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 $\pi h_{9/2}$ band and seven levels of the positive parity $\pi i_{13/2}$ band are found. The extracted transition quadrupole moments for the positive parity $\pi i_{13/2}$ 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 $\pi h_{9/2}$ band shows a stable oblate structure against the increasing spin with an average quadrupole moment ($Q_t$) of ∼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 $\pi i_{13/2}$ and the negative parity $\pi h_{9/2}$ bands in this nucleus. The comparison confirms the shape coexistence structure of the ${}^{189}\mathrm{Tl}$ nucleus in the positive parity $\pi i_{13/2}$ configuration.

• Received 1 February 2006

DOI:https://doi.org/10.1103/PhysRevC.75.054323