Proton scattering on unstable nuclei: study of 40S(p,p') and 43Ar(p,p') reactions, development of detection system MUST

We measured for the first time the elastic and inelastic proton scattering on the 40S unstable nucleus. The experiment was performed in inverse kinematics at the NSCL AT Michigan State University with a 40S secondary beam bombarding a CH2 target at 30 MeV/A. We obtained the elastic scattering angular distribution and two points of the inelastic distribution to the first 2+ excited state found to be located at 860±90 KeV. With a coupled channel analysis, the β2 quadrupolar deformation parameter is found to be equal to 0.35±0.05. This value can be compared to 0.28±0.02 obtained by coulomb excitation. A macroscopic analysis allowed us to extract the neutron and proton transition matrix element ratio Mn/Mp which is equal to 1.88±0.38. This value, greater than N/Z, could indicate an isovector effect in the first 2+ state excitation which could be due to a difference between the neutron and proton vibrations. The microscopic analysis gives the possibility to test the densities and the transition densities to the first 2+ state. The calculated densities for the 40S nucleus show a neutron skin. However the microscopic analysis yields a Mn/Mp ratio of 1.40±0.20. A similar elastic and inelastic proton scattering experiment allowed us to get a deformation parameter of 0.25±0.03 for the 43Ar nucleus. To develop the study of direct reactions induced by radioactive beams at GANIL, we have developed and built, in collaboration with the CEA-Saclay and the CEA-Bruyeres, the new detector MUST.It is based on the silicon strip technology, and is dedicated to the measurement of recoiling light particles emitted in these reactions. The results obtained with a 40Ar beam at 77 Me V/A, have shown the good performances of the detector for the particle identification as well as for the resolutions, and allow us to consider now a large experimental programme concerning these direct reactions induced by radioactive beams. These tests allowed us to establish a reference procedure to calibrate the detector and to analyse the data. We have also developed a simulation code in order to determine the best use conditions of MUST in the future experiments. (author)