Resonances of 6 He via the 8 He ( p , t ) 6 He reaction

We investigated the low-lying spectroscopy of He via the 2-neutron transfer reaction induced by the He SPIRAL beam at 15.4 A.MeV on a proton-rich target. The light charged recoil particles produced by the direct reactions were measured using the MUST2 Si-strip telescope array. Two new resonances were observed above the known 2 state in He, and the angular momentum transfer was deduced through the analysis of the angular distributions. Results are discussed in comparison with the recent calculations of various nuclear structure theories which include the coupling to the continuum technique and to the ones which give an understanding of the cluster correlations in the light weakly-bound nuclei. 1 Motivations and experimental probe New interesting phenomena have been observed close to the neutron drip-line, such as halos or neutron skins, and low-lying resonant states. Progress have been reported in the understanding of these exotic structures, related to the weak binding energies of the neutron-rich light nuclei: they can be explained by the interplay between mean field and shell model configurations with cluster structures [1]. However, the nuclear models disagree for the predictions of the spectroscopy of nuclei far away from the valley of stability, due to the different assumptions on nucleon-nucleon interactions, on few-body and pairing correlations, and to our lack of knowledge on crucial microscopic inputs in the nuclear forces, like the expression of the isospin-dependent terms and of the spin-orbit force. The existence and position of resonant states in the light exotic nuclei may provide crucial information to constrain the models and be used benchmarks to test their validity. For near-drip-line nuclei, like 6,8He or O, the theories all predict resonant states in the low-lying excitation energy region (below 12 MeV). The characteristics of these states, position, width, spin and parity, have not been firmly established and sometimes they have not been clearly observed. In this context, the region of the light nuclei for Z ≤ 8 is particularly attracting: – the nuclear structure offers a rich variety of phenomena related to weakly-bound structures, alphacluster like, extended halo or skin structures [1–4], changes in the shell structure are also observed as compared to the shell ordering in the stable nuclei; – technically, for the systems with a small number of nucleons A ≤ 12, it is feasible for the theories including the so-called ab-initio or realistic interactions (with 2and 3-body terms) to carry out analytical calculations and to obtain accurate results within the Green Function Monte Carlo Method (GFMC) [5]. The No-Core Shell Model (NCSM) calculations can be applied to the light nuclei, including different sets of realistic effective interactions. The model was recently extended to include 3-body interaction for the calculations of the low-lying spectra of the p-shell nuclei [6]; ∗contact e-mail:[email protected] in 2p 300 44 88 24 , v er si on 1 20 J an 2 01 0 Author manuscript, published in "12th International Conference on Nuclear Reaction Mechanisms, Varenna : Italy (2009)"