Final PhD Seminar

Electromagnetic Moments and Emerging Nuclear Collectivity near Z=50

Mr Timothy Gray
Department of Nuclear Physics

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. An overview will be given and four examples of experimental studies in the Z=50 region presented. A Coulomb excitation experiment of radioactive 129Sb allows the measurement of B(E2) excitation strengths, which are 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, and providing valuable insight into the emergence of collectivity. A second experiment studying beta decay into 138Xe aims to measure the magnetic moment of the first-excited 2+ state, and provides a variety of spectroscopic information: gamma-gamma angular correlations, and extensions to the established level scheme. A candidate for the mixed-symmetry 2+ is proposed as the third-excited 2+ 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: the first on 10+ isomeric state in radioactive 130Sn. Whilst this was a low-statistics measurement, the result of g(10+) = -0.31(2) is  unexpectedly large in magnitude. This provides an opportunity to probe core- polarisation effects near 132Sn. The second TDPAD measurement is 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 neutron h11/2 configuration. Possible explanations for g factor are explored, including shape change in Sn isotopes around N=60 and proton excitations across the Z=50 shell closure.

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