Investigation of a large change in deformation for the γ-soft nucleus Sm136

Lifetimes of excited states of the ground-state rotational band in the $^{136}\mathrm{Sm}$ nucleus have been measured up to ${I}^{\ensuremath{\pi}}={20}^{+}$ using the Doppler-shift attenuation method. The states were populated in the reaction $^{107}\mathrm{Ag}$($^{32}\mathrm{S}$, $1p2n$) at 145-MeV beam energy and the $\ensuremath{\gamma}$ rays emitted from the excited states were detected using the Indian National Gamma Array at the Tata Institute of Fundamental Research, Mumbai. The extracted transitional quadrupole moments indicate a reduction of collectivity with increasing spin after the band crossing. The results have been compared with the predictions of the cranked shell model as well as the triaxial projected shell-model calculations and indicate that the nucleus evolves from prolate $\ensuremath{\gamma}$-soft to a stable triaxially deformed shape after the first and the second crossing involving $\ensuremath{\pi}{h}_{11/2}^{2}$ and $\ensuremath{\pi}{h}_{11/2}^{2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}^{2}$ configurations, respectively.