RUI: Shell Structure and Core Polarization in Exotic Nuclei

Ursinus College, Collegeville PA

Investigators

Abstract

It is a well-known fact that nuclei are made up of protons and neutrons. One of the big questions in nuclear science, as identified by the Nuclear Science Advisory Committee, is how nuclei are organized (in terms of the forces between neutrons and protons) as the number of neutrons is increased. The goal of this proposal is to understand the evolution of the structure of neutron-rich nuclei, with a focus on isotopes with 28 neutrons, such as the nucleus Ca-48 (that also has 20 protons). Such nuclei are known to have unusual stability against radioactive decay, but become less stable for isotopes where the proton number is decreased. The experiments, which involve collisions of protons with selected nuclei, will be done at the National Superconducting Cyclotron Laboratory (NSCL). Undergraduate students will participant in the experiments and will gain practical skills in data acquisition, analysis and simulation, and present their work at conferences. The proposed inverse-kinematics proton scattering measurements probe the interplay between collective and single-particle behavior by disentangling the neutron and proton contributions to collective excitations in exotic nuclei. The mechanism(s) driving the erosion of the N = 28 shell approaching Z = 14 is a topic of great interest. It has been suggested that protons drive the onset of collectivity in this region. The experimental determination of the relative contributions of neutrons and protons to collective excitations will provide important insight. Measurements of isotopes S-44 and Si-42 will complete the empirical picture of the relative contributions of neutrons and protons to the first 2+ excitations in the N=28 isotones below Ca-48 down to Z=14, where the N=28 shell collapses. Measurements of isotopes P-41 and P-43 will clarify the degree of coupling of the odd proton to the collective first 2+ excitations of the Si-40 and Si-42 "cores" in these nuclei.