction. The intrinsicenergy Hint ('n MeV) is plotted versus the average densityfor beam energies between 20 and 100 MeV/A, and in thecase of central collisions.

the motion. Introducing the correspondingcollective mass allows to define the localmonopole frequency associated to the evolutionof the system. As soon as the system does enterthe spinodal region this frequency becomesimaginary and one can estimate the transit timein the spinodal region. Values of the frequencyversus time are plotted for the Ar + Ti system inFigure 2 and for two beam energies. At around40 MeV/A one observes a rapid increase in thetransit time, up to about 100 to 120 frn/c, whichcorreponds to the actual explosion of the system.

138

(b=OI

20MeV/U—4. UMeV/U

o so no iso zoo isoTime Ifm/c)

Figure 2Time evolution of the square of the hydrodynamicalmonopole frequency for the Ar + Ti reaction at two beamenergies (20 and 44 MeV/A). In the case at 44 MeV/A thesystems stays more than 100 fm/c in the unstable regionand does explode.

Moreover, rough estimates of I give results invery good agreement with the threshold obtainedin a schematic analysis [5].

In order to go beyond schematic analysisof the explosion mechanism we have performeda microscopic estimate of the way an excitedsystem may go into the unstable spinodal region.We have found that explosion is connected toentering the spinodal region. More precisely thesystem has to stay for about 100 to 120 fm/c inthis region to let instabilities develop up tocompletely disrupt the nucleus. This is in good

• qualitative agreement with schematic studies.

References

[1] G.F. Bertsch and Ph. Siemens, Phys. Lett.B126 (1983) 9[2] Ch. Gregoire, B. Remaud, F. Sebille and L.Vinet, Nucl. Phys. A465 (1987) 317[3] E. Suraud, M. Pi, P. Schuck, B. Remaud, F.Sebille, Ch. Gregoire and F. Saint-Laurent,Preprint GANIL P88-04[4] E. Suraud, D. Cussol, Ch. Gregoire, D.Boilley, M. Pi, P. Schuck, B. Remaud and F.Sebille, Nucl. Phys. A 495 (1989) 73c[5] C.J. Pethick and D. G. Ravenhall, Nucl. Phys.A471 (1987) 19c

139

INTERMEDIATE MASS FRAGMENTS PRODUCED AT LARGE ANGLE IN HEAVY ION REACTIONS

N.H. Papadakis3, N.P. Vodinas3, Y. Cassagnou1, R. Dayrasl, R. Fonte4, G. Imme4,

R. Legrain1, A.D. Panagiotou3, B.C. Pollacco*, G. Raciti4, L. Rodriguez1,

F. Saint-Laurent2, M.G. Saint-Laurent2, N. Saunier1.

1. D.Ph.N/BE, C.E.N. Saclay, 91191 Gif-sur-Yvette Cedex, 2. GANIL, B.P. 5027, 14021 Caen Cedex,

3. University of Athens, Physics Department, Panepistimioupolis 15771 Athens,

4.I.N.F.N. and University of Catania, 57, Corso Italia 1-95129 Catania.

The production of intermediate mass

fragments (5 Z < 25) at large angle (15° <6 < 127°)

was investigated as a function of incident energy

and target mass in the reactions Ne + Ag and

Ne + Au between 20 and 60 MeV/nucleon *). Looking

for possible new processes such as the proposed

"shattering of cold nuclear matter" 2> or "break

up of a hot expanded participant zone into clusters"3 ' , a careful analysis of double differential cross

sections was successful in unfolding the various

sources of emission of every Z-separated fragment.

In energy spectra, angular distributions and

V//-VJ. velocity plots, a contribution relevant

to the well known low energy processes of

evaporation and fission was separated on the ground

of a discrimination between two sources, the

velocities and temperatures of which are favourably

defined in reactions using direct kinematics.

The evolution as a function of incident energy

of the remaining "fragmentation component"

is illustrated in Fig. 1. The two reactions exhibit

10s

10',

| i j i T i |-< 1 ' 1 ' T~r

rr '

| Ne + Auri r

ia

f ,

Ne + Ag

IB

£ <7Z

30 so 7(MeV/A)

Fig. 1 : Yield of Intermediate Mass Fragments from20 up to 60 MeV/nucleon.

trends in sharp contrast: Ne + Ag total fragment

production is unchanging while that of Ne + Au grows

as energy increases. The mass (charge) distribution

of the fragments is an additionnal significant piece

of information. It is usually well reproduced by a

dependence on fragment mass A (charge Z) of the

form 0(2)52-1. Values of T , often found

intermediate between 2 and 3, were considered

at a time as a hint for a liquid-gas phase transition

in nuclear matter4*. Fig. 2 shows values of ^ which

are obtained for the two reactions considering the

t-

1

2e -zua.

I

4.4

4.0

3.6

3.2

2.8

2.4

•) n

,

*

~p

*

' III I' i l l

• 1 i 1 i 1 . 1 > 1

' 1

I

1

I

, ,

1 ' 1 1

Ne+Ag

Ne+Au

I

J_L 1 . ! _ ,

-

_

-

-

Fig.2 Values of T as a function of the maximumavailable energy.

whole set of data in comparison with the only

so-called "fragmentation component" from which

evaporation products, mostly for Ne + Ag, and fission

fragments, for Ne + Au, have been eliminated. The

energy dependence is expressed along the x-axis

of Fig. 2 through the largest available energy per

nucleon calculated as E*/A = / 8

A(.ot= total number of nucleons.

140

Looking first at the unseparated data, the

two reactions again show differences : for Ne + Ag

(A), T is rather constant while, for Ne + Au (.),

one observes a steep decrease of T at low energies,

indicating that more heavy fragments are produced

compared to light nuclei when more energy is

brought in this system. If fragments from fission

and evaporation are removed (*), values of T

for Ne + Au are obtained which are all but

independent of energy as it is still the case for

Ne + Ag. Furthermore, the values of T for gold

in the high energy limit are lower than those for

silver, a trend in relation with the mass of the

system according to theoretical predictions.

In summary, for the reaction Ne + Ag, from

the steadfastness of its total cross section as

well as the constancy of its charge distribution,

the process of fragmentation, whatever it is,

seems well established in the energy range

investigated in this experiment. On the opposite,

the rise of the cross section for gold along with

the variation with energy of its T values show

that the same process, only spotted at the lower

energies through a severe selection of the data

against "relaxed" processes, is in its development

as the incident energy increases.

The most interesting result of inclusive

measurements, such as those just described, lies

in the definition of a domain of angles, velocities

and fragment masses for further investigations

of a possible multifragmentation process (i.e.

simultaneous emission of fragments in opposition

to successive evaporation events). A first attempt

along this line was recently made by measuring

coincidences between intermediate mass fragments

(IMF) and light particles (protons, deuterons, alphas)

detected in a close-to-4» multidetector consisting

in 150 scintillator counters. The dependence on

the impact parameter of IMF production was

investigated in this experiment, the data of which

are presently being analysed 5>.

Hoping for a more powerful detector in

the future making possible the detection of both

heavy and light nuclei in a 4ir geometry, we expect

a complete evidence of the "multifragmentation"

process, understood as an extreme at a high

excitation energy of both evaporation and fusion ;

a glimpse of this process resulted from our previous

experiments. We also expect a valuable insight into

a mechanism inducing a small-sized system, such

as a nucleus, to break into clusters when too much

energy is brought in it.

References

1) N.H. Papadakis et al., to be published.

2) J. Aichelin and 3. Hiifner, Phys. Lett. 136B,

15 (1984).

3) P.J. Siemens, Nature 305, 410 (1983).

4) J.E. Finn et al., Phys. Rev. Lett._49, 1321(1982).

5) G. Bizard et al., C.R. Activite D.Ph.N.,

Note DP4N 89-1, Saclay.

141

Characteriiation of Multifragmcnt Channels in 45 MeV/nucleon B4Kr + 1 M Tb ReactionZ. Majka1*, L.G. Sobotka1, D.W. Stracener1, D.G. Sarantites,1 G. Auger2, E. Plagnol2,

Y. Schut«a, R. Dayras3, J.P. Wieleczko3, 3. Barreto4 and E. Norbeck6

'Department of Chemistry, Washington University, St. Louis, MO 631302GANIL, BP 5027, FI4021 Caen Cedex, France

3CEN-Saclay, F91191, Gif-sur-Yvelte Cedex, France*Institut de Physique, Nucleare-BPOl-91406-Orsay Cedex, France'Department of Physics, University of Iowa, Iowa City, IA 52242

In this work we address the question of the mul-tifragment production mechanism in the reaction of45 MeV/nucleon 84Kr + 159Tb. The experimentwas performed at the GANIL facility and the DwarfBall/Wall detection system was used. This detectorarray consists of 104 fast slow plastic-Cs! elementswith a total solid angle of 90% of 4ft.

Two striking charcteristics of multi-intermediatemass fragment (IMF, I IMF > 2) exit channels wereobserved. These are: 1) high multiplicity of IMF'sis correlated with large light-charged particle (LCP,ILCP < 2) multiplicity. 2) Events with large IMFmultiplicity exhibit no planar character.

7000

6000

TJ

8 5000

i 4000

3000

1000

IMF -

10 30

M,'LCP

Figure 1. Normalized light charged particle (LCP) multiplic-ity distributions as a function of the number of intermediatemass fragments (IMF) are shown as thin solid lines. The LCPmultiplicity distributions corresponding to values of the totaldetected charge 20 < Zoa ^ 45 a n d z£>e« > 7 S a r e shown asdashed and thick solid lines, respectively. The curves are offsetfrom the x axis for display.

The correlation between the multiplicity of theIMF's, M/MF) and the LCP multiplicity, Mr,cp, isshown in Fig. 1. One can see that the MLCP dis-tributions are composed of two parts. When only afew IMF's are detected ths M £ C P distributions peakat rather low values < 8. As the number of de-tected IMFs is increased the MLCP distributions be-come broader and extend to considerably higher val-ues. For still larger M/AJF values, the M^cp distri-butions regain a peaked form but with substantiallygreater mean value than for low M/AJF values. Alsoobserved, but not shown in this report, is the trendthat the total detected charge (Zj)ct) increases withM / M F (the TiDet distributions extend up to 100% ofthe sum of the projectile and the target charge, Zj-0t= 101). Since the detection system is efficient for

the detection of light ions but becomes more ineffi-cient as the charge increases and the ion velocity de-creases (due to absorbers), the events with low valuesof total detected charge must be associated with oneor a few undetected massive heavy ions. This sug-gests that the component of the Micp distributionin coindence with only a few IMF's, which also tendsto have a large deficit in the total detected charge,results from deep inelastic like or fusion events inwhich a massive residue survives but escapes detec-tion. On the other hand, the component of the MJX?Pdistribution in coincidence with several IMF's is as-sociated with well characterized (Zprt > 75) nuclearexplosions with no surviving very massive remnant.This is verified by the Mtcp distributions obtainedby gating on regions of Zoct values, see Fig. 1.

0 10 20 30 0 10 80MuHiplicilg Nln

Figure 2. The multiplicity distributions of LCP's, IMF's and to-tal, when the total detected charge is > 75, are shown in part a).The symbols in part b) correspond to the number of ions, N in

which are within ±45° of the plane defined by the beam and theion with the largest charge. The histograms are the result offolding of the multiplicity distributions of part a) with binomialdistributions with a chance probability of p = 1/2.

Our second observation is that these total explo-sion events show no planar quality. This is demon-strated in Fig. 2. Fig. 2a shows the multiplicity distri-butions for IMF's, LCP's and the total, for the eventsin which the total detected charge exceeds 75% of theTiTot- This selection, of almost complete event re-construction, biases toward large MJMF values. Theevent planarity is investigated by determining thenumber of ions, N,-B, which are within 45° of the planedetermined by the beam axis and the direction of theheaviest IMF of each event. These are shown as sym-bols in Fig. 2b. The mean values are close to 1/2 ofthose found in Fig. 2a suggesting a random distri-bution. This is verified by folding the multiplicitydistributions in Fig. 2a with a binomial distributionwith a. chance probability p = 1/2. These folded dis-tributions are displayed as histograms. The agree-ment is almost perfect indicating that these eventsexhibit no planar character.

*On leave from the Institute of Physics, Jagellonian Uni-versity, PL30059, Krakow, Poland.

142

MULTIFRAGMENT PRODUCTION IN KRYPTON INDUCED COLLISIONS AT 43 MEV/u

R. Bougault, F. Delaunay, A. Genoux-Lubain, C. Le Brun, J.F. Lecolley, F. Lefebvres, M. LouvelJ.C. Steckmeyer - LPC, ISMRA, Universite de Caen, FranceJ.C. Adloff, B. Bilwes, R. Bilwes, M. Glaser, G. Rudolf, F. Scheibling, L. Stuttgg - CRN, Strasbourg, FranceJ.L. Ferrero - Universitad di Valencia, Spain

Motivation

The main purpose of colliding heavy nuclei at energy far above the Coulomb barrier is to study the properties ofnuclear matter in temperature and also density regions out of equilibrium. We then hope to learn about the nuclearmatter equation of state and may be to see some new phenomena. To do that at GANIL energies, we chose tostudy the heavy fragment production, which is expected to be rather large in the Kr+Au, Ag, Th collisions at44 MeV/u , due to either the collision mechanisms which may lead to the formation of several pieces of hotnuclear matter or the deexcitation stage. Indeed for very hot nuclei, the deexcitation through either a sequentialemission of large fragments or a sudden process like multifragmentation has to be studied. Up to now, except forlow statistics emulsion experiments, the multifragmentatipn is rather studied from inclusive experiment than frommultifragment measurements. We used a set up covering with 30 cells, each one made of a parallel plateavalanche counter with localisation and of an ionisation chamber, 5O% of the solid angle between 3°andl5O°with respect to the beam axis.For each fragment we measured the velocity vector and get a Z identification, the threshold were at Z = 8 andv = 3cm/ns for the 3°-30° angular range and at Z= 10 and v = 0,5cm/ns for the 30°-150° angular range.The trigger was done on a number of detected fragments (ND 2 3) requirement. In the forward direction(3° < 8 < 23°) the plastic wall was used to detect light charge particles with high velocity.

Results

Kr * Au43 MeV/u

Number of detectedfragments

Normalized numbersof events

< Z >

< V > cm/ns

Vbe.nT9-Ztot = "

2

84%

25.5

3.56

1 cm/ns Center

5 Center

3

14.5%

22.7

3.31

of mass

of mass

4

1.4%

18.5

3. IS

velocity =2

energy =

5

0.1% 0

16.6

3.1

.77 cm/ns

2.57 GeV

6

.003%*

15.5

2.9

* (which represents 600 recorded events )

TABLE I

The first raw observation is a large occurenceof events with a high number of detectedfragments ; the results are given on Table Iwithout any correction for Gold and Silvertargets.

The production rate for ND = 3 on Gold targetovercomes without corrections 100 mb . Thelargest number of detected fragments, stillsignificative, 5 for silver target and 6 for Goldtarget, exceed 4 , the number given by a doublesequential fission which is known to be possiblewith so large nuclei. The velocity and Z meanvalues decrease as the fragment number NDincreases.To go further in the analysis we have to studyevent by event the correlations between thedetected fragments to be able to separate variousreaction mechanisms, to look for the sources andto study the deexcitation processes. The mostrelevant parameter is the relative velocity

t*rel = | " i ' » j | ) which is an invariant anddepends strongly on the interactions between thefragments at the emission time. As the number ofcombinations of Vrei increases very quicklywith ND (10 values for ND = 5) we will alsouse various moments of the Vrei distributionsand specially the second one which allows tocheck the dispersion around the mean value. Bythis way we separate clearly the sequentialmechanisms where the relative velocities arewidely spread out according to their origin(several sources) from the more simultaneous onewhere the relative velocities are distributedaround a single value. The results of such ananalysis is displayed on the Figure 1 for theKr+Au collisions at 44 MeV/u and forND = 3,4,5 .

FIGURE I

143

The sequential events on the right have two maxima one of them centered around 2.3 cm/ns which is the valuefound for fission events and for coulomb repulsion between two mid sized fragments. In the following, we willdeal with the events displayed on the left where the relative velocities are distributed around the mean value3 cm/ns higher than the coulomb repulsion and the same whatever the number of detected fragments. The meanproperties of these events are given for the three systems under study in the Table II.

Target ND <ZZ> <2P / /C>X MLP <ZLP>

Silver2Z=83

Gold2Z=115

Thorium£ Z = 126

3434

L 5345

u.m.a.3844526373567080

GeV9.8

10.68.09.6

11.08.2

10.111.1

GeV/u.m.a.255240154154152146144140

2.93.02.72.62.82.52.52.4

u.m.a.2.42.42.32.22.22.32.22.1

TABLE H

P//C is the linear momentum of each fragment along the beam axis. The projectile linear momentum is24,3 GeV/c . For each target whatever ND , a large fraction of the initial charge and linear momentum ismissing. But for each target the parallel linear momentum divided by the detected charge remain constant. MLPand < ZLP > are the mean multiplicity and the mean charge per light particle observed in the forward direction.They are the same whatever the target. All these results are a strong indications of a common process for eachtarget and each number of detected fragment. To look for the source of the fragment we plot on Figure II isocontour lines of invariant cross section in the (V//, V x ) plane for Kr + Au and ND = 5 . The fragments

detected in the 30° - 150° angular range behave like if they originated from the same source. Their angularrepartition given by the table associated to the picture shows that > 3 fragments come from the same source witha velocity of 1 cm/ns . We are faced with events where 3 or 4 fragments are coming from the same source andhave relative velocities centered around 3 cm/ns giving a strong indication of a multifragmentation process. Inthis case the whole system is not concerned and it is impossible to clearly identify a source in the forwarddirection.

• IB .u . |m'.v1 dv1dv/,

-from 30* lo 150* 4<—From 3*lo 26-

-3 -2 -1 6 i 2 3 4 5 6 7 8 9

Tcofm ^beaV// |cm;ns|

References

ANGULAR REPARTITION OF ND=5 EVENTS(30°-150° angular rangq, 3°-25° angular range)

(5,0)

5.6%

(4.1)

42%

(3.2)

44%

(2.3)

a%

(1.4)

0.1%

(0.5)

0%

FIGURE I I

- D. Dalili et aL - Zeitschrift fur Physik A316 (1984) 371 .- R. Bougault etal - NIM A259 (1987) 473 .- G. Rudolf et ai - Phys. Review Lett. 57 (1986) 2905.- R. Bougault etaL- NPA 488c (1988) 255c.- Etude experimentale et analyse de remission des fragments corre'les dans la reaction Kr+Au a 35 et 44 MeVpar nucleon ; A. Kamili - these de Doctoral es Sciences Physiques , ULP Strasbourg, 4 juillet 1986.

- Etude des mecanismes de reaction observes dans le systeme Rr+Au a 35 et 44 meV par nucleonJ. Colin - these de 1'Universite de Caen, 5 juillet 1987.

144

ONSET OF MULTIFRAGMENTATION IN 40Ar ON "Al CENTRAL COLLISIONSFROM 25 TO 85 MeV/u

K. Hagel0'', A. Peghaire", G.M. Jina 'h, D. Cussol0, H. Doubre", J. Peter"'6, F. Saint-Laurent", G. Bizard6,R. Brou*, M. Louvel6, J.P. Patry*, R. Regimbart*, J.C. Steckmeyer*, B. Tamain6, Y. Cassagnou",

R. Legrain", C. Lebrund, E. Rosato', J. Natowitz', J.C. Jeong', S.M. Lee', Y. Nagashima', T. Nakagawa',M. Ogihara', J. Kasagp, T. Motobayashi"'6'*

a GANIL, F-14021 Caen-Cedex * LPC Universite, F-14032 Caen-Cedex c CEN DPhN/BE, Saclay d LSNNantes, F-44072 Nantes-Cedex 03 E Dipartimento di Scienze Fisiche, Univ. di Napoli (Italy) ! Cyclotron

Institute, Texas A&M Univ. USA * SUNY, Stony Brook U.S.A. h Inst. of Modern Physics, Lanzhou,China « Institute of Physics, Univ. of Tsukuba, Japan ' Dept. of Physics, Inst. of Technology, Tokyo,

Japan * University Rikkyo, Tokyo, Japan

For the system 40Ar + 27A1 above 20 MeV/u,the production of fusion residues has been studiedin an inclusive experiment [ij. It showed that thecross section of evaporation residues produced in cen-tral collisions vanishes between 32 and 36 MeV/u.The occurrence of multifragmentation is likely. Forthis system, we have made exclusive measurementsof light charged particles and fragments, from 25 to65 MeV/u by 10 MeV/u steps, and also at 85 MeV/u(with a poor statistics).

Since we are dealing with a system in reverse kine-matics, almost all particles with non negligible ve-locities are found in the forward hemisphere. Then,the experimental set-up covered angles from 3.2 to90° with two complementary multidetector systems: Mur [2] and Tonneau [3]. For each particle, onegets the velocity and the nuclear charge. This set-up has several limitations, taken into account in theanalysis.

All events with a multiplicity greater than 1 inthe 132detectors were recorded.

In parallel to the experiment, we have made ex-tensive simulation calculations. Several hypothesisabout the formation and decay of excited fusion nu-clei have been used, at several impact parameter val-ues. The resolution, threshold and geometry of theexperimental set-up have been taken into account tosee how they affect the observables : multiplicity, to-tal momentum, eccentricity...

In the event-by-event analysis, the first step wasto keep only those events where a sufficient informa-tion has been obtained. The criteria has been foundto be the measured total parallel momentum valueP||. The well detected events are kept for the nextstep of analysis. The simulation calculations showthat all central collisions events belong to this group.

Since the largest excitation energies (in thermalor compressional forms are reached in central colli-sions, the next step was to separate central collisionsevents from intermediate impact parameter and pe-ripheral collisions. For a better sorting, we must usethe whole information obtained for each event. Sucha complete information is contained in the sphericitytensor, which is determined by the mass, velocity anddirection of all detected particles [4].

Figure 1 shows the distribution of events in a two-dimensional plot SQS versus t at 45 MeV/u. Similarplots have been obtained at other energies. The con-tinuous evolution of reactions from peripheral to cen-tral collisions is reproduced in the continuous shapeof the distribution. We cut it in 3 areas defined on

the basis of simulation results. Area P correspondsto well detected "peripheral" collisions : their parti-cles are focussed in the c.m. frame (large value of e)at small angles. Fusion reactions not far from com-plete fusion are in areas C (for "central") or M, sincetheir shape is close to a sphere, with the main axisrandomly distributed between 0 and 90°. Reactionsoccuring at intermediate impact parameters belongto area M. (for "mixed").

An expected signature of multifragmentation isthe production of several intermediate mass fragments(i.m.f). The measured probability to emitting 1,2...i.m.f. in central collisions is shown in figure 3 versusthe incident energy, more) fragment We can calcu-late the average probability of emitting i.m.f. : itis the sum of the products in.m.f. x probability toemit v i.m.f. in figure 2. This average probabil-ity is close to 1 (slightly less than 1 at 25 MeV/u,and above 1 at higher energies), in agreement withthe results of ref.[5]. Due to the detection velocitythreshold, the measured proportions shown here areminimum values for vi.m.f. >1, and maximum val-ues for i/i.m.f.< IThe emission of more than 1 i.m.f.strongly increases between 25 and 36 MeV/u andmore slowly above. These events can be attributedto a multifragmentation process. Its onset is locatedjust where the evaporation residues cross section van-ishes [1].

Another results exhibits a strong change in thedecay process for central collisions : the distributionof the total measured charge.lt is shown in figure 3for energies from 25 to 65 MeV/u. A strong shiftis seen between 25 to 36 MeV/u, both on the peaklocation and its width. Above 36 MeV/u, the widthremains constant and the peak shifts slowly. At 25MeV/u, the dominant process is sequential statisti-cal de-excitation of the fusion nucleus, leaving a slowheavy residue [4] with a charge larger than ~13. Thisresidue is undetected or its charge is taken to be 9only ; hence, the total measured charge is low andnever exceeds 22. At higher energies , the break-up in several lighter fragments allows to detect mostfragments and correctly measure their charge ; hence,the total measured charge is much larger and evenreaches the total charge of the system : 31 (note,however, that some events still have a low measuredtotal charge, as at 25 MeV/u, which indicates a co-existence of both processes).

145

[1] G. Auger, E. Plagnol: D. Jouan, C. Guet, D.Heuer, M. Maurel, H. Nifenecker, C. Ristori, F.Schlusser, H. Doubre, C. Gregoire, Phys. Lett.169B. 161 (1986).

[2] G. Bizard, A. Drouet, F. Lefebvrea, J.P. Patry,B. Tamain, F. Guilbault, C. Lebrun, Nucl. Inst.Meth.

[3] A. Peghaire, B. Zwieglinski", E. Rosato», G.M.Jin', J. Kaaagi", H. Doubre, J. Peter, Y. Cas-sagnou, R. Legrain, F. Guilbault, C. Lebrun, tobe published.

[4] J. Cugnon, J. Knoll, C. Riedel, Y. Yariv Phys.Lett. 109B (1982) 167.

[5] E. Plagnol, L. Vinet, D.R. Bowman, Y.D. Chan,R.J. Charity, E. Chavez, S.B. Gazes, H. Han,W.L. Kehoe, M.A. MacMahan, L.G. Moretto,R.G. Stocktad, G.J. Wozniak, G. Auger, ReportLBL-25742 (1988)

.or

.001

.0001

Figure 1 : Distribution of events as a functionof their eccentricity € and the flow angle 9Qs-

collisions centralesArt At

25 35 1.5 55 65 85

energie incidents (MeV/u)

• Figure 2 : Excitation functions for the produc-tion of intermediate mass fragments in centralcollisions events.

S to tS 20 25

Z Mai mesure

30

Figure 3 : Normalized distributions of the to-tal measured charge in central collision events,from 25 to 65 MeV/u.

146

Exp. E32 Event-by-Event Studies ofEnergies.

160 Induced Reactions at GANIL

E32a Multifragmentation Studies in Nuclear Emulsions.

B Jakobsson, G JSnsson, L Karlsson, B NorSn, K SSderstrom,Univ. of Lund, SwedenE Monnand, F Schussler, ISN Grenoble.

These two experiments had the goal tostudy break up properties on an event-by--event basis of central heavy ion collisionsover a variety of energies where criticalbehaviour of nuclear matter could be ex-pected. By placing large enough stacks ofnuclear emulsions of various sensitivitydirectly in vacuum we optimized the condi-tions to determine the collision energy thecharge and energy of each fragment.

We select central collisions by a hightotal charged particle multiplicity (M)trigger. This parameter is not unambigous atlow collision energies but we still believeit to be the best common trigger for centralevents. Fig 1 shows the projected image of ahigh multiplicity 36Ar + 1(nAg collision at32A MeV. By various kinds of ionization +range measurements31 we are able to determinethe charge (Z) of all fragments which leadsto the conclusion that the event in Fig 1 isof multifragmentation type. This kind ofmeasurements are right now in progress withthe help of a recently developed CCD readingsystem for the 36Ar collisions while it isfinished with more conventional microscopictechnique for 1 60 collisions.

Fig 2 shows the complete Z-distributionfor high multiplicity collisions at energiesfrom 25A to 200A MeV. The transition fromfusion-like collisions to multifragmenta-tion-like, finally proceeding towardscomplete vaporization, is obvious. In orderto understand the break-up properties we haveconsidered several kinds of correlationsbetween parameters which are supposed to giveindications about a possible criticalbehaviour. One example of such correlations,namely Y2-n, is shown in Fig 3 <Y2 is theconditional second Z moment*' and n =M/<V"» + Ztarg.t))- The signal for acritical behaviour (the maximum in the curve)is hardly observed in the data which•represents high multiplicity collisionsbetween 15A - 210A MeV.

The type of questions which have beenraised by our results is if this kind ofcomplete 4n data, though it has low statis-tics, can select among models for heavy ionreactions.

Utilizing semi-automatic CCD measurementsystems we hope to increase the statistics bya factor 5-10 and thereby to answer thesequestions better, in the Ar *• Ag material.

Fig 1. A photograph of an 36Ar + 107Agcollision at 32A MeV. The charges areidentified as 5*1,3*2,2*3,5,6,2*8,10,13.

10-'

10"

10'2

TO'3

HIGH MULTIPLICITYJ = 0.1 <TR )

1 S.10 20 30 (0 » 50 » 60

I Z *ZP

1 t=utiftua I |

ZT ZP .T

Fig 2. Charge distribution in high multi-plicity (10£) 0+Ag collisions.

147

*) X Campi, Phys Lett 8208(1988)351.

Publications;

1. B Jakobsson, L Karlsson, V Kopljar,B Noren, K Soderstrom, E Monnand,H Nifenecker, F Schussler.Physica Scripta 38(1988)132.

2. L Karlsson et al.,Proc. of the 3:rd Int Conf onNucleus-Nucleus Collisions, St Malo,1988.

3. B Jakobsson, G Jonsson, L Karlsson,V Kopljar, K Soderstrom, E Monnand, FSchussler, H Nifenecker, G Fai, KSneppen, J P Bonderf, X Campi.Preprint from Univ of Lund, LUIP8812(1988), submitted to Nuclear Physics.

4. B Jakobsson, L Karlsson, B Noren andA Oskarsson.Proc. of the Workshop on Intermediate andHigh Energy Heavy Ion Reactions, Krakow,1987.

2.1

2.0

1.9

1.8

1.7

1.6

1.5

1.4

1.3 -

1.2

1.1

1

16_ 107 .8 0 + 47A9

0 0.1 0.2 0.3 0.4 05 0.6 0.7 0.8 0.9 t o n

• 5% highest multiplicitiesx 10%o 10-40%

Fig 3. The conditional moment versus themultiplicity in high multiplicity (102) andmedium high multiplicity (10-402 highest M)collisions. The curve represents a 3-dperculation calculation.

148

- B 4 -PIONS AND PHOTONS

LINEAR MOMENTUM TRANSFER IN PION ACCOMPANIED 16 0 INDUCED REACTION ON 2232Th AT 95 MeV PER

NUCLEON

B. Erazmus, C. Guet, R. McGrath, M.G. St Laurent, Y. Schutz and J.L. Ciffre-Ganil, F-Caen

M. Bolore, Y. Cassagnou, H. Dabrowski, M. Hisleur, J. Julien and R. Legrain, CEN-Saclay,DPhN, F-91191 Gif sur Yvette

C. Le Brun, J.F. Lecolley and M. Lowel, LPC, ISMRa, F-Caen

Positive pions and protons emitted at 90° were measured in coincidence with two fissionfragments in the reaction 16 0+ 232Th at 95 MeV/nucleon. The linear momentum transferdistribution has been determined from the folding angle between the fragments. The averagemomentum transfer is 2.5 Gev/c + 0.2 Gev/c for a pionic reaction. An analysis in terms ofimpact parameter distribution leads to the conclusion that subthreshold pion productionrequires a large overlap of projectile and target density profiles.

A detailed analysis of impact parameter distributions as predicted by different theoreticalpion production models has been carried out by B. Erazmus in her thesis !) . Although mostmodels do indeed favour central collisions, the best agreement with the empirical impactparameter distribution is given by the statistical model 2' where it is assumed that the pionyield is governed by pure phase space considerations. The results of this experiment(described in detail in ref 3') were soon confirmed by other methods ref.4'.

lj B. Erazmus, thesis, Universite de Caen, Ganil T 88 02 (1988)2) J. Knoll and R. Shyam, Nucl. Phys. A483, 711 (1988) and refs. therein31 B. Erazmus and al, Nude. Phys. A481, 821 (1988)4> S. Aiello et al, Europhysics Lett.ji, 25 (1988)

149

EXPERIMENTAL SEARCH FOR SUBTHRESHOLD COHERENT PION PRODUCTIONB.Erazmus, C.Guet, A.Gilibert, W.Mittig, Y.Schutz, A.C.ViHari, GANIL, BP 5027,F-14021 CAEN CEDEX

W.KUhn, S.Riess.Universitat Giessen, Glessen, W-GerraanyH.Nifenecker, J.A.Pinston, CEN-G.DRF/SPh, F-38041 Grenoble Cedex

E.Grosse, R. Holzman, GSI Darmstadt, W-GermanyJ.P. Vivien, CRN, F-Strasbourg

M. Soyeur CEN-Saclay, IRF-DPhG-SPhT- F 91191 Gif sur Yvette Cedex

It was been suggested that subthreshold pion production in peripheral heavy ion collisions couldoccur because of a coherent excitation of spin-isospin modes through the delta-hole channel ineither projectile of target . Theoretical predictions for the production cross-section and pionspectral shapes have been worked out

i'.'e have performed a first experiment to search for such a process. The reaction we looked for is :

12C t 12C - 12C (I*,1; E X = 15.1 MeV)+n°+ X (1)

at an incident energy of 95 MeV per nucleon* In a peripheral collision the projectile wouldundergo a Gamov-Teller transition (AS = AT = 1) to the well-known 15.1 MeV excited state whichdoninantly decays radiatively to the ground state (Ml transition). A part ( M 5 %) of the relativenotion energy is t-ansferred to the target to produce a neutral pion. A full identification >f theprocess may be achieved by a coincidence measurement of the outgoing ejectile ( C) the 15.1 MeVphoton and the two high energy photons from the TT decay. In this preliminary experiment werestricted ourselves (for obvious reasons) to the following three-fold coincidence measurement :

- The slowed down ejectile was measured around 6 - 0° by the SPEC spectrometer. Its kinetic energywas selected to be between ~ -140 MeV and E -270 MeV (E. is the tocal beam energy).

b o b- The 15.1 MeV photon was detected by a set c-i 5aFp detectors covering a solid angle of 1.6 sr.

- Only one high energy photon (E > 30 MeV) was detected by the same set of BaF detectors.

During a data taking time of about 20 hours, 9 events fullfilling the necessary conditions havebeen observed. Only an upper limit of the total cross-section is inferred by these data. Weestimate that for this specific reaction (1) the total cross-section would be lower than 20 nbarn.A comparison to theoretical predictions is of course premature. However we firmly believe that anuch iroroved and unambigous experiment is feasible by using a larger solid angle BaF0

spectrometer (available in the near future) and getting more beam time.

G.E. Brown and P. Deutchman, Workshop on high resolution heavy ion physics at 100 MeV/N,Sai-lay 1937.

C. Guet, K. Soyeur, J. Bowlin and G.E. Brown. Nucl.Phys. (in print).

150

PION AND PROTON EMISSION IN NUCLEAR COLLISIONS

INDUCED BY 16O IONS OF 94 MeV/U

A. Badala, R. Barbera, S. Aiello, A. Palmeri, G.S. Pappalardo and A. SchillaciG.Bizard, R.Bougault, D. Durand, A. Genoux-Lubain, J.L.Laville, F. Lefebvres,and J.P.Patry.G.M. Jin, E.Rosato

I - INTRODUCTION

I.N.F.N, CATANE

L P C , CAENGANIL, CAEN

The main interest of studying pion production in nuclear collisions stands in the fact that the pion isemitted by the interacting zone. So, it can provide a mean to test the role of collective effects in nuclear collisions.In such a purpose the first difficulty which is encountered is of course to disentangle the proper dynamics of pioncreation among the various final state channels. A way to do that is to explore the final phase spaceconfigurations associated with pion and proton emission when that proton carries out a kinetic energy Ep whichis at least equal to the pion rest mass. In order to establish such a comparison, we studied light charged fragmentsemission triggered either by a pion or by a high energy proton ("HEP", Ep > 140 MeV) in collisions induced by94 MeV/u 16O on Al, Ni and Au targets.

II - EXPERIMENTAL CONSIDERATIONS

Both charged pions and protons were detected using a range telescope in the energy domain between 25and 75 MeV for pions ; and between 40 MeV and 160 MeV for protons. Charged fragments were observedwith two large area multidetectors able to identify elements from Z = 1 to Z = 8 :the first one could work atforward angles between 1°5 and 30° over the whole azimuthal acceptance ; the second one at large anglesbetween 30° and 150° over an azimuthal range of 180° .Fragments velocities were measured by a time-of-flighttechnique.This fragment emission was observed in the multidetectors both inclusively and triggered either by a pion or by a"HEP" detected at three angles : 9 = 70° , 90° and 120° for every target. _

III - GENERAL OVERVIEW OF THE DATA

Charged fragments multiplicities are shownin Fig.l for inclusive ("INC") events, pion ("JI")or proton ("HEP") triggered events, for reactionsinduced on the Al target. "71" multiplicities arerather similar to the "HEP" multiplicities but bothdiffer nettely from the inclusive ("INC") ones. Theincrease of multiplicity when the fragments arecorrelated with a pion or a "HEP" accounts for amore central process in the latter case. Such a trendis confirmed by the fragment velocities. Fig. 2shows velocity spectra for Z > 2 elements(essentially Z = 6, 7, 8) emitted in collisions atforward angles close to the grazing one (6c = 3°)for the 1 6 O+ 1 9 7 Au system : the inclusivecontribution peaks at a value close to the beamvelocity, but the pion-coincident one does notexhibit such a quasi-elastic contribution. Thesefeatures are observable whatever the target ortrigger angle. Especially, we do not observesignificantly any coincident event between a pion ora "HEP" and a projectile-like fragment (Z = 4 to 8)at angles close to the grazing ones corresponding tothe three studied systems (16O+A1, Ni, AU).Such differences are also visible in Fig. 3presenting velocity spectra of Z = 2 particles foreach triggering case : inclusive (3a), "HEP" (3b)and "K" (3c). Two components are clearly exbited :one is centered about the beam velocity and theother one at a lower velocity, corresponding to apossible emission by a slower source. Dashed linesin Fig. 3 represent the two gaussian best fit to thevelocity spectra, the continuous curves being thesum of these two contributions : in the "7C" and"HEP" case (3b and 3c) , the hot sourcecontribution is more pronounced than in the "INC"case (3a) which essentially arises from projectilefragmentation.

'"WC" ,-i

- , r;HEP'!V -T "1

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Figure 2

151

Such a two-component behaviour remains valid for a panicles emitted between 3° and 30° and it evolvescontinuously from the dominance of the high velocity regime at small angles towards its complete cancellation atlarger angles. Fig.4 shows azimuthal distributions of "HEP" triggered a-particles emitted at 4° (4a) and 21°.5(4b) for "HEP"s detected at 70° (Fig.4a)and 90° (Fig.4b) respectively. In the former example, this azimuthaldistribution has been separated according to the velocity partition of Fig. 3b : closed squares correspond to thehigh velocity (H.V.) component; open squares to the low velocity (L.V.) one and diamonds indicate the totalcontribution.

Fig.4b shows (at a large polar angle) a strong asymmetric behaviour and a possible explanation of itcould be the momentum balance effect of a given subsystem which recoils in the opposite direction of thedetected "HEP" carrying out a large transverse momentum (more than 550 MeV/c). All these facts suggest thatboth proton and ot-particle are emitted by a hot intermediate zone. The histogram in Fig.4b results from acalculation assuming a two-step process:1) formation of a hot zone, the size of which is governed by the geometrical concepts of the fireball model;2) sequential decay which is described in the frame of the Weisskopf theory.We can see on Fig.4b that this model - which works without any fitting parameter - reproduces the azimuthaldistribution of the a-particles in shape and absolute magnitude at ?1°5.

3000

2000

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AZIMUTHAL ANOIG (degrees)

180 270 360

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Figure 4

Figure 3

IV - DISCUSSION AND CONCLUSIONS

This calculation gives a good description of the azimuthal distributions of "HEP" triggered a-particles for9a angles larger than ~ 10°.

From these overall data, we can derive that pion emission, like high energy proton emission open the samefinal channels characterized by violent, partially damped collisions and is governed by simple phase-spaceconstraints. The absence of quasi-elastic contribution in pion-coincident events seems to rule out any peripheralcomponent in substhreshold pion production.

REFERENCES

1) Light fragment emission in heavy-ion reactions producing pions and protons in 16O+27A1 collisions at94 MeV/u. Europhysics Letters fi. (1) (1988) p. 25 and XXV Int. Meeting on Nuclear Physics - Bormio(1987)2) Semi exclusive pion production by heavy-ions at energy below 100 MeV/u .Nuclear Physics A482 (1988)p.511c-524c .3) Subthreshold p;.on production in heavy ion collisions : peripheral or central interaction ?, Ill Int. Conf. onnucleus-nucleus collisions. SAINT-MALO (France).4) These: A. SCHFLLACI, Dipartimento di Fisica dellTJniversita di CATANIA (Italic).

152

LOW ENERGY CHARGED PION PRODUCTION IN1 6 O+ X REACTIONS AT 93 MeVAi.

D. Lebrun ' , G. Perrin l , P. de Saintignon ' , C. Le Brun 2 ,

J.E Lecolley 2 , J. Julien 3 , R. Legrain 3 .

1) ISN Grenoble; 2) LPC Caen; 3) DPhN Saclay; France.

J . Introduction.In heavy ion reactions, very low momentum charged

pions can be a tool to study the geometry of the collision.Those pions suffer a strong electromagnetic interactionwhich is very sensitive to the shape of nuclear chargedistribution after the collision took place. In the frame of apositive distorting charge, Coulomb interaction affectsmainly very low momentum panicles, via a well knowndistortion phase space factor ' . Effect results in anenhancement (depletion) of ir( rc+) peaked at zero relativemomentum. These distortions experimentally manifest assingularities in the pion distributions for p-values matchingthose of nuclear products. Such charged pion distributionswere measured here at low transverse momenta (zerodegrees), in reactions induced by a 93 MeV/u oxygen beamon various targets: Li, C, Al, Ni, Ag, Au and Th.

2. Experiment.

To detect low energy subthreshold pions, the requiredmain experimental features are: large solid angle (expectedlow cross sections ), high level particle rejection, efficient andredundant particle identificationfor e, n, p...,compact system(to minimize inflight pion decay losses),variable momentumacceptance with adaptation to very low energy pions, fewpercent momentum resolution, ability to zero degreesdetection, symmetric and simultaneous detection for n+ andTI-, and operation under vacuum due to low energy pions andheavy ion beam. To fullfill these conditions a magneticspectrometer (SPIC, Spectrometre i Pions Charge's) 2 wasdesigned( fig. 1). It consists of a small circular dipole magnet(0 = 46 cm), in the center of which is located the target. Eachof the two symmetric detectors is made of two consecutive x-y multiwire proportionnal chambers allowing trajectoryreconstruction.backed with three plastic scintillators.Detectors have no direct view of the target, eliminating rigidforward panicles.but still have an appreciable solid angle forlow rigidity particles. For one field B= 10 kG, themomentum acceptance was 45-120 MeV/c (T= 7-45 MeV)for particles emitted at 8= 0°il5°. Pions were identified viamomentum, energy loss and time-of-flight combination usualtechniques.

Fig. 1 Schematic view of the pion spectrometer.

3. n / TE+ distributions.

As an observable of nuclear collisions, the ratios ofnegative to positive pions have the advantage of beeingindependent of experimental normalisations. Since, here, thedetection was symmetric and simultaneous for both chargestates, the momentum dependence of these ratios could beeasily extracted. The momentum resolution permitted to

Fig. 2 Distribution of ratios it ~ / it * in three systems.

153

clearly observe strong effects giving rise to very large valuesfor these ratios. These values vary in different regions of p-space and depends on the colliding system. These effects areseen only for the lowest momenta; they disapear for highermomenta corresponding in the present study to pion withkinetic energy above IS MeV. Focussed in this low energydomain, one observes a peak in the distribution of ratios inlight mass systems.(fig.2) This peak is located at momentumcorresponding to pions having the beam velocity. Similareffects were already observed in high energy nuclearcollisions 3. This was explained by Coulomb deflection fromcharged projectile fragments in the fragmentation region. But,as the mass of the system increases, this peak, while stillpresent, is progressively shadowed by another phenomenon.The rise in the spectra is due to the increasing influence of, atleast, one another nuclear charged fragment in the mid-rapidity region. A simple interpretation in term of Coulombinteraction allows to extract in each colliding system, therespective influence of these two contributions. Thus, chargeand velocity of the distorting fragments in the pionic collisioncan be determined. In the mean time, the mid-rapidity chargepresents an isotropic influence on ratios distributions, whilein the fragmentation region the influence of projectile chargedremnants stays peaked near zero degree 2.This indicatesdifferent behaviours of these charges located in differentregions.

4. Pion yield.

In fact, when looked as a function of the size or chargeof the mid-rapidity distorting fragment, the total charged pionyield exhibits a clean linear dependence (fig.3). This givesevidence that the mid-rapidity distorting charge behaves likethe source of pions. Then the origin of these "subthreshold"

pions is located in intermediate system whose charge andvelocity govern the observed cross-sections. Among emittedpions, those travelling near beam velocity are furtherdeflected by projectile remnants in the forward direction. Atlast, in the mid-rapidity moving source frame, pions presentBoltzmann-like radiation spectra, with an apparenttemperature around 8 MeV. This is well below the T valueextracted from the slopes of neutral pions data in similarsystems4. In our experiment these spectra present also a kindof saturation in T for system heavier than 27 Al. Here, thisresult seems to be in agreement with conclusions drawn fromvery different experiment (like particle correlations) in similarheavy ion reaction. In order to complete such studies, twoother beam periods in 1988 were used studying the samecolliding systems: first to look in the transverse momentumregion, secund at zero degree but with 65 MeV per nucleonincident energy. Data reduction and analysis are stil! inprogress.

1) M. Gyulassy and S.K. Kauffmann, Nucl. Phys.(1981) 503.2) D. Lebrun et al.. , Contribution to the third Int. Conf.,Saint Malo, 1988, p. 158; Contribution to the Int. WinterMeeting on Nucl. Phys., Bormio.1989; and submitted forpublication.

3) W. Benenson et al., Phys. Rev. Lett. 43 (1979) 683 and44 (1980) 54.

4) P. Braun-Munzinger and J. Stachel, Ann. Rev. Nucl.Part. Sci.37_(1987)97.

3 0

2 0

10

y1 0 20

Fig. 3 Pion cross-sections versus mid-rapidity distorting charge Zc

154

" PION CONDENSATES " IN EXCITED STATES OF FINITE NUCLEI

R. Blumel and K. Dietrich*

Physikdepartment of the TUM, Garching, FRG

* Guest at the GANIL, August 1984 - March 1985

We investigated the followingquestion : Given a symmetric nucleus(neutron nr N=proton nr Z) far fromshell closure, is it possible thatthere exist low-lying excited statesof such a nucleus with the followingspecific spin-isospin order : theangular momenta of the neutrpns arepointing upward and the ones of theprotons are pointing downward (or v.v.) in the topmost unfilled shells ?

This question was dealt with bymany authors *.see r ef. i to 3 forreferences) for nuclear matter andvery simplified models of a finitenucleus. The answer depends sensiti-vely on the approximations and on thenuclear interactions used. Therefore,we performed a seX.&C.onA4*tzYi£ Hartree-Fock calculation based on Gogny'sinteraction complemented either by acoupling of the nucleons to a pionfield'>2) or by an additional tensorinteraction3). We studied only nucleiin the 2s-ld shell.

The jst theory suffers from thedrawback that the explicit coupling tothe pion field introduces not only a(direct) tensor interaction but alsoan undesired additional interaction ofthe <J.$ •?.•? character which is alreadytaken care of in the Gogny interaction.In the 2nd mentioned model3), thisdouble counting is avoided and, fur-thermore, the exchange contributionsof the tensor force are not neglected.

The following results areobtained :1) In the first-mentioned model1'2),, afinite classical pion field (" pioncondensate ") was never obtained asground state of a nucleus.2) It was, however, found for anoblate excited HF-state of the nu-cleus jfiSig, which mainly containedthe isospins T=l and T=2.3) It was crucial for this result thatthe calculation was AeZjJ-coni-cA eitC(different sign of the quadrupole mo-ment in the excited state and theground state) and that the classicalpion field contained oaty odd compo-mwtb o& tk&- onbiAaJL angu&Vt momen-tum £, essentially only 4=1. Thisimplies that the appearance of thefinite classical pion field is notconnected with parity breaking nucle-onic orbitals !,4) In the 2 n d mentioned model3) weobtained qualitatively the same re-sults, but the tendency to fora anucleonic spin-isospin order was less

pronounced and only occurred for fgSjgwith a lower limit of the excitationenergy of 5.8 MeV.

We note that the lowest Ilr=0+, T=land 1^=0*, T=2 states of ]gS]6 are mea-sured at 7,536 MeV and 12,05 MeV, resp.Furthermore, very low-lying analog sta-tes of 1^=0*, T=l are obtained in 32C&(Ex=0.537 MeV) and 32P (E*=0.513 MeV)and no such low-lying ones in the wholes-d shell. One may take this cautiouslyas an indication that the specific spin-isospin correlations we investigatedmay play a role in certain excited sta-tes of mid-shell s-d nuclei.

The only other favourable case (N=Zand far from shell closure) would be forN=Z isotopes of Cr and Ti. Unfortunately,very little is known experimentally onthe spectra of these (unstable) nucle^L.

1. R. Blumel, K. Dietrich, GANIL P.84-182. R. Blumel, K. Dietrich, Nucl. Phys.

A454 (1986) 6913. R. Blumel, K. Dietrich, Nucl. Phys.

A471 (1987) 453

155

143

Hard Photons from Heavy Ion Collisions at 44 MeV/a

W. Kfihn, Umversitat Giefien(CRNS-GANIL-GieCen-GSI-Krakov Collaboration; E58/E5Cb)

The production of hard photons in heavy ion reactionshas so far mainly been studied in inclusive experiments(for recent reviews, see [1,2]). The photon spectra ex-hibit three components: (i) an exponentially fallingsoft photon component arising from statistical -y-decayafter particle evaporation, (ii) a hard photon compo-nent (slope parameter Ea) , (iii) a lorentzian-shapedexcess yield in the Giant Dipole resonance region.Both statistical emission from hot fragments as wellas nucleon-nucleon bremsstrahlunr have been identi-fied as possible sources of such - -rd photons. Therelative contribution of these two processes dependson the velocity in the nucleon-nucleon center-of-masssystem and on the temperature of the fragments pro-duced in the reaction. Above an incident energy of 30MeV/a, nucleon-nucleon bremsstrahlung is the domi-nant process. Tlus mechanism b of particular interestin that it constitutes a direct, undistorted probe of thephase space distribution in the collision rone [4].At higher bombarding energies, the coexistence of dif-ferent reaction mechanisms leads to severe ambigui-ties in the interpretation of such inclusive data. Toimprove this situation, we have performed two ex-periments where photon emission is studied in coin-cidence with particle detectors. In the first experi-ment, we have used a forward-angle charged particlehodoscope which allowed modest impact parameter se-lection. The second experiment was designed to focuson peripheral collisions with complete identificationof the projectile-like fragment detected in the mag-netic spectrometer SPEG. Both experiments investi-gated the reaction toAr +"* Gd at 44 MeV/a.

BaFj D Absorber

GANJL

Beam

^

\/BaF2

X '

5<r

Target

A

" I f

- 1

JillKPPAC 1 J Plastic-/ scinttlators

PPAC 2

FC

Figure 1: Experimental Setup (ESS) [3], Hard photons aredetected with 7 BaFj detectors. Rough event characteriza-tion is performed by requiring different trigger conditionsin the forward hodoscope.

The setup of the first experiment [3] is shown in fig 1.

Photons were detected with 7 BaFj detectors groupedin 2 blocks positioned at scattering angles of 90° and145° , respectively. Event characterization was madeby requiring a beam velocity heavy ion (peripheral )or a heavy slow fission-like fragment (central ) in theforward hodoscope. Photon identification was madeby time-of-flight with respect to the pulsed beam aswell as exploiting the pulse shape discrimination prop-erties of the BaFj detectors.

10 20 30 <0 50 EO 70

Ej (MeV)Figure 2: Photon spectra with different trigger conditions

Fig. 2 shows the inclusive photon spectrum (a) and thespectra obtained with the above conditions (b,c, re-spectively). Within the tough event characterization,no strong dependence of the slope parameter Ea isobserved. In contrast, the hard photon multiplicity(By > 30MeV) increases by a factor of 2 when goingfrom peripheral to central collisions.

The setup of the second experiment is shown in fig.3. Photons were detected by means of 64 Barium-fluoride (BaFj) detectors grouped in 4 blocks, po-sitioned at laboratory angles between i? = 59° andISO" at 35 cm distance from the target/The projectile-like fragments (PLF) were detected with the magneticspectrometer SPEG. This setup allows to study thehard photon component as a function of the PLF pa-

156

SPEC - Magnet

GANIL Bum

*B*F> -detector*

focal plane detector

Figure 3: Experimental Setup (ES8B). 64 BaFj-detectonare employed to detect hud photon* in coincidence withthe PLF's in SPEC

ramtters such as mass and total kinetic energy loss(TKEL).

BREMSSTRAHLUNG

400 «90TKEL (MEV)

Figure 4: Slope Parameter! for the hard photon componentas a function of the PLF total kinetic energy loss

Fig.4 shows the slope parameter Ea as a function ofthe total kinetic energy loss (TKEL) of the PLF'sdetected in SPEG. No strong dependence is observedindicating that the production mechanism is ratherinsensitive on impact parameter. In contrast, the ob-served multiplicity for T-ray energies above 25 MeVincreases for smaller PLF masses (fig.5). Smaller de-tected fragment masses are associated with larger en-ergy losses and smaller impact parameters. The ob-served increase of the hard photon multiplicity couldbe interpreted as a geometry effect, yielding a largernumber of p-n collisions for smaller impact parame-ters.

The analysis of this experiment if still in progress.From the theoretical point of view, existing VUU-codes are currently extended [5] to predict the frag-ment parameters, thus allowing a direct comparisonto the experiment.

0.0030

0.0029

5 0.0010

0.000016 iO X 30

AVERAGE FRAGMENT MASS

Figure 5: Hard Photon Multiplicity mt • function of thePLF mat*

References[1] V. Metag Nucl-Phys. A488(1988)483c

[2] H. Nifenecker and J.A. Pinston, ISN Re-port 89.09, 1989

[3] R. Hingmann et al., Phys.Rev.Lett.58(759)1987

[4] 1C. Niita et al., Nucl. Phys. A4S2(1988)525c

[5] W. Gassing, U. Mosel, private communi-cation

157

IMPACT PARAMETER DEPENDENCE OF HIGH ENERGY GAMMA-RAY PRODUCTIONIN ARGON INDUCED REACTION AT 85 fvleV/A

M. Kwato Njock, M. Maurel, E. Monnand, H. Nifenecker, P. Perrin, J.A. Pinston, F. SchusslerISN and CEN.Grenoble

andY. Schutz. GANIL_Caen

The production of gamma rays with energiesbetween 30 and 150 MeV has been studiedboth inclusively and exclusively using 3 6 Arnuclei produced by the GANIL accelerator at85 MeV/A.- The inclusive production was studied for 12C,27A|, Natcu, NatAg, 159Tb, 19?Au targets inorder to test the first collision scaling law foundpreviously 1 )• In all cases studied the sourcevelocity was found to be close to the half beamvelocity, thereby confirming previous results.The double differential cross section at 90°was found to be satisfactorily described by theexpression:

P T 5

-vwhere EQ is the inverse slope of theexponential spectrum. Zp,T>, A ^ , Np,T> are

the charge, mass and neutron numbers of thesmallest (largest) nucleus. The massdependence can be deduced rigorously fromthe equal participant model after averaging

over impact parameter 2 ) . Therefore thevalidity of the first collision model implies Py to

be constant. Fig.1 shows that this is realizedwithin 10%, with no systematic trend for theobserved deviations.

- The exclusive experiment consisted inrecording the gamma spectra in coincidencewith a charged particle detector array (CPDA)of 24 detectors covering the angles between 5°and 13°. To optimize the efficiency of theCPDA we have chosen to study the system36Ar+27AI. It is found that the highest particlemultiplicities correspond to the highest gammaintensities. Assuming the validity of the firstcollision model, and after normalization to theinclusive measurement, it was possible toassign an average number of n-p collisions,and thus an average impact parameter, b, toeach of the gamma spectra conditionned by agiven charged particle multiplicity. A variation

of the inverse slope, En, of the spectra was

correlated with the impact parameter as seenon Fig.2 where EQ is plotted as a function of an

overlap distance R-|+R2-b. Also shown onFig.2 are the results of the inclusive experimentwhere the average impact parameter wascomputed from a geometrical model of thereaction . The shape of the En(b) correlation is

interpreted as due to the spatial dependanceof the Fermi momentum distribution of thenucleus.

inO*

0 . "

10

8

6

U

2

0

L *' * ' HL * f ' J•-

- Ar+X 65 M.V/A -

- |

-

0 50 100 150 200TARGET MASS

Fig. 1 - Probability of photon emission in a singlen-p collision versus the target mass.

35

> 3041

iS 2 5

20

-

; ( ) | l1 \ o EXCLUSIVE

• INCLUSIVE

1 I l l i l In n l i i i i l i i i i l i i i i l n i i l i

_

-

-

-

0 1 2 3 4 5 6Rl+R2-b

Fig. 2 - Inverse slope parameter versus theoverlap distance.

158

The results of this experiment are reported inref. 3,4).

1) R. Bertholet et al., Nucl. Phys. A474 (1987)5412) H. Nifenecker and J. P. Bondorf, Nucl.Phys. A442 (1985) 4783) M Kwato Njock et al., Nucl. Phys. A488(1988) 503C4) M. Kwato Njock et al., Nucl. Phys. A489(1988) 368

159

HIGH ENERGY GAMMA-RAY PRODUCTION FROM 44 MaV/A 86|trBOMBARDMENT ON NUCLEI

R. Bertholet, M. Kwato Njock, M. Maurel, E. Monnand, H. Nifenecker, P. Perrin, J. A. Pinston,F.Schussler, CEN-GrenobleD. Barneoud, ISN-Grenoble

C. Guet and Y. Schutz, GANIL-Caen

We report the results from an inclusivemeasurement of gamma rays with energies E~

>20 MeV, produced in 86Kr+12c ,Ag and 19?Auat 44 MeV/A- In the experiments two identicaldetectors were used; one was placed at the fixedangle 6=90° and the other was used to explore

scattering angles between 40° and 160°. Eachdetector consisted of one active BaF2 converter

(60X40X10 mm3) and 3 plastic scintillators (2mmNE102) used to identify the electrons andpositrons of the shower and a large volumeNal(TI) scintillator (15 cm diameter and 20 cmlength). The detection system was completed bya veto plastic scintillator to eliminate the chargedparticles entering the detector and a time-of flightmeasurement between the BaF2 detector and theradio frequency to eliminate the neutrons.Fig. 1 shows the y-energy spectra observed atthree different laboratory angles for the Kr+Aureaction. It is apparent that the spectra have anexponential shape and their relative hardnessincreases as the angle decreases. Thisbehaviour is characteristic of a photon emissionfrom a moving source. The best fit parametersdisplayed in Table 1 were determined from aleast squares fit to the data. It was assumed thatthe differential cross section in the -/-source restframe writes:

and then the same quantity in the Lab. writes:

with X=(1-p COS9Lab) (1-p2)'1/2

In these equations the angular dependence is anisotropic term, plus a dipole term. Since thesource velocity, p, the inverse slope parameter,E0, and the relative amplitude of the E1 , a, do not

show any significant changes for the 3 targets,the mean values of these 3 parameters werededuced. They are reported in table 1 and they

characterize the y-ray emission for Kr inducedreaction at 44 MeV/A incident energy.The results obtained for the Kr induced reactionsat 44 MeV/A are characteristic of those obtainedat beam energies between 20 and 60 MeV/A. Insummary a close inspection of the angulardistribution data shows that:- he velocity of the -/-source for symmetric andasymmetric systems is close to that of thenucleon nucleon c.m.

- in the source c.m. the angular distribution ismainly isotropic. However a small anisotropiccomponent of E1 character has been alsoevidenced. The amplitude, a , of the E1component increases with beam energy andtakes the values cc=0,25 and 40% respectively at30 (ref.1), 44 and 60 MeV/A (ref.2).

10

01

10

0.1

7Au

' 0 0

= 153.5*

3040 SO 60 70

ESL-1Ener9y spectra of hiah ener9y photons for 86Kr +19?Au at

44 MeV/A

1 6 0

- c -DEVELOPMENTS

A number of theoretical studies of incoherentphoton production by nucleon-nucleon collisionshave been reported. The dynamic of the reactionhave been treated within various models, usingthe Boltzmann-Uehling-Uhlenbeck equation 3 ) ,the Boltzmann master equation 4 ) or a transportmodel 5) . In these models a semi classicalapproximation was used for the neutron-protonbremsstrahlung elementary process. These threemodels are in good agreement with the Kr data.The Kr data are reported in ref.6.7).

1) M. Kwato Njock et al.. Phys. Lett. B175 (1986)125

2) E. Grosse et al.. Europhys. Lett. 2 (1986) 93) K. Niita et al., Texas A & M Symp. on HotNuclei (Dec. 1987)4).B.A. Remington and M. Blann, Phys. Rev. C35(1987) 1720

5) J. Randrup, Nucl Phys. A490 (1988) 4186) R. Bertholet, Nucl Phys. A474 (1987) 541

7) R. Bertholet, J. de Phys. 47, C4 (1986) C4-201

Target

CN.,

Ags.," 'Al l

meanvalues

K(nb/MeV-sr)

30.9 ±5.4165±18198±14

e»(MeV)

11.6*0.512.5 ±0.312.1 ±0.212.1 ±0.5

a

0.31 ±0.150.16±0.090.24 ±0.060.24 ±0.06

P

0.170±0.0130.179±0.0100.143 ±0.006O.I64±O.OI5

Table 1

Least squares fit parameters calculated withe eq.{1) of the

text.

30 40 50 60 70 80Er (MeV)

Fig.2

Comparison of experimental differential cross section of

Kr+Ag reaclion with calculations of ref. 5)

161

PROJECTILE gRAGXBITS ISDTOPIC SEPARATIQH :

APPLICATION TO THE LISE SPECTROMETER AT OANIL

J.P. Dufour, R. Del Koral, H. Emmermann, F. Hubert, A. Fleury, D. Jean, C. Poinot, M.S, Pravikoff

CEH Bordeaux, II2P3, Le Haut Vigneau, F~33170 GradignanH. Delagrange, GAHIL, B.P. 5027, F-14021 Caen Cedex

K,-H. Schmidt, GSI, D-6100 Darmstadt

The high intensity beans delivered bythe GAHIL facility at energies around 60JfeV/u allOK the observation of many exoticlight nuclei formed by projectilefragmentation. The spectrometer LISE atGABIL was designed to provide far theseparation of the rare species from theclose to stability isotopes which are ordersof magnitude more produced. An importantstep was to canbine the magnetic rigidityselection with a momentum loss in a solidmaterial located in the intermediate focalplane of the two dipoles (the PFISmethod).The very detailed characteristics ofthe LISE spectrometer and of the PFIS methodcan be found in refs. 1 and 2.

The study of a given nucleus requiresthe tuning of four parameters : the targetthickness, the dégrader thickness, thedipole 1 magnetic rigidity <BpO and thedipole 2 magnetic rigidity (Bpa>.

The selection by the first dipole wouldallow a strict A/Z separation if thevelocity of all the produced fragments wasconstant and if the target was thin. Even atintermediate energies (40-100 KeV/u), thereaction mechanism preserves the beamvelocity rather well in average but asignificant distribution width is observedwhich causes a band A/Z ± MA/Z) of nucleito be simultaneously transmitted as shown infig. 1. In addition the target also inducesa spread since the fragments formed in thefirst or the last matter layers do not exitthe target with identical velocities. Thetarget thickness needs to be optimized tofill the ûp/p acceptance of thespectrometer. The calculation of the optimum

target must be made for each tuningaccording to the A, Z of the fragment andthe opening of the slits defining themomentum acceptance.

The second selection is independent ofthe reaction mechanism. All the ionsentering the dégrader are slowed dawndifferently according to their A, Z and alsotheir velocity. The Bp2 tuning can beanalytically expressed as a function of Bpi,A, Z and d the dégrader's thickness. Theions selected at the same Bp* are thosehaving the same ranges H, (A, Z, Bp,> at theexit of the first dipole. This pointillustrates for example the fact that themethod can separate the A and A + 1 isotopeshaving the same Z and dE/dx but differentranges. The ions not discriminated by thesecond selection are thus those having thesame Z1 »/A 2 5 ratios as shown in fig. 1.

In order the preserve the achromatism ofthe line the dégrader must have a shapegiven by a formula independent of Bpi, Bp2,A and Z. This property is very useful sinceit allows the use of a unique dégrader forseveral tunings on different ions. The onlyreasons to change the dégrader are the massand charge resolutions of the secondselection. In practice this just impliedthat in one experiment with a -*°Ar beam at60 HeV/n, a set of three aluminium degraders(600/J. 900u, 1200>> allowed to select avariety of ions ranging from IEC to *°S.The achieved resolution is shown in fig. 1by a curved band crossing the A/Z ± A(A/Z)band. The width of this second selectionband can be estimated along a constant-Zcut, yielding a (A/ûA)d resolution. It mustbe noted that this momentum-loss resolutionis very different from the usual momentumresolution of a magnet since it bears onions all having the same initial magneticrigidity Bpi. For dégrader thicknesses goingto zero, the (A/AA)d also goes to zero andthe width of the selection 2 in fig. 1increases to infinity. In the case of LISEtuned on S7P, the measured <A/ûA>a resolu-tion reached 100 and was obtained from theratio of the 3 7P image diameter to themeasured interval between the 37P and 3 ePimages in the exit focnl plane.

Pig. 1 : The selection operated by thedégrader is shown here by a band in the H,2plane. The Jightly hatched area represents atypical selected domain which often includesonly one isotope, while in less favorablecases two or three are included.

1 - J.P. Dufour et al., Hucl. Inst. Heth.A248 (1986) 2672 - R. Anne et al., Bucl. Inst. Keth. A257(1987) 215

162

PRODUCTION OF RADIOACTIVE SECONDARY BEAMS

R.BIKBOT, F.CUAPIER, S.DELLA-NEGRA, B.KUBICA,Institut de Physique Nucleaire, F-91406 Orsay

R.ANNE, H.DELAGRANBE, Y.SCHUTZ, G.A.M. I.L., CaenP.AGUER, (3.BASTIN, C.S.N.S.M. Orsay

B.ARDIGSON, A.HACHEM, University de NiceJ.CHAPMAN, J.LISLE, University of Manchester <BB)

Y.GDNO, K.HATANAKA, RIKEN, JapanF.HUBERT, C.E.N. Bordeaux-Bradignan

1. Introduction

Among the recent developments

concerning Nuclear Physics, one o-f the

mast promising seems to be the produc-

tion of radioactive heavy ion beams. An

experimental program centered on this

objective was undertaken at GANIL in

19B4. Secondary beams have been

produced by projectile fragmentation or40transfer reactions using incident Ar

1 R

and 0 ions and light production

targets.

2. Experimental technique

The experimental method implies

the use of the magnetic spectrometer

LISE for selecting the isotopes to be

transmitted.

This spectrometer is composed of

two dipoles, ten quadrupoles and one

sextupole . The radioactive

nuclei emitted around zero degree are

separated from the primary beam, trans-

ported over IB meters, and focused at

the achromatic focal point F.Multiwire

chambers, are used to visualize the

beam profiles in several points. They

are efficient in a broad range of3 8intensities (10 to JO particle per

second). A solid state detector

telescope is used to analyse the

isotopic composition of the secondary

beam.

By interaction of the primary

beam with the production target, a

variety of secondary beams are

produced. Among them, a given beam is

selected by proper adjustment of the

spectrometer magnetic rigidity Bp.

Various characteristics (beam profile,

isotopic composition, intensity) of the

secondary beam have been measured.

The beam profiles in F show a

relatively good focusing, the beam spot

being generally smaller than a 15 mm

diameter circle.

The isotopic compositions of

secondary beams are determined from

AE-E (or AE-time of flight) bidimensio-

nal plots. The isotopes are clearly

separated, as shown in Fig. 1. By

varying the magnetic field in the spec-

trometer, a modification of the isoto-

pic composition is observed. Thus, the

spectrometer can be tuned up to

achieve maximum production of a given

beam.

The influence of target nature,

target thickness and incident energy on

the production rates have been studied.

The characteristics of several se-

condary beams are summarized in Table 1.

Production Mode

Aft44 HeV/u Ar

+

99 mg/cm2 Be

45 KeV/u 1BQ

+

187 mg/cm2 Be

65 MeV/u 1B0

*

567 mg/cmZ Be

39Ar

39C1

3 8S

'*C,'7N

16C

'*B

'*c,17N16C

'*B

Secandar

IMeV/u)

35

34

36

39

3B

39

•it

52

*7

r Beam

3. 10-*

10-*

6.10-6

io-s

=5. 1O"?

S.10-*

10-*

2.10-6

2.10-13

Itpps)

ioe

3.I07

2.10*

io7

5.10S

5.103

10B

2.106

2.10*

" AE/E = ± 3 . * X

Table 1

163

Note? that intensities in the range 1O

- 1O pps are obtained for neutron rich

nuclei three-four mass unite away from

stability. These intensities are low,

but nevertheless usable for inducing

nuclear reactions.

600

500

400

C 300•£S 200

w 100

0C

1 1 1 ,3 1 1 1 1 1

o "C

,„ " ^ " ^ - r ^ * - "c 'k ^ - » B e .^ i e i i i i i i i10 20 30 40 50 60 70 80

AE ( MeV)Fig. 1_ : Bidimensional plat showing theisotopic composition of a secondarybeam obtained in the following condi-tions : 65 MeV/u 1 8D + 1O36 mg/cm2 Be.Bp = 2.248 Tm, ABp/Bp ± 0.22 X.

SUJ

500

100

—i—i—

™c"B

- 0

LI"

M C

12B<*c

20 40 20 40 4O 60 40 60AE (MeVI

Fig. 2 : ] so topic composition of thebeams obtained by purification of thebeam analysed in Fig. 1 ABp/Bp = ±2.7X.a. B2 = 1.O77, b. B2 = 1.O69,c. B2 = 1.057, d. B2 = 1.O43.

tic field (B2), one can achieve a

spectacular purification of thf

secondary beam, leading, in the hc-ii

case to the transmission of nn 1 y nni-a

isotope (Fig. 2).

Secondary beams produced by this

techique have already been uspd for

Nuclear Physics experiments (spin

alignment in fragmentation reactions,

measurement of total reaction cross

sections).

References

1. Production of Secondary RadioactiveBeams from 44 MeV/u Ar Projectiles,ff. Bimbot, S. Della-Negra, P.Aguer,G.Bast in, R.Anne, H.Oelagrange andF.Hubert, Z. Phys. ft322 (1985) 443.

2. Heavy Ion Secondary Beams,R-Bimbot, S.Della-Negra, M.Manasijevic,P.Aguer, G.Bastin,, R.Anne,H.Delagrange, Y.Schutz, F.Hubert,Y.Bono and K.Hatanaka, Int. Conf. onHeavy Ion Nuclear Collisions in theFermi Energy Domain, Caen <May 1906),J. Phys. C4 (1986) 241.

3. Nuclear Physics with Exotic HeavyIon Beams,R.Bimbot, proc. 6th Adriatic Int.Conf. Nucl. Physics. Frontiers ofHeavy Ion Physics, Dubrovnik (june1987), 161.

4. Radioactive Heavy Ion SecondaryBeams,R.Bimbot, Proc. RIKEN-IN2P3 Symposium,Shimoda foct. 19B7), 48,and Report IPNO-DRE-B7-35 (1987).

5. Production of 4OO MeV/u RadioactiveIon Beams for Therapy Purposes,R.Bimbot, Proc. EULIMA Workshop on thePotential Value of Light Ion BeamTherapy and Report IPNd-DRE-BB-37.

An efficient purification of the

secondary beam can be performed by

using a degrader placed between the two

dipoles. In order to preserve, at first

order, the spectrometer achromatism, a

wedge degrsder must be used. Then the

second dipole acts as a dispersive

elt-ment. By setting properly its magne-

164

A TELESCOPIC-MODE USE OF THE LISE SPECTROMETER FOR THESTUDY OF VERY FORWARD ANGULAR DISTRIBUTIONS

Ch. O. Bacri, P. Roussel, R. Anne; M. Bernas, Y. Blumenfeld, F. Clapier,H. Gauvin, J. H<Srault, J.C. Jacmart, A. Latiraier, F. Lelong,

F. Pougheon, J.L. Sida, C. Stephan, T. SuomijarviInstitut de Physique Nucleate, 91406 Orsay Cedex, France

With the acceleration of heavy ions of increasinglylarge masses and high energies, a dominant part ofthe reaction products are found in a more and morereduced forward angular region [1].

The semi-classical grazing angle 6'jf gives an eval-uation of this angular range whereas the inverse ofthe grazing angular momentum t} gives a rough ideaof the minimum size of any angular structure (oscil-lations..) which may result from an ondulatory be-haviour in the collision. In turn this minimum size isrelated to the required experimental angular accuracy.

ProjectileTargetEnergy(MeV/A)C'tmr)l/CVr)

IBQ

"C25

28,59,5

80

8,75,1

19Mu25

2376,0

80

683,0

"Kr"C

25

181,4

80

5,40,7

i«Mu25

1650,9

80

480,5

Table 1 : Grazing angle 9>r and minimum angularsize l/ta, (tir grazing angular momentum) given inthe laboratory for different H.I. collisions at GANIL.

At GANIL (table I) these figures may become ra-ther small. It leads to specific experimental problems,even with the use of magnetic spectrometers whenobservations are performed both at 0° and close tothe magnetic rigidity of the beam in one of its chargestate [1,2,3].

Usually, the target and the focal-plane detectorare linked by a point to point focusing (shifted focus-ing, sometimes used to accomodate finite emittance orkinematical factors, does not alter this basic mode)and angular measurements require ray-tracing tech-

niques. Angular sorting is only achievable (thoughapproximately) by the use of the magnet entrance slitsor/and a beam catcher (Faraday cup). But they can-not any longer be used when they have to be set atvery small angles and that they are submitted to astrong bombardment of scattered particles from thetarget and hence become prohibitive sources of dou-bly scattered fragments. Angular and energy strag-gling of the beam in the target, ghost beams, andelastic scattering on light target contaminants bringother limitations to very-forward measurements.

Drastic improvements as regards all these limita-tions are expected with a zero-dispersion double spec-trometer used in a telescopic mode (the target and thedetector are linked by a parallel to point focusing) [4].This arrangement (fig. 1) allows (i) double B/? and 0sorting, the latter being performed on events alreadyselected by tuning Bp (no strong double scattering)(ii) direct angular measurement from a position in thedetector plane (iii) all other measurements (Bp, AE,E, TOF...) on doubly-sorted events.

This arrangement has been set up on the spec-trometer LISE [Ij. The following limitations werefound. Firstly, the angular dispersion Rje could notbe given high enough a value for an accurate Bp mea-surement, hence, the information on the energy hadto come either from the B/> sorting itself between thetwo dipoles or from a solid state telescope. Secondly,the angular aperture which was expected around ± 30mr was found from ± 28 mr with a low efficiency downto ± 20 mr full efficiency. Within this limitations fo-

beam

axisn,U 1

sltie dsandslils

n*LSOT

n 12= 'BIS

n .U2

i-1

3ij

shields andslils

> 1ing 1

, . _ n

f 0

i e>p1 sorting

RR

R, =6

X.Y.tPPAC

"1 meas. |

33-0

Gm

\ . T O F Xm«as

^ ^

Ion. chamb.or

solid slatetelescopes

P I 1 AE.E3as. I | meas.

«,,*o

Fig. 1 : scheme of theexperiment. DI and Dt arethe two dipoles (quadrupolesare not drawn)

*Ganil, Caen

165

cal lengths between 1.7 and 10 meters (Ru and RS<from 0.17 to 1.0 cm/mr) could be obtained and the1.7 value most often used (fig. 2).

Fig. S : angular calibration in the detector plane(the Y non-dispersive plane is shown)

In the bombardment of C, Al, Ni and Au targetswith a 44 MeV/A i0Ar beam (50 to 200 nAe), angulardistributions of the produced fragments at and around0° have been measured for magnetic rigidities between0.75 and 1.20 of that of the beam (Bp0).

A particular attention was paid to measurementsclose to the beam. With the gold target one could op-erate at 0° and as close as 2.3 °/oo of Bpa while keepinggood ion identification. Finally, by shelding the verycentral angular region of the beam (~ ± 3 mr), thecounting rate was decreased by more than one orderof magnitude and identification still improved (fig. 3).

DEf

Fig. S : results in the study of the collision *°Ar44 MeV/A on the ig7Au target at Of. The angular dis-tribution is obtained with the indicated time of flightwindow. The two (partial) AJE® TOF plots are ob-tained for the indicated angular windows.

The results are being analysed, a sample of themis given here. Fig. 4 shows angular distributions withNi target, at 0.9681 <Bp/Bp0 < 0.9754. We cansee different behaviour (forward-peaked no-forward-peaked) of 40ylr (dashed lines) and SBCl (dotted lines).

I l l

, 3 -

3z

i i i i i i i i i i i i i i-10 10

|mr)15

Fig. 4 •' a sample of angular distribution. Thecontinuous line is obtained with a window only on thetime of flight while the full identification which re-quires A £ is shopped by the granular structure of thesolid state telescope.A technical study is under process in order to increasethe LISE angular acceptance in this telescopic modewhich we have shown to be very efficient for theseforward measurements.

References[lj Ch. O. Bacri, these de 1'Universite Paris VII,

Orsay, 1989

[2] E. Quiniou, these de 3e cycle, Orsay, 1985, IPNO-T-85-02A.C. Mueller and a!., Z. Phys. A 330 (1988) 63

[3] T. Suomijarvi and al, Phys. Rev. C 36 (1987)2691

[4] P. Roussel, GEPL/05/83/58/AE/E, Orsay, 1983

166

DA7 "A Test of Detectors for Recoiling Fragments"

L Lavergne and L Stab, IPN, Orsay.H-A Gustafsson, B Jakobsson, A Kristiansson, A Oskarsson and M Westenius, Univ.of Lund, Sweden.A J Kordyasz, Univ. of Warsaw, Poland.K Aleklett and L Westerberg, Univ. of Uppsala, Sweden.

line Dl. The E300 ym epitaxialwhile the 10 ym

Though this was essentially a test of thinion implanted Si detectors in a realisticexperimental environment some results onfragmentation in 94A MeV ^ 0 inducedreactions were obtained.

We placed three fiE-E Si telescopes in thetime-of-flight chamber in 'detectors were "standard"boron implanted detectorstransmission detectors were of planar typewhere the thinning has been made withelectrochemical technique1'. The 10 ymdetectors were mounted with their junctionsides facing each other and the rear side ofthe uE detector facing the target, therebyprotecting against the strong electron flux.This mounting makes the energy cut-off fallfrom 0.9A MeV for 6Li to 1.4A HeV for e.g.14N while the upper energy limit, set by theE detector thickness is 40 MeV. Fig 1, belowshows the typical charge resolution(62=0.25+0.05 units).

After corrections for the loss of particleinstable isotopes we obtained the yield of 3< Z < 12 fragments with 1.1A < E < 6A MeV for1 60 + "Al and 1 60 + ««Ti reactions at 94AMeV. The yields at various angles for theformer reaction, is shown in Fig 2. Thus weobserve a very high cross-section in aphase-space region which is not connected totrue target fragmentation. An isotropiccomponent - represented by the 180° yield inFig 2 - may be the remnant of such acomponent and it has therefore beensubtracted from the yields at 10°,30°,60° and90° to obtain the second component. We stressthat for 1 60 H- «*Ti we find similar resultsfor the yield in the 3 < Z < 18 region. Thequestion raised by our data is whether thehigh yields of medium heavy fragments couldbe explained by simple many-nucleon transferprocesses or not. So far we have been able toexclude the simplest few-nucleon transferreactions including also the knock-onmechanism and we are at present confrontingour data with more complete dynamical models(e.g. VUU) possibly coupled to statisticalmultifragmentation processes.

5100S

5 6

2.0.105

Fig 1. Charge resolution from projected iE-Ecorrelations in the energy bin 2A - 3A MeV.

10'

- 10°•sf

1,0-

10'3

-94A MeV1.1ASES6A MeV

T \

IBoA \loxtr.H \120°

Z= 3 t 5 6 7 8 9 10 11 12 13

Fig 2. do/dO for 3 < Z < 12 fragments in 94AMeV 1 60 + "Al reactions.

Publications:

1) L Lavergne-Gosselin et al., Nucl. Instr.and Methods in press.

2) H-A Gustafsson et al., Lund InternalReport LUIP 8901(1989)

167

INTERACTIONS OF 2O- 1OO MeV/u HEAVY IONS WITH SOLID AND GASEOUS MEDIASTOPPING POWERS, CHARGE DISTRIBUTIONS, ENERGY LOSS STRAGGLING

AND ANGULAR STRAGGLING

R.BIMBOT, H.GAUVIN, J.HERAULT, B.KUBICAInstitut de Physique Nucleaire, F-9I4O6 - Orsay CedexR.ANNE, G.A.N.I.L., B.P. 5O27, F-14O21 Caen Cedex

G.BASTIN, C.B.N.S.M., B.P.N° 1, F-91406 Orsay CedexF.HUBERT, C.E.N.B.G. , LE? Haut Vigneau, F-3317O Rradignan

1. Motivation

A study of heavy particle pene-

tra.tion through matter has been carried

out at the GANIL facility to cover a

domain of ions and energies for which

no experimental information existed.

This experimental study made it

passible ta measure several essential

parameters, namely :

- stopping powers,

- charge distributions,

- energy loss straggling,

- angular straggling.

The heavy ions which have been16-17^ 40. <K>

O, Ar, Ca,0- 129-132V . ..Mo, Xe in the energy

concerned were :B/*~S6Kr ,

domain available for each of them at

GANIL (20 to 100 MeV/u). This energy

range is of great interest because it

corresponds to a simplification of the

slowing down process when the projec-

tile tends to be totally stripped and

the relativistic effects are still

minor.

Solid and gaseous media have been

used as targets in order to follow at

high energy the gas-solid effect which

had been observed at low energy.

All these experiments have been

performed using the LISE magnetic spec-

trometer. Experimental details and

results are given in references 1 to 9.

2. Stopping powers (ref. 1-2-7-B)

Stopping power data have been

compared with calculations assuming

fully stripped ions. This scaling law

is derived from a relation in which the

HIheavy ion stopping power

4 ++calculated from the He one

nt?

the same velocity, taking into account

is

at

thF* ion t? f 1 EM t i ve? » h._ir'|t' /• :

W ^ f = SHe He7He

(Z. atomic number of the incident ion,

>,, = li 2,. = 2 ) - Far fully strippedHe Heions > = 1 and the relation reduces to

S.,T = S,, Z^/4. S,., is easily calcula-Hi He 1 Hi

ted with known values of S (Zieqler).

Two conclusions can be made :

- This scaling law leads to a fair

agreement for projectiles which are

effectively fully stripped at tht?

target ex it.

- By comparing results from solid and

gaseous media with the same projec-

tiles at the same energies, we observe

a vanishing of the solid-gas effect

when the projectile is totally

stripped. This indicates that the

solid-gas effect is really linked to

the ion charge state.

From the very large set of expe-

rimental values of stopping power S a

new parametrization for the effective

charge >• has been deduced and an impro-

ved semi-empirical procedure to compute

S values has been established (ref. 9).

New tabulations of ranges and stopping

powers of heavy ions in solid media

will be published in the near future.

3. Charge distributions (ref. 1-2-7)

Charge distrit tions have been

obtained for all the projectiles which

have been used especially in Be, Al and

Au degraders. For Kr ions in the energy

range 4-SO MeV/u a semi-empirical

expression has been obtained to predict

the value of the mean charge state O.

Some characteristics of the emission of

168

Auger electrons by ions exiting the

targets have been emphasized.

4. Energy loss straggling (ref.4-5-9)

Experiments have been performed

with solid and gaseous targets to eli-

minate passible discrepancies introdu-

ced by target inhomogeneities.

In the BAN1L velocity regime the

energy loss straggling comes from the

statistical nature of the slowing down

process by electronic collisions and by

statistical fluctuations of the ion

charge.

The energy loss straggling measu-

rements have shown that the calculated

collision straggling (Bohr's theory or

Titeica formulation) cannot explain by

itself the experimental values even

when the projectile is totally

stripped. For partially stripped ions,

the charge exchange effect seems to

account for most of the straggling.

Calculations in some cases where only

two charge states are present show a

goad agreement with experiment.

5. Angular straggling (ref. 3—5)

An original experimental method

using the LISE spectrometer has been

developed for measuring angular distri-

butions of heavy ions exiting solid and

gaseous targets in the energy range

20-9O MeV/u.

The results have been compared to

multiple scattering theories based on

classical or quantum mechanical calcu-

jr

Sigrnuiit] pi nl

~II2-' IKH"M

ItMCl')

11)' I'l2 IU:1 W ' to"*

lations. Using the "redured" Mijynr

parameters ( and r - see rp-f. 3)' i. z

the "universal" scaling Ijw given hy

Sigmund et al has been extended over-

five orders of magnitude according to

the expression u = 1. OO 7-°' '' (see

figure).

References

1. Stopping powers of solids for *°Arand Ca ions at intermediate energies(20-BO MeV/u) ,R.Bimbat, H.Bauvin, I.Orliange, R.Anne,B.Bastin, F..Hubert, Nucl. Instr. andMeth- B17 (19B6) 1.

sol ids for Oenergies (2C—95

2. Stopping powers ofions at intermediateMe?V/u),H.Gauvin, R.Bimbot, J.Herault, R.Anne,G.Bastin, F.Hubert, Nucl. Instr. andMeth. B2B (19B7) 191.

3. Multiple angular scattering of heavyions (t<5"170, 4<?Ar, 8<SKr and ?OOMo> atintermediate energies (20-9O MeV/u),R.Anne, J.Herault, R.Bimbat, H.Gauvin,G.Bastin, F.Hubert, Nucl. Instr. andMeth. B54 (19B8) 295.

4. Interaction of 20-1CO MeV/u heavyions with cold matter,J.Herault, R.Bimbat, H.Gauvin, R.Anne,G.Bastin and F.Hubert, 4th Int.Workshop "Atomic Physics for Ion DrivenFusion", Orsay, 2C—24 June 19BB, J.Phys. C7 (19BB) 33.

5. Etude experimentale du ralentisse-ment d'ions, lourds de 2O a 1OO MeV parnucleons dans la matiere,J.Herault, Thesis, Toulouse <19BB),Drsay Report IPNO-DRE-BB-25 (19BB).

6. Semi—empirical formula for heavy ionstopping powers in solids in the intei—mediate energy range,F.Hubert, R.Bimbat, H.Gauvin, Nucl.Instr. and Meth. (in press). Bordeaux-Gradignan Report CENBG-BB-36 (19BB).

7. Stopping powers of solids for84-8(5,, 1OO 12£»-13Z . ,

Kr, Mo and Xe ions atintermediate energies (2Q-45 MeV/u).Charge state distributions at the equi-1ibrium,H.Gauvin, R.Bimbot, J.Herault, B.KubicaR.Anne, G.Bastin and F.Hubert, DrsayReport IPND-DRE-B9-O3 (19B9).8. Stopping powers of gaseous media forn— *o * ti<s,., . 132., . iO, Ar, Kr and Xe ions at

intermediate energies (25-B5 MeV/u),J.Herault, R.Bimbat, H.Gauvin, B.KubicaR.Anne, G.Bastin and F.Hubert, to bepublished.9. Energy loss stragqling of heavy ions(l<sn *°AT- 40ra " ^ " W 1 0 0n 132ve>>^ LJ, ttr, i_ci, i r, u, Aeiin solid and gaseous media at interme-diate energies (2O-10O MeV/u),J.Herault, R.Bimbat, H.Gauvin, R.Anne,G.Bastin and F.Hubert, to be published.

1 6 9

SEMI-EMP1KICAL FORMULA FOR HEAVY ION STOPP1HG POWERS 111 SOLID

IH THE INTERMEDIATE EHERGY RAHGE

F. Hubert, C.E.JT. Bordeaux, IN2P3. Le Haut Vigneau, F-33170 GRADIGHAHR. Binbot et H. Gauvin, IPH Orsay, B, P. n" 1, F-91405 ORSAY

Stopping-powers and ranges have played acrucial role in many aspects of the heavy-Ion physics. Requirements in NuclearPhysics, Atomic and Solide State Physics,aerospace projects, ionizing radiationeffects an matter have fostered .manyrevisions and new calculations on thesubject. Due to the complexity of atomicstructures all these calculations have toadopt sone approximations in order tosimplify the calculations and/or to rely ona semi-phenomenologlcal treatment. Then theymust be justified by experimental data whenavailable.

In the 2-10 MeV/A energy range thecomparison between experimental stoppingpowers and calculations such as thosetabulated by Jorthcliffe and Schilling" orthe more recently published curves ofZiegler2' have shown significant discrepan-cies, particularly for light stopping mediaand heavy projectiles. In this energy domainthe best agreement is generally obtainedwith the tables of Hubert et al.31. With theOAHIL, the energy domain from 20 to lOOKeV/Ahas become accessible for heavy ions. Thismade it possible to measure stopping powersin this energy range and an extensive studyhas been performed using the GASILspectrometer LISE. In the 20-80HeV/A energyrange, the tabulations of Hubert et al arein reasonable agreement with most of thedata, but overestimate the stopping powersessentially in light stopping media. In thiscase a better agreement is obtained with thevalues from Ziegler.

Slopping powers of Kt iot*.

It appeared then that it was worthreconsidering the prospects for a semi-emplrical calculation of heavy ion stoppingin the light of the considerable body ofheavy ion data now available in the energyrange from 3 to 80HeV/A.

The generalization of heavy ion stoppingpowers was made using the Be the equation :the stopping power S, , 2 of a given medium(2) for a given ion (1) is calculated fromthat of this medium for a particles at thesame velocity (sane E/A value) denoted S . -using the following equation :

111 S1.2= < S « . 2 / 4 >

The effective charge parameter K cannotbe easily deduced from basic principles. Wethen used eq. i 11 to calculate 1 values fromthe available experimental stopping powers.Thus, a set of about 600 effective chargeparameters have been calculated.

The analysis of these data has clearlypointed out the necessity of taking intoaccount the variation of Y with the stoppingmedium. The following formulae have beenderived for the calculation of heavy ioneffective charges in solids which aresuitable for the intermediate energy region.

= l-x,exp <-X;

E/A IMcV/AI

with x, = D+1.658 exp[-0. 05170Z, ]D = 1.764+0.2319 exp(-0. 00430222*)Xa- = 8.144+0.09876 InZiX3 = 0.3140+0.01072 lnZ2X* = 0.5218+0.02521 InZ^

Fig. I gives examples of the agreementobserved in most of the case.

Pig. 1 : Comparison of experimental stoppingpowers with the results of calculations^resented in the text (dotted lines).

1) L.C. Borthcliffe and R.F. Schilling,Hud. Data Tables A7 (1970)2332) J.F. Ziegler, Handbook of stopping crosssections for energetic ions in all elements(Pergamon, lew York, 1980).3) F. Hubert, A. Fleury, R. Bimbot and D.Gardes, Ann. Phys. (France) vol 5S (1980) 1.4) F. Hubert, R. Bimbot, H. Gauvin, acceptedfor publication in HIM B

170

AUTHOR INDEX

AUTHOR INDEX

ADLOFF J.C 48, 64, 67, 77, 143ADORNOA 51AGUERP. 163AIELLOS 151ALAMANOSN 1ALEKLETTK 95,167ALEXANDER J.M 79ANDREOZZIF 97ANNE R 19, 25, 29, 33, 36, 38, 163, 165, 168ARDISSONG 163ARDOUIN D 44, 107, 109, 111ARNOLD E 38ARTUKH A.G 19,25ASAHIK 36AUDIG ' 23AUGER F 1,93AUGER G 142BACRIC.0 165BADALA A 151BARBERAR 151BARNEOUDD 160BARRANCOM 10,130BARRETO J 142BARRETTE J 1, 3, 8, 12, 44, 56, 60, 105BARTEL J 115BASRAKZ 109BASTIN G 163, 168BAZIN D 19, 25, 29, 33, 36, 38BEAUM 27BEAUMELD 3, 12, 44BERATC 8BERLANGERM 58BERNAS M 19, 165BERTHIERB 1,8,12,44,60,93,105BERTHOLETR 102,160BIANCHIL 23BILLEREYR 53BILWESB 64,67,143BILWESR 64,67,143BIMBOTR 36,38,163,168,170BIZARD G 48, 67, 77, 107, 109, 145, 151BLACHOT J 75BLUMELR 155BLUMENFELDY 3,12,44,165BOHLENG 8BOISGARD R 103, 111, 130BONASERA A 51BONCHEP. 119,120,121BONDORF J.P. -. 102BORDERIE B '. 62, 79, 91BORREL V 19, 25, 29, 42BOUGAULTR 67,143,151BRACK M 115BRICAULTP. 33BRONDIA 97

BROUR 48,67,77,145BRUANDETJ.P 38,97BUENERDM 8CABOT C 62,91CASINIG 64CASSAGNOUY 67,71,140,145CAVALLEROS 97CERRUTIC , 58,69,103CHAMBONB 53CHAPMAN J 163CHARITY R.J 64CHARVET J.L '50,53,69,73CHAUVINJ 8CHAVEZ E 60CHBIHIA 53CHEVARIERA 53CHEVARIERN 53CHIODELLIS 58,69CHOMAZ Ph 3, 12, 16, 17, 44CISSEO 60,105CLAPIERF 163,165COCA 40COFFIN J.P. 46,99,113COLEA.J , 113CONIGLIONER 60,105CONJEAUDM 71COSTA G.J 38CRAMER B 73CRANCON J 75CUGNON J 85,128CUNSOLOA 23,93CUSSOLD 138,145DALILID 58DAYRASR 56,60,71,97,105,140,142DE CASTRO-RIZZO D.M 60,105DELAMOTAV 87DE SAINTIGNON P. 153DE SAINT-SIMON M 40,95DEL MORAL R 27,28,35, 101,162DELAGRANGEH 28,29,35,60, 101,105,107,109, 162,163DELAUNAYB 97DELAUNAYF 67,143DELAUNAYJ 97DELLA-NEGRAS 163DEMEYER A 58,69DESBOIS J 103,130DETRAZ C 19, 25, 29, 33, 40DITOROM 51DIETRICH K 18,155DISDIERD 1DOUBREH 1,48,50,73,77,107,109,145DRAIN D 53DUCHENEG 50,53DUEKE 79DUFOUR J.P. 27, 28, 29, 33, 35,101,162DUMONTH 97DURANDD 111,151

D'ONOFRIO A 97ELMASRI Y 38EMMERMANNH 101,162ERA2MUSB 150FAHLIA 46,99,113PAUREB 93FERNANDEZ B 1, 3, 8, 12, 23, 44FERRERO J.L 67,143FINTZ P. 46, 99,113FLEURY A 27, 28, 33, 35, 101, 162FONTE R 140FOTIA .' 23,93FRASCARIAN 3,12,44FREHAUT J 27,50,73FREIFELDER R 64FUCHSH 62,79FUKUDAM , 36GADIF 60,105GADYF 97GALIN J 19,42,50,64,73,79GARDES D 62,79,91GARRON J.P. 3,12,44G ASTEBOIS J 1, 3, 12, 23, 44G ATTY B 42, 73GAUVINH 62,91,165,168,170GEISSELH 28,35GELBKEC.K 107,109GENOUX-LUBAIN A 67,143,151GILLIBERT A 1, 23,150GIRAUDB.G 83GIRAUDETG 27GIZON A 75GLASERM 67,143GLASSERF 38GNIRSM 64GOBBIA 64GOMEZ DEL CAMPO J 56,97GONINM 99GONO Y 36,163GOUJDAMID IllGRANGER 85GRANIERO 69GREGOIREC 23, 51, 55, 69, 79, 81, 87, 89, 107,109,13§GROSSEE 150GUERREAUD 19,25,42,50,64,73,79GUETC 102,115,150,160GUILBAULTF 48,67,77,111.GUILLAUMEG 46,99,113GUILLEMAUD-MUELLER D 19,25,29,33,36,38,40GUIMBALP. 40GUINETD 58,69GULMINELLIF 51GUSTAFSSON H.A 167GVOZDEVB.A , 19HACHEMA 163HAGELK 145HANAPPEF. 38,48,62,77,91

HANELTE 27,28,35HARARS. 71HATANAKAK 163HEITZC 38HERAULT J. 165,168HERRMANN N 64HEUERD 102HEUSCHB 46,60,99,105,113HILDENBRAND K.D 64HOATHS.D , 19HOLZMANR , 150HOSTACHYJ.Y. 8HUBERT P 27,28,29,33,35,101,162,163,168,170ICHIHARAT 36IMMEG 140INGOLDG 50,73ISHIHARAM 36JACMART J.C 12, 19, 25, 29, 33, 44,165JACQUETD 50,62,73,79JAHNKEU 38,50,64,73JAKOBSSONB 147,167JEAND 27,28,35,162JEONG J.C 145JIANG D.X 19,50,73JIN GEN MING 38,67,145,151JOLYS; 53JONSSONG 147JOUAND 62JULIEN J 153JUNDTF 99,113KALININ A.M 19,25KAMANINV.V. 19,25KARLSSONL 147KASAGIJ 36,145KLOTZ-ENGMANN G 71KORDYASZ A.J 167KOXS 38,99,113KRISTIANSSON A 167KUBICAB 163,168KUBOT 36KUHNW 150,156KUTNERV.B 19KWATONJOCKM 158,160KYANOWSKIA 107,109LARANAG 69,97LANGEVINM 19,40LANZANOG 60,105LATIMIERA 165LAUTRIDOUP. IllLAVERGNEL 167LAVILLE J.L 48,77,151LEBRUNC 67,69,143,153LEBFEVRESF 109LEBRUNC 48,55,67,77,111,145LEBRUND , 8,153LECOLLEYJ.F 67,143,153LEES.M 145

LEPEBVRESF 67,107, 143,151LEFORTM 79LEGRAINR 60,67,71, 105,140, 145,153LEJEUNE A 85LELONG F 165LERAYS 58,69,132,134LEVITS 119,120,121LEWITOWICZM 19,25,33LHENORETP. 69LIATARDE 38LILJENZIN J.O 95LIPSV 71LISLE J 163LLERES A 75,101LOCHARDJ.P. 69LOTTB 1,50,73LOUVELM 67,143,145LOVELAND W 95LUCAS R 58,69LUKYANOVS.M 19,25LYNCH W.G 107,109MAGNAGOC 50,53,73MAIERM , 107,109MAKJKAZ 142MALKIA 99MARTIN P. 8MAURELM 102,158,160MAURENZIGP.R 64MAZURC 58,69MERCHEZF 99MERMAZM.C 46,60,105MEYER J 115MICZAIKA A 8MISTRETTA J 99MITTIG W 1, 3, 8, 12, 23, 44, 60, 93, 105, 107, 109, 150MONNAND E 147, 158,160MONNETF 62MONTOYAM 62MORJEANM 23,50,53,69,73MOROR 97MOSTEFAIM 71MOTOBAYASHIT 145MUELLER A.C 19,25,27,29,33,36,38,40NAGARAJANM.A 83NAGASHIMA Y 145NAKAGAWAT 145NATOWITZ J 48,77,103,145NAULINF 19NEBBIAG 69NEMETH J 103,130NESKOVICN 56NEUGARTR 38NGOC 58,69,103,130,132,134NGOH , 134NGUYEN HOAI CHAU 19,25NGUYEN VAN GIAI 17NIELSEN O.B 102

NIFENBCKBRH 75,102,150,158,160NORBECKE 142NORENB 147OESCHLERH 71OGIHARAM ,145OLMIA 64OSKARSSONA 167OSTOJICR 56OUBAHADOU A 55PAGANO A 60,105PALMERIA 60,151PANAGIOTOU A.D 140PAPADAKISN.H 140PAPINEAUL 1PAPPALARDO G.S 151PASCAUD J.M 93PASTOR C 53PATINY 50,53,69,73PATRY J.P. 48,77,145,151PEASLEEG 79PEGHAIRE A 48,53,67,69,77,107,109,111,145PELTED 64PENIONZHKEVICH Yu.E 19,25PERILLOE 97PERRING .' 153PERRINP. 158,160PETER J 48,67,77,91,107,109,145PETROVICIM 64PIM 89PIASECKIE. 73PINSTON J.A 150,158,160PLAGNOLE 93,142POCHODZALLA J 107,109POINOTC 101,162POLLACCOE.C 56,60,71,105,140POUGHEONF 25,29,33,165POUTHAS J 50,73PRANALY 23,50,53,73PRAVIKOFFM.S 27,28,29,33,35,101,162QUEBERTJ 93,107,109,111QUENTINP. 115QUINIOUE 19RACITIG 140RAFFRAYY 81,87RAIMOLDD 64RAMIF 46,64,113RASTEGARB 67RAUCHV. 1REGIMBARTR 145REMAUDB 55,81,87,89,132RIBRAGM , 58,69RICHARD A 19,25,29,33RICHERT J 136RIESSS 150RISTORICH 102RIVET M.F 62,79,91RODRIGUEZ! 140

ROECKL E 19,29ROMANO M 97ROSATOE 48,67,77,145,151ROUSSELP. 165ROUSSEL-CHOMAZ P. 1,3,12ROYERG 55,87ROYNETTEJ.C 3,12,44RUDOLF G 48,64,67,77,143SAINT-LAURENT F 56, 107, 109, 111, 140,145SAINT-LAURENT M.G 19, 25, 29., 38, 97,140SAMADDARS.K 136SARANTITES D.G 142SAUNIERN 140SAUVESTREJ.E 71SCARPACIJ.A 3,12,44SCHEIBLINGF. 1,48,67,77,143SCHILLACI A 151SCHMIDT K.H 27,28,35,162SCHMIDT-OTT W.D 19,25,36SCHUCKP. 87,89,109SCHUSSLERF 102,147,158,160SCHUTZ Y 23,46,113, 142,150,158,160,163SCHWINNE 73SEABORGG.T 95SEBILLEF 55,81,87,89,132SIDAJ.L 165SIHVERL 95SINOPOLIL 53,69SOBOTKAL.G 142SODERSTROMK 147SOKOLOVA 73SOYEURM 150SPERDUTOL 97SPINAM.E 132,134STABL 167STECKMEYER J.C 48,67,77,143,145STEFANINIA.A 64STELZERH 64STEPHANC 1,23,165STERN M 53STILIARISS 8STRACENER D.W. 142STRUMBERGERE 115STUTTGEL 67,143SUMMERERK 27SUOMIJARVIT 3, 12,44,58,69,165SURAUDE 10,89,117,122,138TAMAINB 48,77,107,109,145TAMISIERR IllTARRAGOX 42,79TASSAN-GOTL 1,23TERRASIF. 97THIBAULTC 40TOMASIE 69TOUCHARDF. 40TOULEMONDEM 56TREINERJ 10,123

TROUDETT 126TSANUNGCHAN 38UZUREAUJ.L 50,53,69,73VAGNERONL 69VAUTHERIND 16,17,119,120,121,122,123,124,125,126,127VENERONIM 123VILLARI A.C.C 8,23,150VINETL 81,89VINHMAUN 124,125,126,127VIVIEN 3.P. 150VIYOGIY.P. 107,109VODINASN.P. 140VOLANT C 71VONOERTZENW 6,8WAGNER P. 46,99,113,136WEN LONG Z 23WERNER K 18WESSELS J 64WESTENIUSM 167WESTERBERGL 167WIELECZKO J.P. 56,93,142ZHANG Y.H 33

LABORATORY INDEX

BELGIUM

Service de Physique TheoriqueUniv. Libre de Bruxelles (ULB)Campus de la Plaine C.P. 225Bd. du TriompheB-1050 BRUXELLES

Inst. de Phys. au Sart-TilmanService de Phys. Theor. et Math.Univ. de LiegeBat B5B-4000 LIEGE 1

BRAZIL

Inst. de Fisica (IFUSP)Univ. de Sao PauloCaixa Postal 2051601498 SAO PAULO SPBrazil

CANADA

Foster Radiation LaboratoryMe Gill Univ.MONTREAL PQ, Canada, H3A 2B2

CHINA

Inst. of Modern PhysicsAcad. SinicaP.O. Box 3157 Nanchang RoadLANZHOU (Gansu)China

DENMARK

Niels Bohr Inst.Univ. of CopenhagenBlegdamsvej 17DK -2100 KOBENHAVN

FRANCE

Grand Accelerateur National d'lons Lourds (GANIL)BP 5027F-14021 CAEN Cedex

Centre de Recherche Interdisciplaireavec les Ions lourds (CIRIL)BP 513314040 CAEN Cedex

Laboratoire de Physique CorpusculaireBoulevard du Marechal Juin14032 CAEN Cedex

Laboratoire National Saturne (LNS)Centre d'Etudes Nucleaires de Saclay (CEN)F-91191 GIF sur YVETTE Cedex

Service de Physique Nucleaire (DPhN)Centre d'Etudes Nucleaires de Saclay (CEN)F-91191 GIF sur YVETTE

Laboratoire de Physique TheoriqueUniv. de Bordeaux 1Rue du SolariumF-33170 GRADIGNAN

Centre d'Etudes Nucleaires (CEN)Univ. de Bordeaux 1Le Haut VigneauF-33170 GRADIGNAN

Centre d'Etudes Nucleaires (CEN)DRF/PhNBP 85 XF-38041 GRENOBLE Cedex

Inst. des Sciences Nucleaires (ISN)Univ. de GrenobleAvenue des Martyrs 53F-38026 GRENOBLE Cedex

Laboratoire de Physique NucleaireFac. de Sciences et de TechniquesUniv. de Nantes2, rue de la HoussiniereF-44072 NANTES Cedex 03

IresteLa ChanterieUniversite de NantesF-44087 NANTES Cedex 03

Laboratoire Rene BernasF-9U05 ORSAY

Inst. de Physique Nucleaire (IPN)Univ. de Paris-Sud (Paris XI)B.P. 1F-91406 ORSAY Cedex

Centre de Spectrometrie Nucleaire etde Spectrometrie de Masse (CSNSM)Bat. 104-108F-91504 ORSAY

Centre de Recherches Nucleaires (CRN)B.P. 20 CROF-67037 STRASBOURG Cedex

Inst. de Physique Nucleaire (IPN)Univ. Claude Bernard Lyon 1A3, bd du 11 Novembre 1918F-69622 VILLEURBANNE Cedex

GREECE

Physics Depat.Univ. of AthensPanepistimioupoli-KouponiaGR-157 71 ATHINAI

HUNGARY

Inst. for Theoretical PhysicsEotvos Lorand Univ.Puskin utca 5-7HH-1088 BUDAPEST

INDIA

Sana Inst. of Nuclear Physics92 Acharya Prafulla Chandra RoadCALCUTTA 700 009

ISRAEL

Weissmann Institute of ScienceNuclear Physics Dept.P.O. Box 2676 100 REHOVOTIsrael

ITALY

Dept. di FisicaUniv. di CataniaCorso Italia 571-95129 CATANIA

Dept. di Fisica dell1 Universita (INFN)1st. di Sci. Fis. Aldo PontremoloUniv. Degli Studi di MilanoVia Celoria, 161-20133 MILANO

Ins. Naz. di Fisica Nucleare (Ij-JFN)Mostra d'Oltremare, Pad 201-80125 NAPOLI

JAPAN

Inst. of. PhysicsUniv. of TsukubaIBARAKI 305

Dept. of PhysicsTokyo Inst. of Technology2-12-1 Oh-OkayamaMeguro-KuTOKYO 152

Inst. of Physical and Chemical Research (Riken)2-1 HirosawaWako-ShiSAITAMA 351-01

Dept. of PhysicsRikkyo Univ.Nishi-IkebukuroToshima-kuTOKYO 171

Dept. of PhysicsTokyo Inst. of Technology2-12-1 Oh-okayamaMeguro-kuTOKYO 152

NORWAY

Dept. of PhysicsUniv. of OsloP.O. Box 1048BlindernN-0316 OSLO 3

POLAND

Univ. of Warsawul. HOZA 69PI.00681 WARSZAWAPoland

R.F.A.

Hahn-Meitner-Institut (HMI)Berlin GmbHGlienicker Str. 100D-1000 BERLIN 39

Ges. f. Schwerionenforschung mbH (GSI)Postfach 110 552Planckstr. 1D-6200 DARMSTADT 11

Inst. f. KernphysikTech. HochschuleSchlossgartenstr. 9D-6100 DARMSTADT

Physics Dept. T30Tech. Univ. MunchenJames-Franck-StrasseD-8046 GARCHING BEI MUNCHEN

Inst. f. PhysikUniv. GottigenBunsenstrasse 9D-3400 GOTTINGEN

Physikalisches Inst.Univ. HeidelbergPhilosophenweg 12D-6900 HEIDELBERG

Inst. f. PhysikJohannes-Gutenberg-Univ.Postfach 39 80D-6500 MAINZ

Inst. f. Thor. Phys.Univ. RegensburgPostfach 397Universitatsstrasse 31D-8400 REGENSBURG

ROMANIA

Inst. for Physics and Nuclear Energy (INPE)P.O. Box MG-6R-76900 BUCURESTI-MAGURELE

SPAIN

Dept. de 1'Estructura i Constitucats de la MateriaFac. de FisicaUniv. de BarcelonaDiagonal 64E-08028 BARCELONA

Dept. de FisicaUniv. de les Illes BalearesE-07071 PALMA DE MALLORCA

SWEDEN

Dept. of PHYSICSUniv. of LundSolvegatan 14AS-223 62 LUND

Univ. of UppsalaP.O. Box 533S-751 21 UPPSALA

UNITED KINGDOM

Daresbury Lab.DaresburyHARRINGTON WA4 4AD

U.R.S.S.

Laboratory of Nuclear ReactionsJoint Inst. for Nuclear Research (JINR)Dubna, Head Post OfficeP.O. Box 79101 000 MoskvaDUBNAURSS

U.S.A.

Lawrence Berkeley (LBL)1 cyclotron RoadBERKELEY CA 94720U.S.A.

Center for Theoritical PhysicsMassachusetts Inst. of Technology (MIT)CAMBRIDGE MA 02139U.S.A.

Cyclotron InstituteTexas A and M Univ.COLLEGE STATION TX 77843U.S.A.

Wept, of PhysicsOregon State Univ.CORVALLIS OR 97331U.S.A.

Dept., of PhysicsUniv. of IowaIOWA CITY IA 52242U.S.A.

Physics DivisionOak Ridge National Lab. (ORNL)P.O. Box 2008OAK RIDGE TN 37831-6369U.S.A.

Dept. of ChemistryWashington Univ.Campus Box 11051 Brcokings DrST LOUIS MO 63130-4899U.S.A.

Dept. of PhysicsState Univ. of New York (SUNY)STONY BROOK NY 11794-3800U.S.A.

YUGOSLAVIA

Dept. of Theor. PhysicsBoris Kidric Inst. Vinca (BKI)P.O. Box 522YU-11001 BEOGRAD

IIPUBLICATION LIST

1983-1984

ISOTOPIC DISTRIBUTIONS OF PROJECTILE-I>IKE FRAGMENTS IN 44 MeV/U 4 0 ArINDUCED REACTIONSGUERREAU D . , BORREL V . , JACQUET D . , GALIN J . , GATTY B . , TARRAGO X.IPN - ORSAYPHYS. LETT. B131 (1983) 293.

PERIPHERAL Ar INDUCED REACTIONS AT 44 MeV/U - SIMILARITIES ANDDEVIATIONS WITH RESPECT TO A HIGH ENERGY FRAGMENTATION PROCESSBORREL V . , GUERREAU D . , GALIN J . , GATTY B . , JACQUET D . , TARRAGO X.IPN - ORSAY2. FUR PHYSIK A314 (1983) 191.

ELASTIC SCATTERING OF 4 0 Ar ON 6 0 N i , 1 2 ° Sn AND 2 0 S Pb AT 44 MeV/UALAMANOS N . , AUGER F . , BARRETTE J . , BERTHIER B . , FERNANDEZ B . , GASTEBOIS J . ,PAPINEAU L . , DOUBRE H . , MITTIG W.CEN - SACLAY, GANIL - CAENPHYS. LETT. B137 (1984) 37.

EXPERIMENTAL EVIDENCE OF HIGHLY ENERGY RELAXED PRODUCTS IN THE 35MeV/U 8 4 Kr + 1 9 7 AU REACTIONDALILI D . , LHENORET P . , LUCAS R . , MAZUR C . , NGO C . , RIBRAG M. , SUOMIJARVI T . ,TOMASI E . , BOISHU B. , GENOUX LUBIN A . , LEBRUN C . , LECOLLEY J . F . , LEFEBVRES F . ,LOUVEL M . , REGIMBART R . , ADLOFF J . C . , KAMILI A . , RUDOLF G . , SCHEIBLING F .CEN - SACLAY, LPC - CAEN, CRN - STRASBOURGZ. FUR PHYSIK A316 (1984) 371

FEW NUCLEON TRANSFER VERSUS FRAGMENTATION IN THE PRODUCTION OFPROJECTILE-LIKE FRAGMENTS IN THE 4 0 Ar + 6 8 Zn REACTION AT 2 7 . 6MeV/NUCLEONRAMI F . , COFFIN J . P . , GUILIAUME G . , HEUSCH B . , WAGNER P . , FAHLI A . , FINTZ P .CRN - STRASBOURGZ. FUR PHYSIK A318 (1984) 239

HEAVY PRODUCTS FROM VIOLENT COLLISIONS IN THE 4 0 Ar + n a t Ag SYSTEM AT27 MeV/UBORDERIE B . , RIVET M . F . , CABOT C . , FABRIS D . , GARDES D . , GAUVIN H . , HANAPPE F . ,PETER J .IPN - ORSAY, UNIV. LIBRE DE BRUXELLES, GANIL - CAENZ. FUR PHYSIK A316 (1984) 243

INVESTIGATION OF THE 4 0 Ar + 1 9 7 Au, 2 3 8 U SYSTEMS AT 44 MeV/ULERAY S . , NEBBIA G . , GREGOIRE C . , LA RANA G . , LHENORET P . , MAZUR C . , NGO N . ,RIBRAG M. , TOMASI E . , CHIODELLI S . , CHARVET J . L . , LEBRUN C.CEN - SACLAY, IPN - LYON, CEN - BRUYERES-LE-CHATEL, LPC - CAENNUCL. PHYS. A425 (1984) 345

HIGH MOMENTUM AND ENERGY TRANSFER INDUCED BY 1 7 6 0 MeV 4 0 Ar ON 1 9 7 AuAND 2 3 2 Th TARGETSPOLLACO E . G . , CONJEAUD M. , HARAR S . , VOLANT C . , CASSAGNOU Y . , DAYRAS R . , LEGRAIN R . ,NGUYEN M . S . , OESCHLER H . , SAINT-LAURENT F .CEN - SACLAY, TECHNISCHE HOCHSCHULE - DARMSTADT, GANIL - CAENPHYS. LETT. B146 (1984) 29

INCOMPLETE LINEAR MOMENTUM TRANSFER IN NUCLEAR REACTIONS : ANINTERPLAY BETWEEN ONE-BODY AND TWO-BODY DISSIPATIONGREGOIRE C . , SCHEUTER F .GANIL - CAENPHYS. LETT. B146 (1984) 21

REACHING THE CAPACITY OF A COMPOSITE NUCLEUS FOR ENERGY AND SPINCONTAINMENTJACQUET D . , DUEK E . , ALEXANDER J . M . , BORDERIE B . , GALIN J . , GARDES D . , GUERREAU D . ,LEFORT M. , MONNET M. , RIVET M . F . , TARRAGO X.IPN - ORSAYPHYS. REV. LETT. 53 (1984) 2226

FISSION-FRAGMENT KINETIC-ENERGY DISTRIBUTIONS FROM A TWO-DIMENSIONALFOKKER-PLANCK EQUATIONSCHEUTER P . , GREGOIRE C., HOFMANN H., NIX J .R.GANIL - CAEN, PHYS. DEFT. - MUNICH, LANL - LOS ALAMOSPHYS. LETT. B149 (1984) 303

LIGHT FRAGMENTS PRODUCED IN 4 0 Ar + n a t Ag REACTIONS AT 27 MeV/UBORDERIE B . , RIVET M . F . , CABOT C . , FABRIS D . , GARDES D . , GAUVIN H . , HANAPPE F . ,PETER J .IPN • ORSAY, UNIV. LIBRE DE BRUXELLES, GANIL - CAENZ. FUR PHYSIK A318 (1984) 315

SUBTHRESHOLD PRODUCTION OF NEUTRAL PIONS WITH Ar IONS OF 44 MeV/UHECKWOLF H., GROSSE E., DABROWSKI H., KLEPPER 0., MICHEL C., MULLER tf.F.J., NOLL H.,BRENDEL C., ROSCH W., JULIEN J., PAPPALARDO G.S., BIZARD G., LAVILLE J.L.,MUELLER A.C., PETER J.GSI - DARMSTADT, GSI AND UNIV. OF FRANKFURT, TH - DARMSTADT, CEN - SACLAY,LPC - CAEN, GANIL - CAENZ. FUR PHYSIK A315 (1984) 243

1985

NUCLEUS-NUCLEUS POTENTIAL INSIDE THE STRONG-ABSORPTION RADIUS FROM 1 6

O + 1 2 C ELASTIC SCATTERING AT 94 MeV/uROUSSEL P . , ALAMANOS N. , AUGER F. , BARRETTE J . , BERTHIER B . , FERNANDEZ B. ,PAPINEAU L . , DOUBRE H., MITTIG W.CEN - SACLAY, GANIL - CAENPHYSICAL REVIEW LETTERS 54 (1985) 1779

SOME OPTICAL-MODEL ANALYSES OF THE ELASTIC SCATTERING OF 4 0 Ar AT 1 7 6 0MevEL-AZAB FARID M., SATCHLER G.R.ASSIUT UNIV. (EGYPT), OAK RIDGE (USA)NUCLEAR PHYSICS A441 (1985) 157

DIRECT SURFACE TRANSFER REACTIONS INDUCED BY A 1 1 0 2 MeV 4 0 Ar BEAM ON6 8 Zn TARGET NUCLEUS

MERMAZ M.C., RAMI F. , COFFIN J . P . , GUILLAUME G., HEUSCH B . , WAGNER P . , FAHLI A. ,FINTZ P.CEN • SACLAY, CRN - STRASBOURGPHYSICAL REVIEW C31 (1985) 1972

DIRECT SURFACE-TRANSFER REACTION TO THE CONTINUUM STATES INDUCED BY A1 7 6 0 MeV 4 0 Ar BEAM ON 2 7 A l AND n a t T i TARGET NUCLEIMERMAZ M.C., DAYRAS R., BARRETTE J . , BERTHIER B . , DE CASTRO RIZZO D.M., CISSE 0 . ,LEGRAIN R. , PAGANO A. , POLLACCO E . , DELAGRANGE H., MITTIG W., HEUSCH B . , LANZANO G.,PALMERI A.CEN - SACLAY, GANIL - CAEN, CRN - STRASBOURG, INFN - CATANIANUCLEAR PHYSICS A441 (1985) 129

TRANSFER OF NUCLEONS AT HIGh 'ATIVE VELOCITIESVON OERTZEN W.ISN - GRENOBLE, GANIL - CAENPHYSICS LETTERS 151B (1985) 95

INVESTIGATION OF THE Kr + AU SYSTEM AT 22 MeV/UDALILI D. , BERLANGER M., LERAY S . , LUCAS R., MAZUR C. , NGO C. , RIBRAG N. ,SUOMIJARVI T . ( CERRUTI C. , CHIODELLI S . , DEMEYER A. , GUINET D. , GENOUX LUBIN A. ,LEBRUN C. , LECOLLEY J . F . , LEFEBVRES F . , LOUVEL M.CEN - SACLAY, IPN - LYON, LPC - CAENZ. FUR PHYSIK A320 (1985) 349

STUDY OF THE 4 0 Ar + 6 8 Zn REACTION AT 2 7 . 6 MeV/NUCLEONRAMI F . , COFFIN J . P . , GUILLAUME G., HEUSCH B . , WAGNER P . , FAHLI A. , FINTZ P.CRN - STRASBOURGNUCLEAR PHYSICS A444 (1985) 325

SEMI-CLASSICAL PHASE SPACE APPROACH FOR THE CHARGE EQUILIBRATIONFLUCTUATIONS IN HEAVY ION REACTIONSDI TORO M., GREGOIRE C.GANIL - CAENZ. FUR PHYSIK A320 (1985) 321

HIGH ENERGY FRAGMENTS PRODUCED I N THE 8 4 Kr + 9 3 Kb REACTION AT 3 4 . 5MeV/UCHARVET J . L . , JOLY S . , MORJEAN M., PATIN Y., PRANAL Y. , SINOPOLI L . , UZUREAU J . L . ,BILLEREY R., CHAMBON B . , CHBIHI A. , CHEVARIER A., CHEVARIER N. , DRAIN D. , PASTOR C.,STERN M., PEGHAIRE A.CEN - BRUYERES-LE-CHATEL, IPN - LYON, GANIL - CAENZ. FUR PHYSIK A321 (1985) 701

NUCLEAR FRAGMENTATION PROCESSES I N THE 2 0 Ne + 2 7 A l SYSTEM AT 3 0 MeV/AMORJEAN M., CHARVET J . L . , UZUREAU J . L . , PATIN Y . , PEGHAIRE A . , PRANAL Y . ,SINOPOLI L . , BILLEREY A . , CHEVARIER A . , CHEVARIER N . , DEMEYER A . , STERN M.,LA RANA G., LERAY S . , LUCAS R. , MAZUR C . , NEBBIA G. , NGO C. , RIBRAG M.CEN - BRIJYERES-LE-CHATEL, IPN - LYON, CEN - SACLAYNUCL. PHYS. A438 (1985) 547 .

PRODUCTION AND IDENTIFICATION OF NEW NEUTRON-RICH FRAGMENTS FROM 3 3MeV/U 8 6 Kr BEAM I N THE 1 8 ^ Z £ 27 REGIONGUILLEMAUD-MUELLER D . , MUELLER A . C . , GUERREAU D . , ANNE R. , DETRAZ C . , POUGHEON F . ,BERNAS M., GALIN J . , JACMART J . C . , LANGEVIN M., NAULIN F . , QUINIOU E . , DETRAZ C.GANIL - CAEN, IPN - ORSAYZ. FUR PHYSIK A322 (1985) 415

PRODUCTION OF NEUTRON-RICH NUCLEI AT THE LIMITS OF PARTICLES STABILITYBY FRAGMENTATION OF 4 4 MeV/U 4 0 A r PROJECTILESLANGEVIN M., QUINIOU E . , BERNAS M., GALIN J . , JACMART J . C . , NAULIN F . , POUGHEON F . ,ANNE R. , DETRAZ C. , GUERREAU D . , GUILLEMAUD-MUELLER D . , MUELLER A.C.IPN - ORSAY, GANIL - CAENPHYSICS LETTERS B150 (1985) 71

PRODUCTION OF SECONDARY RADIOACTIVE BEAMS FROM 4 4 MeV/U A r PROJECTILESBIMBOT R . , DELLA NEGRA S . , AUGER P . , BASTIN G. , ANNE R . , DELAGRANGE H . , HUBERT F.IPN - ORSAY, CSKSM • ORSAY, GANIL • CAEN, CEN/BG - GRADIGNANZ. FUR PHYSIK A322 (1985) 443

APPROACH TO THE LIMITS FOR MASSIVE ENERGY AND SPIN DEPOSITION INTO ACOMPOSITE NUCLEUSJACQUET D . , GALIN J . , BORDERIE B . , GARDES D . , GUERREAU D . , LEFORT M., MONNET F . ,RIVET M . F . , TARRAGO X . , DUEK E . , ALEXANDER John M.IPN - ORSAY, DEFT. OF CHEMISTRY - STONY BROOKPHYS. REV. C32 (1985) 1594

STUDY OF THE 4 0 Ar + 6 8 Zn REACTION AT 2 7 . 6 MeV/NUCLEONRAMI F . , COFFIN J . P . , GUILLAUME G. , HEUSCH B . , WAGNER P . , FAHLI A . , FINTZ P .CRN - STRASBOURGNUCL. PHYS. A444 (1985) 325

CENTRAL COLLISIONS I N THE ARGON INDUCED F I S S I O N OF THORIUM I N THETRANSITION ENERGY REGIMECONJEAUD M., HARAR S . , MOSTEFAI M. , POLLACCO E . G . , VOLANT C . , CASSAGNOU Y . ,DAYRAS R . , LEGRAIN R . , OESCHLER H , , SAINT-LAURENT F .CEN - SACLAY, TECHNISCHE HOCHSCHULE - DARMSTADT, GANIL - CAENPHYS. LETT. B159 (1985) 244

INVESTIGATION OF LINEAR MOMENTUM TRANSFER ON THE 3 5 MeV/U A r + U SYSTEMLERAY S . , GRANIER 0 . , NGO C . , TOMASI E . , CERRUTI C . , L'HENORET P . , LUCAS R . ,MAZUR C . , RIBRAG M., CHARVET J . L . , HUMEAU C . , LOCHARD J . P . , MORJEAN M., PATIN Y . ,SINOPOLI L . , UZUREAU J . , GUINET D . , VAGNERON L . , PEGHAIRE A.CEN - SACLAY, CEN - BRUYERES-LE-CHATEL, IPN - LYON, GANIL - CAENZ. FUR PHYSIK A320 (1985) 533

LIGHT PARTICLES AND PROJECTILE LIKE FRAGMENTS FROM 4 0 Ar + 1 2 C REACTIONAT 44 MeV/NUCLEONHEUER D . , BERTHOLET R . , GUET C . , MAUREL M., NIFENECKER H . , RISTORI C h . , SCHUSSLER F . ,BONDORF J . P . , NIELSEN O.B.CEN - GRENOBLE, NBI - COPENHAGENPHYSICS LETTERS B161 (1985) 269

OBSERVATION OF COMPOSITE NUCLEI AT VERY HIGH TEMPERATURESAUGER G. , JOUAN D . , PLAGNOL E . , POUGHEON F . , NAULIN F . , DOUBRE H . , GREGOIRE C.IPN - ORSAY, GANIL - CAENZ. FUR PHYSIK A321 (1985) 243

VANISHING OF THE FUSION-LIKE PROCESS I N HEAVY-ION COLLISIONSBORDERIE B . , RIVET M.F.IPN - ORSAYZ. FUR PHYSIK A321 (1985) 703

1 2 4 S n TARGET RESIDUES I N THE INTERACTION OF HEAVY IONS ATINTERMEDIATE ENERGIESBLACHOT J . , CRANCON J . , DE GONCOURT B . , GIZON A . , LLERES A.CEN - GRENOBLE, ISN - GRENOBLEZ. FUR PHYSIK A321 (1985) 645

HEAVY FRAGMENTS EMISSION I N THE 8 4 Kr ON 1 2 C REACTION AT 3 5MeV/NUCLEONMITTIG W., CUNSOLO A . , FOTI A . , WIELECZKO J . P . , AUGER F . , BERTHIER B . , PASCAUD J . M . ,QUEBERT J . , PLAGNOL E.GANIL - CAEN, CEN - SACLAY, CEN - BORDEAUX-GRADIGNAN, IPS - ORSAYPHYS. LETT. B154 (1985) 259

EMISSION TEMPERATURES I N INTERMEDIATE ENERGY NUCLEAR COLLISIONS FROMTHE RELATIVE POPULATIONS OF WIDELY SEPARATED STATES I N 5 L i AND 8 B ePOCHODZALLA J . , FRIEDMAN W.A., GELBKE C.K., LYNCH W.G., MAIER M., ARDOUIN D . ,DELAGRANGE H . , DOUBRE H . , GREGOIRE C . , KYANOWSKI A . , MITTIG W., PEGHAIRE A . ,PETER J . , SAINT-LAURENT F . , VIYOGI Y . P . , ZWIEGLINSKI B . , BIZARD G. , LEFEBVRES F . , TAMQUEBERT J .NSCL - MICHIGAN STATE UNIVERSITY, GANIL - CAEN, LPC - CAEN, CEN - BORDEAUX-GRADIGNANPHYS. LETT. B161 (1985) 275

NUCLEAR TEMPERATURES AND THE POPULATION OF PARTICLE-UNSTABLE STATES OF6 L i I N 4 0 Ar-INDUCED REACTIONS. ON 1 9 7 AU AT E / A = 60 MeVPOCHODZALLA J . , FRIEDMAN W.A., GELBKE C.K., LYNCH tf.G., MAIER M., ARDOUIN D . ,DELAGRANGE H . , DOUBRE H. , GREGOIRE C . , KYANOWSKI A . , MI1TIG W., PEGHAIRE A . ,PETER J . , SAINT-LAURENT F . , VIYOGI J . P . , ZWIEGLINSKI B . , BIZARD G. , LEFEBVRES F . ,TAMAIN B . , QUEBERT J .NSCL - MICHIGAN STATE UNIVERSITY, GANIL - CAEN, LPC - CAEN, CEN - BORDEAUX-GRADIGNANPHYS. REV. LETT. 55 (1985) 177

REACTION MECHANISM STUDY FOR THE SYSTEM : 2 0 Ne -f 6 0 Hi AT 4 4 MeV/AD'ONOFRIO A. , DUMOOT H. , DELAUNAY B. , DELAUNAY J . , BRONDI A. , MORO M. , ROMANO M.,TERRASI F . , CAVALLARO S . , SPERDUTO L . , SAINT-LAURENT M.G.CEN - SACLAY, INFN - NAPOLI, INFN - CATANIA, GANIL - CAENLETTERE AL NUOVO CIMENTO 42 (1985) 347

SEMICLASSICAL APPROACHES TO PROTON EMISSION I N INTERMEDIATE-ENERGYHEAVY-ION REACTIONSGREGOIRE C . , SCHEUTER F . , REMAUD B . , SEBILLE F.GANIL - CAEN, IPN - NANTESNUCL. PHYS. A436 (1985) 365

THREE-PARTICLE EFFECTS OBSERVED I N TWO-PARTICLE CORRELATIONMEASUREMENTSPOCHODZALLA J . , FRIEDMAN W.A., GELBKE C.K., LYNCH W.G., MAIER M., ARDOUIN D . ,DELAGRANGE H . , DOUBRE H. , GREGOIRE C . , KYANOWSKI A . , MITTIG W., PEGHAIRE A . ,PETER J . , SAINT-LAURENT F . , VIYOGI Y . P . , ZWIEGLINSKI B . , BIZARD G. , LEFEBVRES F . ,TAMAIN B . , QUEBERT J .HSU - EAST LANSING, GANIL - CAEN, LPC - CAEN, CEN - BORDEAUX-GRADIGNANPHYS. LETT. B161 (1985) 256

NUCLEAR COLLISIONS AT INTERMEDIATE ENERGIES : ACHIEVEMENTS ANDPROSPECTS OF GANILDETRAZ C,GANIL - CAENNATURE 315 (1985) 6017

1986

CONTRIBUTION OF DECAY PRODUCTS OF TRANSFER REACTIONS TO HEAVY-IONINELASTIC AND TRANSFER SPECTRABLUMENFELD Y . , ROYNETTE J . C . , CHOMAZ P h . , FRASCARIA N . , GARRON J . P . , JACMART J . C .IPN - ORSAYNUCLEAR PHYSICS A445 (1985) 151

ONE-NUCLEON TRANSFER REACTIONS TO DISCRETE LEVELS INDUCED BY A 7 9 3 MeV1 6 O BEAM ON A 2 0 8 Pb TARGETBERTHIER B . , BARRETTE J . , GASTEBOIS J . , GILLIBERT A . , LUCAS R . , MATUSZEK J . ,MERMAZ M.C. , MICZAIKA A . , VAN RENTERGHEM E . , SUOMIJARVI S . , BOUCENNA A . , DISDIER D . ,GORODETZKY P . , KRAUS L . , LINCK I . , LOTT B . , RAUCH V . , REBMEISTER R . ,SCHEIBLING F . , SCHULZ N . , SENS J . C . , GRUNBERG C. , MITTIG W.CEN - SACLAY, CRN - STRASBOURG, GANIL - CAENPHYSICS LETTERS B182 (1986) 15

ANGULAR EVOLUTION OF PERIPHERAL HEAVY ION REACTIONS AT INTERMEDIATEENERGIES

.BLUMENFELD Y . , CHOMAZ P h . , FRASCARIA N . , GARRON J . P . , JACMART J . C . , ROYNETTE J . C . ,ARDOUIN D . , MITTIG W.IPN - ORSAY, GANIL - CAENNUCLEAR PHYSICS A455 (1986) 357

FRAGMENTATION AND/OR TRANSFER REACTIONS I N THE 3 5 MeV/U A r + Ag SYSTEMBIZARD G. , BROU R . , DROUET A . , HARASSE J . M . , LAVILLE J . L . , PATRY J . P . , PLOYARD G.,STECKMEYER J . C . , TAMAIN B . , DOUBRE H . , PETER J . , GUILBAULT F . , LEBRUN C. ,OUBAHADOU A . , HANAPPE F.LPC • CAEN, GANIL - CAEN, IPN - NANTES, FNRS AND UNIV. LIBRE DE BRUXELLESPHYSICS LETTERS B172 (1986) 301

NUCLEAR CALEFACTIONNGO C . , DALILI D . , LUCAS R . , CERRUTI C . , LERAY S . , MAZUR C . , RIBRAG M . ,SUOMIJARVI T . , BERLANGER M., CHIODELLI S . , DEMEYER A . , GUINET D.CEN - SACLAY, IPN - LYONPROGRESS IN PARTICLE AND NUCL. PHYS. 15 (1986) 171

PROJECTILE LIKE FRAGMENT PRODUCTION I N A r INDUCED REACTIONS AROUND THEFERMI ENERGY : I . EXPERIMENTALS RESULTS - COMPETING MECHANISMSBORREL V . , GATTY B . , GUERREAU D . , GALIN J . , JACQUET D.IPN - ORSAY, GANIL - CAENZ. FUR PHYSIK A324 (1986) 205

PROJECTILE LIKE FRAGMENT PRODUCTION I N A r INDUCED REACTIONS AROUND THEFERMI ENERGY : I I . DIRECT SURFACE TRANSFER REACTIONS?'<?,RMAZ M.C. , BORREL V . , GALIN J . , GATTY B. , JACQUET D. , GUERREAU D.CEN - SACLAY, IPN - ORSAY, GANIL - CAENZ.' FUR PHYSIK A324 (1986) 217

CORRELATED SOURCES OF HEAVY FRAGMENTS I N THE Kr + Au REACTION AT 3 5AND 4 4 MeV/URUDOLF G. , ADLOFF J . C . , KAMILI A . , SCHEIBLING F . , BOISHU B . , GENOUX-LUBAIN A . ,LE BRUN C . , LECOLLEY J . F . , LEFEBVRES F . , LOUVEL M., REGIMBART R . , GRANIER 0 . ,LERAY S . , LUCAS R . , MAZUR C . , NGO C . , RIBRAG M., TOMASI E.CRN • STRASBOURG, LPC - CAEN, CEN - SACLAYPHYS. REV. LETT. 57 (1986) 2905

PERIPHERAL INTERACTIONS FOR 4 4 MeV/U 4 0 Ar ON * 7 A l AND n a t T i TARGETSDAYRAS R . , PAGANO A . , BARRETTE J . , BERTHIER B . , CASTRO RIZZO D.M., CHAVEZ E . ,CISSE 0 . , LEGRAIN R . , MERMAZ M.C., POLLACO E .G . , DELAGRANGE H . , MITTIG W., HEUSCH B . ,CONIGLIONE R . , LANZANO G. , PALMERI A.CEN - SACLAY, GANIL - CAEN, CRN - STRASBOURG, INFN - CATANIANUCLEAR PHYSICS A460 (1986) 299

BETA DECAY OP 1 ? C, " N, 2 2 O, 2 4 F, 2 6 Ne, » 2 A l , 3 4 A l , 3 5 - 3 6 S i ,36-37-38 p , 40 s

DUFOUR J . P . , DEL MORAL R . , FLEURY A . , HUBERT F . , JEAN D . , PRAVIKOFF M . S . ,DELAGRANGE H . , GEISSEL H . , SCHMIDT K.H.CEN - BORDEAUX-GRADIGN'AN, GANIL - CAEN, GSI - DARMSTADTZ. FUR PHYSIK A324 (1986) 487

F I R S T OBSERVATION OF THE EXOTIC NUCLEUS 2 2 CPOUGHEON F . , QUINIOU E . , BERNAS M., JACMART J . C . , HOATH S . D . , GUILLEMAUD-MUELLER D . ,SAINT-LAURENT M.G., ANNE R. , BAZIN D . , GUERREAU D . , MUELLER A . C . , DETRAZ C.IPN - ORSAY, GANIL - CAENEUROPHYSICS LETT. 2 (1986) 505

MAPPING OF THE PROTON D R I P - L I N E UP TO Z = 2 0 : OBSERVATION OF THE T z =_ 5 / 2 S E R I E S 2 3 S i , 2 7 S , 3 1 A r AND 3 5 c aLANGEVIN M., BERNAS M., GALIN J . , JACMART J . C . , NAULIN F . , POUGHEON F . , QUINIOU E . ,MUELLER A . C . , GUILLEMAUD-MUELLER D . , SAINT-LAURENT M.G., ANNE R. , GUERREAU D . ,DETRAZ C . , HOATH S.D.IPN - ORSAY, GANIL - CAEN, UNIV. OF BIRMINGHAMNUCL. PHYS. A455 (1986) 149

MASS MEASUREMENT OF LIGHT NEUTRON-RICH FRAGMENTATION PRODUCTSGILLIBERT A . , BIANCHI L . , FERNANDEZ B . , GASTEBOIS J . , CUNSOLO A . , FOTI A . ,GREGOIRE C . , MITTIG W., PEGHAIRE A . , SCHUTZ Y . , STEPHAN C.CEN - SACLAY, INFN - CATANIA, GANIL - CAEN, IPN - ORSAYPHYSICS LETT. B176 (1986) 317

PROJECTILE FRAGMENTS ISOTOPIC SEPARATION : APPLICATION TO THE LISESPECTROMETER AT GANILDUFOUR J . P . , DEL MORAL R . , EMMERMANN H . , HUBERT F . , JEAN D . , POINOT C . ,PRAVIKOFF M . S . , FLEURY A . , DELAGRANGE H . , SCHMIDT K.H.CEN/BG - GRADIGNAN, GANIL - CAEN, GSI - DARMSTADTNIM A248 (1986) 267

A SEMI-EXCLUSIVE STUDY OF HEAVY-RESIDUE PRODUCTION IN THE 4 0 AR + AGREACTION AT 3 5 MEV/U INCIDENT ENERGYBIZARD G . , BROU R . , DROUET A . , HARASSE J . M . , LAVILLE J . L . , PATRY J . P . , PLOYART G . ,STECKMEYER J . C . , TAMAIN B . , DOUBRE H . , PETER J . , GUILBAULT F . , LEBRUN C . ,OUBAHADOU A . , HANAPPE F .LPC - CAEN, GANIL - CAEN, IPN - NANTES, FNRS AND UNIV. LIBRE DE BRUXELLESZ. FUR PHYSIK A323 (1986) 459

REACTION MECHANISMS IN THE 4 0 AR + 1 9 7 AU COLLISIONS AT 35 MEV/NUCLEONBIZARD G . , BROU R . , DROUET A . , HARASSE J . M . , LAVILLE J . L . , PATRY J . P . , PLOYARD G . ,STECKME^ER J . C . , TAMAIN B . , HANAPPE P . , GUILBAULT F . , LEBRUN C . , OUBAHADOU A . ,DOUBRE H . , PETER J .LPC - ':AEN, FNRS AND UNIV. LIBRE DE BRUXELLES, LNS - NANTES, GANIL - CAENNUCL. PHYS. A456 (1986) 173

CONTRIBUTION OF DECAY PRODUCTS OF TRANSFER REACTIONS TO HEAVY-IONINELASTIC AND TRANSFER SPECTRABLUMENFELD Y . , ROYNETTE J . C . , CHOMAZ P H . , FRASCARIA N . , CARRON J . P . , JACMART J . C . ,CHOMAZ P h .IPN - ORSAYNUCL. PHYS. A445 (1985) 151

DEEXCITATION OF NUCLEI FORMED NEAR THE INSTABILITY TEMPERATURERIVET M.F. , BORDERIE B . , GAUVIN H. , GARDES D . , CABOT C . , HANAPPE F . , PETER J .IPN - ORSAY, FNRS AND UNIV. LIBRE DE BRUXELLES, GANIL - CAENPHYS. REV. C34 (1986) 1282

INCOMPLETE FUSION I N THE 4 0 A r + 6 8 Zn REACTION : TRENDS AND L I M I T SFAHLI A . , COFFIN J . P . , GUILLAUME G. , HEUSCH B . , JUNDT F. , RAMI F . , WAGNER P . ,FINTZ P . , COLE A . J . , KOX S., SCHUTZ Y.CRN - STRASBOURG, ISN - GRENOBLE, GANIL - CAENPHYS. REV. C34 (1986) 161

INVESTIGATION OP THE 8 4 Kr + 9 2 ' 9 8 Mo, n a t Ag AND 1 9 7 Au SYSTEMS AT 22Mev/uDALILI D . , LUCAS R . , NGO C . , CERRUTI C . , LERAY S . , MAZUR C . , RIBRAG M.,SUOMIJARVI T . , BERLANGER M., CHIODELLI S . , DEMEYER A . , GUINET D.CEN - SACLAY, IPN • LYONNUCL. PHYS. A454 (1986) 163

L I M I T S OF THE OBSERVATION OF F U S I O N - L I K E PRODUCTS I N THE 4 0 A r + 2 7 A lSYSTEMAUGER G. , PLAGNOL E . , JOUAN D . , GUET C . , HEUER D . , MAUREL M., NIFENECI&R H . ,RISTORI C . , SCHUSSLER F . , DOUBRE H . , GREGOIRE C.IPN - ORSAY, CEN - GRENOBLE, GANIL - CAENPHYS. LETT. B169 (1986) 161

TARGET RESIDUE CROSS SECTIONS AND RECOIL ENERGIES FROM THE REACTION OF1 7 6 0 MeV 4 0 A r WITH MEDIUM AND HEAVY TARGETSHUBERT F . , DEL MORAL R . , DUFOUR J . P . , EMMERMANN E . , FLEURY A . . POINOT C . ,PRAVILOFF M . S . , DELAGRANGE H . , LLERES A.CEN - BORDEAUX-GRADIGNAN, GANIL - CAEN, ISN - GRENOBLENUCL. PHYS. A456 (1986) 535

M U L T I P L I C I T Y DEPENDENT LIGHT PARTICLE CORRELATIONS I N 4 0 A r 1 9 7 AuREACTION AT E / A = 6 0 MeVKYANOWSKI A . , SAINT-LAURENT F . , ARDOUIN D . , DELAGRANGE H . , DOUBRE H . , GREGOIRE C . ,MITTIG W., PEGHAIRE A . , PETER J . , VIYOGI Y . P . , ZWIEGLINSKI B . , QUEBERT J . , BIZARD G.,LEFEBVRES F . , TAMAIN B . , POCKGDZALLA J . , GELBKE C.K., LYNCH tf.,MAIER M.

. GANIL - CAEN, CEN - BORDEAUX-GRADIGNAN, LPC - CAEN, MSU - EAST-LANSINGPHYS. LETT. B181 (1986) 43

NEW DEVELOPMENTS OF THE GANIL CONTROL SYSTEMLECORCHE E. AND THE GANIL CONTROL GROUPGANIL - CAENNIM A247 (1986) 37

STOPPING POWERS OF SOLIDS FOR 4 0 Ar AND 4 0 Ca IONS AT INTERMEDIATEENERGIES (20 -80 MeV/u)BIMBOT R . , GAUVIN H . , ORLIANGE I . , ANNE R . , BASTIN G. , HUBERT F .IPN - ORSAY, GANIL - CAEN, CSNSM - ORSAY, CEN - BORDEAUX-GRADIGNANNIM B17 (1986) 1

DYNAMICAL DECAY OF NUCLEI AT HIGH TEMPERATURE : COMPETITION BETWEENPARTICLE EMISSION AND F I S S I O N DECAYDELAGRANGE H . , GREGOIRE C . , SCHEUTER F . , ABE Y.GANIL - CAEN, KYOTO UNIV. - KYOTOZ. FUR PHYSIK A323(1986) 437

DYNAMICAL MODEL OF NUCLEAR F I S S I O N WITH SHELL EFFECTSBONASERA A.GANIL - CAENPHYS. REV. C34 (1986) 740

THE FUSION PROCESS IN LANDAU-VLASOV DYNAMICSREMAUD B . , SEBILLE F . , GREGOIRE C . , VINET L.IPN - NANTES, GANIL - CAENPHYS. LETT. B180 (1986) 198

1987

1 6 o + 2 8 8 i E I A S T i c SCATTERING AT E L = 94 HeV/NUCLEONROUSSEL P . , BARRETTE J . , AUGER F . , BERTHIER B . , FERNANDEZ B . , GASTEBOIS J . ,GILLIBERT A . , PAPINEAU L . , MITTIG W., DISDIER D . , LOTT B . , RAUCH V . , SCHEIBLING F . ,STEPHAN C. , TASSAN-GOT L.CEN - SACLAY, GANIL - CAEN, CRN - STRASBOURG, IPN - ORSAYPHYS. LETT. B185 (1987) 29

ONE-NUCLEON STRIPPING REACTIONS TO DISCRETE LEVELS INDUCED BY A 7 9 3MeV 1 6 O BEAM ON A 2 0 8 p b TARGETMERMAZ M.C..BERTHIER B.,BARRETTE J.,GASTEBOIS J.,GILLIBERT A..LUCAS R.,MATUSZEK J . ,MICZAIKA A.,VAN RENTERGHEM E.,SUOMIJARVI T..BOUCENNA A..DISDIER D..GORODETZKY P . ,KRAUS L.,LINCK I.,LOTT B.,RAUCH V..REBMEISTER R..SCHEIBLING F.,SCHULZ N . ,SENS J.C..GRUNBERG C.,MITTIG W.CEN,SACLAY - CRN,STRASBOURG - GANIL,CAENZ PHYS A326 (1987) 353

HIGH EXCITATION INELASTIC SCATTERING INDUCED BY INTERMEDIATE ENERGY 4 0

A r BEAMSFRASCARIA N . , BLUMENFELD Y. , CHOMAZ P h . , GARRON J . P . , JACMART J . C . , ROYNETTE J . C . ,SUOMIJARVI T . , MITTIG W.IPN ORSAY, GANIL CAENNUCLEAR PHYSICS A474 (1987) 253

BINARY BREAK-UP OP A HIGH-VELOCITY HOT SOURCE FORMED I N THE 8 4 Kr + 9 3

Nb REACTION AT 3 4 . 5 MeV/UCHARVET J . L . , DUCHENE G. , JOLY S . , MAGNAGO C. , MORJEAN M., PATIN Y . , PRANAL Y . ,SINOPOLI L . , UZUREAU J . L . , BILLEREY R . , CHAMBON B . , CHBIHI A . , CHEVARIER A . ,CHEVARIER N . , DRAIN D . , PASTOR C . , STERN M., PEGHAIRE A.CEN, BRUYERES-LE-CHATEL - IPN, LYON - GANIL, CAENPHYS. LETT. B189 (1987) 388

FORWARD ANGLE MEASUREMENTS OF 60 MeV/NUCLEON 4 ° A r PERIPHERALINTERACTIONS ON 2 0 8 PbSUOMIJARVI T . , BEAUMEL D. , BLUMENFELD Y. , FRASCARIA N . , GARRON J . P . , JACMART J . C . ,ROYNETTE J . C . , CHOMAZ P h . , BARRETTE J . , BERTHIER B . , FERNANDEZ B . , GASTEBOIS J . ,MITTIG W.IPN ORSAY, CEN SACLAY, GANIL CAENPHYSICAL REVIEW C36 (1987) 2691

ORIGIN OF PROJECTILE-LIKE FRAGMENTS FROM 4 0 A r + 6 8 Zn COLLISIONSBETWEEN 1 5 AND 3 5 MeV/NUCLEON : DIRECT TRANSFER OF NUCLEONS ANDPROJECTILE FRAGMENTATIONRAMI F . , FAHLI A . , COFFIN J . P . , GUILLAUME G., HEUSCH B . , JUNDT F . , WAGNER P . ,FINTZ P . , KOX S . ( SCHUTZ Y. , MERMAZ M.C.CRN, STRASBOURG - ISN, GRENOBLE - GANIL, CAEN - CEN, SACLAYZ. PHYS. A327 (1987) 207

PERIPHERAL COLLISIONS IN THE 8 4 K r + 9 2 ' 9 8 M o , n a t A g AND 1 9 7 A u REACTIONSAT 22 MeV/ULUCAS R..NGO C.,SUOMIJARVI T..BERLANGER M..CERRUTI C..CHIODELLI S..DALILI D . ,DEMEYER A..GUINET D.,LERAY S..MAZUR C..RIBRAG M.CEN,SACLAYNUCL PHYS A 464(1987)172

BETA-DELAYED PROTON DECAY OF THE Tz = - 5 / 2 ISOTOPE 3 1 A rBORREL V . , JACMART J . C . , POUGHEON F . , RICHARD A . , ANNE R . , BAZIN D . , DELAGRANGE H . ,DETRAZ C. , GUILLEMAUD-MUELLER D. , MUELLER A . C . , ROECKL E . , SAINT-LAURENT M.G.,DUFOUR J . P . , HUBERT F . , PRAVIKOFF M.S.IPJV - ORSAY, GANIL - CAEN, CEN BORDEAUX-GRADIGNANNUCLEAR PHYSICS A473 (1987) 331

DIRECT OBSERVATION OF NEW PROTON RICH NUCLEI IN THE REGION 23<Z<29USING A 5 5 A. MeV 5 8 NI BEAM v v

POUGHEON F . , JACMART J . C . , QUINIOU E. , ANNE R., BAZIN D. , BORREL V., GALIN J . ,GUERREAU D. , GUILLEMAUD-MUELLER D., MUELLER A.C. , ROECKL E . , SAINT-LAURENT M GDETRAZ C.IPN, ORSAY - GANIL, CAENZ. PHYS. A327 (1987) 17

MEASUREMENT OF TOTAL REACTION CROSS SECTIONS OF EXOTIC NEUTRON-RICHNUCLEIMITTIG W. , CHOUVEL J .M. , ZHAN WEN LONG, BIANCHI L . , CUNSOLO A. , FERNANDEZ B . ,FOTI A. , GASTEBOIS J . , GILLIBERT A. , GREGOIRE C. , SCHUTZ Y. , STEPHAN C.GANIL - CAEN , CEN - SACLAY , DIPARTIMENTO DI FISICA NUCLEARE - CATANIA ,IPN - ORSAY , INSTITUTE OF MODERN PHYSICS - LANZHOUPHYSICAL REVIEW LETTERS 59 (1987) 1889

NEW MASS MEASUREMENTS FAR FROM STABILITYGILLIBERT A.,MITTIG W.,BIANCHI L.,CUNSOLO A..FERNANDEZ B.,FOTI A..GASTEBOIS J . ,GREGOIRE C.,SCHUTZ Y.,STEPHAN C.GANIL,CAEN - CEN,SACLAY - INFN,CATANIA - IPN,ORSAYPHYS LETT B192 (1987) 39

OBSERVATION OF A BOUND Tz = - 3 NUCLEUS: 2 2 S iSAINT-LAURENT M.G.,DUFOUR J.P.,ANNE R..BAZIN D.,BORREL V..DELAGRANGE H..DETRAZ C. ,GUILLEMAUD-MUELLER D..HUBERT F.,JACMART J.C..MUELLER A.C..POUGHEON F..PRAVIKOFF M.S. ,ROECKL E.GANIL,CAEN - CEN.BORDEAUX - IPN,ORSAYPHYS REV LETT 59 (1987) 33

FORMATION AND DECAY OF HOT NUCLEIVOLANT C..CONJEAUD M..HARAR S..MOSTEFAI M..POLLACO E.C.,CASSAGNOU Y..DAYRAS R.,LEGRAIN R..KLOTZ-ENGMANN G..OESCHLER H.CEN,SACLAY- TECHNISHE HOCHSCHULE,DARMSTADTPHYS. LETT. B195 (1987) 22

GAMMA RAYS FROM PERIPHERAL AND CENTRAL COLLISIONS I N THE REACTION 4 0 A r•f 1 5 8G<2 AT 44 MeV/NUCLEONHINGMANN R., KUHN W., METAG V., MUHLHANS R. , NOVOTNY R., RUCKELSHAUSEN A. ,GASSING W., EMLING H., KULESSA R., WOLLERSHEIM H . J . , HAAS B . , VIVIEN J . P . ,BOULLAY A. , DELAGRANGE H., DOUBRE H., GREGOIRE C. , SCHUTZ Y.GIESSEN UNIV. - GIESSEN, GSI - DARMSTADT, - CRN - STRASBOURG,- GANIL - CAENPHYS. REV. LETT. 58 (1987) 759

HIGH ENERGY GAMMA-RAY PRODUCTION FROM 44 MeV/A 8 6 Kr BOMBARDMENT ONNUCLEIBERTHOLET R., KWATO NJOCK M., MAUREL M., MONNAND E . , NIFENECKER H. , PERRIN P . ,PINSTON J . A . , SCHUSSLER F . , BARNEOUD D. , GUET C. , SCHUTZ Y.CEN - GRENOBLE, ISN - GRENOBLE, GANIL - CAENNUCLEAR PHYSICS A474 (1987) 541

PRODUCTION AND DECAY OF HIGHLY EXCITED NUCLEAR SYSTEMS FORMED IN 8 4 Kr+ 12 c AND 2 7 Al COLLISIONS AT 35 MeV/NUCLEONAUGER F . , BERTHIER B . , CUNSOLO A., FOTI A. , MITTIG W., PASCAUD J .M. , PLAGNOL E . ,QUEBERT J . , WIELECZKO J . P .CEN - SACLAY, INFN - CATANIA, GANIL - CAEN, CEN - BORDEAUX, IPN - ORSAYPHYS. REV. C35 (1987) 190

LIGHT CHARGED PARTICLE EMISSION IN 4 0 A r INDUCED REACTIONS ON 6 8 Z n AT1 4 . 6 , 1 9 . 6 AND 3 5 MeV/NUCLEONFAHLI A.,COFFIN J.P.GUILLAUME G..HEUSCH B..JUNDT F.,RAMI F..WAGNER P..FINTZ P . ,COLE A.J.,KOX S..SCHUTZ Y.,CINDRO N.CRN - STRASBOURG - ISN - GRENOBLE, GANIL - CAEN, RUDJER BOSKOVIC INST. - ZAGREBZ.PHYS A326 (1987) 169

INCLUSIVE TWO-PARTICLE CORRELATIONS FOR l 6 O-INDUCED REACTIONS ON 1 9 7

Au AT E/A = 9 4 MeVCHEN Z . , GELBKE C.K. , GONG W.G., KIM Y.D. , LYNCH W.G., MAIER M.R., POCHODZALLA J . ,TSANG M.B. , SAINT-LAURENT F . , ARDOUIN D. , DELAGRANGE H . , DOUBRE H . , KASAGI J . ,KYANOWSKI A . , PEGHAIRE A . , PETER J . , ROSATO E . , BIZARD G., LEFEBVRES F . ,TAMAIN B . , QUEBERT J . , VIYOGI Y.P .HSU - EAST LANSING, GANIL - CAEN, CEN - BOSDEAUX-GRADIGNAN,VARIABLE ENERGY CYCLOTRON CENTRE - CALCUTTAPHYSICAL REVIEW C36, (1987) 2297

POPULATION OF PARTICLE UNBOUND STATES FOR THE REACTION 1 6 O + Au ATE/A = 9 4 MeVCHEN Z . , GELBKE C.K. , GONG W.G., KIM Y . D . , LYNCH W.G., MAIER M.R., POCHODZALIA J . ,TSANG M.B. , SAINT-LAURENT F . , ARDOUIN D . , DELAGRANGE H . , DOUBRE H . , KASAGI J . ,KYANOWSKI A . , PEGHAIRE A . , PETER J . , ROSATO E . , BIZARD G. , LEFEBVRES F . ,TAMAIN B . , QUEBERT J . , VIYOGI Y .P .NSCL MSU - EAST LANSING, GANIL - CAEN, LPC - CAEN, CEN - BORDEAUX-GRADIGNAN,VARIABLE ENERGY CYCLOTRON CENTRE - CALCUTTAPHYSICS LETTERS B199 (1987) 171

TWO-PARTICLE CORRELATIONS AT SMALL RELATIVE MOMENTA FOR 4 0 Ar-INDUCEDREACTIONS ON 197 A u AT E/A = 60 MeVPOCHODZALLA J . , GELBKE C.K. , LYNCH W.G., MAIER M., ARDOUIN D . , DELAGRANGE H . ,DOUBRE H . , GREGOIRE C . , KYANOWSKI A . , MITTIG W., PEGHAIRE A . , PETER J . ,SAINT-LAURENT F . , ZWIEGLINSKI B . , BIZARD G. , LEFEBVRES F . , TAMAIN B . , QUEBERT J . ,VIYOGI Y . P . , FRIEDMAN W.A., BOAL D.H.NSCL, EAST LANSING - GANIL, CAEN - LPC, CAEN - CEN, BORDEAUX - WISCONSIN UNIV.,MADISON - SIMON FRASER UNIV., BURNABYPHYS. REV. C35 (1987) 1695

SUBTHRESHOLD Pl'ON PRODUCTION AND STATISTICAL MULTIFRAGMENTATION INNUCLEUS-NUCLEUS COLLISIONSBARZ H.W., BONDORF J . P . , GUET C . , LOPEZ J . , SCHULZ H.NIELS BOHR INST. - COPENHAGEN, CENTRAL INST. OF NUCL. RESEARCH - DRESDEN,GANIL - CAENEUROPHYS. LETT. 4 (1987) 997

ACHROMATIC SPECTROMETER LISE AT GANILANNE R . , BAZIN D . , MUELLER A . C . , JACMART J . C . , LANGEVIN M.GANIL, CAEN - IPN, ORSAYNIM A257 (1987) 215

"DELF" A LARGE SOLID ANGLE DETECTION SYSTEM FOR HEAVY FRAGMENTSBOUGAULT R..DUCHON J..GAUTIER J.M..GENOUX-LUBAIN A. ,LE BRUN C..LECOLLEY J . F . ,LEFEBVRES F..LOUVEL M..MOSRIN P. .REGIMBART R.LPC - CAENNIM A259 (1987) 473

STOPPING POWERS OF SOLIDS FOR 1 6 0 IONS AT INTERMEDIATE ENERGIES( 2 0 - 9 5 M6V/U)

GAUVIN H . , BIMBOT R . , HERAULT J . , ANNE R . , BASTIN G. , HUBERT F .IPN - ORSAY, GANIL - CAEN, CSNSM - ORSAY, CEN - BORDEAUX-GRADIGNANNIM B28 (1987) 1 9 1 .

A STUDY OF THE DISINTEGRATION OF HIGHLY EXCITED NUCLEI WITHVLASOV-UEHLING-UHLENBECK EQUATIONVINET L..GREGOIRE C.,SCHUCK P..REMAUD B..SEBILLE F . ,GANIL - CAEN, IPN - NANTESNUCL. PHYS. A468 (1987) 3 2 1 .

DISSIPATIVE EFFECTS IN PROJECTILE FRAGMENTATIONBONASERA A . , d i TORO M., GREGOIRE C.GANIL - CAEN, INFN - CATANIANUCL. PHYS. A463 (1987) 653 .

PERIPHERAL REACTIONS AT INTERMEDIATE ENERGIES IN LANDAU-VLASOV DYNAMICSGREGOIRE C . , REMAUD B . , SEBILLE F . , V1NET L .GANIL - CAEN - IRSTE - NANTESFEYS. LETT. B186 (1987) 14.

SEMI-CLASSICAL DYNAMICS OF HEAVY ION REACTIONSGREGOIRE C . , REMAUD B . , SEBILLE F . , VINET L . , RAFFRAY Y.GANIL - CAEN, IPN - NANTESNUCL. PHYS. A465 (1987) 317.

1988

1 6 O ELASTIC SCATTERING AT E l a b = 94 MeV/NUCLEONROUSSEL-CHOMAZ P . , ALAMANOS N. , AUGER F . , BARRETTE J . , BERTHIER B . , FERNANDEZ B . ,PAPINEAU L . , DOUBRE H . , MITTIG W.CEN - SACLAY, GANIL - CAENNUCLEAR PHYSICS A477 (1988) 345

HIGH SPIN LEVELS POPULATED IN MULTINUCLEON-TRANSFER REACTIONS WITH 4 8 0Mev 1 2 cKRAUS L . , BOUCENNA A . , LINCK I . , LOTT B . , REBMEISTER R . , SCHULZ N . , SENS J . C . ,MERMAZ M.C. , BERTHIER B . , LUCAS R . , GASTEfiOIS J . , GILLIBERT A . , MICZAIKA A . ,TOMASI-GUSTAFSSON E . , GRUNBERG C.CRN - STRASBOURG, CEN - SACLAY, GANIL - CAENPHYSICAL REVIEW C37, 6 (1988) 2529

ONE-NUCLEON STRIPPING REACTIONS TO DISCRETE LEVELS INDUCED BY A 4 8 0MeV 1 2 C BEAM ON A 208 p b TARGETMERMAZ M.C. , TOMASI-GUSTAFSSON E . , BERTHIER B . , LUCAS R . , GASTEBOIS J . , GILLIBERT A . ,MICZAIKA A . , BOUCENNA A . , KRAUS L . , LINCK I . , LOTT B . , REBMEISTER R . , SCHULZ N . ,SENS J . C . , GRUNBERG C.CEN SACLAY, CRN STRASBOURG, GANIL CAENPHYSICAL REVIEW C37, 5 (1988) 1942

APPROACHING PERIPHERAL COLLISIONS IN THE REACTION Ar + AU AT 2 7 . 2MeV/U BY THE ASSOCIATED NEUTRON MULTIPLICITIESMORJEAN M., FREHAUT J . , GUERREAU D . , CHARVET J . L . , DUCHENE G. , DOUBRE H . , GALIN J . ,INGOLD G. , JACQUET D . , JAHNKE U . , JIANG D.X. , LOTT B . . MAGNAGO C . , PATIN Y . ,POUTHAS J . , PRANAL Y . , UZUREAU J . L .CEN BRUYERES-LE-CHATEL - GANIL CAEN - HMI BERLIN - IPN ORSAYPHYSICS LETTERS B203, 3 (1988) 215

MULTIPLE ANGULAR SCATTERING OP HEAVY IONS ( 1 6 / 1 7 O, 4 0 A r , 8 6 Kr AND1 0 0 MO) AT INTERMEDIATE ENERGIES ( 2 0 - 9 0 M e V / u )ANNE R . , HERAULT J . , BIMBOT R . , GAUVIN H. , BASTIN G. , HUBERT F .GANIL - CAEN, IPN - ORSAY, CSNCM - ORSAY, CEN - BORDEAUX-GRADIGNANNIM B34 (1988) 295

STUDY OP THE 1 2 9 Xe + n a t Ag , 1 9 7 AU SYSTEMS AT 2 3 . 7 AND 27 MeV/UGRANIER 0 . , CERRUTI C . , LERAY S . , L'HENORET P . , LUCAS R . , MAZUR C . , NATOWITZ J . ,NGO C . , RIBRAG M., TOMASI E . , IMME G., RACITI G. , SPINELLA G. , ALBINSKI L . , GOBBI A . ,HERMANN N . , HILDENBRAND K.D. , OLMI A.CEN SACLAY - INFN CATANIA - GSI DARMSTADT - INFN FIRENZENUCLEAR PHYSICS A481 (1988) 109

DEEPLY INELASTIC COLLISIONS AS A SOURCE OF INTERMEDIATE MASS FRAGMENTSAT E / A = 2 7 MeVBORDERIE B . , MONTOYA M., RIVET M . F . , JOUAN D . , CABOT C . , FUCHS H. , GARDES D . ,

.GAUVIN H . , JACQUET D . , MONNET F . , HANAPPE F .IPN - ORSAY, FNRS AND UNIV. LIBRE DE BRUXELLESPHYSICS LETTERS B205, 1 (1988) 26

BETA DELAYED MULTI-NEUTRON RADIOACTIVITY OF i 7 B , i 4 B e , 1 9 CDUFOUR J . P . , DEL MORAL R . , HUBERT F . , JEAN D . , PRAVIKOFF M . S . , FLEURY A . ,MUELLER A . C . , SCHMIDT K.H. , SUMMERER K., HANELT E . , FREHAUT J . , BEAU M., GIRAUDET G.CEN - BORDEAUX GRADIGNAN, GANIL - CAEN, GSI - DARMSTADT,TECHNISCHEN HOCHSCHULE - DARMSTADT, CEN - BRUYERES LE CHATELPHYSICS LETTERS B206, 2 (1988) 195

BETA-DELAYED NEUTRON EMISSION OF l 5 B, 1 8 C, 1 9 / 2 0 N / 3 4 , 3 5 A 1 ^^ 39 p

MUELLER A . C . , BAZIN D . , SCHMIDT-OTT W.D., ANNE R . , GUERREAU D . ,GUILLEMAUD-MUELLER D . , SAINT-LAURENT M.G., BORREL V . , JACMART J . C . , POUGHEON F . ,RICHARD A.GANIL - CAEN, IPN - ORSAYZ. PHYS. A330 (1988) 63

EVIDENCE FOR PERSISTING MEAN FIELD EFFECTS AT E/A = 60 MeV FROMPARTICLE-PARTICLE CORRELATION MEASUREMENTS AND THEORETICALINVESTIGATIONS WITH THE LANDAU VLASOV EQUATIONARDOUIN D . , BASRAK Z . , SCHUCK P . , PEGHAIRE A . , DELAGRANGE H . , DOUBRE H . , GREGOIRE C . ,KYANOWSKI A . , MITTIG W., PETER J . , SAINT-LAURENT F . , ZWIEGLINSKI B . , VIYOGI Y . P . ,GELBKE C . K . , LYNCH W.G. , MAIER M. , POCHODZALLA J . , QUEBERT J . , BIZARD G., LEFEBVRES F . , TAMAIN B.

NANTES UNIV.- NANTES, ISN - GRENOBLE, VECC - CALCUTTA, NSCL - EAST LANSING,CEN BORDEAUX - GRADIGNAN, LPC - CAENZ. PHYS. A329 (1988) 505

POSSIBLE DYNAMICAL LIMITATIONS TO EXCITATION ENERGY STORAGE IN NUCLEISAINT-LAURENT F . , KYANOWSKI A . , ARDOUIN D . , DELAGRANGE H . , DOUBRE H . , GREGOIRE C . ,MITTIG W., PEGHAIRE A . , PETER J . , BIZARD G . , LEFEBVRES F . , TAMAIN B . , QUEBERT J . ,VIYOGI Y . P . , POCHODZALLA J . , GELBKE C . K . , LYNCH W., MAIER M.GANIL - CAEN, LPC CAEN UNIV. - CAEN, CENB • GRADIGNAN,VARIABLE ENERGY CYCLOTRON CENTRE - CALCUTTA, NSCL MICHIGAN STATE UNIV. - EAST LAN^WGPHYSICS LETTERS B202 (1988) 190

NEUTRON ENERGY DISTRIBUTIONS IN THE DYNAMICAL COMPETITION BETWEENEVAPORATION AND FISSIONGREGOIRE C . , DELAGRANGE H . , POMORSKI K . , DIETRICH K.GANIL CAEN - TUM GARCHINGZ. PHYS. A329 (1988) 497

PREEQUILIBRIUM EMISSION AND INCOMPLETE FUSION OF LOW AND MEDIUM MASSHEAVY ION SYSTEMSGONIN M . , COFFIN J . P . , GUILLAUME G . , JUNDT F . , WAGNER P . , FINTZ P . , HEUSCH B . ,MALKI A . , FAHLI A . , KOX S . , MERCHEZ F . , MISTRETTA J .CRN - STRASBOURG, ISN - GRENOBLEPHYS. REV. C38, 1 (1988) 135

HIGH ENERGY GAMMA-RAY PRODUCTION IN HEAVY ION REACTIONSKWATO NJOCK M . , MAUREL M . , MONNAND E . , NIFENECKER H . , PINSTON J . A . , SCHUSSLER F . ,BARNEOUD D . , SCHUTZ Y.CEN - GRENOBLE, ISN - GRENOBLE, GANIL - CAENNUCLEAR PHYSICS A482 (1988) 489c

LIGHT-FRAGMENT EMISSION IN HEAVY-ION REACTIONS PRODUCING PIONS ANDPROTONS IN 16 O + 2 7 A l COLLISIONS AT 94 MeV/NUCLEONAIELLO S . , BADALA A . , BARBERA R . , LO NIGRO S . , NICOTRA D . , PALMERI A . ,PAPPALARDO G . S . , BIZARD G . , BOUGAULT R . , DURAND D . , GENOUX-LUBAIN A . , LAVILLE J . L . ,LEFEBVRES F .INFN CATANIA - LPC ISMRA CAENEUROPHYSICS LETTERS 6, 1 (1988) 25

LINEAR MOMENTUM TRANSFER IN PION ACCOMPANIED 1 6 0 INDUCED REACTION ON232 T h A T 95 M e V p E R HUCLEONERAZMUS B . , GUET C . , McGRATH R . , SAINT-LAURENT M . G . , SCHUTZ Y . , BOLORE M . ,CASSAGNOU Y . , DABROWSKI H . , HISLEUR M . , JULIEN J . , LEGRAIN R . , LE BRUN C . ,LECOLLEY J . F . , LOUVEL M.GASIL - CAEN, CEN - SACLAY, LPC ISMRA - CAENNUCL. PHYS. A481 (1988) 821

DISPERSIONS IN SEMICLASSICAL DYNAMICSZIELINSKA-PFABE M., GREGOIRE C.SMITH COLLEGE - NORTHAMPTON, GANIL - CAENPHYSICAL REVIEW C37, 6 (1988) 2594

FLUCTUATIONS OF SINGLE-PARTICLE DENSITY IN NUCLEAR COLLISIONSAYIK S . , GREGOIRE C.TENNESSEE TECHNOLOGICAL UNIVERSITY - COOKEVILLS, GANIL - CAENPHYS. LETT. B212 (1988) 269

LIQUID DROP PARAMETERS FOR HOT NUCLEIGUET C . , STRUMBERGER E . , BRACK H.GANIL - CAEN, REGENSBURG UNIV. - R5GENSBURGPHYSICS LETTERS B205, 4 (1988) 427

1989

TRANSFER REACTIONS AND SEQUENTIAL DECAYS « » THE PROJECTILE-LIKEFRAGMENTS IN THE 60 MeV/NUCLEON 40 Ar + a a t A g , i9"* AU REACTIONSSTECKMEYER J . C . , BIZARD G., BROU R., EUDES P . , LAVILLE J . L . , NATOWITZ J . B . ,PATRY J . P . , TAMAIN B . , THIPHAGNE A., DOUBRE H., PEGHAIRE A. , PETER J . , ROSATO E. ,ADLOFF J . C . , KAMILI A. , RUDOLF G., SCHEIBLING F . , GUILBAULT F . , LEBRUN C. , HANAPPE

F.LPC - CAEN, GANIL - CAEN, CRN - STRASBOURG, LSN - NANTES,FONDS NATIONAL DE LA RECHERCHE SCIENTIFIQUE - BRUXELLESNUCL. PHYS. A500 (1989) 372.

EXCITATION OF GIANT RESONANCES IN 2 ° Ne + 9 0 Zr AND 2 0 8 Pb INELASTICSCATTERING AT 40 MeV/uSUOMIJARVI T . , BEAUMEL D. , BLUMENFELD Y., CHOMAZ Ph . , FRASCARIA N. , GARRON J . P . ,JACMART J . C . , ROYNETTE J . C . , BARRETTE J . , BERTHIER B . , FERNANDEZ B . , GASTEBOIS J . ,ROUSSEL-CHOMAZ P . , MITTIG W., KRAUS L . , LINCK I .IPN - ORSAY, CEN - SACLAY, GANIL - CAEN, CRN - STRASBOURGNUCLEAR.PHYSICS A491 (1989) 314

HEAVY ION CHARGE EXCHANGE REACTIONS TO PROBE THE GIANT ELECTRICISOVECTOR MODES IN NUCLEIBERAT C. , BUENERD M., CHAUVIN J . , HOSTACHY J . Y . , LEBRUN D. , MARTIN P . , BARRETTE J . ,BERTHIER B . , FERNANDEZ B . , MICZAIKA A., MITTIG W., STILIARIS E . , VON OERTZEN W.,LENSKE H., WOLTER H.H.ISN - GRENOBLE, CEN - SACLAY, GANIL - CAEN, HMI - BERLIN,UNIVERSITY OF MUNICH - MUNICHPHYSICS LETCERS B218, 3 (1989) 299

DYNAMICAL ASPECTS OF VIOLENT COLLISIONS IN Ar + Ag REACTIONS AT E/A =27 MeVRIVET M.F. , BORDERIE B . , GREGOIRE C. , JOUAN D. , REMAUD B.IPN - ORSAY, GANIL - CAEN, LPN/UA - NANTESPHYS. LETT. B215 (1988) 55

BETA DECAY OF 2 2 OHUBERT F . , DUFOUR J . P . , DEL MORAL R., FLEURY A., JEAN D. , PRAVIKOFF M.S. ,DELAGRANGE H. , GEISSEL H. , SCHMIDT K.H., HANELT E.CEN - BORDEAUX -GRADIGNAN, GANIL - CAEN, GSI - DARMSTADT,INST. FUR KERNPHYSIK - DARMSTADTZ. PHYS. A333 (1989) 237.

OBSERVATION OF NEW NEUTRON RICH NUCLEI 2 9 F, 35»36 Mg, 3 8/ 3 9 Al, 4 0' 4 1

Si, 43,44 pf 45-47 s / 46-49 c l / J^Q 49-51 A r pRQM THE INTERACTIONS OF55 MeV/u 4 8 Ca + TaGUILLEMAUD-MUELLER D., PENIONZHKEVICH Yu.E., ANNE R., ARTUKH A.G., BAZIN D.,BORREL V., DETRAZ C., GUERREAU D., GVOZDEV B.A., JACMART J.C., JIANG D.X.,KALININ A.M., KAMANIN V.V., KUTNER V.B., LEWITOWICZ M., LUKYANOV S.M., MUELLER A.C.,HOAI CHAU N., POUGHEON F., RICHARD A., SAINT-LAURENT M.G., SCHMIDT-OTT W.D.GANIL - CAEN, JINR - DUBNA, IPN - ORSAY, GOTTINGEN UNIV. - GOTTINGENZ. PHYS. A332 (1989) 193.

SATURATION OF THE THERMAL ENERGY DEPOSITED IN AU AND Th NUCLEI BY ArPROJECTILES BETWEEN 27 AND 77 MeV/UJIANG D.X., DOUBRE H. , GALIN J . , GUERREAU D. , PIASECKI E . , POUTHAS J . , SOKOLOB A. ,CRAMER B . , INGOLD G., JAHNKE U. , SCHWINN E . , CHARVET J . L . , FREHAUT J . , LOTT B . ,MAGNAGO C. , MORJEAN M., PATIN Y. , PRANAL Y., UZUREAU J . L . , GATTY B . ,JACQUET D.GANIL • CAEN, HMI - BERLIN, CEN - BRUYERES-LE-CHATEL, IPN - ORSAYNUCL. PHYS. A503 (1989) 560.

CORRELATIONS BETWEEN PROJECTILELIKE AND TARGETLIKE FRAGMENTS I N THEREACTION 2 7 A l + 44-MeV/NOCLEON 4 0 A rDAYRAS R . , CONIGLIONE R . , BARRETTE J . , BERTHIER B . , de CASTRO RIZZO D.M., CISSE 0 . ,GADI F . , LEGRAIN R . , MERMAZ M.C., DELAGRANGE H . , MITTIG W., HEUSCH B . , LANZANO G.,PAGANO A.CEN - SACLAY, GANIL - CAEN, CRN - STRASBOURG,ISTITUTO NAZIONALE DI FISICA NUCLEARE - CATANIAPHYS. REV. LETT. 62 (1989) 1017.

INCOMPLETE FUSION I N NUCLEUS-NUCLEUS CENTRAL COLLISIONS. STUDY OF40 A r ON 2 7 A l FROM 2 5 TO 8 5 MeV/uHAGEL K. , PEGHAIRE A . , J IN G.M., CUSSOL D. , DOUBRE H . , PETER J . , SAINT-LAURENT F . ,BIZARD G., BROU R . , LOUVEL M., PATRY J . P . , REGIMBART R . , STECKMEYER J . C . , TAMAIN B . ,CASSAGNOU Y . , LEGRAIN R . , LEBRUN C. , ROSATO E . , MACGRATH R. , JEONG S . C . ,

LEE S.M., NAGASHIMA Y. , NAKAGAWA T . , OGIHARA M., KASAGI J . , MOTOBAYASHI T.GANIL-CAEN, TEXAS A&M UNIV.-COLLEGE STATION, LPC-CAEN, CEN-SACLAY, LPN-NANTES,DIPARTIMENTO DI SCIENZE FISICHE-NAPLES, SUNY-STONY BROOK, TSUKUBA UNIV.-IBARAKI-KEN,TOKYO INST.TECH.-TOKYO, RIKKYO UNIV.-TOKYO, INST.MOD.PHYS.-LANZHOUPHYS. LETT. B229 (1989) 20 .

TOTAL CROSS SECTIONS OF REACTIONS INDUCED BY NEUTRON-RICH LIGHT NUCLEISAINT-LAURENT M.G., ANNE R. , BAZIN D . , GUILLEMAUD-MUELLER D . , JAHNKE U . ,J IN GEN MING, MUELLER A . C . , BRUANDET J . F . , GLASSER F . , KOX S . , LIATARD E . ,TSAN UNG CHAN, COSTA G . J . , HEITZ C . , EL-MASRI Y . , HANAPPE F . , BIMBOT R . , ARNOLD E . ,NEUGART R.GANIL - CAEN, ISN - GRENOBLE, CRN - STRASBOURG,FONDS NAT. RECH.SCI. - BRUXELLES AND UCL/ULB - LOUVAIN-LA-NEUVE, IPN - ORSAY,MAINZ UNIV. - MAINZZ. PHYS. A332 (1989) 457.

HEAVY ION COLLISION GEOMETRY AT 9 3 MeV/U FROM P I - / P I + MEASUREMENTLEBRUN D . , CHAUVIN J . , REBREYEND D . , PERRIN G. , DE SAINTIGNON P . , MARTIN P . ,BUENERD M., LE BRUN C . , LECOLLEY J . F . , CASSAGNOU Y . , JULIEN J . , LEGRAIN R.ISN - GRENOBLE, LPC - CAEN, DPHN/CEN SACLAY - GIF SUR YVETTEPHYS. LETT. B223 (1989) 139.

IMPACT PARAMETER DEPENDENCE OF HIGH-ENERGY GAMMA-RAY PRODUCTION I NARGON INDUCED REACTION AT 8 5 MeV/NUCLEONKWATO NJOCK M., MAUREL M., MONNAND E . , NIFENECKER H . , PERRIN P . , PINSTON J . A . ,SCHUSSLER F . , SCHUTZ Y.CEN - GRENOBLE, ISN - GRENOBLE, GANIL - CAENNUCL. PHYS. A489 (1988) 368

A NE213 LIQUID SCINTILLATOR, NEUTRON DETECTOR DESIGNED FOR LIFETIMEMEASUREMENTS OF VERY NEUTRON-RICH NUCLEIBAZIN D . , MUELLER A . C . , SCHMIDT-OTT W.D.GANIL - CAEN, UNIVERSITAT GOTTINGEN -.GOTTINGENNIM A281 (1989) 117 .

A TEST OF NEW POSITION SENSITIVE DETECTORS FOR SPEGVILLARI A . C . C . , MITTIG W., BLUMENFELD Y. , GILLIBERT A . , GANGNANT P . , GARREAU L.INST. DE FISICA DA UNIV. DE SAO PAULO - SAO PAULO, GANIL - CAEN, IPN - ORSAY,CEN SACLAY - GIF SUR YVETTENIM A281 (1989) 240 .

SEMI-EMPIRICAL FORMULAE FOR HEAVY ION STOPPING POWERS IN SOLIDS IN THEINTERMEDIATE ENERGY RANGEHUBERT F . , BIMBOT R. , GAUVIN H.CEN - BORDEAUX-GRADIGNAN, IPN - ORSAYNIM B36 (1989) 357.

COMPRESSION*!. EFFECTS IN HEAVY ION COLLISIONS. 8PINODAL DECOMPOSITIONAND THERMAL ENERGY SATURATIONSURAUD E . , P I M . , SCHUCK P . , REMAUD B . , SEBILLE F . , GREGOIRE C . , SAINT-LAURENT F .ISN - GRENOBLE, LPN - NANTES, GANIL - CAENPHYSICS LETTERS B229 (1989) 359.

DETERMINATION OF NUCLEAR PROPERTIES BELOW NORMAL DENSITY FROM THEMULTIFRAGMENTATION PATTERNSNEPPEN K . , CUSSOL D . , GREGOIRE C.NIELS BOHR INST. - COPENHAGEN, GANIL - CAENPHYS. LETT. B220 (1989) 342.

NUCLEAR DYNAMICS WITH THE (FINITE-RANGE) GOGNY FORCE : FLOW EFFECTSSEBILLE F . , ROYER G . , GREGOIRE C . , REMAUD B . , SCHUCK P .LSN - NANTES, GANIL - CAEN, IRESTE - NANTES, ISN - GRENOBLENUCL. PHYS. A501 (1989) 137.

IllSCIENTIFIC ACTIVITIES

AT GANIL

COLLOQUE GANIL

(GIENS - Var)

I - REACTIONS DE TRANSFERT ET FRAGMENTATION AUXENERGIES INTERMEDIAIRES.(20-25 MAI 1984)

H - COLLISIONS CENTRALES(19-24 MAI 1985)

m - NOYAUX EXOTIQUES(20-23 MAI 1986)

IV - L'EXCITATION DE MODES ELEMENTAIRES DANS LESNOYAUX PAR DES COLLISIONS DIONS LOURDS AUXENERGIES INTERMEDIAIRES.(11-15 MAI 1987)

V - LES INTERACTIONS MATDERE-IONS LOURDS DE GRANDEVITESSE: EFFET DE LA DENSITE DE DEPOT D'ENERGffiINELASTIQUE.(17-20 MAI 1988)

VI - LA PHYSIQUE AVEC LES MULTIDETECTEURS :PERSPECTIVES(22-26 MAI 1989)

I

"Reactions de transfer!et fragmentation aux Energies interme'diaires".

• Diffusion elastique et inelastique d'ions lourds aux energies intermediaires.Aspect experimental: M. Buenerd.Aspect theorique: C. Marty.

• Etude de la disparition des effets de Pauli dans le potentiel optique en fonction de 1'ener-gie. P. Schuck.

' Reactions directes aux energies intermediaires. A. Nagarajan.

• Some ideas and facts on the evolution of direct processes up to 80 MeV/u. W. vonOertzen.

' Experiences sur SPEC aux energies intermediaires. W. Mittig.

' Evolution des reactions a 2 corps et structure nucleaire. L. Kraus.

' Avantages compares des ions legers et lourds pour 1'etude des resonances geantes.B. Bonin.

' Structures et resonances geantes dans les reactions par ions lourds. J.-C. Roynette.

' Nuclear structure studies in grazing heavy ion collisions. G. Pollarolo.

• Calculs microscopiques de 1'excitation de resonances geantes dans les reactions par ionslourds. D. Vautherin.

' Modele theorique pour la reaction Ca + Ca. K. Dietrich.

' Resonances geantes et excitation de multiphonons. Ph. Chomaz.

' Resonances geautes ii temperature finie. N. Vinh Mau.

• Fragmentation du projectile : 1'avant GANIL. R. Legrain.

' Pourquoi etudier la multifragmentation aux energies GANIL ? X. Campi.

' Caractere transitoire de mecanismes de reaction observes dans des collisions 27A1 + 63Cua 13 MeV/nucieon./.-L. Laville.

' Reactions peripheriques induites par un faisceau de 40Ar a 44 MeV/u. Friction et frag-mentation ? R, Dayras.

• Fragments du projectile: distributions isotopiques et taux de production. V. Borrei

• Transfer! de quelques nucleons et fragmentation du projectile aux energies intermediai-res. K Blumenfeld.

' Quelques aspects des mecanismes de la reaction 40Ar -f 68Zn a 27 MeV/u. G. Guittaume.

' Fragmentation et instability thermodynamique./. Cugnon.

' Fragments legers emis ^ grand angle dans la reaction 40Ar + 197Au a 44 MeV/u. Frag-mentation ou condensation. K Cassagnou.

' Liquid-gas phase transitions in nuclear systems. A. Panagiotou,

' Fragmentation de systemes nucldaires chauds. D. Heuer.

• Hodoscope de particules Idgeres & GANIL. Technologic et applications. B. Tamain.

' Fragmentation of target nuclei at 84 MeV/u. Is this an indication of a phase transition.U. Lynen.

• Production de fragments lagers de basse dnergie dans la reaction 20Ne •+ 27A1 a30 MeV/u. M. Moryean,

' Etude des me'canismes d'interaction entre 20 et 100 MeV/u par la mesure des r&sidus dela cible. F. Hubert.

• Formation and decay of localized excitations in nuclei. C. K. Gelbke.

• Effets collectifs et individuels aux Energies intermddiaires. C. Grfgoire.

De courts exposes sur de tres recents requitals ont, par ailleurs, 6t6 presented parN. Alamanos, A. Gamp, A. Demeyer et]. Blachot.

- n -Collisions centrales

• Fission along the mass asymmetry coordinate. An experimental evaluation of the condi-tional saddle masses and of the Businaro Gallone point L. Moretto.

• Emission statistique de fragments lourds a haute temperature. W. Mittig.

' Noyaux k tres haute energie d'excitation par nucleon. E. Plagnol.

' Linear and angular momentum transfer at 8.5 to 35 MeV/u./. Natowitz.

' Residus lourds produits dans les reactions Ar + Ag et Ar + Ho & 27MeV/u. M.-F. Rivet.

' Etude de la fusion complete et incomplete a partir des rgsidus de la cible. / Cranfon.

• Fusion incomplete et transferts de moments dans les interactions du 12C avec des ciblesde masses moyennes et lourdes. M. Pravikoff.

' La systematique des moments lingaires obtenus par la methode des reculs des noyauxradioactifs. J. Jastrzebski

' Effets respectifs du champ moyen et des collisions individuelles aux energies GANIL :approches th^oriques et signatures exp&imentales ? B. Remaud.

• Transfer! d'impulsion et d'energie dans les regions de champ moyen et d'interaction nu-cleon-nucl£on. S. Harar.

• Particules chargers rapides et transfert incomplet d'impulsion. Y. Putin,

• Dynamics of energy thermalisation. E. Pollacco.

' Les particules legeres, sondes de collisions entre noyaux aux energies comprises entre10 et lOOMeV/u./. Galin.

• Transfert incomplet d'impulsion dans les reactions Ar + Au, U Si 20, 35. 44 MeV/u.S. Leray.

• Etude des interactions du Krypton sur des noyaux lourds a 35 MeV/u. C. Le Brun

• Formation et d£sexcitation de noyaux chauds produits au cours de la reaction 40Ai -f238U & 1080 MeV. D.Jacquet.

• Stability des noyaux chauds. P. Bonche.

' Calculs semi-classiques de noyaux chauds. E. Suraud.

' Le modele E.T.F. & temperature finie. M. Brack.

• Multifragmentation de noyaux lourds. N. Carjan.

' Le parametre de densite de niveau en fonction de la temperature avec inclusion de cor-relations dynamiques. P. Schuck.

' Des barrieres de fission en fonction de la temperature - Une approche de goutte liquidechaude.y. Bartel.

• Correlations de pardcules Idgeres emises dans les collisions Ar + Au, Ti a 60 MeV/u.D. Ardouin,

• Interplay between light particle evaporation and fission like reactions around10 MeV/u.y. Alexander.

' Desexcitation du noyau compose par fission et emission de particules : experience ettheorie. Gas de 158Er. P. Grange.

• Competition entre remission de particules legferes et la fission a haute temperature.H. Delagrange.

' Calcul de Hauser-Feshbach pour remission de fragments lourds. / . Gomez del Campo.

• Collisions centrales dans la reaction 40Ar + 68Zn entre 14 et 27 MeV/u. F. Rami.

' Evolution du processus de fusion entre 7 et 38 MeV/n pour le systeme 14N + 27A1.D. Guinet.

• Fusion complete et incomplete dans le systeme Ar + Ag et Ar + Au a 35 MeV/u.C. Lebrun,

• Dispersions en masse et en moment dans les reactions par ions lourds. H. Placard.

' Evolution avec la temperature des barrieres de fission. Ph. Quentin,

' Desexcitation de noyaux chauds. H. Nifenecker.

• Neutron multiplicity as a measure for energy dissipation and momentum transfer.U.Jahnke.

' Fusion ? Excitation ? Changements de phase ? Quelles questions ? Quelles reponses ?M Lefort.

- m -

Noyaux exotiques.

• Etude de noyaux exodques par la methode du champ moyen. P.-H. Heenen.

• LISE : un dispositif experimental pour la production et 1'identificadon de noyaux exoti-ques. D. Bazin.

• Identification en ligne de noyaux legers riches en neutrons. E. Quiniou.

• Calculs H.F. triaxiaux de noyaux Ifigers riches en neutrons. F. Naulin.

• Mesure de masse de noyaux exotiques avec SPEC. A. Gillibert.

• Description microscopique des excitations collectives, des degres de libertes de particu-les et de leur couplage. Ph. Quentin.

• Approche th£orique de noyaux pair-pair lourds. M. Meyer.

• The renormalisation of the axial-vector strength in nuclei: experiments on superallo-wed beta decay. P.-G. Hansen.

• Properties of new beta-delayed proton emitters in the lanthanide region./.-M Nitschke.

• Quelques aspects de la prevision des proprieies nucldaires loin de la stability par la me-thode de Hartree-Fock. F. Tondeur.

• Production de noyaux exotiques dans le modele de la percolation. X. Campi.

• Separation isotopique a LISE - Application aux noyaux tegers riches en neutrons.J.P. Dufour.

• Production et etude de noyaux tres riches en neutrons dans la "fission froide" des isoto-pes de 1'uranium. C. Signarbieux.

' Faisceaux secondaires d'ions lourds. R. Bimbot.

• Les exotiques c6te machine: aujourd'hui, demain et apres-demain. A. Chabert.

• Spectroscopie laser pour des isotopes voisins de Z =50. G. Huber.

• Moments and radii of exotic nuclei by laser spectroscopy in the rare-earth region.R. Neugart.

• Nuclear structure studies of proton rich and neutron rich nuclei at the GSI on line massseparator. E. Roeckl

' Production de noyaux exotiques et mesures de masses dans les reactions par ions lourds.Y. Penimzhkeuich.

' Energy sharing in deeply inelastic reactions. H. Korner.

' Quelq'ies reflexions sur les developpements de la physique des noyaux exotiques auGANIL. C. Detraz.

- I V -

"L'excitation de modes eleinentaires dans les noyaux par descollisions d'ions lourds aux Energies interme*diaires".

• Le r6le du pion en physique nucle'aire.y. Delorme.

• Excitations nucleaires avec un ordre specifique dans 1'espace spin-isospin. K. Dietrich.

• Production de pions sous le seuil: phenomene(s) incoherent(s) et/ou coh6rent(s).B.-E. Erazmus.

' Emission de pions charges en coincidence avec des fragments legers./.-L. Laville.

' Subthreshold pion production with a heavy beam and multiplicity filter. W. Benenson.

• Detection de V Techniques et resultats. E. Grosse.

' Production de photons de haute energie dans les collisions noyau-noyau aux energiesintermediaires. M. Kwato Njock.

' Rayonnement x e"mis dans les collisions d'ions lourds aux Energies intermediaires.N. Alamanos. "

' Rayonnement electromagnetique dans les collisions d'ions lourds aux energies GANIL -Etude theorique. C. Grtgoire.

' Nuclear excitations at finite temperature. F. Bortignon.

• Effets collectifs a 15 et 24 MeV/u. Emission gamma provenant de la decroissance de r€-sonances geantes baties sur des etats excites. ].-]. Gaardhoje.

• Giant resonances in hot rotating nuclei. P. Ring.

' Damping of rotational motion. M. Dossing.

' Calcul semi-classique de resonances geantes & temperature finie. M. Brack.

' Resonances geantes a temperature finie. E. Suraud.

• Collisions peripheiiques : distinction entre transfert et fragmentation par mesure desenergies deposees dans le noyau-cible pour des collisions Ar + U.J. Galin.

• Contribution du mecanisme de transfert evaporation aux spectres d'ions lourds.Y. Blumenfeld.

• Delta excitation with charge exchange reactions. C. Gaarde.

' Reactions d'echange de charge. M Buenerd,

' Charge exchange reactions at 30 MeV/u. H.-G. Bohlen,

• Excitation des resonances geantes par reactions d'echange de charge a GANIL. Les pre-miers resultats. C. Berat.

• La resonance Eldans les reactions d'echange de charge 12C + I2C, 40"44Ca, 90Zr a E,ab =840 MeV. A. Miczaika.

Etats a une particule et fonction de reponse du noyau en reaction de transfert aux Ener-gies intermediaires. S. Gales.

Reactions de transfert direct de 1,2 et 3 nuclebns induites par faisceaux de 16O et 12C aenergie incidente dlev^e. M. Mermaz.

- V -"Les interactions matiere-ions lourds de grande vitesse:effet de la densite1 de depot d'energie inelastique".

• Revue des differents processus de dep6t d'energie lors du passage d'ions dans la matiere.A. L'Hoir.

' Interaction ion-atome. A Salin.

* Aspects collectifs du depdt d'energie dans les solides. N. Cue.

' Ions de recul multicharges de faible energie produits par impact d'ions lourds sur ciblegazeuse.y.-P. Grandin.

' Interference entre excitation elementaire du reseau (processus de capture) et excitationelectronique collective en collisions d'ions Kr36* de haute vitesse avec des cibles solidesminces. K. Wokrer.

* Pouvoirs d'arret, straggling en energie et straggling angulaire aux Energies GANIL.R. Bimbot.

' Etude de la collection de charges dans les semi-conduc, eurs lors de 1'irradiation par ionslourds (tunneling). K Patin.

' Redistribution spatiale de 1'energie d€pos6e. S. Bouffard.

' Interaction d'electrons accElEr6s avec la matiere condensee: experiences et modeles.J. Delaire.

' Ondes de choc engendrees par irradiations laser de grande puissance. R. Fabbro.

• Echelles de temps, interactions electro-phonons, pointe thermique./.-A. Davies.

* Observation de la rdponse transitoire d'un film mince de Nb supraconducteur & 1'impactd'un ion lourd. E. Paumier,

' DEsorption et pulverisation par les ions lourds de haute energie. 5. Delia Negra.

' La conversion d'une excitation electronique en defauts ponctuels. L. Zuppiroli.

• Defauts ponctuels induits dans les cristaux ioniques par forts dep6ts d'energie in61asti-que. A. Perez.

' Relations entre les proprietes conductrices induites par bombardement avec des ionslourds dans le polyimide et les conditions d'hradiation./ Davenas.

' Transitions de phase et formation de composes metastables sous irradiation. G. Martin.

' Contribution des effets de freinage electronique & la creation de defauts dans des me-taux (fer) et des alliages metalliques. A Dunlop.

' Amorphisation sous irradiation d'ions lourds : parallele avec les irradiations laser rapi-de. F. Studer et M. Toulemonde.

• Modifications structurales induites dans quelques ferrites magnetiques: etude par mi-croscopic electronique et spectroscopie Mossbauer. D. Groult.

• Melange par faisceaux d'ions./. Grihle.

• Trace dans les isolants./.-P. Duraud.

• EfFet de 1'irra'diatioon sur la viscosite des verres. A. Barbu.

• Modification de la structure electronique du quartz sous bombardement d'ions de hauteenergie. F.Jollet.

• R6le du (dE/dx) electronique sur le dopage du Hg, Cd, Te par implantation.C. Blanchard,

»

• Spectroscopie d'emission et d'absorption resolue a 1'echelle des femto-secondes.A Antonetti.

- V I -

La physique avec les multide'tecteurs : perspectives(22-26 mai 1989)• Noyaux chauds : production, desexcitation, temperature limite. D. Guerreau.

' Decay of hot nuclear systems formed in dissipative collisions. K.-D. Hildenbrand,

' Formation et desexcitatibn de noyaux chauds. K. Dietrich.

• Que devient la fission lorsqu'elle n'existe plus ? M. Louvel.

' Noyaux chauds : emission de fragments complexes et transfert de moment lineaire.E.G. Pollacco.

' Intermediate mass fragment and energetic particle emission from equilibrated sources.M. Di Two.

' Quelques resultats obtenus avec des emulsions nucleaires. F. Schussler.

' La resonance geante dipolaire : une sonde de la collectivite et de la forme des noyauxchauds. N. Alamanos.

' Quelles signatures pour une transition de phase ? Ph. Chomaz.

• Emission de photons lors des collisions peripheriques 40 Ar + 158 Gd at 44 Mev/u.W. Kuhn.

' Description du multidetecteur PACHA - Analyse des resultats et description des metho-des de simulation utilisees./.A Scarpad.

' New results with the MSU 4?r array. G. Westfall.

• Sur les detecteurs 4 n et les analyses des donnees aux energies relativistes. D. L'Hote.

• Presentation du projet ENDRA : la physique et les choix techniques retenus.E. Plagnol

• Reaction dans le domaine de 1'energie de Fermi: persistence des collisions fortementrelaxees, fragmentation dissipative et une soludon non thermique du dilemme des tem-peratures. H. Fiichs.

• Sur les observables dans la fragmentations. X. Campi.

• Multifragmentation et proprietes de la matiere nucleaire./. Cugnon.

' Multifragmentation - Revue experimentale et resultats recents pour des systemeslourds./F. Lecolley.

' Multifragmentation du systeme Ar + Al de 25 Mev/u a 85 Mev/u. D. Cussol.

• Correlations entre fragments a 44 MeV/u. L. Stugge.

• Caracteristiques globales de la multifragmentation dans les systemes 20Ne + Ag et 20Ne+Au a 60 MeV/u. C. Volant.

' Using streamer chambers for intermediate energy heavy ions./ Sullivan.

' Transition entre la fission et la multifragmentation. H. Oeschler.

• Vous avez dit multifragmentation ? E. Surraud.

• Etude dynamique des modes d'instabilite" du noyau. B. Remand.

' Correlations et formation d'amas dans les systemes de spins finis excites: une contribu-tion a 1'etude de la fragmentation./ Richert.

• Production of intermediate mass fragments - the onset of multifragmentation.U. Lynen.

' Multifragmentation dans le cadre d'un modele d'agregation restructuree. S1. Leray.

• A BaF2 crystal ball to study heavy ion collisions. E. Migneco.

• EMRIC : un detecteur pour la mesure des correlations entre 7 et 200 Mev/u. S. Kox.

' Interferometrie de particules avec multidetecteur Ji ICs. P. Lautridou.

' Qu'apportent les multidetecteurs de particules chargees dans les mesures de correlationentre particules legeres ? F. Saint-Laurent.

' Complex fragment emission from Mb + Au reactions at 50 to 100 MeV/u.N. Namboodiri.

' Le multidetecteur 4 TT Amphora. A. Giorni.

• Calorimetrie neutronique appliquee a 1'etude des collisions dissipatives. M. Motjean,

• Eden, un outil pour etudier des structures de grande energie. A Van der Woude

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