Experimental study of shape coexistence in 189Tl

The shape coexistence phenomena in Tl-189 are investigated by measuring the lifetimes of the high spin states through the recoil distance LIFETIME measurement technique. For this study, the Ho-165(Si-28, 4n)Tl-189 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 similar to 2.6 e b. To better understand the shape coexistence phenomena in Tl-189 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 Tl-189 nucleus in the positive parity pi i(13/2) configuration.