Fusion of the Most Neutron-Rich Nucleus 8He: Recent Results from GANIL
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EPJ Web of Conferences 17, 01003 (2011) DOI: 10.1051/epjconf/20111701003 © Owned by the authors, published by EDP Sciences, 2011 Fusion of the most neutron-rich nucleus 8He: Recent results from GANIL A. Lemasson1;2;a 1 NSCL, Michigan State University, East Lansing, MI 48824, USA 2 GANIL, CEA/DSM - CNRS/IN2P3, Bd Henri Becquerel, BP 55027, F-14076 Caen Cedex 5, France Abstract. Re-accelerated radioactive ions beams of 8He from the SPIRAL facility at GANIL have been used to perform the first experimental investigations at energies around the Coulomb barrier of fusion and direct reactions with the most neutron rich 8He. Two experiments (in the 8He+65Cu and 8He+197Au systems) have been performed, using the complementarity of in-beam and off-beam techniques, to measure integral and differential cross sections for elastic scattering, fusion and neu- tron(s) transfer. These results, compared with Coupled Reaction Channels calculations as well as phenomenological approaches allowed to probe and understand the different facets of the interplay between the intrinsic structure of 8He and reaction mechanisms. 1 Introduction study of the relative importance of 1n and 2n transfer [6,7]. Recent advances in accelerator and isotope-production In this talk we present the first investigations of technology, like the Isotope Separation On-Line (ISOL) reaction mechanisms at energies around the Coulomb technique, provides new opportunities to study and barrier involving the 8He nucleus. The heaviest He- understand reactions involving rare short-lived isotopes lium isotope, with the largest neutron/proton ratio [1{4]. These nuclei far from the β-stability line, char- (3/1) of known bound nuclei and a lifetime of only acterised by small binding energies and unusual ratio 0.12 s, is a weakly-bound nucleus (S2n = 2:1 MeV) ex- of neutron to proton numbers, have a variety of novel hibiting an neutron halo/skin and a Borromean struc- properties like extended and therefore diluted mat- ture. The increase of the two neutron separation en- ter distributions (halo), Borromean structures (three- 8 ergy (S2n) in the more neutron-rich He compared to body bound systems, where any of its two-body sub- 6He is different from the known behavior for other systems are unbound) or large breakup probabilities. isotopic chains. The helium isotopic chain, where the Theses new intrinsic properties, that differs signifi- neutron emission threshold varies from 20.5 MeV to cantly from those of stable nuclei, are expected to in- 0.9 MeV, represents an ideal laboratory for studying fluence reaction mechanisms at energies around the the effect of the weak binding and of the changing in- Coulomb barrier and are also relevant to reactions trinsic structure with increasing number of neutrons of astrophysical interest [5]. The large probability of on reaction mechanisms. Moreover, if Helium isotopes breakup associated with the weak binding of the va- are considered as composite objects of interacting (α lence nucleon(s) led to the expectation that breakup core) and non interacting (valence neutrons) objects of the projectile would be a dominant channel and with respect to the electrostatic barrier, the fusion re- also influence the reaction mechanism for these nuclei action of Helium isotopes can be seen as a ideal case [1{3]. Attempts have been made to understand the in- for studying the quantum tunnelling of composite ob- fluence of the breakup process on the fusion and elas- jects [8,9]. tic scattering of beams of both stable and radioactive The SPIRAL facility at GANIL provides the most weakly bound nuclei using various approaches. How- 8 ever, the loosely bound valence nucleon(s) could also intense beams of He presently available, giving a unique imply large transfer cross sections from the projectile opportunity to study neutron correlations and the ef- to the target. This in turn would also be expected to fect of Borromean structure on the reaction mecha- influence the reaction mechanism. In particular, Bor- nisms at energies around the Coulomb barrier. How- ∼ 5 romean nuclei (in which the binding is essentially aris- ever, measurements with low intensity ( 10 pps) ing from pairing correlations) represents a unique tool and low energy re-accelerated beams of rare short- to understand the dynamical aspects of pairing cor- lived isotopes represent an experimental challenge due relations between loosely bound nucleons through the to the million time lower intensity compared to stable beams. Hence, the development of innovative sensitive a e-mail: [email protected] and selective methods is required to reach new mea- This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20111701003 EPJ Web of Conferences b) c) Elab = 19.9 MeV E* (MeV) 100 a) 103 49 54 59 64 69 CRC 1n+2n 3 -1 10 2 R 10 σ (mb/sr) 10 69 / Ω Ga σ 68 Bare Ga / d -2 70 σ 1 10 CRC Ga d 10 2 66 (mb) Cu 10 Elab = 19.9 MeV σ 65 Cu 0 -3 68 10 10 E = 30.6 MeV Zn 100 lab 67Cu 3 69 10 CRC 1n+2n Zn -1 101 10 2 R Fusion σ (mb/sr) 10 / -2 3 Neutron transfer Ω 10 10 σ CRC Fusion / d σ 1 d (mb) 10 CRC Transfer 10-3 σ Bare Elab = 30.6 MeV CRC 0 -4 102 10 30 40 50 10 15 20 25 30 35 θ 20 30 40 50 60 70 Lab (deg) θ (deg) ECM (MeV) CM Fig. 1. (a) Heavy residue, fusion, and neutron- transfer excitation functions for the 8He+65Cu system: (upper panel) Measured partial residue cross sections as a function of the center-of-mass energy. The lines are obtained using the statistical model code CASCADE. (lower panel) Fusion and neutron-transfer cross sections for the 8He+65Cu system. Predictions from Coupled Reaction Channels calculations for fusion (continuous lines) and neutron transfer (dashed lines) are also shown. (b) 1n+2n transfer angular distributions at Elab = 19:9 MeV (upper panel) 30.6 MeV (lower panel) along with corresponding CRC calculations for 1n + 2n transfer. (c) Elastic-scattering angular distributions obtained at Elab = 19:9 MeV (upper panel) and 30.6 MeV (lower panel) along with no-coupling (bare) calculations (dotted line) and CRC calculations including couplings to 1n and 2n transfer (full line). surement limits and also to measure and disentangle and 8He+197Au employing two independent setups re- the different reaction processes. spectively using in-beam and off-beam techniques are With the above motivations, two independent ex- presented in the following paragraphs. periments were performed with low intensity radioac- tive ions beams of 8He. The complementarity of in- beam and off-beam sensitive and selective experimen- 2.1 Complete reaction studies above the Coulomb tal techniques was used to obtain integral and differ- barrier in the 8He+65Cu system ential cross sections for elastic scattering, neutron(s) 8 transfer and fusion of the He projectile on two differ- 8 ent targets. These various experimental results com- Reaction mechanisms with He were investigated by means of in-beam measurements performed in the 8He bined with Coupled Reaction Channels calculations 65 and with a phenomenological analysis, represent a sig- + Cu system [10]. This experiment aimed at the nificant step toward a consistent understanding of re- simultaneous measurement of fusion and direct reac- action mechanisms with 8He at energies around the tions (elastic scattering and neutron(s) transfer) at en- Coulomb barrier. These different aspects of the talk ergies above the Coulomb barrier. Simultaneously, the are presented in the following sections. First, the ex- goal was to improve the sensitivity of in-beam mea- perimental results for 8He+65Cu and 8He+197Au sys- surements with low-intensity ISOL beams. The ex- tems are presented respectively in the section Sect. 2.1 perimental setup consisted of 12 Compton suppressed and 2.2. A discussion of these results is presented in Clover γ-ray detectors (EXOGAM array), a Silicon Sect. 3 using Coupled Reaction Channels calculations stripped ∆E-E annular detector and 45 neutron detec- and a systematic study of fusion and transfer excita- tors (Neutron Wall array). At two beam energies (19.9 tion functions in the Helium isotopic chain. and 30.6 MeV), inclusive and exclusive measurements of γ-rays from target-like residues, charged particles (4;6;8He) and neutrons were used to obtain integral fusion and neutron(s) transfer cross sections as well 2 Recent experimental investigations with as differential cross sections for elastic scattering and 8He at GANIL SPIRAL facility neutron(s) transfer. The individual residue cross sections were obtained Radioactive ion beams of 8He were produced by frag- using the corresponding intensities of the well-known, mentation of a 75 MeV/nucleon 13C beam on a thick low-lying γ transitions from the measured inclusive graphite target, then fully purified and re-accelerated γ-ray spectra. Figure 1(a) shows the residue cross sec- by the CIME cyclotron at the SPIRAL facility at tions as a function of the center-of-mass energy for GANIL. The 8He beam, with an energy resolution bet- the 8He+65Cu system. The lines in the figure are the ter than 2×10−3, a beam spot size of ∼ 5 mm diameter results of statistical model calculations for the evap- and an average intensity of (2 − 4) × 105 particles/s. oration residues formed in the decay of 73Ga using Two experiments were performed in the 8He+65Cu the statistical model code CASCADE [11].