REVISTA MEXICANA DE FfslCA 47 SUPLEMENTO 2. 78-82 DICIEMBRE 2001

Reaction cross sections for 6He+209Bi below the Coulomb barrierO. Lizcano, E.E Aguilera, and E. Martínez-Quiroz

Departamefllo del Acelerador, Instituto Nacional de Investigaciones NuclearesApartado posla/18-102?, 11801 México, D.F, Mexico

Universidad Aut6noma del Estado de México.Av. IlIslill<lo Lilerario No. 100,50000 To/uca. Edo. de México. Mexico

].J. Kolata, G. Rogachev, H.Y. Lee, and L.O. LammPhysics Department, University o/ NOlre Dame

NOlre Dame, IlIdialla 46556 USA

Á. HorválhDepartmenr o[ Alomic Physics, Eotvos Loránd Uni\'ersity

PávllállY P. SIIIY. l/A. Budapesl, HUlIgary, H-III?National Superr:mulllcting CyclotronÚlboratory, Afic/zigan 5tate University,

Easl Lallsillg, MI 48824 USA

E Becchetti, T. O'Oonnell, and O. RobertsPhysin Department, University o/ Mic¡'igan

AIIII Arbor. Michigall 48109 USA

P.A. DeYoungPhysics Department, Hope College

Hollalld, MI 49422 USA

J.D. HinnefeldPhysics Deparlmem, Indiana University.South lJend

Soulh Belld. IlIdialla 46615 USA

Recibido el 30 de enero de 2001; aceptado el 27 de junio de 2001

In a rcccnt experimento we have for Ihe firsl time mcasured near.barrier and sub.barricr transfer andlor breakup yie1ds for an exotic Borromeannuc1eus 6Ue, on a 209Bi targcr. An isolated 4He group was observed al an efTective Q-value of approximately -2.5 MeV whose integratedcross section was exceptionally large. greatly exceeding the fusion yield both above and below the barrier. Simullaneously measured elastjc-scaltcrjng angular distributions required total reaction cross sections that continned this large yield. Prc1iminary coupled-channels calcula.tions suggestcd that the reaction mechanism can best be describcd by direct breakup and neutron transfer to unbound states in 211Bi. Thecalculations also predicted an enhancement in the sub-barrier fusion yield due to coupling (o the transfer andlor breakup channel, whichstrongly suggestcd that Ihis is the doorway state (hat accounts for the remarkable suppression of the fusjon barrier that was observed in aprevious expcrimenl. In this work we extcnd these measurements to even lower energies, down to 14.7 MeV. which is nearly 6 MeV bclowthe harriero It is found that the 4 He group completc1y dominates the total reaction yicld at these energies and sorne evidence is given thal anew process might become important at low energics.

Key",ordJ: Exotic beam; near-barrier fusion; neutron transfcr

En un experimento reciente, hemos medido por primera vez la transferencia de neutrones y/o rompimiento del proyectil cerca y abajo dela barrera, para el núcleo "Borromeano" exótico 6He, sobre un blanco de 209Bi. Se observó un grupo aislado de 4He a una Q efectiva deapro.'úmadamente -2.5 MeV. cuya sección eficaz integrada fue excepcionalmente grande, excediendo la sección de fusión tanto arriba comoahajo de la barrera. Las distribuciones angulares elásticas. medidas simultáneamente, requirieron de una sección eficaz total que confirmóestos resultados. Cálculos preliminares de canales acoplados sugirieron que los mecanismos de reacción pueden ser mejor descritos comorompimiento directo y transferencia de neutrones a estados no ligados en 211 Bi. Los cálculos también predijeron un acrecentamiento en lasección de fusión abajo de la barrera debido al acoplamiento del canal de transferencia y/o rompimiento, lo cual fuertemente sugiere queéste es el "cstado pucrta" que explica la gran disminución de la barrera de fusión obscrvada en un experimento previo. En este trabajo,extendemos estas medidas a energías aún más bajas. hasta 14.7 McV, la cual está alrededor de G MeV ahajo de la barrera. Sc encontró queel grupo de 4He domina completamente la sección de reacción total a estas energías y sc present<l.evidencia de que un nuevo proceso puedeestar tomando importancia a bajas energías.

Descriptores: Haz exótico; fusión cerca de la barrera; transferencia de ncutrones

PAes: 25.60.-1; 25.60.Gc; 25.60.Jc; 27.20.+n

D. LIZCANO. E.E AGUILERA. AND E. MARTfNEZ.QUIROZ 79

l. Introduction

In this work we report 00 OUT measurements of a-particlesproduced in (he 6He+209Bi rcactioo al energies below theCoulomb barrier. Similar measurements al two energies,one aboye and one below the barrier, have beco previ-ously reported 11]. lnteresl in 6He-indueed reaelions origi-nated, among other things. from sorne recent theoretical stu.dies aboul fusion near lhe barrier of lhe IlLi+2oBpb sys-tem, which generated sorne controversies. The Borromeanradioaelive nucleus 11 Li (half life of 8.7 ms). whieh canbe seeo as a structure consisting of two nculroos weaklyhaund (300 keY) around a 9Li eore, has been eonsidered asa neulron skin" 12] or a "neulron halo" 13] nucleus. Sornegroups [4...j)] have reporled lhal in lhe fusion of lhis sys-lem, lhe eoupling of lhe breakup ehannel lo lhe fusion ehan-oel reduces the fusioo cross seclion leading lo sorne struc-ture in the excitation function in the barriee regioo. How-ever, other authors [7] reported only an enhancement in thefusioo yield, cven in the presence of strong breakup chan-neis. Since the competition between the projectile breakupand the sub.barrier fusion is very important in the forma-tion of superheavy elements via fusion with exotic projec.tiles, it is important to salve this controversy. At the presenttime, the llLi+208pb system is not available because ofthe low intensity of 11 Li beams and bad energy resolu.tion. However, the 6He nucleus, with two neutrons weaklybound (0.97 Me Y) around a '!le eore, is radioaetive (half Iifeof 806 ms) and has also a "neutro o skin" structure, so that itcould be expected that it should show sorne effects similarto those mentioned aboye for 11 Li. In addition, 6He is thesimplest Borromeao nucleus, and this provides ao unusualoportunity to study three.body interactions in the nucleus.

In particular, if a neutron skin nucleus, such as 6He, ap-proaches a stable nucleus, neutmos in the skio can touch thetarget befare protoos in the core can do so, because those ncu-trons are distributed outside protoos. Since lhe Fenni energyis so different between the two nuclei, neutmns io the skinmay tlow into the stable nucleus thus enhancing the reactioocross section.

Another possibility that has becn considcred is the exis-tence of soft-dipole mode excitations in which the weaklybound neutron halo performs collective oscillations againstthe residual nuclear coreo This could contribule to enhancethe fusion probability al sub-barrier energies because the po-¡arizalion of lhe projeelile indueed by lhe Coulomb ficldof lhe targel brings lhe neulron halo closer lo lhe larget.The corresponding "neutron now" has been vicwed as "neu.lron avalanehe" by sorne aUlhors [8]. This same polarizalionmechanism has the potential to induce scparations of the halofrom lhe .1He eore thal slreleh beyond lhe poinl lhal can besustained by the weak residual farces that hold them togcther.The relevance 01' this break-up process at the bombardingconditions that may lead to fusion has generatcd considera-ble controversy recently for the case of the neutron-halo nu-cleus 11 Li, as mentioned aboye. The possiblc effects of the

10' .10'

10'

10'

f10' I'"-~.•.10'

10'!

10';.'

15 20 25 30Ee• (M.V)

FIGURE l. Fusion excitarion functions of 5.4He+209Bi (Kolata, elal. [g}, Barnet, el al. [lO}). Curves are BPM calculations using theparabolic (-) or the WKB (- - -) appcoximation. From thesc fits.for the 6Hc+209Si system Vbf ;;;: 20.3 MeY.

exotic structure of 6He over reactions with 209Bi are experi-mentally investigated in this work.

2. Antccedents

In a recent wark [91. we reported our measurements of nearand sub-barrier fusion of 6IIe+209Bi. The corresponding ex-citation function is reproduced in the upper pan of Fig. 1,along with the result of one-dimensional barrier penetra-lion model (BPM) ealculaIions, whieh lend a fusion bar-rier Ví)f = 20.3 MeY. For comparison purposes, corrcspond.ing dala for lhe 'lIc+209Bi syslem are also shown 110]. inlhe lower part of lhe figure. The curves eorrespond lo lheparabolie approximalion (-) and lo lhe WKB approxima-tion (- - -). The lalter should give belter resulls for lhese lighlsystems at the lowest energies sincc the one-dimensionalpOlenlial barrier can hardly be approximaled by a simpleparabula here. While lhe data for lhe lighler syslem showno eohacement al all, those for the heavier system show astriking enhacement of the fusion cross section with respectlo lhe BPM predielions. Clearly. lhis large enhaeemenl muslbe related to sorne reaction mechanism that appears becauseof thc exotic structure that the additional two neulrons con-fer to the projectile. i.e., the "neutron.skin" or "neutron-halo"structure of 6He. We also ootice that there is no evidence offusion suppression or structure in the rusion excitation func-tion at the bamer energy for 6He+209Bi so that, in case thatprojeelile breakup is presenl. il is nol having lhe effeels lhalwere mentioned aboye. Similar conclusions were obtained inRef. 1111 forthe 6He+238U syslem. lt is lherefore importanlto measure the breakup channel in the {iHe+209Bi rcaction.

A partieularly enlighlening way lo presenl lhe fusiondala has been inlrodueed by Slelson el al. 1121 who nOledthat many rusion excitation functions near the barricr havethe property that (aE"n)1/2 is linear, even in (he presenceof large cnhancemcnts relative to potential-model estimates,leading lo lhe formula a(E) = 'Ir R2(E",,,-Tj2 /[4(F0-T)]'/2.

Re>' Mex. n,. 47 S2 (200 1) 78-82

80 REAcrIO~ CROSS SECTIONS FOR 6He+209Bi BELOW THECOULOMB BARRIER

where "o is lhe nominal barrier and R is lhe barrier radius.They further show lhal this behavior results fmm lhe ¡111m-

duction of a distribution of barriers with uniform weight ex-lending from sorne lhreshold energy T lo 2Vo - T, By makinga fit of thi~ fonnula lo lhe 6He+209Bi rusion data, a thresh-old barrier Tr = 15.4 MeY was dedueed in Ref. 9, meaningthal there is a 25% dynamic reduction in lhe barrier heightcaused by sorne reaction mechanism thal becornes importantal an energy neae Tf• According (o Stelson el al., this couldprobably be interpreted as due lo neck formation promolcd by"neutron now", a view thal is further supported by Ihe posi-t¡ve Q-value [oc one- and two-neutron transfer in lhis sys-temo 11 would appear, therefore, that neck formation via neu.tron flow is a good can didate lo explain the observed largesub-barrier fusion enhacement. Clearly, it is also imp0rlantto measure the neutron transfer channels in this syslcm.

In Ihe one-neutron transfer channel, the residual 5Herapidly decays (2 x 10-21 s) into an o:.parlicle plus a neu-trono The two.neutron transfer channel directly gives a resi-dual o.particle and so does the breakup channel. The com-mon signature of all three channels is then the emission ofan o-particle per evenL \Ve thus designed an experiment tomeasure the o-particles produced in this system.

3. Experimental procedure

The 'He beam used in lhe experimenl was produeed by lheTwillSo1 radioactive nuclear beam facility at Ihe Universityof NOlre Dame [13,141, The primary beam was 7Li whiehreacts with a primary target of 9Be losing a proton to pro-duce the secondary beam of 'He, Two large supereonduelingsolenoids act as thick lenses to colleet and focus the secon-dary beam onto a spot in the secondary larget which was typ-ically 5 mm full width al half maximum (l'WHM), The high6He-beam purity and good resolution from contaminants ob-lained by Twinsol has been reported in Ref. 14, The see-ondary target was a 3.2 mglcm2 Bi layer evaporated anta alOO Itg!cm2 polyethylene baeking, The reaetion events andal50 elastically scatlered particles were detected with a setof ó..E - E silicon lelescopes placcd at various angles on ei-Iher side of the beam. Figure 2 presents a typical spectrumtaken at E1ab :;:= 22.5 McV and 0lah:;::= 135°, where the elas-tic and the o-particle groups are c1early distinguishcd.

The low energy lail of lhe 'He group is produeed by reae-tions in the backing of the target, as dctermined from a sepa-rate spectrum taken with a backing foil withoul Bi. Since thesecondary beam is contaminated wilh ions having the samemagnetic rigidity as the desired 6He beam, we used time-of-flight tcchniques to idcntify and climinate n's producedby any contaminant bcam. except by {ritans. Tritons of {hesame magnetic rigidity as GHe have also the same velocityso that they can not be discriminated by time-of.t1ight mea-surements. It tums out that the contaminant tri ton beam hasjust the right energy to produce, in rcactions with the ni tar-get, o:'s which wauld fal! in the region of the peak in Fig. 2.

"" '11l8'"lO ~.

o; S 2 •

< "<o -20 .10 o 10~

w " a vlllue (lMtv)

o

" 1,-'He,J d ~.

20~,

P -----'He

o .O lOO 200 300 "'"

Energy (channel)

FIGURE 2. ~E VS. Etotal spectrum taken at e1ab = 1350, at alaboratory 6Hc cnergy of 22.5 MeY. Thc cncrgy calibrationis 80 kcVlchannel.A Q.value spectrum forthc 4Hcgroup is shownin Ihe ¡nset (11.

In arder lo eliminate this possibility we did a separate ex-perimenl wilh a 3H beam of the appropriale energy (one-halflhe 'He energy), whieh showed no evenls in this region,The a partides in lhe peak of Fig, 2, whieh have a meanenergy aboul 2 MeY below lhat of lhe elastie group, are lhenproduced by actual reactions of6He with 209Bi.

It is worthwhile to notice that the rneasurements,from 22,5 MeY lo 14,7 MeY, were developed in two experi-menls, The firsl one, previously reported [11, started at anenergy of 22,5 MeY for "He (whieh is abo ve lhe barrier)and then a 6.54 mg/cm2 mylar foil was used to reduce itlO 10,0 MeY, whieh is below lhe barrier. In lhe seeond ex-periment, the bombardment was started at 19.2 MeV and,by inserting a 2.47 mg/em2 polypropylene foil, an ener-gy of 17,0 MeY was reaehed; 16,2 MeY and 14,7 MeYwere lhen reaehed by inserting lhe mylar and mylar pluspolypropylene absorbers, respectively, The lhiekness of lheBi largel, polyethylene backing and mylar and polypropyleneabsarbers was delermined in separate experiments by energyloss measurements using alpha-particle sources.

4. Resulls and discussion

Thc alpha angular distributions obtained at the four energiesin the second pan of the experiment are shown in Fig. 3.along with gaussian fits to the data. The centroids and widlhsof lhe gaussians are presented in Table 1. The lasl eolumnin lhe Table gives the 10lal alpha eross seetions, oblained byintegration of the gaussians over the whole salid angle. Themost striking feature of these measurements is the very largemagnitude obtained for these cross seclions. which exceedby faf Ihe fusion cross sections al lhe same encrgies. Even atLhe lowest mesurcu cnergy, which is nearly ti MeV below [hebarrier, the cross section is still large.

The elastic angular distributions are shown in Fig. 4. Thesol id curves are the result of an optical model lit where allparameters were kept fixed except for the imaginary diffuse-ness, which had the simple linear variation shown in the cap-

Re" Mex, Fís, 47 S2 (2001) 78-82

D. L1ZCAND. E.E AGUlLERA. AND E. MARTfNEZ.QUIROZ 81100

10' o .10''-4.7 MeV o o

10'I

.10~16.2M.V o

101

~ .A! 17.9MeV ~ •

•• 10'~_19.0MeV ~

• 192MeV• 179MeV

80 16.2PJreV14.7PJreV

'i~60e..,~: J~~L~'~,..,

O' .•••.•... : .•~/' -..::.::

o ~ ~ 60 60 l001~1~160160ftc,.(deg.)

FIGURE 3. Angular distributions of o.particJes from the6He+209Si rcaclion and gaussian fits to the data. Energies aregiven in the lahoratory sys(cm.

10'

'He + '"81 Ela.t1c

~

.22.0 MeV .

o

20 -40M ~1001201401M1809cII (deg.)

2422

.. -.-. BPM

---- Optlcal ModeI-Stelson

1614

10'

10'

18 20E,.(MeV)

FIGURE 5. Total reaction cross scctions for lhe 6He+<lUYSi syslemand sorne model prediclions.

FIGURE 4. Experimental elaslic scattering angular C1ls1ributionsand oplical model fits lo lhe dala (~). The oplical poten-tial paramelers are V=150MeV. R=7.!J5fm, a=O.68fm.!VI; 25.0 McV,RI ;9.38 Cm, a,; 1+0.0438 (22.0- E,.u) Cm.The dotted curve is a calculation made with a potential approprialefor 4He+ 209Si (10]. but with a radius appropriate for 6He (seetext).

become importanl at around 15 MeV and this produces thethreshold energy Tr for the barrier dislribution of lhe Stel.son mode!. Finally, the solid curve is a Slelson model fil tolhe (otal reacrion data. with lhe aboye values of Vbr and Rbr.This leads to a new lhreshold barrier T, = 10.7 MeV which,following the previous line of reasoning, could mean that anew process is taking over al this lower energy. The validityof this speculation. of course, remaios to be seen. And thistakes us to lhe conclusions of this work.

Transfer andlor breakup yields were measured, through lherespective alpha particles, for lhe borro mean nucleus 6He ona 209Bi target in an energy interyal going from nearly 2 MeV

5. Summary and conc1usions

(mb)773 (31)637 (70)643 (42)528 (38)296 (31)189(27)

FWHM(dcg)

119.6 (5.6)114.4 (80.9)131.8 (19.7)100.8 (36.5)86.9 (41.7)89.9(58.8)

TABlE l. Pararnclers ofthe gaussian filS lo the dala shown in Fig. 3and to those reported in Ref. l. The valucs labcled wirh (.) are takcnal50 [rom this Referencc. The last column givcs the total cross scc.tion [oc [he rncasured alphas.

E1ab Cenlroid(MeV) (deg)22.5' 86.2 (2.5)19.2 122.0 (16.9)19.0' 116.6 (5.3)17.9 121.0 (8.8)162 127.0 (92)14.7 130.0 (13.0)

tion. Even though we emphasized hcre fits to lhe total rcae-tion eros s sections (discussed below). lhe comparison withexpeliment is very nice, except perhaps at 14.7 MeV. ThedOHed¡¡oc al 22.5 MeV is lhe aplical model prediction usingparameters obtained from 4He scattering, but with radius pa-rameter incrcased to correspond to {he larger size of 6He, ac-cording to an A 1/3 scaling. This iIIustrates the expeclationfor elastic scattering if 6He were a "normal" nucleus (withno halo).

Figure 5 shows the total raction cross scctions, whichwere taken as the sum of fusion plus alphas, for the fiye mea-surcd energies. The dashcd curve rcpresents the optical modelpredictions mentioned abo ve, and we can see that it givcs areally nice fit to the data. In arder to have sorne additional in-sight we applied sorne simple models lo lhese data. Since lhereaction cross section below the barrier should vary mainlybecause of barrier-penetration effects, one can expect that abarrier penelration model should be appropriate. A fit to thedala of lhe c1assical BPM gave Ihe dolted line, Wilh a barrierheighl Fu, of 14.5 MeV and a radius Ru, of 10.3 fm. NOlelhal Vu, is remarkably c10se to lhe value for the thresholdbarrier, T( = 15.4 MeV, deduced from lhe Slelson-model tltto the fusioo data. A possible interpretation for this is that themeasured alpha channels, which give the main contribulionto the reaction cross section (and that can be seen as a "newprocess" with respect to the simple assumptions ofthe BPM),

Rev. Mex. Frs. 4752 (2001) 78-82

82 REACflON CROSS SECfIONS FOR 6He+209Bi BELOW TBECOULOMB BARRIER

above lhe barrier, down lo about GMeY below il. By addinglhese to lhe previously measured fusion yields, lotal reae-tion cross sections were deduced. Simuhaneously measuredelastie sealtering angular distributions eould be deseribed byusing un aplical potential with a simple energy variation,which did al50 nicely reproduce the total reaction cross sec-tiaos. A simple barrier penetratían model, applied la the to-tal reaction cross sections, gave a barrier height remarkablyclase lO the previously deduced threshold barrier [oc fusioo(from lhe Stelson model), indiealing lhat the measured alphaehannels are mosl probably responsible fOl the large fusionenhaneement previously observed. When the Slelson modelwas in lime applied lo lhe tOlal reaelion dala, a new thresholdbarrierT, = 10.7 MeY was obtained, whieh eould be an indi-

1. E.E Aguilera el al .• P/¡ys. Re ••.!.RII. 22 (2000) 5058 .2. 1.Tanihala el al .• Phys. !.RII. B 289 (1992) 261.3. M.V.Zhukov el al.• P/¡ys. Rep. 231 (1993) 151.4. N. Takigawa, M. Kuratani. and H. Sagawa. Phys. Rel: e 47

(1993) R2470.5. M.S. Hussein, M.P. Pato, L.E Canto, and R. Donangc1o, Phy.\'.

Re ••.C46 (1992) 377; 47 (1993) 2390.6. M.S. fiussein, Nuel. P/¡ys. A 588 (1995) 85c.7. C. Dasso and A. Vitturi, P/¡ys. Re ••.C50 (1994) R12.8. N. Fukunishi,T.Olsuka. and 1.Tanihala.P/¡ys. Re ••.C48 (1993)

1648.

catian that a new process is taking ayer al this )ow energy. Inoeder lo investigate this possibility, it would be worthwhile loextend rhe measurements down lo this energy. which is aboutane halfthe nominal [usion barrier.Given the high cross sec-tioos obtained even at the lowest measured energy. this en-deavour seems lo be reachable.

6. Acknowledgments

This WOlk was partially supported by CONACyT (Mexieo)and by the Nalional Seienee Foundation under grants No.(USA) PHY99-01133, PHY98-70762, PHY98-04869 andPHY97-22604.

9. J.J. Kolalael al., P/¡ys, Re>' !.RII. 81 (1998) 4580.

10. A.R. Bamett and J.S. Lilley, Phys. Re ••.C9 2010 (1974).

11. M. Trotta el al .. P/¡ys. Re ••.!.RII. 8-1(2000) 2342.

12. P.H.Stelson el al .. Phys. Re>'.C 41 (1990) 1584.

13. M.Y. Lee et al., in 14-th Internarional Conlerence 011 the Ap-plication 01Acceleralors in Research alld lndustry: Al? Con!Proc., Denton, Texas 1996, edited by J.L. Duggan and I.L. Mor-gan, (AIP Press, New York, 1997),Vol.392, p. 397.

14. M.Y. Lee et al., Nucl. lnstnu". Methods in Phys. Research A422 (1999) 536.

Re ••.Mex. Fls. 47 S2 (2001) 78-82