Reactions with the double-Borromean nucleus 8He A. Lemasson, A. Navin, N. Keeley, M. Rejmund, S. Bhattacharyya, A. Shrivastava, Dominique Bazin, D. Beaumel, Y. Blumenfeld, Avhishek Chatterjee, et al. To cite this version: A. Lemasson, A. Navin, N. Keeley, M. Rejmund, S. Bhattacharyya, et al.. Reactions with the double-Borromean nucleus 8He. Physical Review C, American Physical Society, 2010, 82, pp.044617. 10.1103/PhysRevC.82.044617. in2p3-00531199 HAL Id: in2p3-00531199 http://hal.in2p3.fr/in2p3-00531199 Submitted on 3 Nov 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Reactions with the double-Borromean nucleus 8He A. Lemasson,1 A. Navin,1, ∗ N. Keeley,2 M. Rejmund,1 S. Bhattacharyya,1, 3 A. Shrivastava,4 D. Bazin,5 D. Beaumel,6 Y. Blumenfeld,6 A. Chatterjee,4 D. Gupta,6, y G. de France,1 B. Jacquot,1 M. Labiche,7 R. Lemmon,7 V. Nanal,8 J. Nyberg,9 R. G. Pillay,8 R. Raabe,1, z K. Ramachandran,4 J.A. Scarpaci,6 C. Simenel,10 I. Stefan,1, x and C.N. Timis11 1GANIL, CEA/DSM - CNRS/IN2P3, Bd Henri Becquerel, BP 55027, F-14076 Caen Cedex 5, France 2Department of Nuclear Reactions, The Andrzej So ltan Institute for Nuclear Studies, ul. Ho_za69, PL-00-681 Warsaw, Poland 3Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata 700064, India 4Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India 5National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan, 48824, USA 6Institut de Physique Nucl´eaire,IN2P3-CNRS, 91406 Orsay, France 7CRLC, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, U.K. 8Department of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Mumbai 400005, India 9Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden 10IRFU/Service de Physique Nucl´eaire, CEA Centre de Saclay, F-91191 Gif-sur-Yvette, France 11Department of Physics, University of Surrey, Guildford, GU2 7XH, U.K. (Dated: October 15, 2010) Differential cross-sections for elastic scattering and neutron transfer reactions along with cross- sections for fusion in the 8He+65Cu system are reported at energies above the Coulomb barrier (Elab = 19.9 and 30.6 MeV). The present work demonstrates the feasibility of using inclusive mea- surements of characteristic in-beam γ rays with low-intensity (∼ 105 pps) radioactive ion beams to obtain the residue cross-sections for fusion and neutron transfer. Exclusive measurements of γ rays in coincidence with light charged particles have been used to further characterize the direct reactions induced by this double-Borromean nucleus. Coupled reaction channels calculations are used to illustrate the important role played by the transfer channels and to help in understanding the influence of the structure of 8He on the reaction mechanism. PACS numbers: 25.60.-t, 25.70.Hi, 25.70.Jj I. INTRODUCTION binding of the valence nucleon(s) led to the expectation that breakup of the projectile would be a dominant chan- nel and also influence the reaction mechanism for these The advent of re-accelerated beams using Isotope Sep- nuclei [1{3]. In particular, the influence of breakup on aration On Line (ISOL) techniques provides new oppor- fusion reactions around the barrier has been of consider- tunities to study and understand reactions with nuclei able interest [1, 8]. Attempts have been made to under- far from stability [1{4]. The neutron separation ener- stand the influence of the breakup process on the fusion gies of neutron-rich nuclei near the drip line are con- and elastic scattering of beams of both stable and ra- siderably smaller (Sn ∼ 1 MeV) than those for typical dioactive weakly-bound nuclei using various approaches stable nuclei (Sn ∼ 8 MeV). In addition to the large [9{13]. However, the loosely bound valence nucleon(s) spatial extent (halo) of the weakly-bound valence nu- could also imply large transfer cross-sections from the cleon(s), these nuclei can also exhibit Borromean struc- projectile to the target. This in turn would also be ex- ture [5] (three-body bound systems in which none of the pected to influence the reaction mechanism. Large trans- two-body subsystems are bound). Typical examples in- fer cross-sections have been reported for 6He beams on clude 6;8He, 11Li and more recently 22C[6], which can 65Cu [14, 15], 64Zn [16], 197Au [17], 209Bi [18, 19] and be viewed as three-body systems consisting of a core nu- 238U targets [20, 21]. Refs. [14, 19] also showed that cleus and two valence neutrons. These properties are the transfer cross-sections were even larger than those expected to influence the reaction processes and are also for breakup. More recently, large transfer cross-sections relevant to reactions of astrophysical interest [7]. The (1n+2n) were also reported in the 8He+197Au system at large probability of breakup associated with the weak energies below and above the Coulomb barrier [22]. The effect of transfer on other channels was theoretically in- vestigated using Coupled Reaction Channels (CRC) cal- culations [2]. In order to have a complete understanding ∗Corresponding author email : [email protected] of the reaction mechanisms for neutron-rich nuclei, mea- yPermanent address: Dept. of Physics and Centre for Astroparticle surements of all the reaction channels are necessary. Physics and Space Science, Bose Institute, Kolkata 700091, India. Experimental investigations involving low-energy zPermanent address: Instituut voor Kern- en Stralingsfysica, K.U. Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium. (around the Coulomb barrier) and low-intensity ra- xPermanent address: Institut de Physique Nucl´eaire, IN2P3- dioactive ion beams (RIB) have brought about new CNRS, 91406 Orsay, France. challenges. In general, investigations with low-intensity 2 precise cross-sections to be obtained using off-beam (a) ∼ 5 -20 measurements with beam intensities of 10 pps have also been reported [22]. 5.8 5.6 7.5 8 With a half-life t1=2=119.1 ms, He is the most -25 - 2.9 5/2- neutron-rich bound nucleus known today and is an ideal B. E. (MeV) 1.3 1/2 5.4 2.65 - candidate to study the influence of nuclear structure on - + 4.4 1 3/2 1.8 2 1.3 1/2- the reaction mechanism around the Coulomb barrier. Ac- 0+ 5 3.1 2+ 3/2- 4 He 0+ curate measurements of its charge radius [28] and mass He 7 -30 6 He [29] were recently reported. A significant reduction in He 6 8 0+ charge radius from He to He was interpreted as a 8 He change in the correlations of the valence neutrons. The (b) helium isotopic chain is an ideal laboratory to study the 21 effect of the dramatic variation of the intrinsic properties 20 on the reaction mechanism. Fig. 1(a) shows the low- 3 lying energy levels of the helium isotopes. The neutron 2 th 1 emission threshold has the largest variation for a single E (MeV) 4 0 isotopic chain, from 20.5 MeV for the tightly-bound He -1 to 0.9 MeV for 6He (see Fig. 1(b)). Additionally, this 4 5 6 7 8 is the only isotopic chain where the two-neutron separa- A tion energy S2n increases as the neutron drip line is ap- proached. The structure of 8He has been investigated us- ing the direct reactions 8He(p,d)7He [30] and 8He(p,t)6He FIG. 1: (a) Low-lying states in 4−8He isotopes (Refs. [23{26]). [31], quasi-free scattering [32], and knock out reactions (b) Lowest particle emission thresholds in the helium isotopic 8 chain. Full (open) symbols represent the bound (unbound) [33, 34]. The low-lying spectrum of He was also re- 3 6 8 isotopes. The line is to guide the eye. cently studied by means of the H( He, p) He reaction [35]. The ground state configuration of 8He (involving 4 2 2 ν3=2 [36] or ν3=2ν1=2 [31, 32, 37]) is still an open ques- tion. The ground state of 8He can be considered to be ∼ 5 radioactive ion beams ( 10 pps) have been restricted composed of a 4He+4n or 6He+2n structure. The possi- due to both the limited availability of ISOL beams bility of a significant contribution from the 6He (2+) first and the need for a substantial increase in experimental excited state to the 8He ground state has also been in- sensitivity given the typical reduction by a factor of vestigated [31, 38]. As 6He is a Borromean nucleus, 8He ∼ 6 10 in intensity compared to stable beams. The direct could be considered to be a \double-Borromean" nucleus. measurement of residues produced in reactions induced The study of transfer reactions induced by neutron-rich by light projectiles at these energies is difficult given nuclei like 8He, associated with predictions of possible their low recoil velocities. Measurements of reaction dineutron correlations [37, 39], could be used as a tool to products using the in-beam γ-ray technique are much probe pairing in finite fermionic systems [40, 41]. more challenging compared to stable beams due to the relatively lower yields and the additional background arising from the radioactive decay of the beam particles. The motivation of this work is to measure for the first The first inclusive γ-spectroscopy measurement using time elastic scattering, transfer and fusion cross-sections a 109 pps re-accelerated RIB was performed by Cat- for 8He at energies around the Coulomb barrier and un- ford et al.