Étude du mécanisme de la réaction ( 3He, α)à217 (205) MeV et de l'excitation d'état de trous dans les couches profondes

Abstract

The ( 3He, α) reaction on 12C, 16O, 28Si, 58Ni, 90Zr, 118, 124Sn and 208Pb targets has been studied at E 3He = 217 Me V(or205 MeV) in order to investigate the reaction mechanism at high energy and large momentum transfer. The reaction yields large cross sections at very forward angles and strongly enhances the largest orbital momentum transfers. The main mechanism is direct pick-up although two-step processes are quite important for some transitions. The alpha optical potential choice is of critical importance to reproduce the pick-up angular distributions shapes in the framework of the DWBA-ZR analysis. Very good fits are obtained for the studied nuclei and l-transfers with a unique empirical potential: if V α( E inc) = V 3He(3/4 E inc) + V n(1/4 E inc) . The excitation energy spectra have been measured up to 100 MeV in the residual light and medium nuclei, and up to about 16 MeV in heavy nuclei. A survey of the main transitions is given. In addition to the generally well-known low-lying levels, peaks or broad structures are observed for each nucleus at higher excitation energies. They are attributed to pick-up from inner shells: 1s( 11C and 15O), 1p( 27Si), 1d 5/2 + 1p( 57Ni), 1f 7/2( 89Zr), 1g 9/2( 117, 123Sn)and1h 11/2( 207Pb). The corresponding strengths are evaluated.