Fast lifetime measurements as a probe of nuclear shapes in 188Pt and 190Pt

Abstract

This thesis presents research on the application of fast-timing techniques with LaBr3 detectors for gamma-ray detection to measure state lifetimes and probe nuclear shapes in 188Pt and 190Pt. These nuclides lie at the boundary between the stable platinum isotopes that may have triaxial or γ-soft shapes, and the lighter isotopes that show evidence of shape coexistence. This study presents the measured lifetimes of the first 2+, 5−, 7− and 12+ states in 188Pt and 190Pt, populated through 176Yb(16O,4n) and 176Yb(18O,4n) reactions, respectively. The Generalised Centroid Difference method was employed to determine the 2+ and 5− state lifetimes. This required knowledge of the Prompt Response Difference (PRD), which was deduced using a novel 3D fitting method that enables deduction of the PRD without the requirement generally used in the literature of manually adjusting data points to have a common decay energy reference. The 12+ and 7− state lifetimes were determined by fitting time-difference spectra with a decaying exponential convolved with a Gaussian prompt response. While the evaluated 2+1 lifetime for 190Pt is confirmed, a revision of the 2+1 lifetime for 188Pt is suggested. These new results align with other observations of the evolving nuclear behaviour with increasing mass number. However, an evaluation of the B(E2; 2+1 → 0+1 ) strength across the mass chain suggests that remeasurements of 2+1 lifetimes in lighter platinum nuclei are required. Interpretation of the structural and shape changes in these nuclei is further investigated through General-Collective-Model calculations for 186,188,190Pt. The results for high-spin states in 188Pt and 190Pt highlight significant discrepancies with previous literature, particularly for the lifetimes of the 12+ and 7− states. Updated values show closer alignment with more recent measurements for 190Pt, but differ for 188Pt. Additionally, new measurements of decay branching ratios, derived from high-quality γ-γ coincidence data, yield updated transition strengths that can be used in the future to challenge existing interpretations of the nuclear structure.