α-particle production in the scattering of 6He by Pb-208 at energies around the Coulomb barrier

arXiv:0705.1678v2 [nucl-ex] 29 Nov 2007 α -partile pro dution in the sattering of 6 He by 208 Pb at energies around the Coulomb barrier D. Esrig a , A.M. Sánhez-Benítez b,c , A. M. Moro d,∗ , M.A.G. Álvarez d , M.V. Andrés d , C. Angulo c , M. J. G. Borge a , J. Cabrera c , S. Cherubini e , P. Demaret c , J.M. Espino d , P. Figuera e , M. Freer f , J.E. Garía-Ramos b , J. Gómez-Camaho d , M. Gulino e , O.R. Kakuee g , I. Martel b , C. Metelko f , F. Pérez-Bernal b , J. Rahighi g , K. Rusek b,h , D. Smirnov i , O. Tengblad a , V. Ziman f a Instituto de Estrutura de la Materia, CSIC, E-28006, Madrid, Spain b Departamento de Físia Apliada, Universidad de Huelva, E-21071, Spain c Institut de Physique Nuléaire and Centre de Reherhes du Cylotron, Université atholique de Louvain, B-1348 Louvain-la-Neuve, Belgium d Departamento de Físia Atómia, Moleular y Nulear, Universidad de Sevil la, Apdo. 1065, E-41080 Sevil la, Spain e INFN Laboratori Nazionali del Sud, I-95123 Catania, Italy f Shool of Physis and Astronomy, University of Birmingham, B15 2TT Birmingham, United Kingdom g Van de Graa Laboratory, Nulear Researh Centre, AEOI, PO Box 14155-1339, Tehran, Iran h Department of Nulear Reations, Andrzej Soltan Institute for Nulear Studies, Hoza 69, PL-00681 Warsaw, Poland i Instituut voor Kern-en Stralingsfysia, University of Leuven, B-3001 Leuven, Belgium. Abstrat New exp erimental data from the sattering of 6 He+ 208 Pb at energies around and b elow the Coulomb barrier are presented. The yield of breakup pro duts oming from pro jetile fragmentation is dominated by a strong group of α partiles. The energy and angular distribution of this group have b een analyzed and ompared with theoretial alulations. This analysis indiates that the α partiles emitted at bakward angles in this reation are mainly due to two-neutron transfer to weakly b ound states of the nal nuleus. Key words: Nulear Reations, Sattering Theory, Halo Nulei, Breakup Reations Preprint submitted to Elsevier 1 February 2008 PACS: 24.10.Eq, 24.50.+g, 25.70.Mn, 25.70.Hi 1 Intro dution Reations involving nulei far from stability at energies around the Coulomb barrier provide an exellent to ol to study the novel prop erties of exoti systems. The inrease in intensity of radioative ion b eams ahieved along the past two deades has ontributed deisively to the improvement of the auray of these exp eriments. One of the exoti nulei that has reeived more attention in reent years is the 6 He. Its interesting Borromean struture, onsisting of an α ore plus two-weakly b ound halo neutrons, and its relatively long half-life (807 ms) makes this nuleus an exellent andidate for this kind of exp eriments. Reations indued by 6 He on several targets [1,2,3,4,5,6℄ at energies around the Coulomb barrier exhibit some ommon features, suh as a remarkably large ross setion for the pro dution of α partiles. This eet is learly asso iated with the weak binding of the halo neutrons, that favors the disso iation of the 6 He pro jetile in the nulear and Coulomb eld of the target. To plae our work in the appropriate ontext, we rst review some reent exp eriments with 6 He at Coulomb barrier energies. In the work of Aguilera et al. [1℄ a simultaneous analysis of the elasti and two-neutron removal hannels for the 6 He+ 209 Bi reation revealed that, at energies b elow the Coulomb barrier, the reation ross setion is almost exhausted by the α hannel while the omplete fusion ross setion is very small. In a more reent measurement of the same reation [2℄ in whih neutronα oinidenes were reorded, the authors onluded that more than half of the α partiles pro dued b eyond the grazing angle arise from two-neutron transfer to unb ound states of the 211 Bi residual nuleus. Similar onlusions were ahieved in an exp eriment done by Di Pietro et al. [3℄, where ab out 80% of the measured α partiles oming out from the reation 6 He+ 64 Zn were identied as oming from transfer or breakup. As a further example, we mention the measurements of Navin et al. for 6 He+ 65 Cu at Elab = 19.5 and 30 MeV [4℄. The 66 Cu yield is largely underestimated by statistial mo del alulations, suggesting that an imp ortant fration of these pro duts have an origin dierent from fusion evap oration. The observation of the harateristi γ rays from heavy pro duts (suh as 65 Cu) in oinidene with pro jetile-like partiles, onrm that these other pro esses ould b e 1n and 2n transfer followed by evap oration. ∗ Corresp onding author Email address: morous.es (A. M. Moro). 2 Reently, Raab e et al. [5℄, measured ssion fragments for the reation 6 He+ 238 U at energies around the fusion barrier. Based up on kinematial onsiderations, they onluded that the large observed yield for ssion b elow the barrier is entirely due to a diret pro ess, the two-neutron transfer. Their onlusions were supp orted by alulations p erformed in the distorted wave Born approximation (DWBA) for the two-neutron transfer to exited states of the 238 U target. In this work, we present new data for the breakup of 6 He on 208 Pb at energies around the Coulomb barrier, measured at the CYCLONE RNB faility at the Centre de Reherhe du Cylotron (CRC) of the Université atholique de Louvain (UCL), Louvain-la-Neuve, Belgium. The elasti sattering data from the same reation has b een analysed and presented b efore [7,8,9℄. A omprehensive optial mo del analysis of these data revealed the existene of a long-range absorption eet, whih is a lear indiation of the presene of reation mehanisms that remove ux from the elasti hannel at distanes well b eyond the strong absorption radius. The same eet has b een also rep orted in other reations indued by 6 He [10℄ and by other weakly b ound nulei, suh as 17 F+ 208 Pb [11℄. The inlusion of a dynami p olarization p otential (DPP) in the phenomenologial pro jetile-target interation showed that part of this long-range absorption eet arises from the distortion pro dued in 6 He due to the intense dip ole Coulomb interation [12℄. The phenomenologial optial mo del required a very large imaginary diuseness in order to repro due the elasti data even after the inlusion of the DPP. Thus, the nulear interation ontributes also to this long-range absorption. Given the dominane of the 4 He hannel in these low energy reations, it is plausible to suggest that the mehanisms resp onsible for the pro dution of these fragments are also resp onsible for the long-range absorption eet. Guided by this motivation, in this work we present an analysis of the twoneutron removal hannel measured in the same exp eriment. In this exp eriment, the energy and sattering angle of the α partiles emitted at bakward angles were reorded. The purp ose of this work is to understand the reation mehanisms whih are relevant in the ollision of 6 He on 208 Pb, by examining the angular and energy distributions of the α partiles pro dued in the ollision. To do this, for eah b eam energy, the angular distribution as well as the angle-integrated energy distribution of the α partiles have b een evaluated and ompared with theoretial alulations. The pap er is organized as follows. In setion 2 we desrib e the exp erimental setup and analysis metho d, and disuss general features of the measured observables. In setion 3, we ompare these observables with theoretial alulations p erformed with the transfer to the ontinuum metho d, diret breakup alulations using ontinuum disretized oupled-hannels alulations, and neutron transfer alulations in DWBA. In setion 4 we disuss our results 3 and ompare them with previous works. Finally, in setion 5 we present the summary and outlo ok of this work. 2 Exp erimental setup and analysis pro edure The sattering of a 6 He b eam on a 208 Pb target was studied at the radioa- tive b eam faility of the CRC/UCL at Louvain-la-Neuve. The 6 He b eam was pro dued by the 7 Li(p,2p) 6 He reation in a LiF p owder target with a graphite ontainer. The atomi b eam was ionized in an ECR soure, puried by magneti separation and reaelerated in the CYCLONE110 ylotron at the CRC/UCL. This tehnique provides a highly pure b eam, essentially free of ontaminants. The only ontaminant ever observed in this b eam has b een rep orted in the the work of Miljani et al. [13℄ where 4 He 1 H + 2 ions were observed as an impurity in a 17 MeV 6 He b eam with a ratio of intensities of 1:5400. For the angular range overed in the present exp eriment, the ontribution of this impurity turned out to b e negligible. The 6 He b eam was pro dued at lab oratory energies of 14, 16, 18 MeV within the rst harmoni of the aelerator with an average intensity of 4 · 10 6 ions p er seond and at 22 MeV at the lowest limit of the seond harmoni with an intensity of 1.5 · 10 5 ions p er seond. A high intensity 4 He b eam at lab oratory energy of 12 MeV was used for normalization of the elasti ross setion. The elasti data obtained from this exp eriment are published elsewhere [9℄. We briey review here the main features of the setup. For a more detailed desription of the exp erimental setup we refer to previous publiations [7,9℄. The size of the 4,6 He b eams were redued by passing the b eam through a set of two ollimators of 5 mm and 7 mm diameter with the latter at 400 mm from the target. The targets onsisted of self-supp orting foils of enrihed 208 Pb (87%) mounted on a movable ladder with a thikness of 0.950 mg/m 2 for the 12 MeV 4 He and 14,16,18 MeV 6 He b eams and 2.080 mg/m 2 for the 22 MeV 6 He b eam. The latter was used to omp ensate for the low intensity of the b eam. The reation pro duts were measured using four LEDA detetors in the standard form and six LEDA detetors in the LAMP onguration overing angles in the forward diretion from 5 ◦ to 65 ◦ . For a detailed desription of the p erformane and eieny of these detetors see [14℄. In the bakward diretion, the most relevant for this analysis, the DINEX telesop e array [15,7℄ was plaed at a distane of 37 mm and 42 mm from the 0.950 mg/m 2 and 2.080 mg/m 2 thik 208 Pb targets resp etively. The DINEX telesop e overed therefore different lab oratory angles for the dierent targets ranging from 136.3 ◦ to 166.6 ◦ for the thin target and from 131.8 ◦ to 164.5 ◦ for the thik target of 2.080 mg/m 2 used only for the 22 MeV 6 He b eam. The DINEX array onsisted of 4 four quadrants forming a CD [15℄, eah omp osed of single sided Si strip detetors ( ∆ E) 40 µ m thik with sixteen radial strips staked in a 500 µ m thik single PAD Si-detetor. Eah strip subtended an angle of ab out 2 ◦ , although this value dep ends on the sattering angle. The energy alibration was p erformed using a triple alpha soure for the front single sided Si strip detetor of the telesop e. The telesop e as a whole was alibrated using the elasti sattering p eaks of the 4 He and 6 He b eams from the 208 Pb target at dierent energies b elow the Coulomb barrier where the elasti p eak at bakward angles still has signiant statistis. To evaluate the energy losses of the b eam and ejetiles in the dierent media, i.e. target thikness and dead layers of the front and bak detetors, a simulation programme develop ed by R. Raab e [16℄ and adapted to the geometry of our setup was used. The energy losses of the 4,6 He ions were alulated in tables using SRIM [17℄ and inserted into the simulation programme. The fat that the interation region had a nite size, was also taken into aount in the simulations. In order to obtain the energy of the ejetile we rst added the signals from the two detetors of the telesop e, ∆ E and E. The latter was multiplied by a mathing onstant α(θ) that dep ends on the relative gain of the dierent eletroni hains (see [9℄ for more details). Then we used the energy of the elasti p eaks of 4 He at 12 MeV and 6 He at 14, 16 and 18 MeV to alibrate the total telesop e signal. The DINEX telesop es allow mass and harge separation of the reation pro d- uts. A typial mass sp etrum, ∆ E versus total energy, ET , obtained with the DINEX array for a 6 He b eam energy of 22.0(1) MeV and θlab = 144◦± 1 ◦ is shown in Fig. 1. Using these alibrations and the mass and harge separation obtained with the telesop es we are able to identify the energy distribution of the breakup pro duts as well as the elasti ounts in every ring ( θi ), as shown in this gure. The energy loss in the 40 µ m thik ∆ E detetor is displayed versus total energy. The elipsoids selet the elasti and breakup events used in the analysis. The low energy seletion orresp onds to protons. The bakground events are remarkably low onsidering that no ondition b eyond oinidene b etween the front and bak detetors of the telesop e is applied. We obtained the breakup ross setion by making use of the ratio of breakup to elasti events seen in the telesop es. To have reasonable statistis, we group ed the breakup events in 1 MeV bins. Then, we evaluated for eah detetor strip θi and for eah energy bin Ei the ratio of breakup to elasti events Nbu(θi, Ei)/Nel(θi) . Note that unertainties asso iated with b eam intensity, solid angle, eieny of the eletroni hain or target thikness disapp ear in this ratio. When we add these ratios for all the energy bins, we obtain the ratio of breakup to elasti ross setions as a funtion of the sattering angle. These ratios are shown in Fig. 2, as a funtion of the lab oratory sattering (LAB) 5 Fig. 1. Two dimensional plot of the 22 MeV 6 He sattered from a 2.08 mg/m 2 208 Pb target at lab oratory angle of 144(1) ◦ . The regions enlosed by solid lines orresp ond to the 6 He and 4 He events. The plot inludes all events in oinidenes with multipliity ≥1 to stress the low bakground level. Therefore one an see events fully stopp ed in the ∆E detetor orresp onding to α 's from the grand-daughter of 210 Pb and 234 Th present in the target in very small amount. The ount rate was less than two α partiles every 10 min for this angular overage. angle , at several b eam energies. It is notieable that, at Elab = 22 MeV, the yield of α fragments exeeds the elasti ones by a fator of ten. As disussed in the intro dution, this large α ross setion has b een rep orted for other rea- tions indued by 6 He on several medium-heavy targets, suh as 65 Cu [4℄, 64 Zn [3℄, and 209 Bi [1℄. The breakup double dierential ross setion, with resp et to the angle and the energy of the α partile, dep ends on energy and angle, and an b e related to the ratio of the numb er of ounts and to the elasti dierential ross setion by the following expression: d2σbu dEdΩEj,θi =Nbu(θi, Ej) Nel(θi) 1 ∆Ej dσel dΩ!θi (1) where ∆Ej= 1 MeV is the bin width. The dierential elasti ross setion for the dierent lab oratory angles are taken from [9,18℄. If this expression is integrated over the energy of the breakup fragment, one obtains the dierential breakup ross setion, as a funtion of the sattering 6 angle, whih is given by: dσbu dΩθi ≈ j=Emax X j=Emin Nbu(θi, Ej) Nel(θi) dσel dΩ!θi . (2) These dierential ross setions are presented in Fig. 3. Errors bars orresp ond to statistial errors. It should b e noted that the breakup dierential ross se- tions are highest around the Coulomb barrier ( E= 18 MeV), and they derease b oth at lower energies and higher energies. However, for 14 MeV, whih is well b elow the barrier, the breakup ross setions are still sizeable. This indiates that the mehanism pro duing alpha partiles is eetive for pro jetile-target separations as large as 17.3 fm, whih orresp onds to the distane of losest approah for a head-on ollision at this energy. This mehanism, indeed, will b e a soure of the long range absorption whih we have seen in the analysis of elasti data. Similarly, if, for eah energy bin of the α partiles, the double dierential ross setions are integrated with resp et to the angle, over the angular range overed by the CD detetors, one gets a breakup dierential ross setion as a funtion of the energy, whih is given by dσbu dE Ej ≈ i=16 X i=1 2πsin(θi)∆θi ∆Ej Nbu(θi, Ej) Nel(θi) dσel dΩ!θi . (3) These ross setions are presented in Fig. 4. Our purp ose is to understand whih mehanism is resp onsible for the large pro dution of α partiles. The fat that the α partiles are pro dued with relatively large energies, that inrease with the pro jetile energy, leads us to onlude that the pro ess should b e a diret one, and not a omp ound nuleus formation. Within the diret mehanism piture, we an onsider three mehanisms: a) Transfer to the ontinuum: As the 6 He nuleus gets lose to the target, it leaves the two neutrons with low kineti energy with resp et to the 208 Pb target, and the remaining α partile esap es. If this is the ase, we would exp et that the α partile would have an energy distribution entered around the energy of the elastially sattered 6 He. b) Diret breakup: The 6 He nuleus breaks up in the eld of the target and it go es to a ontinuum state with low exitation energy. In this ase, we would exp et that the α partile (and the neutrons) to have a similar velo ity to the elastially sattered 6 He, and hene its energy would have a broad distribution around 4/6 of the energy of 6 He. ) Neutron transfer: One of the neutrons of 6 He is transferred to the target, 7 130 140 150 160 170 θlab (deg.) 0.1 1 10 σbu/σel 22 MeV 18 MeV 16 MeV 14 MeV Fig. 2. Ratio b etween the measured 4 He and 6 He events, as a funtion of the lab oratory sattering angle, at several b ombarding energies. The angular range at 22 MeV is dierent to that overed at energies b elow the Coulomb barrier due to dierent p ositioning of the target. See setion 2 for more details. Errors bars orresp ond to statistial errors. pro duing a b ound state of 209 Pb and leaving 5 He in a broad resonane, that rapidly deays pro duing 4 He. In this ase, the kineti energy of the 5 He resonane, although dep endent on the Q-value, would b e similar to that of the elastially sattered 6 He (for Q≃0 ), and the alpha partiles would have a broad distribution around 4/5 of the energy of 5 He. It should b e noted that, in our work, diret breakup, 2n transfer and 1n transfer orresp ond to dierent approahes to desrib e the mehanism that pro due alpha partiles, rather than to dierent reation hannels. From the theoretial p oint of view, these three mehanisms desrib e the removal of the valene neutrons in 6 He, but they do not lead neessarily to dierent nal states. They should b e seen as dierent approahes to a very diult 4-b o dy problem ( α + n+ n+ 208 Pb) whih annot b e solved aurately. Eah of these metho ds emphasize a dierent way in whih the fragmentation is pro dued. In the diret breakup metho d, one assumes that the 6 He is broken up by exiting the neutrons to ontinuum states with low relative energy with resp et to the α ore, and hene a representation in terms of the 6 He ontinuum states is used. In the transfer to the ontinuum approah, it is assumed that the fragmentation o urs by transfer of the valene neutrons to weakly b ound states of the target, and hene a target representation is preferred in this ase. Finally, the 1n transfer orresp onds to an intermediate situation, in whih one assumes that one of the neutrons is transferred to states of low relative energy with resp et to the target, while the other remains in a low energy state with resp et to the alpha ore. Then, these approahes should b e understo o d as 8 0 50 100 dσ/dΩ (mb/sr) 0 50 100 dσ/dΩ (mb/sr) 0 50 100 dσ/dΩ (mb/sr) 0 30 60 90 120 150 180 θlab (deg) 0 50 100 150 200 dσ/dΩ (mb/sr) TC DBU NT E=14 MeV E=16 MeV E=18 MeV E=22 MeV Fig. 3. Angular distribution of α partiles arising from 6 He fragmentation, in the lab oratory frame, for several inident energies. Exp erimental angular distributions are ompared with transfer to the ontinuum (TC) alulations (solid lines). The distribution obtained at 22 MeV is also ompared with a diret breakup (DBU) alulation, p erformed within the CDCC approah (dashed line), and a DWBA alulation for the one neutron transfer (NT) leading to b ound states of the 209 Pb nuleus (dotted-dashed line). extreme pitures that emphasize dierent degrees of freedom of the breakup pro ess, and not dierent reation hannels. For instane, it has b een shown [19℄ that, if a large basis is inluded in b oth the diret breakup and transfer to the ontinuum representations, there is a strong overlap b etween the states p opulated in b oth metho ds. As a onsequene, the ross setions alulated within the two approahes an not b e simply added to obtain the total breakup ross setion. It is apparent from Fig. 4 that the p osition of the energy p eak is not onsistent 9 we have p erformed CDCC alulations for the Elab = 22 MeV ase. Again, we assumed a simple di-neutron mo del for 6 He. Partial waves s , p and d were inluded for the 2n- 4 He relative motion. For eah partial wave, the 6 He ontinuum was divided into energy bins, aording to the sheme detailed in [31℄. In analogy with the presription used in the transfer to the ontinuum alulations, the eetive two-neutron separation energy S2n= 1.5 MeV was used to generate the 6 He ground state and ontinuum wavefuntions. With this presription, the elasti angular distribution is very well repro dued. The breakup angular and energy distributions obtained from this alulation are represented in Figs. 3 and 4 by the dashed line (b ottom panel). It an b e seen that the exp erimental data are underestimated by almost an order of magnitude, at all the measured angles. The inlusion of higher partial waves did not solve the disrepany. Hene, as we antiipated, the diret breakup piture is inadequate to desrib e the present data. From the gure, we see also that the CDCC distribution dominates the small angle region, with a pronouned p eak around 30 ◦ . At these angles, the transfer to the ontinuum urve is very small, suggesting that for the desription of the α partiles emitted at forward angles the diret breakup sheme should provide a more suitable representation than the TC. We note that a prop er CDCC alulation for the present reation would require a three-b o dy desription of 6 He, thus giving rise to a four-b o dy sattering problem. This kind of alulations has b een reently rep orted by the Kyushu group for 12 C [38℄ and 209 Bi [32℄ targets. Although our results will b e mo died to some extent if a realisti three-b o dy desription of 6 He is used, we b elieve that our simplied alulations retain the essential physis to illustrate the inadequay of the CDCC metho d to desrib e the present data. 3.3 One neutron transfer Another hannel that ould ontribute to the pro dution of α partiles is the one neutron stripping, 208 Pb( 6 He, 5 He) 209 Pb. This pro ess will pro due 5 He in a resonant state, whih will eventually deay into n+ 4 He. The alulations for this pro ess were p erformed within the DWBA approximation. Due to Q- value onsiderations, this pro ess p opulates mainly b ound states of the 209 Pb nuleus, similar to what o urs in the 208 Pb(d,p) 209 Pb reation [39℄. Thus, we inluded the known b ound states for the 209 Pb nuleus, and assumed unit sp etrosopi fators, sine these are known to b e mainly single-partile states. Conerning the sp etrosopi fator for 6 He →5 He+n, in a strit shell mo del piture, with maximal pairing, sp etrosopi fator for neutron transfer is equal to numb er of valene neutrons, what would give 2. We adopt however the value 1.60 for this sp etrosopi fator, rep orted by Nemets et al [40℄, and 16 obtained using the metho d detailed in Ref. [41℄. For the entrane hannel, 6 He+ 208 Pb, the eetive p otential obtained from CDCC alulations was used. For the exit hannel, 5 He+ 209 Pb, the p otential was alulated by folding the n+ 209 Pb p otential from the ompilation of Varner et al. [42℄ and the α + 208 Pb p otential of Goldring et al. [43℄ with the ground state wave funtion of 5 He. The latter was represented by an energy bin of a width 1.2 MeV plaed at an exitation energy of 0.8 MeV ab ove the 5 He →4 He+n breakup threshold. The binding p otential for 4 He+n was taken from Ref. [44℄. The alulated angular distribution is shown by the dotted-dashed line in the lowest panel of Fig. 3 ( Elab =22 MeV). It should b e noted that, in this ase, the sattering angle orresp onds to the enter of mass of the 5 He* system, rather than the 4 He angle. At intermediate angles ( θlab ≈90◦ ), our alulation predits a signiant ontribution of the 1n transfer, although the α yield is still dominated by the 2n transfer. This result is in agreement with the exp erimental data of De Young et al. [2℄ in whih the 2n transfer was identied as the main mehanism pro duing α partiles at these angles, with a small ontribution arising from 1n transfer. We note however that the ross setions rep orted in [2℄ for the 1n and 2n transfer pro esses are inferred from the α partiles observed at 90 and 120 degrees, while in our exp eriment the observed α partiles orresp ond to larger angles (ab ove 130 ◦ ). Therefore, the ross setions rep orted in Ref. [2℄ annot b e readily extrap olated to our ase. Moreover, even if we ompare the same angular range, we do not exp et to get the same quantitative results, b eause the states p opulated in b oth reations (namely, 209 Pb and 210 Bi) are dierent. It an also b e seen that at the angles of interest of our exp eriment ( θlab > 130◦ ), the ontribution of the 1n transfer is negligible and the pro dution of α partiles is essentially due to 2n transfer. Moreover, the 1n transfer annot explain the energy of the α partiles sine the maximum kineti energy of the α partiles emitted in this pro ess, whih orresp onds to a transfer to the 209 Pb ground state, is ab out 17.5 MeV, while the exp erimental energy distribution extends b eyond 20 MeV. Therefore, we onlude that the one neutron stripping hannel is not resp onsible for the underestimation of the data at these angles. 4 Disussion The results found in this work are onsistent and omplementary to our previous analysis of the elasti sattering for the same reation [9℄. The optial mo del analysis p erformed in the previous work revealed the imp ortane of a long range absorption mehanism. With the present analysis, one an onlude 17 that this mehanism is presumably related to the two-neutron removal pro ess disussed in this work. In order to draw more denite onlusions it would b e desirable to obtain data in a wider angular range. Furthermore, in order to understand more learly the imp ortane of dierent reation mehanisms, it would b e very useful to p erform a similar exp eriment in whih the neutrons ould b e deteted in oinidene with the α partiles. Given the small detetion eieny for neutrons, and the fat that the two neutrons an b e emitted, in priniple, in arbitrary diretions, this exp eriment is also very hallenging from the exp erimental as well as from the theoreti- al p oints of view. Despite these diulties, a similar exp eriment has b een reently p erformed for the reation 6 He+ 209 Bi at 22 MeV [2℄. By measuring neutronα oinidenes it is onluded that approximately 55% of the observed α yield around and b eyond the grazing angle is due to two-neutron transfer to unb ound states of the 211 Bi nuleus. Our results are also onsistent with other exp eriments on 6 He indued rea- tions on several targets, suh as 64 Zn [3℄, 65 Cu [4℄ and 238 U [5℄, for whih large 1n and 2n transfer ross setions have b een inferred using dierent exp erimental tehniques. In the ase of the reation using the 238 U target, these onlusions are supp orted by the reent alulations of Cárdenas et al. [45℄. Using a shemati oupled-hannels alulations, the authors show that the 2n removal ross setion from 6 He an b e well aounted for by an inoherent sup erp osition of several transfer pro esses with dierent Q values. Similarly, the alulations in this work involve also an inoherent sum of 2n transfer to nal states with a wide range of Q values. 5 Summary and outlo ok In this pap er we have presented new exp erimental data for the reation 6 He+ 208 Pb at energies around and b elow the Coulomb barrier. The reation ross setion at bakward angles is dominated by a prominent 4 He group, whih was interpreted as oming from pro jetile fragmentation. For eah sattering energy, the angular and energy distributions of the 4 He fragments have b een analyzed and ompared with transfer to the ontinuum alulations, in whih the two-neutrons are assumed to b e transferred as a luster to b oth b ound as well as unb ound states of the target nuleus. A- ording to these alulations, most of the observed α yield omes from the transfer of the valene neutrons to highly exited states of the target in the proximity of the two-neutron breakup threshold. By ontrast, diret breakup 18 alulations and one neutron transfer alulations fail to explain the present data. This analysis suggests a senario in whih the 6 He nuleus is broken up in the eld of the target, and the valene neutrons are left with a small relative energy with resp et to the target. By energy onservation, the kineti energy lost by the neutrons is transferred to the α partiles, whih are therefore aelerated with resp et to the b eam velo ity. These onlusions are onsistent with previous measurements for other reations indued by 6 He on several targets, for whih large transfer ross setion have b een also rep orted [1,2,3,4,5℄. Further measurements for this reation, inluding omplete kinematis, wider angular overage, detetion of ssion and evap oration pro duts, et, would b e very useful to disentangle more learly the imp ortane of dierent reation mehanisms, and improve our understanding of the pro esses that take plae in the sattering of exoti nulei at energies around the Coulomb barrier. Some of these measurements have b een already p erformed and the subsequent analysis is underway. Despite the reasonable quantitative agreement with the data, the transfer to the ontinuum alulations an b e improved in b oth the struture and reation asp ets. Conerning the struture mo del, the main approximation involved in our alulation is the assumption of the validity of the di-neutron mo del. Fourb o dy DWBA alulations have b een p erformed by Chatterjee et al. [46℄ for the sattering of 6 He on Pb and Au at high energies, using a realisti three-b o dy desription of the 6 He nuleus. By omparing with a onventional three-b o dy DWBA alulation based up on a di-neutron mo del of 6 He, they nd that the latter alulation gives to o large breakup ross setion. The disagreement is attributed to the bigger rms radius in the di-neutron mo del, as ompared to a realisti three-b o dy mo del. In our alulations this eet is aounted for by inreasing the 2n separation energy in 6 He. Also, the desription of the nal states in 210 Pb ould b e improved by using a more realisti level density for this nuleus. Conerning the desription of the reation mehanism, these alulations ould b e improved by inluding ouplings among nal states in 210 Pb, by means of the CCBA metho d. These ouplings ould b e generated in a luster mo del, by folding the 2n- α and α + 208 Pb interations with the internal wavefuntions for the 210 Pb states. Also, ouplings b etween the transfer/breakup hannels and the elasti hannel ould b e inorp orated b eyond the rst order, thus p erforming a oupled-reation hannels alulation. This alulation would p ermit p ermit an assessment of whether the expliit inlusion of these hannels an explain the long-range absorption eet enountered in the analysis of the elasti data. If this is the ase, the optial p otential required to reprodue the elasti data in presene of transfer hannels should have a smaller 19 diuseness, as ompared to the phenomenologial optial p otential derived in absene of these hannels. Notie that, in the DWBA approah, these higher order eets are aounted for in an eetive way by an appropriate hoie of the phenomenologial optial p otentials used to desrib e the inoming and outgoing distorted waves. These CCBA and oupled reation hannels al- ulations are b eyond the sop e of this work, and hene they have not b een attempted. Aknowledgements This work has b een supp orted by the Spanish MCyT pro jets FPA2005-04460, FPA2005-02379, FPA-2000-1592-C03-02, FPA2003-05958, and FPA2002-04181- C04-02/03, by the Europ ean Community-Aess to Researh Infrastruture ation of the Improving Human Potential Program, ontrat numb er HPRICT-1999-00110 and the Belgian program P5/07 on interuniversity attration p oles of the Belgian-state Federal Servies for Sienti, Tehnial and Cultural Aairs. We are grateful to J. Kolata for useful disussions. A.M.M. a- knowledges nanial supp ort by the Junta de Andaluía. D.E. aknowledges nanial supp ort by the MEC. Referenes [1℄ E. F. Aguilera et al. , Phys. Rev. Lett. 84 (2000) 50585061. [2℄ P. A. DeYoung et al. , Phys. Rev. C71 (2005) 051601. [3℄ A. Di Pietro et al. , Phys. Rev. C 69 (2004) 044613. [4℄ A. Navin, et al., Phys. Rev. C 70 (2004) 044601. [5℄ R. Raab e, et al., Nature (London) 431 (2004) 823. [6℄ J. F. Liang, C. 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