Monday, October 27, 2014

Intermediate-energy Coulomb excitation of neutron-rich chromium isotopes

Travis Baugher (Rutgers)

In the nuclear shell model, the magic numbers, caused by large gaps in the nuclear energy levels, are well-established for nuclei near stability, but have been observed to change in the exotic regime. Traditional shell gaps can be reduced or disappear altogether while new ones can emerge. This can be explained by shifts in the single-particle energy levels due to the monopole components of the proton-neutron tensor interaction, for example. This shell evolution can lead to new regions of deformation and rapidly changing nuclear structure far from stability. The region below 68Ni has been of interest recently since the enhanced 2+ energy and small quadrupole transition probability in 68Ni suggested the possibility of an N=40 sub-shell gap, while nearby the iron and chromium isotopes were observed to be collective approaching N=40. Recent experiments using the National Superconducting Cyclotron Laboratory’s highly-segmented HPGe array, SeGA [1], and the 192-element high-efficiency CsI(Na) scintillator array, CAESAR [2], combined with detailed simulations, have quantified the quadrupole collectivity in the iron isotopes out to N=42 and in the chromium isotopes out to N=40 [3,4]. The results pose sensitive benchmarks for state-of-the-art large-scale shell-model calculations and a recent effective interaction developed for this region and emphasize the importance of the 0g9/2 and 1d5/2 neutron orbitals beyond the N=40 sub-shell gap for describing nuclear structure in these isotopes.