Electromagnetic Moments and Emerging Nuclear Collectivity near Z=50

Abstract

The isotope 132Sn appears to be one of the most robust examples of a double shell closure. As a result, the surrounding region provides excellent opportunities to test shell-model predictions and investigate the emergence of collectivity as protons and neutrons are added to or removed from 132Sn. In this thesis emerging collectivity is investigated through indications of wave function fragmentation, enhanced B(E2) strengths, angular momentum sharing between protons and neutrons, the onset of deformation, and the weakening of shell closures. Both static and dynamic electromagnetic moments are used to identify examples of these phenomena in nuclei at or near Z=50.

B(E2) excitation strengths were measured by the Coulomb excitation of radioactive 129Sb. B(E2) values were compared against particle-core coupling and state-of-the-art shell-model calculations. The results indicate a significant enhancement in the quadrupole excitation, in stark contrast to the predictions of the long standing particle-core coupling scheme --- this is a strong experimental fingerprint of emerging collectivity. A second experiment studying beta decay into 138Xe aimed to measure the magnetic moment of the first-excited 2+ state, which is sensitive to the proton versus neutron balance --- a key indicator of emerging collectivity. The experiment also provided a variety of spectroscopic information: gamma-gamma angular correlations, and extensions to the established level scheme. A candidate for the mixed-symmetry 2+ level is proposed as the 2_3+ state, and the Kumar-Cline sum rules are applied to shell-model calculations of N=84 isotones to investigate how nuclear shapes develop with emerging collectivity. Two time-differential perturbed angular distribution (TDPAD) measurements are presented for h_11/2 isomeric states in Sn isotopes. The g factors measured allow us to probe configuration mixing and core-polarization effects across the Sn isotopic chain. The first was on the 10+ isomeric state in radioactive 130Sn. While this was a low-statistics measurement, the result of g(10+) = -0.31(2) is unexpectedly large in magnitude. The second TDPAD measurement was on the 11/2- isomer in 111Sn. The measured value of g = -0.214(4) deviates significantly from the (quenched) Schmidt value of g = -0.243 expected for a pure vh_11/2 configuration. Possible explanations for this g factor are explored, including shape change in Sn isotopes around N=60 and proton excitations across the Z=50 shell closure. This result provides evidence for increased g_9/2 holes in the vicinity of 111Sn, an indication of the Z=50 shell weakening --- another sign of emerging collectivity.