Speaker
Description
The semi-magic Sn nuclei, extending beyond the $N=50$ and $N=82$ shell closures, present one of the most-studied isotopic chains on the nuclear chart. $^{118}_{50}$Sn$_{68}$ and $^{120}_{50}$Sn$_{70}$ lie in the neutron mid-shell, where shape coexistence was proposed with the signature of deformed excited $0^+$ states intruding into the seniority-like spherical yrast bands. However, transition strengths studies were hindered because only limits were available in the literature on the lifetimes of the excited $0^+_3$ states. Notably, the lack of electric monopole strengths between the $0^+_3$ and $0^+_2$ states, $\rho^2(E0;0^+_3\rightarrow0^+_2)$, obscured the shape difference and mixing amplitudes between the excited $0^+$ states.
These $0^+_3$ lifetimes were recently measured for the first time in a thermal-neutron capture experiment at the Institut Laue-Langevin. The world's highest-flux thermal neutron beam of $10^8$~neutrons/cm$^2$/s was delivered onto enriched $^{117}$Sn and $^{119}$Sn targets, respectively. Low-spin states in $^{118,120}$Sn were populated up to the $\approx 9$-MeV neutron separation energies, and the decaying gamma-ray cascades were detected with the Fission Product Prompt Gamma-ray Spectrometer (FIPPS) comprised of eight Compton-suppressed HPGe clovers coupled to an array of 15 LaBr$_3$ fast scintillation detectors.
In total, $\approx 4\times10^9$ counts were recorded in the $\gamma\gamma\gamma$ cube for each isotope, where two LaBr$_3$ events were in coincidence with one HPGe.
Monopole transition strengths from the lifetime measurements for the $0^+_3$ states in $^{118,120}$Sn will be presented along with theoretical interpretations employing MR-CDFT calculations without adjustable parameters.