Nuclear structure below tin-132: An investigation of neutron-rich nuclides via beta and isomeric decay.

摘要

Almost everything that is known about nuclear shell structure has been derived from experimental work on stable nuclides or nuclides very close to stability. This is largely a consequence of accessibility---historically the means did not exist to produce and study exotic nuclides very far from stability. The well-known magic proton and neutron numbers have been validated, in certain regions, as experiment has pushed further out, but it should not be assumed that the magic numbers will remain magic in nuclides with extreme ratios of neutrons to protons. In fact, the smaller neutron magic numbers ( i.e. 8, 20) have already been observed to disappear in some neutron-rich nuclides [1-3].; The effect of the reduction, or quenching, of neutron shell gaps in neutron-rich nuclides has been known in theoretical calculations since the late 1970s [4]. The consequence of neutron shell quenching in neutron-rich nuclides in an astrophysical context has been used in attempts to understand the significant departures of calculated rapid neutron-capture process (r-process) yields from the observed solar r-process abundances. Since the theoretical phenomenon of quenching is strongly model-dependent, unambiguous experimental indicators of the shell structure of very neutron-rich nuclides are important. The results of recent experiments on the nuclide C13048d82 [5,6] have been interpreted as evidence of a weakening of the N = 82 shell closure just below Z = 50.; This thesis describes an investigation of the experimental signatures for the Bryan Earl Tomlin persistence of the N = 82 shell closure, or alternatively the emergence of N = 82 shell quenching, for neutron-rich 46Pd, 47Ag, and 48Cd nuclides.; An experiment was performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University to study the low-energy structure of neutron-rich transition-metal nuclides with 44 Z 50 and N 82 in the region near doubly-magic S13250n82 . Exotic nuclides were produced by projectile fragmentation of a 136Xe49+ beam at 120 MeV/nucleon. The NSCL Beta Counting System (BCS), employing a double-sided Si strip detector, was used to identify secondary beam fragments and correlate implantation events with subsequent beta-decay events, on an event-by-event basis [7]. In addition to the BCS, twelve auxiliary HpGe gamma-ray detectors were employed to measure both beta-delayed gamma rays, as well as, prompt gamma rays emitted following isomeric decay.; New spectroscopic data were obtained for neutron-rich isotopes of 44Ru, 45Rh, 46Pd, 47Ag, 48Cd, and 49In. These new data include isomeric-transition and beta-decay half-lives and gamma-ray energies and relative intensities. Deduced level schemes were used to extend the systematics of Pd, Ag and Cd isotopes to higher mass numbers. In this work, the neutron-rich even-even 46Pd E( 2+1 ) and E( 4+1 ) systematics were extended up to 120Pd74, and no evidence of a reduced N = 82 shell gap in this isotopic series was found. Additionally, the partial level schemes that were deduced for 123,125Ag and 125-127Cd have been interpreted as demonstrating single-particle character, indicative of intact Z = 50 and N = 82 shell gaps.