Experimental study of shape coexistence inTl189

Abstract

The shape coexistence phenomena in $^{189}\mathrm{Tl}$ are investigated by measuring the lifetimes of the high spin states through the recoil distance LIFETIME measurement technique. For this study, the $^{165}\mathrm{Ho}$($^{28}\mathrm{Si}$, $4n)$$^{189}\mathrm{Tl}$ reaction at a beam energy of 138 MeV was used. In this measurement, the lifetimes of four levels of the negative parity $\ensuremath{\pi}{h}_{9/2}$ band and seven levels of the positive parity $\ensuremath{\pi}{i}_{13/2}$ band are found. The extracted transition quadrupole moments for the positive parity $\ensuremath{\pi}{i}_{13/2}$ band show interesting changes with increasing level spin. This band starts with a small oblate shape (${Q}_{t}=1.1$ $e$ b) at low spin ($17/{2}^{+}$) and attains a strongly deformed prolate shape (${Q}_{t}=8.8$ $e$ b) at high spins ($33/{2}^{+}$). On the other hand, the negative parity $\ensuremath{\pi}{h}_{9/2}$ band shows a stable oblate structure against the increasing spin with an average quadrupole moment (${Q}_{t}$) of \ensuremath{\sim}2.6 $e$ b. To better understand the shape coexistence phenomena in $^{189}\mathrm{Tl}$ nucleus, the experimental results are also compared with the results of the total Routhian surfaces calculations, for the positive parity $\ensuremath{\pi}{i}_{13/2}$ and the negative parity $\ensuremath{\pi}{h}_{9/2}$ bands in this nucleus. The comparison confirms the shape coexistence structure of the $^{189}\mathrm{Tl}$ nucleus in the positive parity $\ensuremath{\pi}{i}_{13/2}$ configuration.