Lifetime measurements of the 2^{+}_{1} states in even-even platinum isotopes with A = 180-186

Abstract

The neutron-deficient even-mass platinum isotopes have long been of interest for their transitional shapes and the emergence of shape coexistence. The more neutron-rich ^{190-198}Pt isotopes have weakly deformed, oblate structures in their ground states and show some evidence of triaxiality. In contrast, the lighter mid-shell isotopes ^{178-186}Pt exhibit prolate ground states and clear evidence of shape coexistence, with ^{188}Pt as a transition point between two regions. These platinum isotopes present a notably complex evolution of shapes and shape coexistence that would benefit from further investigation.

The lifetimes of 2^+_1 states and the B(E2; 2^+_1\rightarrow0^+_1) reduced transition strengths are valuable to give insight into the magnitude of the nuclear deformations and their shape. However, there are significant discrepancies in 2^+_1 lifetimes for the shape-coexistence region, ^{180-186}Pt, from the Evaluated Nuclear Structure Data File (ENSDF) database and more recent measurements using methods such as the recoil distance method (RDM) and the Generalised Centroid Difference (GCD) method.

To address this puzzle, we performed direct-timing lifetime measurements of the 2^+_1 states in ^{180-186}Pt using exponential-convoluted Gaussian fitting and GCD methods applied to \gamma-fast timing data taken with LaBr_3(Ce) detectors at the Australian Heavy Ion Accelerator Facility (HIAF) in August and September 2023. The 2^+_1 lifetimes obtained from both the convolution and GCD methods are in agreement, yielding results of 533(26)~ps, 561(14)~ps, 479(6)~ps and 350(4)~ps for ^{180-186}Pt, respectively. The measurements for ^{180}Pt and ^{186}Pt in the present study are consistent with ENSDF, and those for ^{182}Pt align with a recent result. However, a discrepancy is observed between our measurement and the ENSDF data for ^{184}Pt, suggesting to revisit the lifetime measurement for this isotope.

Our experimental values were then compared to the results obtained from an empirical multi-band-mixing model to clarify the underlying shape assignments. The calculation considering shape coexistence reproduced the experimental B(E2; 2^+_1\rightarrow0^+_1) values well, thereby further emphasising the presence of shape coexistence in these regions.

A new passive-shielding array for LaBr_3 detectors for future lifetime measurements has been designed, built and incorporated into the CAESAR gamma-ray detector array. The first test experiments with the new shields were performed in October 2025, and the results are currently being analysed. Show more