Proton scattering on radioactive proton sd-shell nuclei and comparison to Coulomb excitation

Abstract

Systematic measurements of the first excited states in neutron-rich nuclei in the π(sd)-shell have been performed in the last two years using the technique of intermediate energy Coulomb excitation. These experiments were designed to study the evolution of nuclear shell structure as the neutron-dripline is approached and to provide experimental data which can be easily compared to nuclear model predictions. These measurements indicated a breakdown of the N=20 shell closure for 32Mg and a weakening of the N=28 shell closure below 48Ca, as well as a region of moderate collectivity around 40,42S. However, this experimental technique is sensitive only to the electromagnetic response of the nucleus. To probe the neutron distributions we performed a series of proton scattering experiments in inverse kinematics on neutronrich sulfur and argon isotopes. Proton scattering at energies of about 30 MeV/nucleon is more sensitive to neutrons than to protons by about a factor of three. The combination of the proton scattering results with the Coulomb excitation measurements allows a determination of the ratio of the neutron transition matrix element Mn to the proton transition matrix element Mp. Deviations from the homogeneous collective model expectation Mn/Mp=N/Z can be indicative of isovector deformation predicted to occur for neutron-rich sulfur isotopes. Proton scattering experiments and Coulomb excitation experiments on the same isotopes have been performed at the NSCL with radioactive beams produced in the A1200 fragment separator and protons detected in an array of Si strip–Si-CsI telescopes.