Shape coexistence in the neutron-deficient isotope 187Tl

Abstract

A triple shape coexistence manifests in 188Pb via the presence of three characteristic isomers, and the observation of two isomers in 187Tl with microsecond lifetimes and energies comparable to those in 188Pb was the main motivation behind this new study. A heavy-ion fusion evaporation reaction involving a nanosecond-pulsed 154 MeV 32S beam incident on a 159Tb target was used to populate excited states in 187Tl, observed via time-correlated gamma-ray spectroscopy with Gammasphere. Subsequent analysis resulted in a greatly extended level scheme for 187Tl, including the verification of the 11/2-[505] structure, the observation of the unfavoured signature of the h9/2 prolate band, and possible evidence of enhanced deformation structures associated with the prolate i13/2 band. New decay paths from the two microsecond isomers in 187Tl have been found, but are in one case still incomplete, so that no firm assignment on their configurations can be made. The best possible configuration for either of the isomers is p11/2-[505] x2n{7/2-[514] x 9/2+[624]}, being a combination of the known 8- isomer identified in 188Pb with the 11/2-[505] proton, that is a low-lying prolate structure in 187Tl. Potential energy surface calculations suggest that the nominally prolate single-particle states are triaxial, and there is some evidence to support the triaxiality of the 11/2-[505] state in terms of a signature splitting that decreases at the i13/2 neutron alignment. The proposed configurations for the isomers were also calculated to be triaxial, and an interesting example of shape coexistence between oblate, prolate, and triaxial shapes in 187Tl may have been observed.