Beta-decay studies of neutron-rich nuclides and the possibility of an N = 34 subshell closure

An inspection of periodic trends in ionization energies is one manifestation of electronic shells in atoms. Analogously, trends observed in nuclear masses led to the development of the nuclear shell model. Areas of increased stability at the traditional magic nucleon numbers 2, 8, 20, 28, 50, 82, and 126 have been associated with large energy gaps encountered in the filling of shell-model orbitals. The reordering of levels within the shell model is an important question in nuclear structure studies as it may lead to the development of new magic numbers far from stability.; The experimental characterization of the N = 32 subshell closure in Cr, Ti, and Ca isotopes stimulated shell model calculations, using a new effective interaction labeled GXPF1 [1], that also indicated the possibility of an N = 34 subshell closure in the 22Ti and 20Ca isotopes. The N = 32 and suspected N = 34 subshell closures were attributed to a strong proton-neutron monopole migration of the nuf5/2 state as protons are removed from the pif7/2 level. One indication for the subshell closure at N = 34, predicted by GXPF1 calculations, is an energy of ∼1500 keV for the first excited 2+1 state in 56Ti, similar to the energy of the 2+1 state in 56Ti at the N = 32 subshell closure.; beta decay studies have been performed at the National Superconducting Cyclotron Laboratory on neutron-rich pf-shell nuclides to investigate the possibility of a subshell closure at N = 34. The nuclides that were studied included 56Sc, 57Ti, 58,59V, and 60Cr and were produced through the fragmentation of a 140 MeV/nucleon 86Kr beam on a 9Be target. The nuclides were implanted into a Double-sided Si Strip Detector which was used to detect both implanted ions and decays. Decays were correlated with implanted ions on an event-by-event basis and gamma rays were monitored with 12 detectors from the Segmented Germanium Array. For all nuclides studied, half-lives and level schemes were deduced.; The experimental study focused on the beta decay of 56Sc, to determine the energy of the 2+1 state in T5622i34 . A value of 1129 keV was determined for the energy of the 2 + → 0+ transition in 56Ti, 400 keV lower than predicted, suggesting the absence of a shell closure in the 22Ti isotopes. The ground state Jpi assignments for 56Sc and 57Ti along with other nuclides around the N = 32 subshell closure were used to infer the migration of the neutron f5/2 state with the removal of protons from the f7/2 level using an extreme single-particle model. While the migration of the f5/2 agrees qualitatively with calculations, the increase in energy of the nuf 5/2 with the removal of protons from the f7/2 seems to be overestimated in GXPF1. However, there still exists the possibility that with the removal of the last two protons from the f7/2 state, the N = 34 subshell closure develops.