SHAPE COEXISTENCE IN TL-185 AND TL-187 - INVESTIGATION OF THE DEFORMED MINIMA

High spin gamma-ray spectroscopy of 185Tl and 187Tl has been performed with the reactions Gd-154(Cl-35, 4n) and Tb-159(S-32, 4n). Positive g amma-ray identification with the thallium isotopes was made via X-ray coincidences, and supported by mass selected gamma-ray spectra, the la tter obtained with the reactions Gd-154(Ar-36, p4n) and Gd-155(Ar-36, p3n). Rotational bands associated with both prolate and oblate shape w ere observed. The bandheads of the proposed oblate 13/2+ [606] states were found to be isomeric, with meanlives of 12 +/- 2 ns in Tl185 and 1.0 +/- 0.2 ns in Tl187. Prolate deformed i13/2 bands were observed in both nuclei, while in Tl187, bands due to h9/2 and f7/2 protons coupl ed to the prolate shape are also assigned. An h9/2 band is tentatively assigned in Tl185. The observation of these rotation-aligned bands at low excitation energy implies that the development of prolate deforme d minima in the odd nuclei is not necessarily blocked by occupation of a single deformation-driving orbital. Equilibrium deformation calcula tions for intrinsic states in a range of thallium nuclei are presented . Experimental trends with mass number are reproduced, but absolute ex citation energies, and energy differences between the prolate and obla te states are not, continuing the persistent discrepancy between theor y and experiment in the mercury region. Theoretical calculations of in truder orbital occupation probabilities show a correlation between pro late deformation and h9/2 and f7/2 proton pair population, in particul ar of the 1/2- [541] orbital from the h9/2 proton shell. They also sho w that blocking of the 1/2- [541] orbital significantly suppresses the prolate deformation. Implications for the structure of the prolate de formed mercury and thallium isotopes are considered, leading to the co nclusion that the prolate mercury core nuclei consist of a mixture of low-OMEGA proton intruder excitations.