GNPN A Ediboard - NuPECC

NuclearPhysics

News

Vol. 16, No. 2, 2006, Nuclear Physics News 1

Nuclear Physics News is published on behalf of theNuclear Physics European Collaboration Committee(NuPECC), an Expert Committee of the EuropeanScience Foundation, with colleagues from Europe,America, and Asia.

Volume 16/No. 2

Editor: Gabriele-Elisabeth Körner

Editorial BoardJ. D’Auria, Vancouver R. Lovas, DebrecenR. F. Casten, Yale S. Nagamiya, TsukubaA. Eiró, Lisbon H. Ströher, JülichM. Huyse, Leuven (Chairman) T. J. Symons, BerkeleyM. Leino, Jyväskylä C. Trautmann, Darmstadt

Editorial Office: Physikdepartment, E12, Technische Universitat München, 85748 Garching, Germany, Tel: +49 89 2891 2293, +49 172 89 15011, Fax: +49 89 2891 2298,

E-mail: [email protected]

CorrespondentsArgentina: O. Civitaresse, La Plata; Australia: A. W. Thomas, Adelaide; Austria: H. Oberhummer, Vienna; Belgium:C. Angulo, Lauvain-la-Neuve; Brasil: M. Hussein, São Paulo; Bulgaria: D. Balabanski, Sofia; Canada: J.-M. Poutissou,TRIUMF; K, Sharma, Manitoba; C. Svensson, Guelph: China: W. Zhan, Lanzhou; Croatia: R. Caplar, Zagreb; CzechRepublic: J. Kvasil, Prague; Slovak Republic: P. Povinec, Bratislava; Denmark: K. Riisager, Årnus; Finland: M. Leino,Jyväskylä; France: G. De France, GANIL Caen; B. Blank, Bordeaux; M Guidal, IPN Orsay; Germany: K. D. Gross, GSIDarmstadi; K. Kilian Jülich; Greece: E. Mavromatis, Athens; Hungary: B. M. Nyakó, Debrecen; India: D. K. Avasthi,New Delhi; Israel: N. Auerbach, Tel Aviv; Italy: E. Vercellin, Torino; M. Ripani, Genova; L. Corradi, Legnaro;D. Vinciguerra, Catania; Japan: T. Motobayashi, RIKEN; H. Toki, Osaka; Malta: G. Buttigieg, Kalkara; Mexico:J. Hirsch, Mexico DF; Netherlands: G. Onderwater, KVI Groningen; T. Peitzmann, Utrecht; Norway: J. Vaagen, Bergen;Poland: T. Czosnyka, Warsaw; Portugal: M. Fernanda Silva, Sacavém; Romania: V. Zamfir, Bucharest; Russia: Yu.Novikov, St. Petersburg; Spain: B. Rubio, Valencia; Sweden: P.-E. Tegner, Stockholm; Switzerland: C. Petitjean, PSIVilligen; United Kingdom: B. F. Fulton, York; D. Branford, Edinburgh; USA: R. Janssens, Argonne; Ch. E. Reece,Jefferson Lab; B. Jacak, Stony Brook; B. Sherrill, Michigan State Univ.; H. G. Ritter, Lawrence Berkeley Laboratory;S. E. Vigdor, Indiana Univ.; G. Miller, Seattle.

Copyright © 2006 Taylor & Francis Group, LLC. Reproduction without permission is prohibited.All rights reserved. The opinions expressed in NPN are not necessarily those of the editors or publishers.

Nuclear Physics News ISSN 1050-6896

Advertising Manager Maureen M. Williams, 28014 N. 123rd Lane, Peoria, AZ 85383, USATel: +1 623 544 1698Fax: +1 623 544 1699E-mail: [email protected]

Circulation and SubscriptionsTaylor & Francis Inc.325 Chestnut Street8th FloorPhiladelphia, PA 19106, USATel: +1 215 625 8900Fax: +1 215 625 8914

SubscriptionsNuclear Physics News is supplied free of charge to nuclear physicists from contributing countries upon request. In addition, the following subscriptions are available:

Volume 16 (2006), 4 issues Personal: $81 USD, £49 GBPInstitution: $665 USD, £403 GBP

NuclearPhysics

News

2 Nuclear Physics News, Vol. 16, No. 2, 2006

Volume 16/No. 2

Contents

Editorial............................................................................................................................................................... 3

Laboratory PortraitA Unified Approach to Nuclear Science at Florida State University

by Paul Cottle ................................................................................................................................................. 4

Feature ArticlesEnhanced Electron Screening in Metals: A Plasma of the Poor Man

by Claus Rolfs.................................................................................................................................................. 9

Laboratory Studies of Stardustby Ernst Zinner .............................................................................................................................................. 12

Lattice QCD at Non-Vanishing Temperatures and Chemical Potentials by Z. Fodor and S. D. Katz ............................................................................................................................ 20

Facilities and MethodsExperiments with Stored Relativistic Exotic Nuclei at the FRS-ESR Complex

by Wolfgang R. Plaß and Christoph Scheidenberger.................................................................................... 27

Meeting Reports12th Euroschool on Exotic Beams, 25 August–2 September 2005, Mainz, Germany

by Christoph Scheidenberger ....................................................................................................................... 33

The 12th International Conference on Capture Gamma-Ray Spectroscopy and Related Topicsby Dr. Andreas Woehr and Prof. Ani Aprahamian ...................................................................................... 34

Report on the International Conference Frontiers in Nuclear Structure, Astrophysics and Reactions—FINUSTAR

by Rauno Julin and Sotirios V. Harissopulos ............................................................................................... 36

Workshop on the Physics of Compressed Baryonic Matterby Bengt Friman and Peter Senger .............................................................................................................. 37

Physics Opportunities with EURISOLby Robert Page, Angela Bonaccorso, and Nigel Orr ................................................................................... 39

News and Views ................................................................................................................................................ 40

Calendar ............................................................................................................................................................ 44

editorial


The views expressed here do not represent the views and policies of NuPECC except where explicitly identified.

Sunny days ahead for nuclear sci-ence? Don’t throw away yourumbrella quite yet.

This has been an unusually dampseason in Northern California. Thesecond wettest March on record hasbeen followed by an equally dismalApril. Spring is on hold and pedestri-ans mutter darkly about global climatechange, as they dodge the puddles.Only the owners of ski resorts arecheerful, with enough snow to keepthe lifts running until mid-summer.The weather has been on my mind aswell. It provides apt metaphors forreflection on the state of nuclear sci-ence in the early 21st century.

Reviewing recent accomplish-ments, we can bask in the warm sun-shine of exceptional productivity. Wesee real advances in knowledge ofnew forms of nuclear matter, ofnuclear currents, of neutrinos, and ofthe properties of nuclei far from stabil-ity. There is tremendous excitement atnuclear physics conferences aroundthe world.

When we look to the future, how-ever, the outlook has been much morecloudy. The funding situation inrecent years has been unsettled to saythe least, especially here in the UnitedStates. Budget cuts have limited beamtime at user facilities, and the jobmarket has shrunk for young scien-tists coming in to the field. This illwind has not been restricted tonuclear physics; it has affected all ofthe physical sciences, the foundationof the technological society in whichwe live.

Fortunately, there are some signs ofa thaw in funding here. Most importantwas the publication by the NationalAcademy of Sciences, in late 2005, of acommittee report entitled “Risingabove the Gathering Storm: Energizingand Employing America for a BrighterEconomic Future.” Many reports arewritten in Washington, D.C., but thisone gained unusual attention. The com-mittee, which was chaired by NormanAugustine, the former chairman ofLockheed-Martin, was both diverseand distinguished. It included univer-sity presidents, Nobel laureates, andCEOs of Fortune 500 corporations. Theparticipation of these corporate andacademic heavyweights gave the com-mittee more clout and a broader per-spective than would have been case fora group of scientific experts alone, andmitigated the usual charges of self-interest. The report included recom-mendations to greatly increase thenumber of science teachers in theschools, to sustain and strengthen thenation’s commitment to basic research,and to make the U.S. an attractive set-ting to study and perform research.

The report has already had consid-erable impact. In his State of theUnion speech earlier this year, Presi-dent Bush introduced the “AmericanCompetitiveness Initiative,” whichincorporated many of the committee’srecommendations, and included,among other things, a call to doubleinvestment in research in the physicalsciences over the next decade. This isa significant development. Of course,we must remember that like the

weather, political conditions in theU.S. are unsettled, changeable, andhard to forecast. The negotiation andcompromise necessary to bring thisinitiative to fruition are just beginning.Nevertheless, there is a sense of opti-mism in the scientific community thathas not been felt in many years.

It is in this warming political cli-mate that the Department of Energyand the National Science Foundationrecently announced they will ask theirNuclear Science Advisory Committee(NSAC; the equivalent of NuPECC)to prepare a new long-range plan. Thisis particularly timely because, with thepossibility of a growing budget, it isessential that we have a fresh, well-articulated vision of our future inorder to make a strong case for ourfield.

By definition, NSAC plans arenational in scope. However, thenuclear physics community is verymuch an international one. It is myhope that NSAC will make everyeffort to place its new long-rangeplan in a global context with anemphasis on international collabora-tion. In an era with so many politicaland social challenges, we in the sci-entific community have a specialresponsibility to work together to doour best to ensure a sunny and fairoutlook for our children and grand-children.

JAMES SYMONS

Nuclear Science DivisionLawrence Berkeley

National Laboratory

1

2

laboratory portrait


A Unified Approach to Nuclear Science at Florida State University

The Superconducting LinearAccelerator Laboratory at FloridaState University is a busy, teemingplace with the ongoing developmentof a radioactive beam facility, the con-struction of advanced detector sys-tems, nuclear spectroscopy using asophisticated array of Compton-sup-pressed clover gamma-ray detectors,and experiments with the finest sourceof polarized lithium beam in theworld. However, for FSU’s nuclearexperimentalists, the laboratory alsoserves as a home base for scientificprograms at the National Supercon-ducting Cyclotron Laboratory atMichigan State University, GANIL,Gammasphere, the Thomas JeffersonLaboratory, and the RelativisticHeavy Ion Collider. In engaging thefield of nuclear science at energiesranging from keV to TeV, and instudying physics from the spectros-copy of heavy and light nuclei to had-ron science and to the search for thequark-gluon plasma, FSU’s nuclearphysicists cross-fertilize each other’sstrengths in experimental innovation,theoretical vision, and education.

The members of the laboratoryhold leadership positions throughoutthe nuclear physics community. KirbyKemper was elected to the Rare Iso-tope Accelerator (RIA) Steering Com-mittee in 2002 and reelected in 2005.Sam Tabor was chair of the Gammas-phere Users’ Executive Committee in2002. Tony Frawley was Run Coordi-nator for Run 2 of the PHENIX detec-tor at the Relativistic Heavy IonCollider in 2001–2002, serves on thePHENIX Executive Council, and co-chairs the committee assembled by

Brookhaven management to write upthe physics case for RHIC II in prepa-ration for the next long-range plan.Mark Riley served on the U.S.Nuclear Science Advisory Committeefrom 1999 to 2002, was chair of the2001 Gordon Conference on NuclearChemistry, has served on the GRETA/GRETINA Steering Committee sinceits inception, and was chair of the APSPublications Oversight Committee in2001. He is presently chair of theGammasphere Users’ Executive Com-mittee. Paul Eugenio serves on theGlueX Collaboration Executive Boardat the Thomas Jefferson Laboratory, ischair of the CLAS Hadron Spectros-copy Physics Working Groups, andserves on the CLAS Executive Coor-dinating Committee. He is alsospokesperson for Jefferson LabHyCLAS experiment. Ingo Wieden-hoever held an Outstanding JuniorInvestigator Award from the U.S.Department of Energy in 2002–2005,and is presently serving as a memberof the National SuperconductingCyclotron Laboratory Users’ Execu-tive Committee. Paul Cottle chairedthis committee in 2004. Volker Credeis one of the spokespersons for the Jef-ferson Laboratory FROST program.

The members of the FSU Labora-tory are fortunate to benefit from alongstanding fruitful collaborativerelationship with the Physics Depart-ment’s nuclear theory group, whichconsists of four faculty members and anumber of postdoctoral fellows andgraduate students. The collaborationsbetween FSU’s experimentalists andtheorists cover the entire range ofenergies from nuclear structure phys-

ics to hadronic and relativistic heavyion physics. The two groups share aseminar series and a coffee room aswell as a floor of the Physics Depart-ment’s main building.

As of this writing, 152 studentshave completed their Ph.D.s at theFSU Nuclear Physics Laboratory.

Laboratory History The FSU Accelerator Laboratory

began operation in 1960 following theinstallation of an EN Tandem Van deGraaff accelerator. It was the second ofits type in the United States. Thefirst useful acceleration of negativelycharged helium ions was achieved atFSU in 1961, and isobaric analogue res-onances were identified at the facility inproton-induced reactions in 1963 [1].

The laboratory entered its seconddevelopment stage in 1970 with theinstallation of an Super-FN TandemVan de Graaff accelerator. At that time,the research program turned to anemphasis on heavy-ion beams. A super-conducting linear post-accelerator wasfunded by the National Science Founda-tion in the mid-1980s to double beamenergies, with the first experiment onthe completed facility run in 1987.

The Laboratory Facility The Super-FN tandem is injected

by either a National ElectrostaticsSNICS-II cesium sputter ion source ora laser-pumped polarized lithium ionsource. Among the beams availablefrom the sputter source is the radioac-tive isotope 14C. The FSU lab is theonly one in the world presently using a14C beam. In addition, helium beams

laboratory portrait


can be produced using an rf-dischargeand the same cesium charge exchangecanal as the polarized lithium source.The FN tandem is equipped with aPelletron charging system. Both theusual carbon foil strippers and aturbo-pumped recirculating gas strip-per are located at the terminal of theFN tandem.

The superconducting linear accel-erator consists of 12 accelerating reso-nators installed in 3 cryostats, plusbuncher and re-buncher each locatedin their own cryostats. The resonatorsare niobium-on-copper “split-ring”resonators produced by ArgonneNational Laboratory. The cryostatswere designed and built at FSU. Allthe resonators are designed for β =0.1, except for the buncher, which isdesigned for β = 0.06.

The Development of a Radioactive Beam Facility—RESOLUT

The laboratory has constructed anin-flight radioactive beam facilitynamed RESOLUT. The in-flight tech-nique allows the production of largequantities of exotic ions without thelimitations of the efficiency of an ionsource or the chemistry of the beammaterial. One of the concerns associ-ated with an in-flight facility—onethat can be particularly important inexperiments near the Coulomb bar-rier—is that the energy definition ofthe secondary beam is relatively poorbecause of the kinematic broadeninginduced in the production reaction.However, K. E. Rehm addressed thisissue at Argonne by placing a super-conducting resonator downstream ofthe production target. This resonator isused to sharpen the energy resolutionof the secondary beam and in effectgives all reaction products the samevelocity. This technique was adaptedand improved for RESOLUT, which

provides an increased angular accep-tance for the recoil products and uses adispersive magnetic spectrometer toselect the beam of choice morecleanly, creating in effect a novel typeof mass spectrometer.

A photograph of the RESOLUTfacility is shown in Figure 1, and aschematic drawing of its componentsis displayed in Figure 2.

The experimental program at RES-OLUT, which began in 2005, isdirected toward studying astrophysicalproblems of nucleosynthesis experi-mentally and to study high-isospinstates in stable nuclei. A neutron wallconsisting of plastic position-sensi-tive scintillators has been constructedfor use at the end of RESOLUT. Theneutron wall is particularly useful fordetecting neutrons from inverse kine-

matics (d,n) reactions with the radio-active beams produced by RESOLUT.

Polarized Lithium Source The FSU optically pumped polarized

lithium ion source (OPPLIS) uses anargon-ion laser pumped ring-dye laserat 671 nm to optically pump a lithiumatomic beam in a weak magnetic fieldinto the MF = 3DF = 3DI + 1/2(MS = 3D1/2, MI = 3DI) hyperfinesubstate. An electro-optic modulatoris used to generate two strong compo-nents from the single dye laser fre-quency to interact with atoms in bothground-state hyperfine levels. RFtransitions downbeam of the opticalpumping region can be used to selectother MF states. The lithium atoms arethen thermionically ionized on a hottungsten strip, extracted, accelerated

Figure 1. Members of the RESOLUT group in front of the major components ofRESOLUT. Group members include Prof. Ingo Wiedenhoever (front andcenter) and (from left to right) Patrick Peplowski, Eric Diffenderfer, RobertReynolds, Allison Bernstein, Simon Brown, and Dr. Lagy Baby.

laboratory portrait


to 5 keV and the passed through acesium charge exchange cell to pro-duce nuclear spin polarized Li− forinjection into the tandem. The sourcehas been used for producing polarized7Li for measuring nuclear reactionanalyzing powers of all three tensorranks.

A schematic for the polarized lith-ium source is shown in Figure 3, andthe source is described in more detailin Ref. [2].

Recently, OPPLIS was used todemonstrate that for low momentumtransfers (<1.3 fm−1) the analyzingpowers and cross-sections for the elas-tic scattering of 7Li by a variety of tar-gets depend only on the properties ofthe projectile—and not on those of thetarget [3]. This result shows that theproperties of loosely bound radioactivebeams such as 6He, 8B, and 11Li can beobtained from elastic scattering no mat-ter what target is used as long as themomentum transfers are not too large.

FSU g-ray Detector Array The FSU Compton-suppressed γ-

ray detector array consists of threeCompton-suppressed “Clover” seg-mented germanium detectors and tenCompton-suppressed single-crystalgermanium detectors. The array presentlyalso includes several Compton-sup-pressed germanium systems from theHolifield Radioactive Ion Beam Facil-ity at Oak Ridge National Laboratory.

A segmented silicon particle detectorarray, liquid scintillator neutron detec-tor, and plunger for Recoil DistanceMeasurements are available as auxil-iary detectors. The array is shown inFigure 4.

The γ-ray detector array and parti-cle detector array were recently usedto find five new states in the neutron-rich nucleus 20O using the laboratory’s14C beam and a radioactive 10Be targetin the 10Be(14C,α)20O reaction [4].Shell model calculations suggestedthat most of the newly observed statesresulted from promotions of nucleonsacross the Z = 8 and N = 8 shellgaps, and that the sizes of these gapsare decreasing steadily as the neutrondripline is approached.

Work at Major Radioactive Beam Facilities and Gammasphere

The FSU Laboratory has a largeresearch effort at the National Super-conducting Cyclotron Laboratory(NSCL) at Michigan State Universityand is involved with several of themajor research groups there. Membersof the FSU Laboratory led a collabora-tion that included the NSCL GammaGroup, the University of Surrey, andthe University of California—Berke-ley in a study of the near-neutron drip-line nucleus 42Si and two neighboringnuclei using knockout reactions. Theresults, which were published inNature [5], established the existenceof the Z = 14 subshell closure in thisnucleus and challenged the notion thatthis nucleus is deformed because ofthe hypothesized collapse of the N =28 major shell closure.

Another collaboration led by FSU(with the NSCL β-decay group, theUniversity of Tokyo, RIKEN and JAERI)led to the observation that whileintruder configurations (promotions ofpairs of neutrons across the N = 20major shell closure) do not appear inthe low-energy level structure of the

Figure 2. A schematic of RESOLUT.

Figure 3. A schematic of the FSU Optical Polarized Lithium Source.

laboratory portrait


N = 17 nucleus 28Na, they do occur atlow energies in the N = 18 isotope29Na [6]. This suggests that the N =20 shell closure narrows significantlyin going from N = 17 to N = 18, her-alding the arrival of the island ofinversion, which is highlighted by thestrongly deformed N = 20 isotones30Ne, 31Na, and 32Mg.

The NSCL’s new sweeper magnet,which is used to study reactions inwhich neutrons are stripped from neu-tron-rich nuclei, was constructed atFSU’s National High Magnetic FieldLaboratory. One of the detection sys-tems used to detect the stripped neu-trons is MoNA, the Modular NeutronArray. The array was constructed by aunique collaboration including theNSCL and FSU and several otherinstitutions that emphasize undergrad-uate education.

The FSU laboratory also has activecollaborations with GSI and GANIL.Members of the laboratory have beeninvolved with the observation of proton-

proton correlations observed in the two-proton radioactivity of a high spin iso-mer in 94Ag at GSI [7] as well as theconfirmation of a strong N = 14 sub-

shell closure in the near-neutron driplinenucleus 22O using inverse kinematicsproton scattering at GANIL [8].

The FSU laboratory continues tobe heavily involved in work using theGammasphere array. FSU led anexhaustive study of lifetimes of highspin states of strongly deformed nucleiin the mass 135 light rare earth regionusing the Doppler Shift AttenuationMethod [9]. The work eliminated thesystematic errors that commonlyoccur when such measurements areperformed separately and demon-strated the validity of the additivity ofsingle-particle quadrupole moments inthis mass region.

Hadronic and Relativistic Heavy Ion Physics

The laboratory also hosts the FSUHadronic Physics Group, which hasbeen deeply involved with the CEBAFLarge Acceptance Spectrometer(CLAS) since its conception. FSU isvery active with the study of excited

Figure 5. The gas vessel for the PHENIX RICH shown prior to departurefrom Tallahassee with the machinists who constructed it.

Figure 4. The FSU Gamma-Ray Detector Array with members of the Gamma-Ray Spectroscopy Group.

laboratory portrait


nucleon states and the search for so-called missing baryon resonances.These missing states are predicted byquark models based on three effectivequark degrees of freedom, but havenot been established experimentally.The group is presently preparing forpolarization experiments using polar-ized beams and a frozen-spin butanoltarget (FROST). These measurementswill provide the necessary constraintsto disentangle the broad and overlap-ping baryon states. The group is alsopreparing for a high statistics searchfor gluonic hybrid mesons andstrangeonia using CLAS. These mea-surements are scheduled back-to-backwith the polarization experiments andform the next major projects at Hall-B(CLAS) of Jefferson Lab. The searchfor new forms of matter drives FSU’sefforts for GlueX, a $40-milliondetector that will be constructed forthe upgraded 12 GeV CEBAF facilityat Jefferson Laboratory. The HadronicPhysics Group continues to be activewith several other experiments,including E852 at the BrookhavenAGS and the Crystal Barrel at Bonn.

The long history of heavy-ion phys-ics at the FSU accelerator led to thelaboratory’s involvement in PHENIX,one of the two major detectors at the

Relativistic Heavy Ion Collider. ThePHENIX Ring Imaging Cherenkov(RICH) detector, the primary electronidentifier, was built in the mid to late1990s by a collaboration of FSU,SUNY Stony Brook, KEK, WasidaUniversity, CNS (Tokyo), NagasakiIAS, and ORNL. FSU was responsiblefor all but the photon detector and itselectronics. The RICH gas vessel isshown in Figure 5 prior to its departurefrom Tallahassee for BrookhavenNational Laboratory. FSU has beenheavily involved in the development ofLevel 2 triggers for PHENIX. A mem-ber of the FSU group currently serveson the PHENIX Executive Council andheads the Level 2 Trigger effort. FSUprovided the PHENIX Run Coordina-tor for the first full energy RHIC run in2001/2002. The FSU group works pri-marily on J/ψ production in heavy ioncollisions as a signature of deconfine-ment, and is presently leading the anal-ysis of J/ψ → e+e− data from Cu+Cucollisions in Run 5.

References 1. J. D. Fox, C. F. Moore, and D. Robson,

Phys. Rev. Lett. 12, 198 (1964). 2. E. G. Myers et al., Nucl. Instr. And

Meth. B56/57, 1156, (1991). 3. O. A. Momotyuk etal. (to be published).

4. M. Wiedeking et al., Phys. Rev. Lett.94, 132501 (2005).

5. J. Fridmann et al., Nature 435, 922(2005).

6. V. Tripathi et al., Phys. Rev. Lett. 94,162501 (2005).

7. I. Mukha etal., Nature 439, 298 (2006). 8. E. Becheva et al., Phys. Rev. Lett. 96,

012501 (2006). 9. R. W. Laird et al., Phys. Rev. Lett. 88,

152501 (2002).

PAUL COTTLE1

feature article


Enhanced Electron Screening in Metals: A Plasma of the Poor Man

CLAUS ROLFS Experimentalphysik III, Ruhr-Universität Bochum, Bochum, Germany

For the astrophysically important class of charged-parti-cle-induced fusion reactions, there is a repulsive Coulombbarrier in the entrance channel of height Ec = Z1Z2e

2/r,where Z1 and Z2 are the integral nuclear charges of theinteracting particles, e is the unit of electric charge, and r isthe radius. Due to the tunneling effect through the Coulombbarrier at energies E < Ec, the fusion cross section σ(E)drops nearly exponentially with decreasing energy E:

σ(E) = S(E) E−1exp(−2πη), (1)

where η = 2πZ1Z2e2/hv is the Sommerfeld parameter, h is

the Planck constant, and v is the relative velocity of theinteracting nuclides. The function S(E) contains all nucleareffects and is referred to as the nuclear or astrophysicalS(E) factor. In this parametrization of the cross-section it isassumed that the Coulomb potential of the target nucleusand projectile is that resulting from bare nuclei. However,for nuclear reactions studied in the laboratory, the targetnuclei and the projectiles are usually in the form of neutralatoms or molecules and ions, respectively. The electronclouds surrounding the interacting nuclides act as a screen-ing potential: the projectile effectively sees a reduced Cou-lomb barrier, both in height and radial extension. This, inturn, leads to a higher cross section for the screened nuclei,σs(E), than would be the case for bare nuclei, σb(E). Thereis an enhancement factor [1,2],

flab(E) = σs(E)/σb(E) ≈ exp(πηUe/E) ≥ 1, (2)

where Ue is an electron-screening potential energy. Thisenergy can be calculated from the difference in atomicbinding energies between the compound atom and the pro-jectile plus target atoms of the entrance channel, or alterna-tively from the acceleration of the projectiles by the atomicelectron cloud. For energy ratios E/Ue > 1000, shieldingeffects are negligible, and laboratory experiments can beregarded as essentially measuring the bare cross-section:σ(E) ≡ σb(E). However, for E/Ue < 100, shielding effectsbegin to become important for understanding and extrapo-

lating low-energy data. Relatively small enhancementsarising from electron screening at E/Ue ≈ 100 can causesignificant errors in the extrapolation of cross-sections tolower energies, if the curve of the cross-section is forced tofollow the trend of the enhanced cross-sections, withoutcorrection for the screening. Note that for a stellar plasma,the value of the bare cross-section σb(E) must be knownbecause the screening in the plasma could be quite differentfrom that in the laboratory nuclear-reaction studies, that is,σp(E) = fp(E) σb(E), where the plasma enhancement factorfp(E) together with σb(E) must be explicitly included foreach situation. A good understanding of electron-screeningeffects in the laboratory is needed to arrive at reliable σb(E)data at low energies (for independent σb(E) data, see indi-rect methods such as the Trojan-Horse-Method [3]). Animproved understanding of laboratory electron screeningmay also help eventually to improve the correspondingunderstanding of electron screening in stellar plasmas, suchas in our sun. According to Eq. (2) one expects an exponen-tial enhancement at low energies in the laboratory, whichcan be described by a single parameter Ue.

Experimental studies of fusion reactions involving lightnuclides ([4] and references therein) have shown theexpected exponential enhancement of the cross-section atlow energies, which could be described by a single valuefor Ue. However, the observed enhancements were in sev-eral cases larger (up to about a factor 2) than could beaccounted for from available atomic-physics models, thatis, the adiabatic limit Uad.

Recently, the electron screening in d(d, p)t has beenstudied for deuterated metals, insulators, and semiconduc-tors, that is, 58 samples in total ([6–8] and referencestherein). As compared to measurements performed witha gaseous D2 target (Ue = 25 ± 5 eV [5]; Uad = 2 × 13.6eV = 27.2 eV), a large screening was observed in all met-als (of order Ue = 300 eV, i.e. a factor 10 higher than Uad),whereas a small (gaseous) screening was found for theinsulators and semiconductors.

Suggested solutions of the large enhancements includ-ing aspects such as stopping power, beam intensity, thermal

feature article


motion, channeling, solid state properties, and Fermi shut-tle acceleration mechanism were not successful [7]. Finally,an explanation of the large screening in metals was sug-gested [7] by the Debye plasma model applied to the quasi-free metallic electrons. The electron Debye radius aroundthe deuterons in the lattice is given by

RD = (εokT / e2neff ρa)1/2 = 69 (T /neff ρa)

1/2 [m] , (3)

with the temperature T of the quasi-free electrons inunits of K, neff the number of these electrons per metallicatom, and the atomic density ρa in units of atoms/m3. Withthe Coulomb energy of the Debye electron cloud and a deu-teron projectile at RD set equal to Ue ≡ UD, one obtains

UD = 2.09 × 10−11(neff ρa/T)1/2 [eV]. (4)

For T = 293 K, ρa = 6 × 1028 m−3, and neff = 1 one obtainsa radius RD, which is about a factor 10 smaller than theBohr radius of a hydrogen atom; furthermore, one obtainsUD = 300 eV, the order of magnitude of the observed Ue

values. A comparison of the calculated and observed Ue

values led to neff values, which were for most metals of theorder of one. The acceleration mechanism of the incidentions leading to the high observed Ue values is thus theDebye electron cloud at the rather small radius RD.

The neff values have been compared with those derivedfrom the Hall coefficient: they agreed within experimentaluncertainties for all metals with known Hall coefficient.Another critical test of the Debye model is the predictedtemperature dependence, UD ∞ T−1/2, that is, a decrease ofUD with increasing temperature, which was experimentallyverified (Figure 1).

Furthermore, the Debye energy UD should scale withthe nuclear charge Zt of the target atoms. The predictionwas verified [9–11] in 7Li(p,α)α and 6Li(p,α)3He (Zt = 3),9Be(p,α)6Li and 9Be(p,d)8Be (Zt = 4), 50V(p,n)50Cr (Zt =23), and 176Lu(p,n)176Hf (Zt = 71), always for pure metalsand alloys. The data demonstrate that the enhanced elec-tron screening occurs across the periodic table and is notrestricted to charged-particle-induced reactions among lightnuclides studied so far [4]. The 7Li and 6Li data (Figure 2)demonstrate with high precision the isotopic indepen-dence of the electron screening effect, that is, the same Ue

value for the 7Li and 6Li nuclides, particularly in the casesof the Li metal and PdLix alloys. The two reactions withneutrons in the exit channel demonstrate further that theelectron screening is an effect in the entrance channel of

the reaction and not influenced by the ejectiles of the exitchannel, that is, by the charged particles of the exit chan-nel studied so far [4].

Finally, the Debye model predicts a dependence on thenuclear charge of the ion, UD ∞ Zi; the prediction was ver-ified in the d(3He, p)4He studies in metals (Zi = 2): taking atypical value of Ue = 300 eV for the d+d fusion reaction inmetals at T = 290 K, one expects for d(3He,p)4He theDebye value UD = ZiUe(d+d) = 600 eV, consistent withobservation Ue = 680 ± 60 eV.

It should be noted that the Debye model is used to cal-culate the effects of electron screening on fusion reactionsin a stellar plasma, fp(E). Using a metallic plasma theDebye model was tested successfully with respect to allparameters entering the model. One may thus call metals “aplasma of the poor man.” An improved theory is highlydesirable to explain why the simple Debye model appearsto work so well. Without such a theory, one may considerthe Debye model as a parametrization of the data, with anexcellent predictive power.

There is another important prediction of the Debyemodel concerning radioactive decay of transuranic nuclidesin a metallic environment. In general, for the α-decay andβ+-decay one expects a shorter half-life due to the accelera-tion mechanism of the Debye electrons for these positivelycharged particles similar as for the protons, deuterons or

Figure 1. The observed values Ue(T) for Pt is shown as afunction of sample temperature T. The dotted curverepresents the prediction of the Debye model and the solidcurve includes the observed T-dependence of the Hallcoefficient, that is, neff (T).

feature article


3He in the fusion reactions, whereas for the β−-decay and e-capture process one predicts a longer half-life (here: decel-eration of the negatively charged particles). For example, if theα-decay 210Po ⇒ α + 206Pb with Eα = 5.30 MeV and T1/2 =138 days occurs in a metal cooled to T = 4 K, one arrivesat UD = ZαZtUe(d+d)(290/4)1/2 = 2 × 82 × 300eV × 8.5= 420 keV, where we used again a typical value of Ue =

300 eV for the d+d fusion reaction in metals at T = 290 K.The enhancement factor then gives flab = 265, and thus thehalf-life is shortened to 0.5 days. For the biologically dan-gerous transuranic waste 226Ra ⇒ α + 222Rn (Eα = 4.78MeV, T1/2 = 1600 years) an analogous calculation leads toT1/2 = 1.3 years. Experiments are in progress to test thesepredictions. If these predictions should also be verified, onemay have a solution to remove the transuranic waste(involving all an α-decay) of used-up rods of fission reac-tors in a time period of a few years. Finally, a reduced half-life of α-emitters such as 238U and 232Th in a metallic envi-ronment may have important corrections in their use ascosmo-chronometers [2] (i.e., the age of the elements) aswell as in understanding the flux of geo-neutrinos using theKamland detector [12] (i.e., the energy source of the earth).

References 1. H. J. Assenbaum et al., Z. Phys. A327, 461 (1987). 2. C. Rolfs and W. S. Rodney, Cauldrons in the Cosmos (Univer-

sity of Chicago Press, 1988). 3. C. Spitaleri et al., Phys. Rev. C69, 055806 (2004). 4. F. Strieder et al., Naturwissenschaften 88, 461 (2001). 5. U. Greife et al., Z. Phys. A351, 107 (1995). 6. F. Raiola et al., Eur. Phys. J. A13, 377 (2002). 7. F. Raiola et al., Eur. Phys. J. A19, 283 (2004). 8. F. Raiola et al., Eur. Phys. J. A31, 1141 (2005). 9. J. Cruz et al., Phys. Lett. B624, 181 (2005). 10. D. Zahnow et al., Z. Phys. A359, 211 (1997). 11. K. U. Kettner et al., Phys. Lett. (submitted). 12. T. Araki et al., Nature 436, 499 (2005).

Figure 2. Astrophysical S(E) factor of 7Li(p,α)α and6Li(p,α)3He for different environments: Li2WO4 insulator,Li metal, and PdLi1% alloy. The solid curves through thedata points include the bare S(E) factor (dotted curve) andthe electron screening.

1

2

feature article


Laboratory Studies of Stardust

ERNST ZINNER Laboratory for Space Sciences and Physics Department, Washington University, St. Louis, Missouri 63130, USA

Introduction Since the 1950s it has been established that carbon

and all heavier elements are produced in stars and thatthese elements are produced in different stellar sourceswith very different isotopic ratios [e.g., 1]. Althoughmany stellar sources must have contributed material tothe solar system, it was believed that this material hadbeen thoroughly mixed during solar system formationand resulted in very uniform isotopic ratios. As a con-sequence, signatures of individual stars had been com-pletely obliterated and the solar system or “cosmic”abundances of elements and isotopes, although provid-ing an important touchstone for stellar nucleosynthesis,represented only an average of distinct stellar sources.This situation has been dramatically changed with thediscovery in 1987 that primitive meteorites contain tinygrains of pristine stardust. These grains condensed inthe outflows of evolved stars and in supernova ejecta,survived interstellar travel and solar system formation,and are preserved in certain meteorites [2–5]. Theirpresolar, stellar origin is indicated by their isotopiccompositions, which encompass a vast range and arecompletely different from that of the solar system. Thegrains can be located in and extracted from their mete-oritic hosts and studied in detail in the laboratory.Because a given grain is a piece of a star, it can provideinformation on stellar evolution and nucleosynthesis,galactic chemical evolution, physical conditions in stel-lar atmospheres, dust processing in the interstellarmedium, and condition during solar system formation.Since the discovery of the first presolar grains, theirstudy has grown into a new kind of astronomy, comple-menting traditional astronomical observations. After ageneral overview and discussion of laboratory analysistechniques I will concentrate on issues of nucleosyn-thesis and topics of nuclear physics interests (sections5–8). The reader interested in obtaining more detailedinformation is referred to some reviews on presolargrains [2–7].

Isolation of Presolar Grains The first hints of the survival of presolar signatures in

solar system materials came from isotopic anomalies, isoto-pic ratios different from those dominating the solar system,in hydrogen and the noble gases neon and xenon. However,these hints were largely ignored and it was not until the dis-covery of anomalies in oxygen, a major rock-forming ele-ment [8], that the idea of survival of presolar material inprimitive meteorites was taken seriously. However, itturned out that the solids exhibiting isotopic anomalies inoxygen (and, as it was soon found, in many other elements)had formed in the solar system and only inherited presolarsignatures from their precursors. It took more than a decadeto find bona fide stardust that had condensed in stellarsources. This feat was achieved by Ed Anders and his col-leagues at the University of Chicago by, as Anders put it,“burning down the haystack to find the needle” [9]. In thisapproach, chemical dissolution and physical separationtechniques were used to track the carriers of anomalous, so-called exotic, noble gas components and led to the separa-tion of presolar diamond [10], silicon carbide (SiC) [11,12],and graphite [13]. These phases are not only high-tempera-ture phases that must have had a condensation origin, butare also chemically resistant and thus could be isolated byharsh chemical treatment.

Although these carbonaceous phases carried exoticnoble gases, which aided in their discovery, and althoughalmost all SiC and graphite grains are of stellar origin, theidentification of presolar oxide grains is more difficult. Thereason is that the solar system is oxygen-rich (i.e., has O >C), leading to the formation of oxygen-rich minerals fromprocessed, that is, isotopically homogenized, material,which constitute a large background. Identification ofpresolar O-rich grains requires isotopic measurements ofindividual grains in the ion microprobe (see section 4). Sep-aration of oxide phases such as corundum (Al2O3) andspinel (MgAl2O4) by chemical processing still helps,because the fraction of presolar grains among these phasesis much higher (1–2%) [14,15] than among silicates, where

feature article


only one grain out of 5,000 grains, and that only for grainssmaller than 1 μm, is of presolar origin [16].

Types of Presolar Grains In spite of the grains’ low abundance, their small

size, and the background of isotopically normal grains ofsolar-system origin, an ever increasing number of differenttypes of presolar minerals have been identified. Table 1lists presolar grain types, their abundances, sizes, stellarsources, as well as nucleosynthetic signatures carried by thegrains. Nanodiamonds are the most abundant, but they areonly ~2.5 nm in size, precluding analysis of individualgrains. Their presolar nature rests on the fact that they carryanomalous Xe and Te, but their average C isotopic ratio isnormal (i.e., solar). Thus it cannot be ruled out that only afraction of the diamonds have a stellar origin.

All other grain types are large enough that they can beanalyzed as single grains for their isotopic compositions.Although silicates have the second-highest abundances,they have been discovered only in the last couple of years

because of the overwhelming presence of isotopically nor-mal silicates [16,17]. The abundance of oxides is also rela-tively high but only among sub-μm grain in the mostprimitive meteorites. Silicon carbide is the best studiedgrain type because almost pure SiC separates can be pro-duced by chemical processing of meteorites, and becausetrace element concentrations are high enough so that manyelements can be analyzed in addition to C and Si. Averagegrain sizes are less than 1 μm but grains up to 20 μm havebeen found. Figure 1 a shows an SEM image of an unusu-ally large grain. Analysis in the ion microprobe has shownenormous ranges in the isotopic compositions of individualgrains (Figure 2) and has led to the classification of differ-ent sub-types according to the C, N, and Si isotopic ratiosof the grains [18]. Mainstream, Y, and Z grains most likelyoriginated in C-rich Asymptotic Giant Branch (AGB) stars.Grains of type X come from supernovae, grains of typeA+B probably from J stars and/or from post-AGB stars thathave undergone a very late thermal pulse, and a few grainsappear to have a nova origin. Silicon nitride grains are

Table 1. Presolar grain types.

*Abundances (in parts per million) vary with meteorite type. Shown here are the maximum values. #In low-to-intermediate-mass stars: Core H: Core H burning followed by first (and second) dredge-up. Shell H: Shell H burning during the RG and AGB phase. Shell He: He burning during thermal pulses of AGB phase followed by third dredge-up. CBP: Cool bottom processing. HBB: Hot bottom burning. s: s-process, neutron capture at low neutron density, followed by third dredge-up. In supernovae: H, He, O: H, He and O burning in different stellar zones in the massive star before explosion. e: s-process taking place in several zones. e: equilibrium process, leading to the Fe-Ni core. n-burst: Neutron capture at intermediate neutron density. r: Neutron capture at high neu-tron density. p: p-process, photo disintegration and proton capture. In novae: Ex H: Explosive H burning.

Grain type Abundance* ppm Size mm Stellar sources Nucleosynthetic processes# exhibited by grains

Nanodiamonds 1400 0.002 SNe r, p

Silicates in IDPs ~900 ≤1 RGB and AGB Core H

Silicates in meteorites 180 ≤0.5 RGB and AGB Core H, CBP

Oxides 110 0.15–2 RGB, AGB, SNe Core H, CBP, HBB, Shell H, He, s

Mainstream SiC 14 0.3–20 AGB Core H, Shell H, Shell He, s

SiC type A+B 0.25 0.5–5 J stars? Shell He and H

SiC type X 0.15 0.3–5 SNe H, He, O, e, s, n-burst

Graphite 1 1–20 SNe, AGB H, He, O, e, s, n-burst; Core H and He

Nova grains (SiC, gr.) 0.001 ~1 Novae Ex H

Si nitride 0.002 ≤1 SNe He, O

TiC ~0.001 0.01–0.5 SNe, AGB He, O, e

feature article


found in SiC-rich residues. They are extremely rare andhave the isotopic signatures of SiC X grains, thus have aSN origin.

The separation of graphite is more complicated that thatof SiC [19]. Most presolar graphite grains are larger than1 μm (Figure 1b) and range up to 20 μm in size. They havebeen separated according to density [19]. Low-densitygrains have isotopic signatures that indicate a SN origin[20], whereas high-density grains seem to have an origin inC-rich AGB stars of low metallicity, that is, stars that wereborn with low abundances of “metals” (all elements heavierthan He) [21]. Many graphite grains contain tiny sub-grainsof titanium-, zirconium-, and molybdenum-rich carbides,cohenite (Fe3C), kamacite (Fe-Ni) and elemental iron[22,23]. These grains must have condensed before thegraphite and in some cases apparently acted as condensa-tion nuclei (Figure 1c).

Analytical Techniques Presolar grains have been analyzed for their size and

morphology (SEM), internal structure (TEM), elemental(SIMS, Synchrotron XRS) and isotopic (SIMS, RIMS)compositions. Measurements of isotopic ratios are mostimportant and by far most efforts have been devoted tothem. Two basic types of isotopic analysis techniques havebeen applied, “bulk” analysis, the analysis of collections oflarge numbers of grains, and single grain analysis.

In spite of the low abundances of diamond, SiC andgraphite in meteorites, chemical and physical separationprovides essentially pure samples with enough grains forbulk analysis. Bulk analysis has been performed by GasMass Spectrometry for C, N, and the noble gases [24,25]and by Thermal Ionization Mass Spectrometry (TIMS) forthe heavy elements Sr, Ba, Nd, Sm, and Dy [18]. Althoughonly averages over many grains are obtained by these mea-surements, they make it possible to determine isotopicratios of trace elements, which cannot be obtained on singlegrains. Measurements can be done on grain size and densityseparates, for gas MS by stepwise heating (pyrolysis) orcombustion in an oxygen atmosphere.

Information on individual stars can be obtained by theanalysis of single grains and correlations of isotopic ratiosof several elements can serve to obtain the stellar history ofa given grain. The technique of choice is Secondary IonMass Spectrometry (SIMS) with the ion microprobe. Singlegrain analysis revealed a tremendous range of isotopicratios (see Figure 2). It also led to the identification of newgrain types such as corundum and spinel [26] and silicon

Figure 1. Secondary electron (a and b) and transmissionelectron (c) microscope images of presolar grains. (a)This large SiC grain shows euhedral features. (b)Graphite grain with smooth, shell-like surface (“oniontype”). (c) TEM micrograph of a microtome slice of apresolar graphite grain. The TiC grain in the center of thegraphite spherule apparently served as a condensationnucleus. Scale bars in a and b are 1 μm, in c 100 nm.

feature article


nitride [27], as well as of rare sub-populations of SiCgrains. Although most isotopic measurements in the pasthave been made on >1 μm grains, a new type of ion micro-probe, the NanoSIMS (Figure 3) allows analysis of grainsdown to 100 nm in size. It was instrumental in the discov-ery of presolar silicates in interplanetary dust particles [17]and primitive meteorites [16]. Laser ablation and ResonantIonization Mass Spectrometry (RIMS) has been applied tothe isotopic analysis of the heavy elements Sr, Zr, Mo, Ru,and Ba in single presolar SiC and graphite grains [28–30].The unique advantage of this technique is its high ioniza-tion efficiency and the fact that a chosen element can beionized selectively at the exclusion of any isobaric interfer-ences. Thus it is possible to measure Zr isotopes in the pres-ence of Mo and vice versa. Single grain measurements of

He and Ne isotopes have been made by laser heating andgas MS [31].

s-process Nucleosynthesis The agreement of the distribution of 12C/13C ratios

found in carbon stars [32] with that in mainstream SiCgrains indicates an origin in such stars. Further evidence isprovided by the grains’ N isotopic ratios and the presenceof radioactive 26Al (deduced from excesses in its daughter26Mg) and 22Ne [24]. However, the most convincing evi-dence is obtained from the s-process patterns exhibited byall the heavy elements whose isotopic compositions havebeen measured to date (Figure 4). AGB stars have longbeen implicated as the main source of nuclei produced bythe s-process, the slow capture of neutrons at neutron densi-ties that are low enough that unstable isotopes can decaybefore another neutron is added [e.g., 33,34]. The evolutionof the heavy elements thus progresses along the valley ofstability. Two reactions provide neutrons in AGB stars:13C(α,n)16O and 22Ne(α,n)25Mg. The first occurs underradiative conditions in the intershell between the H- andHe-burning shells between thermal pulses. It is responsiblefor most of the s-process production of the heavy elements.

1E+0

1E+1

1E+2

1E+3

1E+4

14N

/15N

1E+0 1E+1 1E+2 1E+3 1E+4

12C/13C

Mainstream ~93%

A+B grains 4-5%

Nova grainsX grains ~1%

Y grains ~1%

Z grains ~1%

Solar

Sola

r

Figure 2. Nitrogen and carbon isotopic ratios ofindividual presolar SiC grains. Because rare graintypes have been identified by special automaticimaging searches, the number of grains of differenttypes do not correspond to their relative abundances inmeteorites. These abundances are given in the legend.The dotted lines indicate the solar (terrestrial) isotopicratios.

Figure 3. NanoSIMS, a new type of ion microprobe withhigh sensitivity and high spatial resolution. In thisinstrument a primary ion beam (Cs+ or O−) is focused ontothe sample and by sputtering produces secondary ions.These ions are accelerated, separated according to theirmass in a double-focusing magnetic mass spectrometer,and simultaneously detected in five different electronmultipliers.

feature article


The second reaction occurs during the thermal pulses, lastsfor a much shorter time, and results in much higher neutrondensities [for details see Ref. (33)].

Figure 4 shows the isotopic patterns of heavy elementsmeasured in bulk samples of SiC, which are dominated bymainstream grains. These patterns are characteristic of thes-process and for all elements except Dy agree very wellwith theoretical models of s-process nucleosynthesis inlow-mass AGB stars [29,35]. The isotopic patterns allowthe determination of different stellar parameters such asneutron exposure, temperature, and neutron density [18].These parameters in turn depend on stellar mass and metal-licity as well as on the neutron source. For example, themeasured Ba patterns indicate a neutron exposure half of

that inferred for the solar system. Another example is pro-vided by the abundance of 96Zr, which is sensitive to neu-tron density because of the short half life (64 d) of 95Zr. Thelow 96Zr measured in individual grains indicates that the22Ne(α,n) neutron source must have been weak, excludingintermediate-mass (M > 3Mö) AGB stars as parent starsof mainstream SiC grains [29].

One interesting consequence of isotopic measurementsin presolar SiC grains was that some results motivatednuclear astrophysicists to determine neutron-capture cross-sections with high precision. Discrepancies between Ba andNd isotopic patterns measured in presolar SiC and theresults of model predictions led to the suggestion that thecross-section used in the theoretical calculations were

-100

-80

-60

-40

-20

0

20

126 128 129 130 131 132 134

Xe

-100

-80

-60

-40

-20

0

20

132 134 135 136 137 138

Ba

KJC

KJE

Dev

iatio

n fr

om s

olar

rat

io in

%

-100-80

-60-40

-200

20

4060

80100

80 82 83 84 86

KJA

KJGKr

KJA

KJG-100

-80

-60

-40

-20

0

20

84 86 87 88

KJA

KJE

Sr

-100

-80

-60

-40

-20

0

20

160 161 162 163 164

Dy

Mass-100

-80

-60

-40

-20

0

20

142 143 144 145 146 148

Nd

Mass

-100

-80

-60

-40

-20

0

20

94 95 96 97 98 100

Mo

-100

-80

-60

-40

-20

0

20

99 100 101 102 104

Ru

-100

-80

-60

-40

-20

0

20

147 148 149 150 152

Sm

MassFigure 4. Isotopic patterns of heavy elements measured in presolar SiC grains. The isotopic ratios are relative to thereference isotope indicated by a filled circle and are plotted as deviations from the solar ratios in percentages. The zig-zagpatterns seen are typical of production of the isotopes by the s-process, slow neutron capture, and in first order reflect theneutron capture cross-sections.

1

2

feature article


incorrect. This suspicion was confirmed by subsequentimproved cross-section determinations that successfullyresolved the discrepancies [36–38].

Grains from Supernovae For a given presolar grain the stellar source is

unknown and must be inferred from the grain’s isotopiccomposition. From astronomical observations it wasclear that RGB and AGB stars and supernovae were themost likely sources of the grains. A rare sub-type of SiC,the X grains have isotopic signatures that indicated a SNorigin. These grains have large excesses of 28Si relativeto the heavier Si isotopes and most have 12C and 15Nexcesses (see Figure 2). Such signatures are predictedfor different layers of core-collapse (Type II) superno-vae [39,40]. The smoking gun for a SN origin of Xgrains was provided by the finding that they contain evi-dence for the presence of radioactive 44Ti (T1/2 = 60 y)and 49V (T1/2 = 337 d) at the time of their formation[41,42]. Both of these isotopes are only produced insupernovae, mostly in a layer that contains almost pure28Si. Evidence for a SN origin is also found in low-den-sity graphite grains in the form of 15N, 18O, and 28Siexcesses as well as evidence for the initial presence of41Ca and 44Ti [20]. Silicon carbide X grains exhibit a Moisotopic pattern that is completely different from thatfound in mainstream grains (Figure 5). Interestingly, it isnot the pattern expected for the r-process (rapid additionof neutrons at very high neutron densities) but has beensuccessfully explained by a neutron-burst model at inter-mediate neutron densities [43]. A neutron burst is pre-dicted to occur in a narrow O-rich zone of Type IIsupernovae and can account for the Mo pattern in Xgrains [40].

Although the overall isotopic signatures of grains areconsistent with theoretical predictions, the grain datapresent fundamental problems. One is that these signaturesare found in completely different layers of the supernovae:the high 26Al/27Al ratios found in X grains in the He/Nzone#, high 12C/13C and 18O/16O ratios in the underlying He/C zone, high 15N/14N ratios at the bottom of this zone, highneutron densities (n-burst) at the top of the underlying O/Czone, 28Si, 44Ti, and 49V in the Si/S zone (44Ti and 49V alsoin the underlying Ni core). It is still unclear how these dif-ferent layers can be mixed together to produce the isotopicsignatures observed in the grains, in particular in view of

the fact that a huge layer consisting mostly of O liesbetween the C-rich layers and the Si/S layer. Another puz-zle is why most of the SN grains identified so far are car-bonaceous (SiC and graphite) and why only a handful of O-rich grains of a SN origin have been identified (Figure 6).Supernovae are predicted to contain far more O than C.Furthermore, theoretical models fail to predict in detailmany isotopic ratios found SN grains [20]. Thus it is clearthat the study of presolar grains provides new and funda-mental challenges to nuclear astrophysicists.

Short-lived Radioisotopes Many presolar grains contain evidence for the initial pres-

ence of short-lived, now extinct isotopes in the form of large#The zones are named according to the two most abundant elements.

1000

500

0

-500

-1000100989796959492

Type X grain29Si=–309±10‰30Si=–565±10‰

12C/13C=304±2414

N/15

N=76±10

Mainstream grain29Si=13±15‰30Si=33±23‰

12C/13C=99±414

N/15

N=2059±566

Isotopic Mass Number

δδ

δδ

Figure 5. Isotopic patterns of Mo measured in amainstream and a type X SiC grain. The plotted ratios areδ-values, deviations from the solar ratios relative to 96Mo inpermil (‰). Also given are the C, N, and Si isotopic ratiosof the two grains. Figure courtesy of Andy Davis.

1

feature article


excesses in the daughter isotopes of these radionuclides. Inmany cases these excesses are enormous so that there is littledoubt that they are of radiogenic origin. I have already men-tioned 44Ti and 49V. They are only produced in supernovae,both by α-rich freeze-out, 44Ti also by Si burning, in the Si/Sand Ni zones. Both are of general astrophysical interest for dif-ferent reasons, 44Ti because there is a chance that γ-rays fromits decay can be detected in remnants of rece nt SN explosions,49V because the initial presence of this isotope with a half-lifeof only 337 days implies grain condensation in SN ejecta on atime scale of a couple of years. Because neutron capture in theHe/C zone can also produce 49Ti excesses [39,40], this signa-ture by itself is no proof for initial 49V. However, the correla-tion between 49Ti excesses and the V/Ti ratio established itspresence beyond any doubt [42].

Two other short-lived isotopes for which evidence wasfound in SN grains are 41Ca (T1/2 = 1.05 × 105 y) [44] and26Al (T1/2 = 7.3 × 105 y) [for a summary see Ref. (3)]. Incontrast to 44Ti and 49V, these two radionuclides are alsoproduced in AGB stars and have, in fact, been detected ingrains with an AGB origin. Inferred 41Ca/40Ca (from 41Kexcesses) and 26Al/27Al (from 26Mg excesses) ratios are muchhigher in SN grains than in grains from AGB stars. In SNgrains the former range up to 1.6 × 10−2, in agreement withtheoretical predictions for n-capture production in the He/C,

C/O, and the O-rich zone of SNII [39,40]. The latter range upto 0.6 in X grains; the highest ratios are predicted for the He/N zone, where 26Al is produced by proton capture an 25Mg.41Ca/40Ca measured in hibonite (CaAl6O19) grains from AGBstars [45] range up to 2 × 10−4, in good agreement with theo-retical predictions for n-capture production in the He shell.Inferred 26Al/27Al ratios in mainstream SiC grains (up to 2 ×10−3) agree with predictions for production in the H shell ofAGB stars [e.g., 46]. However, ratios in many oxide grain(corundum, spinel, hibonite, silicate) are much higher, as willbe discussed in the next section.

A short-lived isotopes for which evidence has been onlyfound in grains from AGB stars is 99Tc (T1/2 = 2.1 × 105 y).RIMS analysis of Ru isotopes in single mainstream SiC grainsrevealed s-process patterns with depletions in all Ru isotopesrelative to s-process-only 100Ru [30]. Whereas all measuredratios are in good agreement with AGB models, 99Ru showssystematic excesses, which have been successfully explainedby incorporation and decay of 99Tc in the grains. It is fittingthat a signature of this elements, whose presence in stars [47]was the first astronomical evidence for stellar nucleosynthesis,has now been found in stardust studied in the laboratory.

Evidence for Stellar Mixing The isotopic analysis of presolar grains provides evidence

for mixing processes in their stellar sources. I already men-tioned the fact that SN grains carry isotopic signatures thathave their origin in very different stellar zones. Their presencein individual grains implies mixing of these zones, althoughthe details of these mixing processes are still not understood[20]. Another example where isotopic ratios measured inpresolar grains indicate mixing in stars is provided by the Oisotopic ratios of O-rich grains (Figure 6). The O isotopic com-positions of Group 1 grains can be explained by core H burn-ing and the first (and second) dredge-up whereby different17O/16O ratios indicate different stellar masses [48]. Group 3grains most likely come from low- and Group 4 grains fromhigh-metallicity stars. However, the large 18O depletions inGroup 2 grains cannot be produced by standard models and anextra mixing process called cool bottom processing (CBP) hasbeen proposed to explain them [49]. In this process, assumedto occur on the RGB and AGB, material from the convectiveenvelope is believed to circulate to hot regions close to the H-burning shell where 18O is destroyed by 18O(p,α)15N. Suchextra mixing has also been invoked to explain low 12C/13Cratios and 7Li and 3He anomalies in RGB stars [50].

Additional evidence for CBP is given by the inferred 26Al/27Al ratios found in many oxide grains, which range up to 0.1

1E-4

1E-3

1E-2

17O

/16O

1E-5 1E-4 1E-3 1E-218O/16O

Corundum

Spinel

HiboniteSilicates

solar

sola

r

Group 2

Group 1

Group 3

Group 4

Figure 6. Oxygen isotopic ratios measured in individualO-rich presolar grains. The four groups defined by Nittleret al. [14] are indicated. The dotted lines indicate the solar(terrestrial) isotopic ratios.

feature article


[14,45,51]. However, standard models of H shell burning inAGB stars result in ratios of only ~2 × 10−3 [46,52]. Conse-quently, CBP has also been invoked to account for the high 26Al/27Al ratios in oxide grains. In their model Nollett etal. [53] usetwo parameters to characterize CBP, the circulation rate, whichmostly affects the 18O/16O ratio in the envelope, and the temper-ature reached by the circulated material, which mostly affectsthe 26Al/27Al ratio. Not all AGB stars experience CBP and oneof the subjects of current research is which stars did and whichones did not. The parent stars of mainstream SiC grains appar-ently did not. Is it possible that CBP prevented O-rich stars frombecoming carbon stars, so that evidence for effects of CBP on26Al/27Al ratios are only seen in O-rich but not C-rich grains?

Conclusions The study of presolar dust grains in the laboratory has

become a new branch of astronomy. The grains provideinformation on isotopic ratios that could not be obtainedfrom stars. Of special interest are results that do not agreewith stellar models and thus trigger the introduction of newmodels (e.g., CBP), the measurement of cross sections (Ba,Nd), or the search for new processes not considered before(neutron burst). The field is vigorously expanding and tech-nical advances are expected to yield new surprises.

References 1. G. Wallerstein et al., Rev. Mod. Phys. 69 (1997) 995. 2. E. Zinner, Ann. Rev. Earth Planet. Sci. 26 (1998) 147. 3. E. Zinner, in: Meteorites, Planets, and Comets (Ed. A. M. Davis),

Vol. 1, Treatise on Geochemistry (Eds. H. D. Holland and K. K.Turekian), Elsevier-Pergamon, Oxford, (2004), pp. 17.

4. L. R. Nittler, Earth & Planet. Sci. Lett. 209 (2003) 259. 5. K. Lodders, S. Amari, Chem. Erde 65 (2005) 93. 6. T. J. Bernatowicz, E. Zinner (Eds.), Astrophysical Implications of

the Laboratory Study of Presolar Materials, AIP, New York (1997). 7. D. D. Clayton, L. R. Nittler, Annu. Rev. Astron. Astrophys. 42

(2004) 39. 8. R. N. Clayton et al., Science 182 (1973) 485. 9. E. Anders, E. Zinner, Meteoritics 28 (1993) 490. 10. R. S. Lewis et al., Nature 326 (1987) 160. 11. T. Bernatowicz et al., Nature 330 (1987) 728. 12. M. Tang, E. Anders, Geochim. Cosmochim. Acta 52 (1988) 1235. 13. S. Amari et al., Nature 345 (1990) 238. 14. L. R. Nittler et al., Astrophys. J. 483 (1997) 475. 15. E. Zinner etal., Geochim. Cosmochim. Acta 67 (2003) 5083. 16. A. N. Nguyen, E. Zinner, Science 303 (2004) 1496. 17. S. Messenger et al., Science 300 (2003) 105. 18. P. Hoppe, U. Ott, in: Astrophysical Implications of the Lab-

oratory Study of Presolar Materials (Eds. T. J. Bernatow-icz and E. Zinner), AIP, New York, (1997), pp. 27.

19. S. Amari et al., Geochim. Cosmochim. Acta 58 (1994) 459. 20. C. Travaglio et al., Astrophys. J. 510 (1999) 325. 21. M. Jadhav et al., New Astron. Rev. (2005) (submitted). 22. T. J. Bernatowicz et al., Astrophys. J. 472 (1996) 760. 23. T. K. Croat etal., Geochim. Cosmochim. Acta 67 (2003) 4705. 24. R. S. Lewis etal., Geochim. Cosmochim. Acta 58 (1994) 471. 25. S. Amari etal., Geochim. Cosmochim. Acta 59 (1995) 1411. 26. L. R. Nittler et al., Nature 370 (1994) 443. 27. L. R. Nittler et al., Astrophys. J. 453 (1995) L25. 28. G. K. Nicolussi et al., Astrophys. J. 504 (1998) 492. 29. M. Lugaro et al., Astrophys. J. 593 (2003) 486. 30. M. R. Savina et al., Science 303 (2004) 649. 31. R. H. Nichols, Jr. etal., in: Advances in Analytical Geochemistry

(Eds. M. Hyman and M. Rowe), JAI Press Inc., (1995), pp. 119. 32. D. L. Lambert et al., Astrophys. J. Suppl. 62 (1986) 373. 33. M. Busso et al., Ann. Rev. Astron. Astrophys. 37 (1999) 239. 34. M. Lugaro et al., Astrophys. J. 586 (2003) 1305. 35. R. Gallino et al., in: Astrophysical Implications of the Lab-

oratory Study of Presolar Materials (Eds. T. J. Bernatow-icz and E. Zinner), AIP, New York, (1997), pp. 115.

36. K. H. Guber et al., Phys. Rev. Lett. 78 (1997) 2704. 37. P. E. Koehler et al., Phys. Rev. C 57 (1998) R1558. 38. K. Wisshak et al., Nuclear Physics A621 (1997) 270c. 39. S. E. Woosley, T. A. Weaver, Astrophys. J. Suppl. 101 (1995) 181. 40. T. Rauscher et al., Astrophys. J. 576 (2002) 323. 41. L. R. Nittler et al., Astrophys. J. 462 (1996) L31. 42. P. Hoppe, A. Besmehn, Astrophys. J. 576 (2002) L69. 43. B. S. Meyer et al., Astrophys. J. 540 (2000) L49. 44. S. Amari et al., Astrophys. J. 470 (1996) L101. 45. L. R. Nittler et al., Lunar & Planet. Sci. XXXVI (2005)

Abstract #2200. 46. N. Mowlavi, G. Meynet, Astron. Astrophys. 361 (2000) 959. 47. P. W. Merrill, Astrophys. J. 116 (1952) 21. 48. A. I. Boothroyd, I.-J. Sackmann, Astrophys. J. 510 (1999) 232. 49. G. J. Wasserburg et al., Astrophys. J. 447 (1995) L37. 50. C. Charbonnel, Astrophys. J. 453 (1995) L41. 51. E. Zinner et al., Geochim. Cosmochim. Acta 69 (2005) 4149. 52. A. I. Karakas, J. C. Lattanzio, Publ. Astron. Soc. Australia

20 (2003) 279. 53. K. M. Nollett et al., Astrophys. J. 582 (2003) 1036.

ERNST ZINNER

3

feature article


Lattice QCD at Non-Vanishing Temperatures and Chemical Potentials

Z. FODOR AND S. D. KATZ Department of Physics, University of Wuppertal, Germany and Institute for Theoretical Physics, Eotvos University, Budapest, Hungary

Introduction Quantum Chromo-Dynamics (QCD) describes the physics

of strong interactions. These phenomena are in many casesnon-perturbative. A particularly interesting sector of the stronginteractions is at extreme conditions. With increasing tempera-tures (T) we expect a transition at some T = Tc. The dominantdegrees of freedom are hadrons in the low temperature phaseand colored objects in the high temperature phase. Present lat-tice results suggest a cross-over and a critical point at somenon-vanishing T and chemical potential (m).

Because we are mostly interested in the physics aroundTc, non-perturbative methods are necessary among which lat-tice QCD is the most systematic. There are at least two seri-ous difficulties with lattice simulations. The first one isconnected to the lightness of the quark masses. The cost ofcomputations increases strongly as the quark massesdecrease, therefore most lattice results were obtained withunphysically large quark masses. The second difficulty isconnected to the continuum limit. Calculations are alwaysperformed at a finite lattice spacing (a). In order to get physi-cal results, we have to take the a → 0 limit. Because, forexample, for the equation of state (EoS) the computationalcosts scale as a−13 it is not surprising that up to very recentlymost results were obtained only at one set of lattice spacings.

The situation is much easier in the case of the puregauge theory. The first problem does not exist because thequark masses are infinite. There are continuum extrapo-lated results, for example, for the equation of state, bothwith unimproved and improved lattice actions and theyshow nice agreement [1–3]. There are also numerous EoSresults for the full theory with dynamical quarks [4–7],which will be discussed in the following.

For a long time it was believed that no physical answercan be given to questions with non-vanishing baryonic den-sities. The reason for that is the infamous sign problem,which spoils any Monte-Carlo method based on importancesampling. Recently, new techniques were developed, whichare able to cover small to moderate baryonic chemical

potentials at non-vanishing temperatures (chemical poten-tial is used to set the baryonic density).

In this article recent results on non-vanishing densitiesand the determination of the EoS when approaching thephysical quark mass and continuum limit are presented.

Lattice Formulation, Non-Vanishing Temperatures and Densities

Thermodynamical quantities can be obtained from thepartition function, which can be given by a Euclidean path-integral:

where U and , are the gauge and fermionic fields and SE

is the Euclidean action. The lattice regularization of thisaction is not unique. There are several possibilities to useimproved actions that have the same continuum limit as theunimproved ones. The advantage of improved actions isthat the discretization errors are reduced.

Usually SE can be split up as SE = Sg + Sf, where Sg isthe gauge action containing only the self interactions of thegauge fields and Sf is the fermionic part. The gauge actionhas one parameter, the b gauge coupling, whereas theparameters of Sf are the mq quark masses and mq chemicalpotentials. For the fermionic action the two most widelyused discretization types are the Wilson and staggeredfermions.

For the actual calculations finite lattice sizes of Ns × Nt

are used. The physical volume and the temperature arerelated to the lattice extensions as:

Therefore lattices with Nt >> Ns are referred to as zerotemperature lattices whereas the ones with Nt > Ns are

Z U S UE= −∫D D DΨ Ψ Ψ Ψexp{ ( , , )} (1)

Ψ Ψ

V N a TN as

t

= =( ) , .3 1 (2)

feature article


finite temperature lattices. Because the gauge coupling bhas the largest influence on the lattice spacing, it essentiallydetermines the temperature (increasing b increases T).

For large homogeneous systems the pressure is propor-tional to the free energy density. Unfortunately the freeenergy density (−T/V logZ) cannot be measured directly.We can only measure the derivatives of logZ with respect tothe parameters of the action. Then, with an integration wecan obtain the pressure. This method is known as the inte-gral method for calculating the pressure. In order to removethe divergent zero-point energy we have to subtract thepressure measured on zero temperature lattices. Furtherthermodynamical quantities can be derived directly fromthe pressure. For example, the energy density (ε), entropydensity (s), and speed of sound (cs) have the following rela-tion with the pressure:

Although QCD at finite chemical potential (m, which asalready mentioned, is used to set non-vanishing baryonicdensity) can be formulated on the lattice [8], standardMonte-Carlo techniques cannot be used at m ≠ 0. The rea-son is that for non-vanishing real m the functional mea-sure—thus, the determinant of the Euclidean Diracoperator—is complex. This fact spoils any Monte-Carlotechnique based on importance sampling. Several sugges-tions were studied earlier to solve the problem. Unfortu-nately, none of them was able to give physical answers fornon-vanishing densities. About three years ago new tech-niques appeared, with which moderate chemical potentialscould be reached on the lattice.

One of the most popular ideas [9,10] was to produce anensemble of QCD configurations at m = 0 and at the corre-sponding transition temperature Tc (or at any other physi-cally motivated point for which importance samplingworks). Then one determined the Boltzmann weights [11]of these configurations at m ≠ 0 and at T lowered to thetransition temperatures at this non-vanishing m. An ensem-ble of configurations at a transition point was reweighted toan ensemble of configurations at another transition point.

Line of Constant Physics Lattice calculations of the EoS are usually performed

with a fixed Nt and then, because in a fixed temperaturerange Nt is inversely proportional to the lattice spacing,the continuum limit can be approached by increasing Nt.

Keeping Nt constant means that the temperature can only bevaried by changing the lattice spacing. This is usuallyachieved by varying the gauge coupling. If we want tokeep, for example, the quark masses constant then thedimensionless lattice mass parameters (amq) have to betuned accordingly. This defines the line of constant physics(LCP) in the parameter space.

If we keep the mass parameters constant and do not fol-low the LCP—which is the case in most EoS lattice stud-ies—then we have to face the following unphysicalsituation. Cooling down two systems, one at 3Tc and one atTc to zero temperature, the quarks in the former case will be3 times heavier. In this approach not mq but mq /T is keptconstant.

Previous Results on the Equation of State There are numerous lattice results for the EoS using

dynamical quarks. However, in all cases the quarkmasses—for computational reasons mentioned in the intro-duction—were set to higher values than their physical one.The first results were obtained with staggered fermions.Calculations were performed by the MILC collaboration[4,5] and by Karsch, Laermann, and Peikert from Bielefeld[6]. The first calculation with Wilson fermions was done bythe CP-PACS collaboration [7].

Staggered results are shown on Figure 1. No LCP wasused in these cases, which means that the curves corre-spond to constant mq /T, that is, increasing quark masseswith increasing temperature. Figure 2 shows the EoSobtained with Wilson fermions for Nt = 4 and 6. The low-est quark mass used here corresponds to a pion mass of 500MeV. The LCP was used in this analysis.

In the last years small nonzero chemical potentials[9,10] have also been used to determine the EoS [12–15].

Recently, at the lattice conference both the MILC col-laboration [16] and the RBC–Bielefeld collaboration [17]reported on their ongoing work in QCD thermodynamics.

Although the published results all apply QCD withdynamical quarks they still have several weaknesses:

1. In all cases, unphysical quark masses were used, whichresults in unphysical pion masses. Because the transi-tion temperature is higher than the physical mass of thepion, but smaller than the pion masses used in these cal-culations, it might be important to use physical values.

2. The works with staggered fermions did not use the lineof constant physics, which results in an unphysicaldependence of the hadron masses on the temperature.

ε ε= ∂∂

− = + =Tp

Tp s p T c

dp

ds, ( ) , .2

ε(3)

feature article


3. A known problem with staggered fermions is the tastesymmetry violation, which causes a non-physical non-degeneracy of the pion masses. This non-degeneracydisappears in the continuum limit, but it is still large forthe lattice spacings used in these calculations.

4. The approximate R algorithm [18] was used for thecalculations with 2, 3, or 2 + 1 flavors of staggeredfermions. This algorithm has an intrinsic stepsize thatleads to systematic errors in the results. In order toeliminate this systematics an extrapolation to zerostepsize should be performed. None of the previousworks have done such an extrapolation. It should bementioned that due to the subtraction in the calcula-tion of the pressure the error coming from the typi-cally used finite stepsizes is comparable with theresult itself.

5. The discretization errors are still probably large. This isespecially true for temperatures around and below Tc

where the lattice spacing of Nt = 4 lattices can be aslarge as 0.3 fm.

6. The determination of the physical scale is not alwaysunambiguous. Ref. [6] uses, for example, the string

tension, which is—strictly speaking—not an existingquantity in full QCD because at large distances thestring breaks and a meson pair is produced.

New Results with Physical Quark Masses, Equation of State

In the following the new results obtained in collabora-tion with Y. Aoki and K.K. Szabó are presented. Details ofthis work are found in Ref. [20]. We have determined theEoS for two sets of lattice spacings, Nt = 4 and 6. Weimproved on all points listed earlier.

Lattice action, LCP The lattice action we used was a combination of the

tree-level Symanzik-improved gauge action and the stout-improved fermionic action [21]. The stout improvement isknown to reduce the taste symmetry violation significantly.

As mentioned earlier, using an approximate algorithmwithout performing the necessary extrapolations is danger-ous. Instead we used the exact rational hybrid Monte-Carlo(RHMC) algorithm [22,23].

Figure 1. Left: The pressure (lower symbols) and energy density (upper symbols) with 2 flavors of unimproved staggeredfermions on Nt = 4 (diamonds) and Nt = 6 (squares and circles) lattices for different mq /T ratios. [5]. Right: The pressurewith 2, 2 + 1, and 3 flavors of p4 improved staggered fermions on Nt = 4 lattices [6].

1

feature article


The quark masses were set to their physical values sothat the meson masses agree with their physical values upto a few percent. Moreover, the physical quark masses werekept constant while increasing the temperature, that is, wefollowed the LCP.

In order to give the EoS in T/Tc units, we had to find theratio of the scales at the different simulation points. For thiswe matched the static quark–antiquark potential for the dif-ferent points at an intermediate distance. Tc was defined asthe turning point of the isospin number susceptibility [20].The precise determination of Tc, that is, connecting thescale to physical quantities, will be the subject of a subse-quent publication.

Results In order to present the Nt = 4 and 6 results on the same

plots we rescaled all quantities in the following way. Atinfinite temperatures all quantities should approach theirfree Stefan-Boltzmann limit (c). This limit is, however dif-ferent in the continuum (ccont) and on lattices with somefixed Nt(cNt

). Therefore all results are scaled with a factorccont/cNt

so that they could be compared with the continuumStefan-Boltzmann limits.

Figure 3 shows the pressure and the energy density nor-malized by T4. For comparison, the Stefan-Boltzmann limitis also shown. Similarly, one can determine the entropy

density, speed of sound, and quark number susceptibilities[20].

New Results with Physical Quark Masses, Critical Endpoint

A critical point is expected in QCD on the temperatureversus baryonic chemical potential plane. Our goal in thissection is to determine the location of this critical point.

The lattice action we used was the unimproved stag-gered action with physical quark masses (it means, that thepion and kaon masses take approximately their physicalvalues).

The partition function of lattice QCD with nf degeneratestaggered quarks is given by the functional integral of thegauge action Sg at gauge coupling b over the link variablesU, weighted by the determinant of the quark matrix M,which can be rewritten [9] as

Z m DU S U

M m U DU S U

g

ng w

f

( , , ) exp ( , )

[det ( , , )] exp ( ,/

β μ β

μ β

= −⎡⎣ ⎤⎦

= −

∫4 ))

[det ( , , )]

exp ( , ) ( , )

det ( ,/

⎡⎣ ⎤⎦

×

− +⎡⎣ ⎤⎦

∫

M m U

S U S U

M mw wn

g g w

fμ

β β4

μμμ

, )

det ( , , )

/U

M m Uw w

nf⎡

⎣⎢

⎤

⎦⎥

⎧

⎨⎪⎪

⎩⎪⎪

⎫

⎬⎪⎪

⎭⎪⎪

4

(4)

Figure 2. The pressure (left) and ε − 3p (right) obtained from �(a) improved Wilson fermions for several pion masses onNt = 4 (filled symbols) and Nt = 6 (open symbols) lattices [7].

2

feature article


where m is the quark mass, m is the quark chemical potentialand nf is the number of flavors. For non-degenerate massesone uses simply the product of several quark matrix deter-minants on the 1/4-th power. Standard importance sam-pling works and can be used to collect an ensemble ofconfigurations at mw, bw, and mw (with, e.g., Re(mw) = 0 ornon-vanishing isospin chemical potential). It means wetreat the terms in the curly bracket as an observable—whichis measured on each independent configuration—and therest as the measure. By simultaneously changing severalparameters, for example, b and m one can ensure that eventhe mismatched measure at bw and mw samples the regionswhere the original integrand with b and m is large. In prac-tice the determinant is evaluated at some m and a Ferren-berg-Swendsen reweighting [11] is performed for the gaugecoupling b. The fractional power in Eq. (4) can be taken byusing the fact that at m = mw the ratio of the determinants is1 and the ratio is a continuous function of the chemicalpotential. The details of the determinant calculation can befound in Ref. [10].

In the following we keep m real and look for the zeros ofthe partition function on the complex b plane. These are theLee-Yang zeros [19]. Their V ® ¥ behavior tells the differ-ence between a crossover and a first order phase transition.At a first order phase transition the free energy µ log Z(b)isnon-analytic. Clearly, a phase transition can appear only in

the V → ¥ limit, but not in a finite V. Nevertheless, the par-tition function has Lee-Yang zeros at finite V. These are at“unphysical” complex values of the parameters, in our caseat complex b-s. For a system with a first order phase transi-tion these zeros approach the real axis in the V ® ¥ limit(the detailed analysis suggests a 1/V scaling). This V ® ¥limit generates the non-analyticity of the free energy. For asystem with crossover the free energy is analytic, thus thezeros do not approach the real axis in the V ® ¥ limit.

Figure 4 shows Im(b ¥0) as a function of m enlarged aroundthe endpoint mend. The picture is simple and reflects the physicalexpectations. For small m-s the extrapolated Im(b ¥0) is incon-sistent with a vanishing value, and the prediction is a cross-over. Increasing m the value of Im(b ¥0) decreases, thus the tran-sition becomes consistent with a first order phase transition.

Setting the scale leads to the final results of the analysis.As we already discussed, the quark masses, used to deter-mine the endpoint, correspond approximately to their phys-ical values. The pion to rho mass ratio, extrapolated to ourT ¹ 0 parameters, is 0.188(2) (its physical value is 0.179),whereas the pion to K mass ratio in the same limit is0.267(1) (its physical value is 0.277).

Figure 5 shows the phase diagram in physical units, thusT as a function of mB, the baryonic chemical potential(which is three times larger then the quark chemical poten-tial). At m = 0 the transition between the hadronic and

Figure 3. Left: the pressure p, as a function of the temperature. Both Nt = 4 (red, upper curve) and Nt = 6 (blue, lowercurve) data are obtained along the LCP. They are normalized by T4 and scaled by ccont/cNt

. In order to lead the eye linesconnect the data points. Right: the energy density (e), red (upper), and blue (lower) for Nt = 4 and 6, respectively. Thisresult was obtained directly from the pressure.

feature article


quark-gluon plasma phases is a cross-over. As we increasethe chemical potential the transition temperature decreases,but the transition itself remains a cross-over. At a givenendpoint chemical potential the transition is a second orderone. For even larger chemical potentials the transition tem-perature further decreases and the transition becomes a firstorder one. The curvature of the crossover line separating theQGP and the hadronic phases is given by T/Tc = 1−Cm2B/Tc

2

with C = 0.0032(1). The endpoint is at Th = 162 ± 2 MeV, mE = 360 ± 40

MeV.

Summary Previous results using either staggered of Wilson fermi-

ons were discussed. They suffer from several weaknesses. New results on the EoS were presented. Our analysis

attempted to improve on previous analyses by several means.We used for the lightest hadronic degree of freedom the physi-cal pion mass. We used two different sets of lattice spacings(Nt = 4, 6). The system was kept on the line of constant phys-ics (LCP) instead of changing the physics with thetemperature.Due to our smaller lattice spacing and particularly due to ourstout-link improved fermionic action the unphysical pion masssplitting was much smaller than in any previous staggeredanalysis. An exact calculation algorithm was applied.

We presented results for the pressure and energy den-sity. In a similar way results for the entropy density, speedof sound, and the isospin and strangeness susceptibilitiescan be obtained.

Although a continuum extrapolation could already beperformed with the current data, because the Nt = 4 lattices

are rather coarse (especially around and below Tc) it wouldbe safer if the EoS on even finer lattices (Nt = 8) wereobtained. Such an analysis would be a major step towardthe final results for the equation of state.

We also discussed the overlap-improving multiparam-eter reweighting technique, in order to calculate physicalobservables at non-vanishing temperatures and chemicalpotentials. A critical point is expected in QCD on the tem-perature versus baryonic chemical potential plane. Usingthe above lattice method for m ≠ 0 we studied dynamicalQCD with nf = 2 + 1 staggered quarks of physical masseson Nt = 4 lattices. We used physical quark masses in thisanalysis. Our result for the critical point is TE = 162 ± 2MeV and mE = 360 ± 40 MeV. The continuum limitextrapolation is also missing in this case.

Acknowledgments This work was partially supported by OTKA Hungarian

Science Grants No. T34980, T37615, M37071, T032501. andby the EU Hadron physics project RII3CT-20040506078.

The computations [25] were carried out at Eötvös Uni-versity on the 330 processor PC cluster of the Institute forTheoretical Physics and the 1024 processor PC cluster ofWuppertal University.

Figure 4. Im(b¥0) as a function of the chemical potential.

Figure 5. The phase diagram in physical units. Dotted lineillustrates the crossover, solid line the first order phasetransition. The small square shows the endpoint. Thedepicted errors originate from the reweighting procedure.Note, that an overall additional error of 1.3% comes fromthe error of the scale determination at T = 0. Combiningthe two sources of uncertainties one obtains TE = 162 ±2 MeV and μE = 360 ± 40 MeV.

feature article


References 1. G. Boyd et al., Nucl. Phys. B 469 (1996) 419 [arXiv:hep-lat/

9602007]. 2. M. Okamoto et al., Phys. Rev. D 60 (1999) 094510

[arXiv:hep-lat/9905005]. 3. Y. Namekawa et al., Phys. Rev. D 64 (2001) 074507

[arXiv:hep-lat/0105012]. 4. T. Blum et al., Phys. Rev. D 51 (1995) 5153 [arXiv:hep-lat/

9410014]. 5. C. W. Bernard et al. [MILC Collaboration], Phys. Rev. D 55

(1997) 6861 [arXiv:hep-lat/9612025]. 6. F. Karsch, E. Laermann, and A. Peikert, Phys. Lett. B 478

(2000) 447 [arXiv:hep-lat/0002003]. 7. A. Ali Khan et al., Phys. Rev. D 64 (2001) 074510 [arXiv:hep-

lat/0103028]. 8. P. Hasenfratz and F. Karsch, Phys. Lett. B 125 (1983) 308; J.

B. Kogut, H. Matsuoka, M. Stone, H. W. Wyld, S. H. Shenker,J. Shigemitsu, and D. K. Sinclair, Nucl. Phys. B 225 (1983) 93.

9. Z. Fodor and S. D. Katz, Phys. Lett. B 534 (2002) 87[arXiv:hep-lat/0104001].

10. Z. Fodor and S. D. Katz, JHEP 0203 (2002) 014 [arXiv:hep-lat/0106002]; JHEP 04 (2004) 050 [arXiv:Xep-lat/0402006].

11. A. M. Ferrenberg and R. H. Swendsen, Phys. Rev. Lett. 61(1988) 2635; A. M. Ferrenberg and R. H. Swendsen, Phys.Rev. Lett. 63 (1989) 1195.

12. Z. Fodor, S. D. Katz, and K. K. Szabo, Phys. Lett. B 568(2003) 73 [arXiv:hep-lat/0208078].

13. R. V. Gavai and S. Gupta, Phys. Rev. D 68 (2003) 034506[arXiv:hep-lat/0303013].

14. C. R. Allton et al., Phys. Rev. D 68 (2003) 014507 [arXiv:hep-lat/0305007].

15. F. Csikor et al., JHEP 0405 (2004) 046 [arXiv:hep-lat/0401016].

16. C. Bernard et al., arXiv:hep-lat/0509053. 17. C. Jung, arXiv:hep-lat/0510035. 18. S. A. Gottlieb, W. Liu, D. Toussaint, R. L. Renken, and R. L.

Sugar, Phys. Rev. D 35 (1987) 2531. 19. C. N. Yang and T. D. Lee, Phys. Rev. 87 (1952) 404; T. D. Lee

and C. N. Yang, Phys. Rev. 87 (1952) 410. 20. Y. Aoki, Z. Fodor, S. D. Katz, and K. K. Szabo, hep-lat/0510084 21. C. Morningstar and M. J. Peardon, Phys. Rev. D 69 (2004)

054501 [arXiv:hep-lat/0311018]. 22. M. A. Clark, B. Joo, A. D. Kennedy, Nucl. Phys. Proc. Suppl.

119 (2003) 1015 [arXiv:hep-lat/0209035]. 23. M. A. Clark and A. D. Kennedy, Nucl. Phys. Proc. Suppl. 129

(2004) 850 [arXiv:hep-lat/0309084]. 24. F. Karsch, E. Laermann, and A. Peikert, Nucl. Phys. B 605

(2001) 579 [arXiv:hep-lat/0012023]. 25. Z. Fodor, S. D. Katz, and G. Papp, Comput. Phys. Commun.

152 (2003) 121 [arXiv:hep-lat/0202030]. 3

facilities and methods


Experiments with Stored Relativistic Exotic Nuclei at the FRS-ESR Complex

Introduction For almost three decades, storage

and cooler rings have been indispens-able tools for atomic, nuclear, andhigh-energy physics. While electro-static storage rings (ELISA at Aarhus,Denmark; CSR at Heidelberg,Germany) are specialized for the low-est beam kinetic energies, electromag-netic rings cover the energy rangefrom the Coulomb barrier (ASTRID atAarhus, Denmark; TSR at Heidelberg,Germany; CRYRING at Stockholm,Sweden) up to the relativistic regime(CELSIUS at Uppsala, Sweden;COSY at Jülich, Germany; IUCF atBloomington, USA). The past LEARmachine, the present AD (both locatedat CERN in Geneva, Switzerland), andthe future FAIR-facilities HESR andFLAIR will provide antiproton beamsat relativistic and low energies (from afew MeV down to rest).

The experimental storage ringESR of GSI at Darmstadt, Germany, isthe only existing storage-ring facilityfor experiments with radioactive,highly charged ions. This is due to theworldwide unique combination withan in-flight separator, the FRagmentSeparator FRS [1]. The investigationof exotic nuclei in a heavy-ion storageand cooler ring opens up a new fieldof precision experiments. Already thefirst experiments showed the discov-ery potential for nuclear structure andastrophysics [2–4] The high chargestates strongly affect those decay chan-nels of radioactive ions, which involvethe atomic cloud, such as electron con-version and electron capture branches,but also led to the discovery of newdecay modes like the beta-decay to

bound states of the released electron[5]. Mapping of wide ranges of themass surface benefits from the largevariety of cocktail beams provided bythe FRS and the ESR’s capability toact as a high-resolution multi-turnmass spectrometer.

Planned in the 1980s, the FRS-ESRcomplex has stimulated many technicalforefront developments and yieldedseveral physics highlights since thebeginning of its operation in the earlynineties. Some of the importantachievements of the first-generationexperimental program at FRS-ESR willbe presented in the following.

Novel Methods Energetic secondary beams are

produced by projectile fragmentationin peripheral collisions of relativisticheavy-ion beams and by fission ofuranium beams after electromagneticexcitation in the production target ofthe FRS. This low excitation-energyfission is particularly interestingbecause it leads to hitherto inaccessi-ble, neutron-rich nuclides [6]. Subse-quent separation in-flight allows oneto inject mono-isotopic beams or well-defined cocktail beams of exoticnuclei into the ESR. At typical ener-gies of 400–800 MeV/u the ions arefully stripped off their electrons whenpenetrating through matter, becausethe velocity is similar to or evenexceeding the orbital electron veloci-ties, thus leading to bare nuclei or tofew-electron ions. The beam qualityof the separated fragments can be con-siderably improved by stochastic pre-cooling [7] and electron cooling in theESR [8]. While stochastic cooling is

best suited for beams with large longi-tudinal momentum spread (of theorder of several permille to percent),electron cooling leads to highestphase-space densities and a final rela-tive velocity spread of the order of 10−7.Cooling times of few seconds can bereached, see Figure 1. At intensitiesbelow a few thousand ions Coulomb-ordered, crystalline beams [8,9] areformed. Depending on the residual gascomposition and pressure and on thecooler electron current, stored beamshave typical lifetimes of about onehour (for the heaviest species such asbare uranium nuclei) up to about oneday (for lighter nuclei such as neon)and can thus be observed for longtimes.

In the last years the experimentalprogress was intimately connected withthe development of new and challengingmeasurement and detection methodssuch as time-resolved Schottky spec-troscopy [11], the operation of the ESRin the isochronous mode [12] (a modethat is usually avoided for circular accel-erator machines, because it lacks phasefocusing and leads to unstable opera-tion), and a fast, ultra-high-vacuumcompatible time pick-up detector [13]including a data acquisition system thatemploys ultra-fast signal sampling(sampling rates up to 20 giga-samplesper second are used for the recording ofsignals from the time pick-up detector).Both, the time pick-up detector andtime-resolved Schottky spectroscopyreach the ultimate sensitivity for singleions and are therefore ideal diagnostictools for experiments with weak second-ary beams. The Schottky noise power isproportional to the beam intensity.



Decay studies can be performed with atime resolution of about one second.Based on these developments, two new,complementary techniques, have been,for example, pioneered for direct massmeasurements: Schottky Mass Spec-trometry (SMS) [14,15] and Isochro-nous Mass Spectrometry (IMS) [16,12].SMS is based on cooled beams andemploys time-resolved Schottky spec-troscopy for analysis of the intensitiesand revolution frequencies of the ions,see Figure 2. The cooling time is thelimiting parameter for the accessiblehalf-lives of the order of about one sec-ond. IMS fully profits from the shortseparation times of an in-flight system(few 100ns) and allows the time-of-flight mass measurement within fewmicroseconds.

Example Results

Mapping the Mass Surface Both methods, SMS and IMS, are

capable of mapping large areas of thenuclear mass surface with typical

accuracy between 30 . . . 100 keV. Fig-ure 3 shows a part of those nuclides,whose mass was experimentallydetermined at FRS-ESR for the firsttime. In one of the runs, neutron-defi-cient nuclei were produced by bis-muth fragmentation [11], and withSMS the masses of almost 465 differ-ent isotopes were directly determined,more than 170 of them were previ-ously unknown. Some additional 107new masses could be obtained byusing the links of α-chains, leading toone of the most important results ofthese experiments: the location of theproton-drip-line in the region of fran-cium [15,17]. Moreover, the shell gapenergy of neutron-deficient lead iso-topes was probed and yielded newinsights on the deformation effects inthis area [17,18].

IMS is suited to investigate veryshort-lived nuclides such as uraniumfission products with millisecondhalf-lives or even less. With primary-beam intensities of 2·109 uraniumions per spill the measurements reachpresently out up to e. g. 135Sn. The

new mass data in the vicinity ofclosed shells (N = 50.82, Z = 28.50)and in particular in the vicinity ofdouble shell closures contribute to theinvestigation of isospin dependenceof shell and pairing effects [19] andthe long-standing question of possibleshell quenching. The potential pros-pects of combined FRS-ESR experi-ments are illustrated with the newisotope 235Ac in the right part of Fig-ure 2, indicating its simultaneous firstidentification, mass determinationand half-life measurement [20].

Half-Lives, New Decay Modes Some nuclear decays involve atomic

electrons and are thus altered bychanges of the ionic configuration. Themost dramatic modifications occur in163Dy, a stable nucleus when dressed,but radioactive when fully ionized. Itdecays to 163Ho3+ with a half-life ofT1/2 = 48 ± 5 days [5]. The reason is thechanging Q-value from a negative to apositive value. Sounding paradoxical,the difference of atomic ionization ener-gies of the now missing shells of mother

Figure 1. Stochastic pre-cooling and electron cooling of fragment beams as indicated. Left: time-resolved frequencyspectrum starting with the beam injection into the ESR. Right: reduction of the relative momentum width of the 207Tl81+

due to both cooling methods.



and daughter ion is additionally avail-able for the nuclear decay. This effectled to the discovery of a long predictednew decay mode: the beta-decay tobound states [21], where the emittedelectron becomes bound in an inneratomic shell of the daughter nucleus.This two-body beta-decay is the time-reversed orbital electron capture.Together with the FRS, these basicexperiments were recently extended tosecondary beams, which ideally allowthe choice of the best-suited candidates(in terms of Q-value, half-life, branchingratios, etc.). Studying the bound-statebeta-decay (βb) of bare 207Tl81+ it is pos-sible to obtain a wealth of unique infor-mation [22]: total and partial βb-decayrates, the Qβb-value, and the “Fermi-function,” which is the ratio of boundand continuum electron wave functionat the origin. This function has beenprobed for β−- and electron-capturedecay, but never before for β−-decay.For the first time a branching ratio ofthis new decay mode was measured, theresult is shown in Figure 4. The newlyavailable phase-space of the emptyelectron shells leads to an additionaldecay branch and a half-life which isshorter by 11%.

Besides decays involving leptons,also hadronic decays such as proton-

and α-radioactivity are expected tovary characteristic properties. Themissing electron screening of thenuclear charge distribution results inan increased Coulomb-barrier heightand thus to reduced decay probabili-ties. Depending on the Q-value, thiseffect may alter the decay rates by upto 100%, which will be an issue forcoming studies.

Isomeric Beams The complete blocking of “usu-

ally” open decay channels due tothe absence of atomic shell elec-trons has been studied in isomericstates with low transition energies,where internal conversion domi-nates. For instance the isomericstate in neutral 151Er (T1/2 = 0.58 s,excitation energy E* = 2.568 MeV)decays with 95% probability byinternal conversion via a 58 keVE3-transition. When fully ionized,the residual 5%-branch for β+-decay leads, together with minorcorrections (for details see Ref.[23]), to an almost 30-foldincreased half-life, in excellentagreement with the experimentalvalue.

One of the future goals is to pro-vide purely isomeric beams, whose

decay properties can be studied incases, where other detection meth-ods fail, for example, due to toolong half-lives.

Applications in Nuclear Astrophysics Origin and evolution of the

chemical elements and the explana-tion of the observed stellar, in partic-ular the Solar abundance pattern(s)belong to those questions in astro-physics, where nuclear data add keycontributions. Charged-particlefusion reactions account for theabundances of nearly all metals upto the iron group, whereas almost allisotopes beyond iron are createdfrom captures of free neutrons. Oncethe macroscopic parameters (seedmaterial, densities, temperature, entropy,etc.) of quiescent burning phases ofstellar plasmas and thermonuclearrunaways are settled, the nucleosyn-thesis pathways, their endpoints,time scale, energy production andthe final abundance pattern aremainly governed by nuclear reactioncross sections, mass-differences, andbeta-decay rates. Masses play a cru-cial role because, for instance, the r-process follows a trail in the chart ofnuclei, which is characterized by aneutron-separation energy of ≈2 MeV.

Figure 2. Left: broad-band Schottky spectrum of cooled uranium projectile fragments with known calibration masses(bold labels) and masses determined for the first time (outlined letters). Right: The time-dependent Schottky-noise signalof the new isotope 235Ac (in hydrogen-like charge state) from which mass and half-life are determined.

1



Network calculations rely on dataof up to about 6,000 nuclei.Schottky and Isochronous MassSpectrometry, capable of probinglarge fractions of the mass surfacerather than isolated spots, areextremely valuable to test mass pre-dictions over a wide range of protonand neutron numbers, to stimulateimprovements and to allow forextrapolations into unknown terri-tory. It is also obvious, that at typi-cal temperatures kT ≈ 30 keV (s-process) or >100 keV (r-, rp-pro-cess), nuclear disintegrations occurmainly from bare nuclei or highly-charged ions, leading to signifi-cantly changed decay rates, inextreme cases decay channels mayeven become totally blocked, othersmay appear newly. Age-dating tech-niques and nuclear cosmo-chronom-eters, mostly used to put lower limitson the age of the Galaxy, are clearlyaffected by these environmentaleffects and still suffer from someuncertainties. Reliable data are thusneeded.

Summary and Outlook The tremendous research potential

of storage-ring experiments withunstable beams has been pioneered atthe FRS-ESR facility. Novel tech-niques, such as electron and stochasticcooling for relativistic heavy ions,Schottky and Isochronous Mass Spec-trometry, first tests for elastic scatter-ing off internal targets and novelexperimental studies probing the di-electronic recombination, decay-studiesof bare exotic nuclei and high-preci-sion atomic-structure and QED-stud-ies with highly charged ions [24] areonly the most prominent examples tomention. Further exciting results areexpected from new combined frag-ment-separator-storage-ring facilities,which will become operational soon,such as the HIRFL project at Lanzhou,China, and at RIKEN, Japan. TheFRS-ESR facility described hereserves as a model for these new facili-ties and simultaneously as the proto-type for the upcoming NUSTARfacility comprising of the fragment-separator and storage-ring complex at

FAIR [25,26]. The intensity gain ofthe new synchrotrons, the increasedtransmission of the SUPER-conduct-ing FRagment Separator SUPER-FRS[1] for fission fragments (while pre-serving the characteristic mono-isotopicselectivity and single-particle sensitiv-ity) and improved acceptance andcooling capabilities will allow nuclear-structure studies on nuclei which arepresently out of reach, such as r- andrp-process nuclei, and neutron-richdrip-line nuclei up to the nickelregion.

Increased performance is expectedfrom a system of collector and stor-age-cooler rings, each of themequipped with unique instrumentationand specialized for dedicated experi-ments. The fast stochastic cooling actson a time scale of typically 100 ms andleads to efficient storage and coolingof neutron-rich fission fragments. TheCR collector ring is optimized for effi-cient capture and fast phase-spacereduction of the fragment beams fromSuper-FRS and for Isochronous MassSpectrometry with “hot” secondary

Figure 3. Newly measured masses from SMS (large squares) compared with models. Absolute deviations from the HFB(left), HF+BCS (center) and FRDM (right) models are illustrated on the chart of nuclides [10].



beams, thus allowing to measuremasses of species with half-livesdown to a few microseconds with aprecision of typically δm ≈ 50 keV.The stochastically pre-cooled beamswill be transferred into the new exper-imental storage ring NESR for preci-sion mass measurements (δm≈10keV,lower half-life limit T1/2 ≈ 50 ms) anddecay studies employing time-resolved Schottky Mass Spectrometry.Pure isomeric beams can be producedin favourable cases (half-lives exceed-ing ≈ 1 s, excitation energies in excessof ≈ 1 MeV).

Reaction studies in storage ringsare typical second-generationexperiments, which require theincreased luminosities. A novelheavy-ion-electron collider willprovide purely electromagneticallyinteracting probes for the Coulombexcitation of exotic nuclei [27].Electrons permit single-step excita-tion to well-defined states. With

conventional methods such experi-ments are restricted to stable nuclei,whereas the collider technique pro-vides much cleaner conditions andgives access to exotic nuclei. Theexperiments will probe dipolestrength distributions, shell struc-ture, charge distributions and formfactors by means of elastic andinelastic electron scattering. Newopportunities for nuclear-structurestudies arise from hyperfine inter-action and isotope-shift measure-ments, for example, by theinteraction with monochromaticphotons from lasers or by the di-electronic recombination [28] ofnuclei in an electron target of theNESR. Antiprotons colliding withexotic nuclei are a new hadronicprobe to unravel nuclear matter dis-tributions. Thus, new methods andnew probes will open up a new fieldfor precision experiments. Anexciting new era is to be expected.

References 1. H. Geissel et al., Nucl. Instr. Meth.

B204, 71 (2003). 2. H. Geissel et al., Phys. Rev. Lett. 68,

3412 (1992). 3. H. Irnich et al., Phys. Rev. Lett. 75,

4182 (1995). 4. T. Radon et al., Phys. Rev. Lett. 78,

4701 (1997). 5. M. Jung et al., Phys. Rev. Lett. 69,

2164 (1992). 6. M. Bernas et al., Phys. Lett. B 415, 111

(1997) and Nucl. Phys. A 616, 352(1997).

7. F. Nolden et al., Nucl. Instr. Meth.A532, 79 (2004).

8. M. Steck et al., Nucl. Instr. Meth.A532, 357 (2004).

9. R. W. Hasse, Phys. Rev. Lett. 86, 3028(2001).

10. H. Geissel et al., Nucl. Phys. A 746,150c (2004).

11. Yu. A. Litvinov et al., Nucl. Phys. A756, 3 (2005).

12. M. Hausmann et al., Nucl. Instr. Meth.A 446, 569 (2000).

13. J. Trötscher et al., Nucl. Instr. Meth. B70, 455 (1992).

Figure 4. Left: first direct observation of bound-state beta-decay: traces of bare mother nuclei 206 m,gTl81+ and hydrogen-like bound-beta-decay daughter 206Pb81+, recorded and observed with time-resolved Schottky spectroscopy forapproximately 20 min. Right: branching ratio of bound-state and continuum-state beta-decay as a function of Q-valuefor a nucleus with atomic number Z=81 (solid line) compared with the experimental value for the decay of bare 207Tl81+

(data point, error bars are smaller than the symbol size).



14. B. Franzke, H. Geissel, and G.Münzenberg, “Experiment proposal forthe SIS-FRS-ESR facilities,” GSI (1987).

15. T. Radon etal., Nucl. Phys. A 677, 75(2000).

16. H. Wollnik et al., “Experiment pro-posal for the SIS-FRS-ESR facilities,”GSI (1986).

17. Yu. N. Novikov et al., Nucl. Phys. A697, 92 (2002).

18. M. Bender etal., Eur. Phys. J. A 14, 23(2002).

19. Yu. A. Litvinov et al., Phys. Rev. Lett.95, 042501 (2005).

20. H. Geissel et al., nucl-ex/0510009. 21. F. Bosch, Phys. Scripta T59, 221 (1995). 22. T. Ohtsubo et al., Phys. Rev. Lett. 95,

052501 (2005). 23. Y. Litvinov etal., Phys. Lett. 573, 80

(2003). 24. T. Stöhlker et al., Lecture Notes in

Physics 627, 115 (2003). 25. T. Nilsson and B. Rubio, submitted to

Nucl. Phys. News in December 2005. 26. See http://www.gsi.de/forschung/kp/

kp2/nustar.html 27. G. Münzenberg et al., Nuclear Physics

at Storage Rings, AIP, 293 (2000). 28. C. Brandau et al. Phys. Rev. Lett. 91,

073202 (2003).

WOLFGANG R. PLAßJustus-Liebig-University Gießen

Germany

CHRISTOPH SCHEIDENBERGER

GSI, DarmstadtGermany

2

meeting reports


12th Euroschool on Exotic Beams, 25 August–2 September 2005, Mainz, Germany

In summer 2005, the 12th Euro-school on Exotic Beams was held inMainz, Germany. This school seriesstarted out in the 1990s at Leuven,Belgium. It is intended for doctoralstudents and post-docs working onphysics and techniques related toradioactive ion beams. Since 2000 ithas travelled to various places inEurope (Jyväskylä, Les Houches,Valencia, and Surrey). The nextschool will be held at ECT* in Trento,Italy, 11–15 September 2006.

This year’s school at Mainzbrought together more than 70 stu-dents from 19 different, mostlyEuropean, countries, with both gen-ders almost equally represented. It is agood practice of the school that lectur-ers and students live at the same loca-tion. Besides the lectures, there isample time and opportunity for dis-cussions and the exchange of ideas.The lectures were given by well-known specialists from universitiesand research laboratories in Europeand America. Among the main courseswas a basic introduction on heavy-ionaccelerators and on specific aspects oftheoretical and experimental nuclearphysics with exotic nuclei. A specialsession was devoted to the sustainabil-ity of nuclear energy and a comparisonof various electricity supply options.During a poster exhibition studentspresented their own research work andcould learn from each other. The one-day visit to a European large-scale

research facility led the school thisyear to GSI at Darmstadt. Here, theattendees inspected the acceleratorand experimental facilities. In particu-lar, they learned about specific advan-tages of cancer therapy with heavy-ionbeams. They recognized their futureopportunities with the broad physicsprogram addressed by the upcomingFAIR project.

A boat trip to the most scenicpart of the Rhine river betweenMainz and Koblenz provided a rec-reational break during the lectureprogram. The atmospheric farewellevening combined typical local foodwith live folksongs from different

countries, presented by both lectur-ers and students. This balancedcombination of hard work and lei-sure made the Euroschool on ExoticBeams an unforgettable event forall participants.

For more information on theMainz school, including the programand the given lectures, see http://www-linux.gsi.de/~scheid/euroschool-05/home.htm. The Euroschool onExotic Beams is funded by the EUunder contract number HPCF-2001-00101-01.

CHRISTOPH SCHEIDENBERGER

GSI Darmstadt

meeting reports

34 Nucluear Physics News, Vol. 16, No. 2, 2006

The 12th International Conference on Capture Gamma-Ray Spectroscopy and Related Topics

The 12th International Conferenceon Capture Gamma-Ray Spectroscopyand Related topics (CGS-12) washosted by the University of NotreDame from Monday, 5 September toFriday, 9 September 2005. The Uni-versity of Notre Dame du Lac,founded in 1842 by a priest of theCongregation of Holy Cross, is anindependent, national Catholic univer-sity located in a rather picturesque partof Indiana adjacent to the city of SouthBend and approximately 90 milessoutheast of Chicago. The conferencewas preceded by a one-day workshopon Nuclear Isomers organized byProfessor Yang Sun on Sunday, 4September 2005. The conference pro-gram was lively and interactive with alarge number of women and youngscientists giving invited talks. Theconference theme varied across cur-rent topics in Nuclear Structure,Nuclear Symmetries, Nuclear Reactionswith stable as well as radioactive ionbeams, Nuclear Astrophysics, Toolsof Nuclear Science from instrumenta-tion to facilities, as well as Applica-tions of Nuclear Science. The lastmeeting of the series, CGS-11, washeld in Pruhonice near Prague in theCzech Republic in 2002. Some of theother conferences were held in SantaFe, NM, USA (1999); Budapest, Hungary(1996); Fribourg, Switzerland (1993);Asilomar, USA (1990); Leuven,Belgium (1987); Knoxville, TN, USA(1984); Grenoble, France (1981);Brookhaven, USA (1978); Petten, theNetherlands (1974); and Studsvik,Sweden (1969).

More than 150 scientists from 23countries attended this conference.

The program was rather denselypacked for five and a half full dayswith many excellent contributionsfrom the speakers as well as those whopresented their posters, making thisconference a big success.

There are enormous changes in thefield of interest for this conferenceover time. A main emphasis of thefield has shifted toward NuclearAstrophysics and toward other appli-cations of nuclear science. New facili-ties have come on line specificallyusing neutrons with n_TOF at CERN,Geneva and DANCE at Los AlamosNational Laboratory. Another pleasantsurprise for us was the participation ofsignificant numbers of women at thismeeting.

The conference program at CGS-12 was scheduled to avoid any parallelsessions enabling all the participantsto be a part of the entire program. In aspecial evening session—assisted byother spirits—we included a sessionon “Data for Nuclear- and Astrophys-ics Application” to address the needsof the collection, compilation and dis-semination of nuclear data in our field.This was perhaps one of the liveliestsessions of the conference. An emerg-ing trend is the shift toward web-baseddata bases and compilations. In addi-tion to the existing nuclear data bases

on nuclear structure and reactioncross-sections, new databasesaddressing are emerging for thespecial needs of reaction-rates fornuclear astrophysics.

A special session of the conferencewas dedicated to the memory of ourdear friend, colleague, and one of thefounding members of neutron capturespectroscopy, Dr. David D. Warner.Dave Warner was a member of theInternational Advisory Committee,one of the main invited speakers toCGS-12 and the first person to haveregistered on-line to attend CGS-12.Dave Warner was not only influentialin developing the program in NuclearPhysics in the UK but also in Europeas well as the USA. He was on theadvisory and executive committees ofevery facility in Europe, including theGSI-FAIR project, NuPecc, and theinitial stages of planning for the Rare

1

2

3

meeting reports

Vol. 16, No. 2, 2006, Nucluear Physics News 35

Isotope Accelerator in the US. Davewas a great friend, mentor, and col-league and he is missed more than wecan say in words.

Professor Steven Yates of theUniversity of Kentucky in Lexingtonfunded a prize of $500 to be awardedto the best poster presented by a post-doctoral fellow or a graduate student.The Founder’s Award was inaugu-rated in honor of the memories ofJean Kern, Raman Subramanian,and Gabor Molnar. All three of themhave played crucial roles in establish-ing the CGS series of conferencesas well as hosting one of the former

symposia for CGS. The best posterswere chosen by the selection commit-tee consisting of Professors Art Cham-pagne (University of North Carolina atChapel Hill), Alejandro Frank (UNAM—Mexico City, Mexico), and Jan Jolie(University of Koeln, Germany). Thecommittee had a tough time decidingon a winner and they suggested split-ting the prize among two graduate stu-dents. The winners were Hye YoungLee and Smarjit Triambak; both arepresently graduate students at the Uni-versity of Notre Dame Nuclear Struc-ture Laboratory. Smarjit Triambakis presently located at the University

of Washington in Seattle workingwith his advisor Alejandro Garcia.Hye Young Lee works in NuclearAstrophysics at the University ofNotre Dame with her advisor MichaelWiescher.

The CGS-12 welcoming receptionwas held in the courtyard of the Uni-versity of Notre Dame Snite Museumof Art with live music provided by theNuclear Jazz Quarktet. The collec-tions of the Snite Museum of Art placeit among the finest university artmuseums in the USA. The gallerieswere open for the conference partici-pants and a special exhibit was heldfor CGS-12. The Fritz & MillieKaeser Mestrovic Studio Gallery ofthe museum featured the BRAN-CACCI PROJECT—PHASE ONE.

In this series of murals, BillSandusky, professor of art at SaintMary’s College (Notre Dame) hadreinterpreted the fresco cyclepainted in the Brancacci Chapel ofSanta Maria del Carmine in Flo-rence, Italy by Masaccio, Masolino,and Filipino Lippi, painted betweenabout 1424 and 1480. Prof.Sandusky was available in the gal-lery on Sunday evening for a gallerytalk and questions.

The conference excursion was toview the architectural styles ofChicago. We traveled by coach fromNotre Dame to Chicago where weboarded “Chicago’s First Lady.” The

meeting reports


boat took us on an architecturalroundtrip on the Chicago river and wegot a humbling view of the skyscrap-ers of the Windy City. The conferencedinner was served on the boat and thereturn from the lake was facingChicago’s silhouette at night fromLake Michigan.

The conference ended with animpressive performance of “A Uni-verse of Dreams” with the EnsembleGalilei with National Public Radio’sNeal Conan. The performance con-sisted of music, poetry, and storieswith projected images from the

Hubble Space Telescope capturingour imagination.

The members of the advisoryand program committees met fordinner in the Golden Dome at NotreDame. The outcome was a unani-mous decision to have the next con-ference in Cologne, Germany forthe 2008 meeting. Professor JanJolie of the “Universität zu Köln”has agreed to organize the next con-ference. We are looking forward to13th International Symposium onCapture Gamma-Ray Spectroscopyand Related Topics (CGS-13)!

DR. ANDREAS WOEHR AND

PROF. ANI APRAHAMIAN

Chair of CGS-12

Report on the International Conference Frontiers in Nuclear Structure, Astrophysics and Reactions—FINUSTAR

The international conference entitledFrontiers In NUclear STructure, Astro-physics and Reactions (FINUSTAR)was held on the Aegean island of Kos,Greece, on September 12–17, 2005.The venue was the Kos InternationalConvention Center (KICC) at the Kip-riotis Village Resort. It was organizedby the Institute of Nuclear Physics ofthe National Center for ScientificResearch “Demokritos,” Athens, theDepartment of Physics of the Univer-sity of Jyväskylä, Finland, and theDepartment of Physics of the NationalTechnical University of Athens.

FINUSTAR covered a wide spec-trum of research activities, both theo-retical and experimental, in nuclearstructure, nuclear astrophysics, andnuclear reactions. Although theseresearch directions refer to differentsub-communities of nuclear physics,the interplay between these fields hasbeen strengthened over the last years

mainly by utilization of commoninstrumentation and research facili-ties. The aim of FINUSTAR was togather researchers from these scien-tific “brotherhoods” into closer con-tact in order to discuss commonproblems, present recent results, getinformed about the latest develop-ments in theory as well as themost recent achievements in instru-mentation. To meet these goals,FINUSTAR included a long list oftopics:

• Nuclear structure at the extremes • Collective phenomena and phase

transitions • Nuclear masses and ground state

properties • Synthesis and structure of heaviest

elements • Mean field theories, shell model,

cluster models, and moleculardynamics

• Nucleosynthesis in the cosmos andnuclear physics aspects

• Weak-interaction processes • Nucleon scattering as a probe for

nuclear structure • Nuclear reactions off stability and

indirect methods • Reaction dynamics at low and

intermediate energies • Radioactive and exotic beams • Facilities and instrumentation for

the future

The conference was attended by160 physicists from all 5 continentswith a fair representation of all themajor nuclear physics laboratoriesin the world. It featured 144 contri-butions, 80 of them given as oraland 64 as poster presentations.Ninety-two of them were based onexperimental and 52 on theoreticalresults. The presentations were ofexcellent quality and invoked lively

meeting reports


and instructive discussions. Alsoworth mentioning is the impres-sively high percentage of youngresearchers among the speakers.The proceedings are to appear inAIP Conference Proceedings, Vol. 831.

In light of the enthusiasticresponse of the international nuclearphysics community to FINUSTAR,we are planning to organize the sec-ond one of the series in the autumn of2007.

RAUNO JULIN

Department of Physics,University of Jyväskylä

SOTIRIOS V. HARISSOPULOS

Tandem Accelerator FacilityInstitute of Nuclear Physics, NCSR

“Demokritos”

Workshop on the Physics of Compressed Baryonic Matter

About 130 theorists and experi-mentalists from 16 countries partici-pated in a workshop on compressedbaryonic matter that took place at GSIon December 15 and 16, 2005. Thegoal of the workshop was to reviewthe physics of strongly interactingmatter under extreme conditions, andto discuss future perspectives, theoret-ical and experimental, of the physicsat high baryon density.

Tetsuo Hatsuda (Tokyo) openedthe scientific session with a reviewof the present understanding of theQCD phase diagram. He identifiedfour major phases: the hadronicphase at low temperatures T andsmall baryon chemical potential μB

where chiral symmetry is broken;the deconfined Quark-Gluon Plasma(QGP) phase at high T and /or largeμB where the quarks are asymptoti-cally free; a phase of pre-formedpairs bound by strong residual forceslocated between the hadronic andthe QGP phase; and a color super-conducting phase at very low T andvery large μB.

Recent developments in LatticeQCD calculations at finite baryonchemical potential μB were reported byFrithjof Karsch (Brookhaven/Bielefeld)and Zoltan Fodor (Wuppertal). In the

calculations—which correspond to apion mass well above the physicalvalue—one finds a critical endpoint atabout μB = 300–400 MeV, a first-orderphase transition for larger and a cross-over for smaller values of μB. Clearlythe discovery of the first-order phasetransition and/or the critical endpointwould represent a major milestone onthe way toward a quantitative under-standing of strongly interacting matter.

Wolfram Weise (Munich) pre-sented an effective field theoreticalmodel for QCD thermodynamics,which includes features of both decon-finement and chiral symmetry restora-tion. In comparison with lattice QCDresults he finds good agreement forobservables like the quark numberdensity for nonzero quark chemicalpotential.

The structure of compressed bary-onic matter in the interior of compactstars was discussed by Fridolin Weber(San Diego). At baryon densitiesabove 2 or 3 times saturation densityone expects exotic states like hyperonmatter, kaon or pion condensed mat-ter, or quark matter. It was stressedthat the composition of a pulsar coredepends both on mass and on spin fre-quency. Dirk Rischke (Frankfurt) dis-cussed conditions at low temperatures

and very high quark chemical poten-tials (μq > 1 GeV) where quark matteris predicted to form a color supercon-ductor due to the attractive quark–quark interaction.

Ralf Rapp (Texas A&M Univer-sity) addressed theoretical aspects ofanother fundamental goal of heavy-ion physics: the search for the onset ofchiral symmetry restoration in hot anddense matter. He discussed possibleconsequences of chiral symmetry res-toration on the in-medium spectralfunction of low mass vector mesons,which is relevant for dilepton spec-troscopy in heavy-ion collisions.Recent experimental results were pre-sented by Romain Holzmann (GSI)for the HADES collaboration, byOliver Busch (GSI) for the CEREScollaboration, and by Gianluca Usai(Cagliari) for the NA60 collaboration.Of particular interest is the invariantmass distribution of dimuon pairsmeasured by NA60, which indicatesthat the in-medium spectral functionof the rho meson is broadened but notshifted in mass.

Burkhard Kämpfer (Dresden) dis-cussed the properties of mesons andbaryons using QCD sumrules. Heexplored the competition between thequark condensate and the 4-quark

meeting reports


condensate in different channels. Inparticular, he finds that the in-mediumproperties of the D-meson aresensitive to changes of the quarkcondensate.

A crucial ingredient needed for aquantitative interpretation of datafrom heavy-ion reactions are phe-nomenological models that describethe dynamics of nucleus–nucleuscollisions. Christian Fuchs (Tübin-gen) and Elena Bratkovskaya(Frankfurt) reviewed the status ofkinetic transport theory. The calcu-lations predict that high baryon den-sities of up to 10 times nucleardensity and energy densities of 5–6 GeV/fm3 are reached in centralAu+Au collisions already at beamenergies of 25 AGeV. The transitionfrom hadronic matter to deconfinedmatter of quarks and gluons isexpected above a critical density ofabout 1 GeV/fm3.

Adrian Dumitru (Frankfurt) dis-cussed the status of non-equilibriumchiral hydrodynamics calculations. Inthe case of a first-order phase transi-tion he predicted inhomogeneities inthe energy density distribution thatshould effect the collective flow andhadron abundances. Yuri Ivanov(Moscow) presented results of multi-fluid hydrodynamics calculations. Heshowed trajectories in the QCD phasediagram for central Au+Au collisionsat different beam energies. Accordingto the calculations, the conditions inthe fireball are close to the criticalpoint (as predicted by QCD lattice cal-culations) at beam energies around30 AGeV.

Claudia Höhne (GSI), who pre-sented an overview on strangeness

production data measured at theCERN-SPS, discussed the excitationfunction of the kaon-to-pion ratio mea-sured in Pb+Pb collisions that exhibitsa peak at a beam energy of 30 AGeV.This observation is not reproduced bytransport calculations or by statisticalmodels. A similar but less pronouncedstructure is found in the lambda-to-pion ratio reflecting the transition frombaryon to meson dominated matteraround 30 AGeV. Moreover, the NA49collaboration measured the kaon-to-pion ratio event-by-event and observednonstatistical fluctuations at a beamenergy around 20 AGeV. Such fluctua-tions are expected to occur in systemsclose to the critical point, in analogywith critical opalescence in macro-scopic systems.

Volker Koch (Berkeley) consideredfluctuations to be a key observable bothfor the critical point and for a first-orderphase transition. In the second case, oneexpects spatial fluctuations (“blob for-mation”) due to spinodal instabilities inthe phase coexistence region. Moreover,he stressed the role of strangeness-baryon number correlations as a signa-ture for the nature of the deconfinedphase (“simple” QGP versus boundstate QGP).

Volker Friese (GSI) presentedthe plans for the heavy-ion collisionprogram of the Compressed Bary-onic Matter (CBM) experiment atthe future Facility for Antiprotonand Ion Research (FAIR). The CBMmeasurements will focus on rareprobes containing heavy quarks, forexample, open and hidden charm,and on penetrating probes, for exam-ple, dilepton pairs from light vectormeson decays. Hadronic probes,

including (multi strange) hyperons,as well as fluctuations and correla-tions of bulk matter particles such aspions and kaons, will also be mea-sured. The goal is to obtain a com-prehensive picture of the high net-baryon density region of the phasediagram of strongly interacting mat-ter. In this sense the CBM researchprogram is complementary to theheavy-ion experiments at RHIC andLHC where matter at high tempera-tures and small net baryon densitiesis explored.

The final session of the work-shop was devoted to the discussionof a planned White Book on thephysics of dense baryonic matter.The aim of this document is toreview the status of theory andexperiment, to discuss the relationbetween physics questions andobservables, and to map out a strat-egy for future research. MishaStephanov (Chicago), Jorgen Randrup(Berkeley), and David Blaschke(Rostock) reported on a first draftof the book that was prepared bythree working groups. The nextdraft of the White Book will be dis-cussed at the workshop on “ThePhysics of High Baryon Density,”which will take place at the ECT*in Trento from May 29 to June 2,2006.

The presentations of the work-shop can be found at http://www-aix.gsi.de/conferences/cbm2005-Dec/

BENGT FRIMAN

PETER SENGER

Gesellschaft fürSchwerionenforschung, Darmstadt

meeting reports


Physics Opportunities with EURISOL

The proposed EURISOL radioac-tive beam facility is an ambitious leapbeyond the capabilities of any currentISOL facility in the world (see http://www.eurisol.org). EURISOL is intendedto produce and accelerate isotopesspanning the broadest possible rangeof isospin and masses. It promises toopen up an entire new vista for nuclearphysics and have a significant bearingon many other fields of science,including condensed matter, atomic,particle, and astrophysics.

The EURISOL Design Study, a4-year project funded under the Euro-pean Union’s 6th Framework Pro-gramme, is part of the roadmaptoward the construction of the finalfacility. The Design Study is under-taking preparatory work on the vari-ous elements of the facility (high-intensity proton driver, target-ionsources, postaccelerator, etc). As sucha major project as EURISOL has to bedriven by science, an integral part of

the Design Study is the “Physics andInstrumentation” Task. As part of theplanned work program of this Task, aworkshop was held at ECT* in Janu-ary. The aim of the workshop was toidentify some of the areas of physicsin which EURISOL could uniquelyadvance our knowledge. To achievethis, the workshop set out to attractspeakers and participants representingdiverse fields at the frontiers ofnuclear physics research.

Over 40 scientists from 12 differentcountries participated in the workshopin the stimulating atmosphere of theECT* in Trento. Throughout the work-shop a considerable fraction of the timewas given over to lively open discus-sions of the ideas presented by thespeakers, many of whom were amongthe younger members of our commu-nity. Time was also devoted to debatingmore general issues, including variousaspects of the user needs (especially interms of the postaccelerated beams) and

a proposal to form an ISOL user groupto draw together the future EURISOLresearch community.

The detailed program of the work-shop, including a slide report, can befound at http://ns.ph.liv.ac.uk/eurisol.Although the topics addressed in thepresentations covered a broad spectrumof the nuclear physics research that islikely to figure at the EURISOL facil-ity, it is evident that the science pro-gram is continuously evolving. Newideas are always welcome, so if youhave something to contribute or wantto get involved, please get in touch!

ROBERT PAGE

Liverpool

ANGELA BONACCORSO

Pisa

NIGEL ORR

Caen

news and views


Proton–Proton Correlations Observed in Two-Proton Radioactivity of 94Ag

Since 1960, the spontaneousbreak-up of atomic nuclei by emis-sion of two protons was expected toproceed via simultaneous emissionof a pair of protons coupled into a2He cluster. Recently, Ivan Mukhaand colleagues made the surprisingobservation [1] of a case of two-proton radioactivity characterizedby emission of the protons either inthe same or opposite directions. Atthe GSI research center in Darms-tadt, Germany, the lightest knownisotopes of silver, 94Ag, was synthe-sized by using nuclear reactionsbetween accelerated 40Ca ions and58Ni atoms. After purification byon-line mass separation the 94Agnuclei were implanted into acatcher positioned in a highly seg-mented array of silicon and germa-nium detectors. The simultaneoustwo-proton emission was identifiedfor a long-lived (0.4 s), high-spinstate of 94Ag. This (21+) isomer isalso known to undergo one-protondecay [2], thus making it the firstnucleus exhibiting one- as well astwo-proton radioactivity. Both dis-integration modes were unambigu-ously identified by “tagging” onγ rays that are known to de-excitethe high-spin states populated in thedaughter nuclei 93Pd and 92Rh,respectively.

The observed two-proton decay isunexpectedly fast. This result as wellas the directional correlation observedfor the emission of the two protons areinterpreted as indicating a very large,prolate (cigar-like) deformation of theparent nucleus, with the emission ofprotons occurring either from the

Figure 1. Upper panel: Partial two-proton decay half-life (T1/2) of 94Ag (21+) asa function of the nuclear deformation parameter a, which is the ratio of the longto the short axes of the ellipsoid. Model estimates for the two-proton decayproceeding as simultaneous three-body breakup (black curves) and quasi-classical 2He decay (grey curves) are shown for the angular momenta L = 6, 8,10. The nuclear shapes corresponding to the derived deformations aredisplayed for the L = 6 calculation. The experimental T1/2 value is shown by thedotted line (marked as “Exp.”), the grey region indicating the experimentaluncertainty. Lower panel: Intensity W predicted for the two-proton emission asa function of the angle q between one emitted proton and the long ellipsoid axis(thick grey curve) [1].

1

news and views


same or from opposite ends of the“cigar.” This first measurement ofcorrelation data in two-proton radio-activity calls for further experimentalstudies of the properties of this trulyexotic isomer as well as for a morequantitative theoretical description of

the observed two-proton decaybehavior.

References 1. I. Mukha etal., Proton–proton Correla-

tions Observed in Two-proton Radioac-tivity of 94Ag, Nature 439, 298 (2006).

2. I. Mukha et al., Observation of ProtonRadioactivity of the (21+) High-SpinIsomer in 94Ag, Phys. Rev. Lett. 95,022501 (2005).

ERNST ROECKL

GSI DARMSTADT

The South East European Nuclear Physics Network (SEENet)

In view of the key role that nuclearscience plays in the technologicaldevelopment of our society, a group of16 nuclear physicists from 10 Institutesand Departments from South-EastEuropean (SEE) Research Centres andUniversities agreed to establish theSouth East European Nuclear PhysicsNetwork (SEENet). The first SEENetmeeting was held on September 16,2002 at the Physics Department of theUniversity of Sofia, Bulgaria. A secondmeeting took place six weeks later atthe Faculty of Sciences of the IstanbulUniversity, Turkey, where a Memoran-dum of Understanding (MoU) wassigned by the representatives of (a) theFaculty of Physics of the University ofSofia, Bulgaria, (b) the Institute ofNuclear Research and Nuclear Energy(INRNE), Sofia, Bulgaria, (c) the Rud-jer Boskovic Institute (RBI), Zagreb,Croatia, (d) the Physics Department ofthe University of Zagreb, Croatia, (e)the Institute of Nuclear Physics of theNational Centre for Scientific Research“Demokritos,” Athens, Greece, (f) theNational Institute for Physics andNuclear Engineering (NIPNE), Bucha-rest, Romania, (g) the UniversityPolitechnica of Bucharest, Romania,(h) the Vinca Institute of Nuclear Sci-ence, Belgrade, Serbia-Montenegro, (i)the Department of Physics of the Univer-sity of Novi Sad, Serbia-Montenegro,

and (j) the Faculty of Sciences of theIstanbul University, Turkey.

According to the signed MoU,SEENet aims at

• Creating a users network of SEEgroups that carry out research inSEE nuclear physics installationsand European Large Scale Facilities(LSF) and enhancing SEE regionaland European collaborations.

• Performing mapping studies onNuclear Science and Technologyin SEE according to NuPECC andother European organization’sguidelines.

• Promoting both independentnuclear research activities in SEEand activities complementary tothose at LSF.

• Facilitating training in NuclearScience and Technology forgraduate students and young sci-entists by providing access toSEENet member laboratoriesand LSF.

• Facilitating the access to thefunding instruments of the Euro-pean Commission for networkmembers.

• Enhancing the interaction of thescientific groups involved by orga-nizing workshops, conferences,and schools on Nuclear Scienceand Technology.

SEENet has a Members Council(MC) consisting of representatives of allmember institutions as well as a SteeringCommittee (SC) comprising the nationalrepresentatives (one scientist from eachSEENet country). The Steering Commit-tee elects a Chair and a Vice-Chair for a2-year period that is renewable.

So far, SEENet meetings havebeen organized at the Department ofPhysics of the University of Novi Sad,Serbia-Montenegro (May 2003), NIPNE(Bucharest, Romania, May 2005) andthe Institute of Nuclear Physics ofNCSR “Demokritos,” Greece (Athens,November 2005). In addition, SEENethas grown and now includes threeadditional members: (a) the Faculty ofApplied Physics of the National Tech-nical University of Athens (NTUA),Greece, (b) the Physics Department ofthe Aristotle University of Thessalon-iki, Greece, and (c) the Department ofAtomic and Nuclear Physics of theUniversity of Bucharest, Romania.The SEENet meetings paved the wayfor forming the proposal for a South-Eastern Nuclear Physics networkactivity as an integral part of EURONSI3 [1]. Another important outcomewas the decision to take specific mea-sures for the integration of SEEnuclear physics groups in the Euro-pean research area both in terms ofresearch activities and funding.

news and views


As mentioned earlier, SEENet isnow a network activity of EURONSI3. As such it has undertaken to pro-vide the European nuclear physicscommunity and the EC with tworeports. One will include an accountof the research activities carried out bySEE nuclear physics groups at Euro-pean LSF. The other will feature amapping of nuclear science in the SEEregion. For the preparation of thereports, SEENet has formed threeworking groups on Physics, Infra-structures and Education, respectively.Each working group consists of sixmembers, one from each SEENetcountry, acting as national conveners.The purpose of the two reports is notonly to highlight the broad scientificpotential of the SEE nuclear physicscommunity, which includes more than400 nuclear physicists, but also toemphasize the problems related to theoperation of the existing SEE infra-structures and the lack of propermobility schemes. These problemshave been identified by all SEENetmembers and are listed in the “Posi-tion Letter On Nuclear Science inSouth-Eastern Europe” that was pro-duced during the last SEENet meetingin Athens (November 2005). This let-ter can be found on the SEENet web-site [2] together with the MoU and theminutes of all SEENet meetings.

Looking back at the course ofevents following the first SEENetmeeting in Sofia and the firstexpression of concern about thefuture of Nuclear Science in SEE(article by Dimiter Balabanski [3]),one cannot but appreciate the deter-mination of the SEENet communityto contribute decisively to the effortsof the broader European nuclearphysics community. In fact, morethan 100 nuclear physicists fromSEE are actively involved in key

nuclear physics experiments runningin European LSF, and many othersare participating in design studies offuture nuclear physics instrumenta-tion and infrastructures. At the sametime, SEENet is keen on promotingcollaborations between SEE nuclearphysics groups performing measure-ments in the regional acceleratorlaboratories, which provide researchopportunities and training for youngphysicists. One such example is thejoint effort of the 5 MV TandemAccelerator Facility of the GreekNational Research Centre“Demokritos,” Athens, the 9 MV FNTandem Laboratory of the NationalInstitute for Physics and NuclearEngineering (NIPNE) in Bucharest,Romania, the University of Sofia,Bulgaria, and the University ofIstanbul, Turkey, both on an experi-mental and theoretical level. Thisintense collaboration is based on along-term research program onNuclear Astrophysics and NuclearStructure. Another significant SEEevent worth mentioning is the Bal-kan School on Nuclear Physics,which has been organized in differ-ent countries of the region for thepast 10 years. The 5th BalkanSchool will be held in Brasov,Romania [4] this summer.

From a personal viewpoint, themost important achievement ofSEENet is that it has managed to makefunding agencies in the South-EasternEuropean countries aware of the sci-entific potential of the nuclear physicscommunities in the region. As a result,three SEE countries: Greece, Roma-nia, and very recently Croatia, joinedNuPECC in 2005. This provided anopportunity for the broader Europeannuclear physics community to becomeacquainted with the problems the SEEscientific community faces related to

funding and decaying infrastructures.In recognizing these problems andappreciating the high-level scientificpotential and the vital role of the SEEregion in future activities of the inter-national nuclear physics community,NuPECC has embarked on a series ofdiscussions on possible actions andinitiatives that would lead to upgrad-ing the nuclear physics installations inSEE and would enhance the involve-ment of SEE groups in fundingschemes within the upcoming 7thFramework Program (FP7).

As SEENet and NuPECC member,I look forward to a promising futurefull of challenges and opportunitiesfor nuclear science in SEE and in thisdirection the support of the entireEuropean nuclear physics communityis imperative.

References 1. http://www.gsi.de/EURONS 2. http://www.inp.demokritos.gr/

SEENet 3. D. Balabanski, Nuclear Physics News,

12(4), 4 (2002). 4. http://www.nipne.ro/school/index.html

SOTIRIOS V. HARISSOPULOS

SEENet ChairInstitute of Nuclear Physics, NCSR

“Demokritos,” Aghia Paraskevi,Athens, Greece

news and views


IUPAP Young Scientist Prize in Nuclear Physics

This prize was established byIUPAP in 2005 at the time of the Gen-eral Assembly in Capetown, SouthAfrica.

The purpose of this prize, which con-sists of $1,000, a medal, and a certificateciting the recipient’s contributions, is:

To recognize and encourage very promisingexperimental or theoretical research in nuclearphysics, including the advancement of amethod, a procedure, a technique, or a devicethat contributes in a significant way to nuclearphysics research, by a scientist within eightyears of obtaining the Ph.D. (or equivalent)degree.

Nominations by one or two nominators(and distinct from the nominee) are opento all experimental and theoreticalnuclear physicists. Three prizes willordinarily be awarded at the time of the

tri-annual International Nuclear PhysicsConference. However, the selectioncommittee may, given the circum-stances, decide to award only two prizesor in a special situation only one prize,in which cases the monetary award willbe inversely proportionally larger.

Nominations are due October 1 ofthe year preceding the InternationalNuclear Physics Conference and arevalid only until then. It will beextremely helpful to the selection com-mittee to receive at least two additionalletters supporting the nomination thatdetail the expected significance of thecontributions of the nominee to nuclearphysics. It is also appropriate to submitadditional materials such as publishedarticles that underline the expected sig-nificance of the nominee’s contribution

to nuclear physics. It is important thatthe selection committee has the specificinformation that allows it to determinewhat the nominee has contributed andhow this contribution is expected toimpact the field.

Nominations are to be sent by thedeadline to the chair of the IUPAPCommission of Nuclear Physics (C12).For particulars please check the IUPAPwebsite: www.iupap.org under “com-missions.” The next InternationalNuclear Physics Conference will be heldJune 3–8, 2007, in Tokyo, Japan.

Next deadline for nominations:October 1, 2006. Nominations shouldbe sent to: Dr. Walter F. Henning,Chair of the IUPAP Prize SelectionCommittee, GSI, Planck Strasse 1,D-64291 Darmstadt, Germany.

Documents

GNPN A Ediboard - NuPECC