REVISTA MEXICANA DE FÍSICA 46 SUPLEMENTO 1, 113-115

Proton halo in 8B

J.1. Kolata, V. Guimaracs, D. Pctcrson, P. Santi, R.H. \Vhitc-Stcvcns, and S.M. VinccntPhysics Deparrment, Universiry o/ Norre Dame

Norre Dame, JI/dial/a 46556-5670 USA

F.D. Becchetti, M.Y. Lee, T.W. O'Donnell, O.A. Roberts, and lA. ZimrnerrnanPhysics Department, University 01Michigan

AI/I/ ArIJO!: Michigal/48J09-JJ20 USA

Recibido el 14de febrero de 2000; aceptado el4 de abril de 2000

SEPTIEMBRE 2000

Thc angular distribution for the breakup or8B on a 58Ni target has been measureJ at suh-Coulomb energies. Thc data are inconsistent withtirst-order rcaclion theories but are remarkably well dcscriheJ hy caJculations including higher~ordercffects. The comparison with theoryilluminates the exotic proton halo structure al' 8B.

Kl'.I"tnm/s: Heavy ion reaclions; nuclear fragmenlation; proton halo

Se presentan mcdiciones de la distribución angular para el rompimiento de 8B en un blanco de 58Ni a energías sub-coulombianas. Los datosno son consistentes con las teorías de reacciones a primer orden de aproximación. pero son descritos muy bien por cálculos que incluyenefeclos de m<lsalto orden. La comparación con la teoría permite entender la estructura exótica de halo de protones del 8S.

f)C'scripl0rC's: Reacciones de iones pesados; fragmentación nuclear; halo de protones

rACS: 24.10.-i: 25.60.-1: 27.20.+n

In a recent experirnenl, von Sehwarzenberg er al. [1] mea-sured the brcakup 01' 8B on a .58Ni target at sub-Coulombcnergies. For a low-Z larget such as this, multiple Coulombexcitation is expected to contribute only minimally, ano the£2 componenl is enhanced relative to El in lhe breakupprocess. The very small cross seetion reported in ReL 1,whieh was less than lhat predicted by any reasonablc struc-ture model for 8B [2], has generated considerable interes!.Nunes and Thompson (2) and Dasso, Lenzi, and Villuri [3)independently suggeslcd that lhe explanation for lhis resultmight be slrong oestructive nuclear-Coulomb interference ef-feels, dcspite the faet that at the angle whcre the measure-ment was made the c1assical distan ce 01' closesl approachwas far outside the range 01' the nuclear force for a "nor-mal" nuclear system. A strong nuclear-dominated peak in theJiITerenlial cross section at a center-of-mass angle well in-side that expecled for the onsel 01"nuclear breakup 01'a nor-mal nucleus was predicted by both 01' these groups. How-evcr. the cotTesponding ealculations are only first-order inlhe nuclear and Coulomh f¡elds and mighl he modified hyhigher-order effects. It was also suggested in Ref. 2 lhat evenpure Coulomh exeitation would he considerahly modifiedfrom Ihal expecled in lhe normal "point-Coulornb" approxi-mation which ignores Ihe extended size 01'the valence protonwave-function. This leads to a further reduction in thc cal-cubtcd breakup cross seetion. Both these effecls are directlyatlributahle lO the exotic "halo" structure 01'88, and ncithcr\vould he presenl I"ora "normal" nuclear projectile. A !lewexperiment has now been carried oul in an attempt to verifyIhe implicalions of these calculations.

Thc cxperimenl was earried oul at the Nuclear StructureLahoralory 01"the University 01' Notre Dame with a low-

encrgy radioactive 8B heam from the TwinSol facility [4].A gas target containing I allll of :lHc was bombarded bya high-inlcnsily, nanosecond-hunchcd 6Li bearn. The sec-ondary healll from the 6LieHe,ni~B direct transfer reactionwas focused onto an isotopically-enriched .5HNi target. Thelahoralory energy of the HB bealll al the centcr of this targelwas 25.75 MeV. which is below the Coulomb harrier for a Nilarge!. The beam had a Illaximum angular divergence of::I: 4°amI a spot size of 4 mm FWHM. Pulse-pileup tagging with aresolving time ol' 50 ns was used to eliminate pileup events.The 8B breakup cvents, and also elastically-scattered parti-eles, were detected wilh Iwo Si ¿;E-E leleseopes. Eaeh lele-scope had a circular collimator that subtended a solid angle01'41 msr. corresponding to a overall effcctive angular res-olul;on of ahout 110 (FWHM), computed hy folding in theacceptance of the collimator with the spot size and angulardivergence of the heam.

Unambiguous separation 01"the 7Be fragments resultingfrom 8R breakup from 'Be contamination in the direct heamelastically scattered hy the G8Nitarget was crucial for the suc-cess 01' lhis experiment. The contaminanls in the heam wereidentified using time-of-l1ight (TOF) techniques. The timeresolut;oll 01'heller than 3 ns (FWIIM) was adequate lo sep-arale IBe from HB, as illustratcd in Fig. 1. At sub-Coulombenergies, il is I~oteasy lo carry out a coincidence measure-mcnl hctwecn thc 7Be fragment and the proton, as was doneat higher energies [5. G], Juc to the much reduced kinelllaticfocusing and lower energy of thc prolons. Essentially, onewould require a 4JT prolon delector with a threshold oelow100 kcV. Thus, we determined only the integrated IBe yield.The experimental angular distribution is prcsented in Fig. 2,as a function 01' thc center-of-mass (cm) angle of the detec-

b)20 -,,

•~~T•-L

250

FIGURE 1. (a) Thc uE vs. ETOT/\I. ~pcctrum lakcn al 8LA[J :;::45°. The 7Be and ,liS gules are 5hown. (b) TOF-L\E spcctrulllillustrating the scparation bctwccn the 7Re brcakup cvcnts amIclastically-scattered 7Be in the direct bcam. This spcctrum corre-sponds lo cvcnts in the gates shown in Fig. la. Thc brcakup evcnlsare cmphasilcd with largcr dols. Thc cncrgy calibration ami limecalibrallon are 20 keV/channcl and 0.50 nslchanncl. rcspcctivcly.

o l""",1,,,I U," l. jI U 'U..L...........iiU..J..U..'-'..u.I..LJuu,'-'u ¡,'" '" ••L•.••U.Lii L..."",jO 10 20 30 40 50 60 70 80 90

0eM (deg.)

PIGURE 2. Experimental angul;lr distribution for 8B breakup asmcasurcd in this work. compared with the calculatiol1s presenLedin Ref. 7. The various curves are discusscd in the text

calculation that followed the time evolution of the valenceproton wave function to aH arders in the Coulomb and nu-clear f1elds of the targct. Their results are compared withour data in Fig. 2. The dotted curve corresponds to pureCoulomb breakup while the dashed curve, whieh ineludesnuclear effeels. can be direetly eompared with the ealeula-tions presented in Refs. 2 and 3. 1l can be seen thal h¡gher-order eouplings have eomplelely eliminated the large-anglepeak predieled by these first-order lheories. Nevertheless. itis also clear that Coulomb. nuclear interference at very largedistances, due to the extended nature of the "proton halo" inti B, still plays an important role and is essential to reproducethe experimental data.

Two other curves also appear in Fig. 2. The lhin salid lineillustrates purc Coulomb excitation under the usual ..poin[.likc" assumption. The dotted curve, which is much doserto the experimental result, is the correet pure.Coulomb-e:<.citation calculation which takes aceount of the extendedsize of the valence proton orbital of the projectile. It appearsthat this is the majar part of the explanation for the verysmall breakup cross section observed in the experiment ofvan Schwarzcnberg el al. [11. This result emphasizes the ¡m.ponance of incorporating the unusual structure of 8B in aUaspects of the reaction dynamics as first discussed in Ref. 2.One should also note tAat the "point.like" approximation inCoulomb excitation calculations is still valid far neutron.halo

400

•l.

300200

TOF

','

.~/BS breakup events

100

100

200 8S_

ted 7Be. We used the 8B elastic-scattering Jacobinn lo trans-form the laboratory angJes la the cm frarne. The normaliza-tion was obtained from a measurement of 8B elastic scatter-ing whieh is expeeted lo be purely Rutherford at forward an-gles.

h is obvious from inspcction of the experimental angu.lar distribution (F:g. 2) that our data are complctely incon-sistent with a large amplitude peak in the vicinity of 700_900 which was a prominent feature of both first-order the-orles [2,31 mentioned aboye. Very recently, however. twoealeulations [7,8J lhat ineorporate higher-order effeets havebeen published, and lhey display a mueh different large-anglebehavior. Esbensen and Semeh [7J perfonned a dynamieal

Rn'. Mex. Fí.<. 46 SI (2000) 113-1 t5

PRafON HALO I:"l8B l1S

nuclei since the relevant distance in this case is that betweenlhe eore and lhe eenler-of-mass of lhe halo nucleus, whieh issmall,

The lhiek salid eorve in Fig, 2 includes lhe effeel of nu-cleon transfer from the projectile to the target. This is theca1culation that is most appropriate ror comparison with ourdata, since we do not detect the pro ton and therefore do notdelermine ilS ullimale fale, The large-angle peak is partiallyrestored. However, transfer was not included in the ca1cu.latioos presented in Refs. 2 and 3 so the computed transferyield should be added lo lhe angular dislributions presenledthere. The overall agreement between theory and experirncntis remarkable, espeeially eonsidering lhal lhere has been norenormalization of the predicted absolute cross section.

Nunes and Thompson [8] have also ineluded higher-ordereffects, using the coupled discretized. continuum channels(CDCC) melhod eombined wilh lhe slruelure model of Es-bensen and Bertseh lO], The advanlage of lhis approaeh waslhal lhey were able lo explieitly show lhal lhe vanishing oflhe large-angle peak resulls direelly from lhe eoupling amongcontinuum states. Nunes IIO] has repeated the ca1culation us-ing lhe slruelure mode] of Kim el al. [11] whieh has bolh alarger El and E2 componen!. In general, lhe dala favor lheCDCC ea!culalion using lhe wave funetion of ReL 9, bul lhedifferenees are smalI.

In conclusion, lhe angular dislribulion of lhe breakup of8B into 7Be+p on a 58 Ni target was measured ayer a widerange of angles al sub-barrier energies. The data are com-pletely inconsistent with first-order reaction thearies [2,3]which predict a large-amplitude, nuclear-dominaled peak inthe cross seetion at a cm angle of 70°-90°. Hawever, re-eent ealeulations [7,8, lO) ineorporaling higher-order effeelsare in exeellent agreement with experiment. In these cal-eulations, the spurious peak is eliminated by continuum-

1. 1. von Schwarzenbcrg el al., Phys. Re ••.'c 53 (1996) R2598.2. F.M. Nuncs and!.J. Thompson, Phys. Rey. C 57 (1998) R2818.

3. CH. Dasso, S.M. Lenzi, and A. Vitturi, Nud. Phys. A639(1998) 635.

4. M.Y. Lec et al .. Nud. /llStrum. Methods ill Phys. Resean:hA422 (1999) 536,

5. T. Motobayashi el al., Phys. Rev. !.RII. 73 (1994) 2680.

6. T. Kikuehi el al., Phys. Lm, 8391 (1997) 261.

7, H. Esbenscn and G,F. Bcrtsch, Phys, Ret: C 59 (1999) 3240.

eonlinuum eouplings. Coulomb-nuclear interferenee al verylarge distances, and the need to aceount ror the extended sizeof lhe valenee prolon wave funetion in eompuling Coulombbreakup, are importanl fealores of bolh ealeulalions. Neilhereffect would be present in the sub-barrier interactions of nor-mal nuclear syslems. Thus, lhe presenl dala may well be lhebest evidence yet for the exotic "proton halo" structure of8B. This has been a malter of sorne controversy, sinee re-action cross sectian measurements at relativistic energies byTanihala el al. [12J displayed liuJe or no enhaneemenl, whilesimilar measurements at intermediate energies by Warnerelal. [13] and Negoila el al. [14J showed a ralher subslantial en-hancement. Enhancements in (he reactian cross sectians werelhe firsl signalure of lhe neulron halo. The present dala il-lustrate that finite-size effects in the Coulomb excitation ofweakly-bound prolon-rieh nuclei, and nuclear-Coulomb in-terference at very large distances, well outside the "normal"range of the nuclear force, are crucial features for the under-standing of 8B reactions at near-bamer and sub-bamer cner.gics.

Finally, lhe inleraelions of exolie, wcakly-bound nueleiat near- and sub-barricr energies will increasingly be investi-gated as the next generatian of radioactive ion beam facilitiesusing lhe ISOL leehnique beeome available. It is eomfortingthat there exist at least twa successful theoretical approacheslo lhe diffieult problem of underslanding lhe low-energy re-action dynamics of weakly bound nuclei.

One of us (V.O.) was supported by lhe Funda<;ao de Am-paro a Pesquisa do Eslado de Sao Paulo-Brazil while onleave from lhe Universidade Paulista (UNIP). This work wassupported by lhe National Seienee Foundalion under OranlsNo. PHY94-0]76], PHY95-12199, PHY97-22604, PHY98-04869 and PHY99-01 133.

8. F.M. Nunes and!.J. Thompson, Phys. Rev. C 59 (1999) 2652.9. H. Esbcnscn and G.F. Bertsch, Phys. !.RII. 8359 (1995) 13;

Nud. Phys A600 (1996) 37.

10. F.M. Nuncs, private eornmunication (1999).

11. K.H. Kim, M.H. Park, and B.T. Kim, Phys. Rev. C 35 (1987)363.

12. 1.Tanihala el al., Phys. !.RII. 8206 (1988) 592.

13. R.E. Wamer el al., Phys. Rev. C 52 (1995) RII66.14. F. Ncgoila el al., Phys, Rev. C 54 (1996) 1787,

Rev. Mex. Fis. 46 SI (2000) 113-115