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 LaBr$_3$ detectors for gamma-ray detection to measure state lifetimes and probe nuclear shapes in $^{188}$Pt and $^{190}$Pt. These nuclides lie at the boundary between the stable platinum isotopes that may have triaxial or $\gamma$-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 $^{188}$Pt and $^{190}$Pt, populated through $^{176}$Yb($^{16}$O,$4n$) and $^{176}$Yb($^{18}$O,$4n$) 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 $^{190}$Pt is confirmed, a revision of the $2_{1}^+$ lifetime for $^{188}$Pt is suggested. These new results align with other observations of the evolving nuclear behaviour with increasing mass number. However, an evaluation of the $B(E2; 2{_{1}^+}\rightarrow 0{_{1}^+})$ 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 $^{186,188,190}$Pt. The results for high-spin states in $^{188}$Pt and $^{190}$Pt 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 $^{190}$Pt, but differ for $^{188}$Pt. Additionally, new measurements of decay branching ratios, derived from high-quality $\gamma$-$\gamma$ coincidence data, yield updated transition strengths that can be used in the future to challenge existing interpretations of the nuclear structure.