Shape coexistence in ¹⁸⁵Tl and ¹⁸⁷Tl - investigation of the deformed minima

Abstract

High spin gamma-ray spectroscopy of 185Tl and 187Tl has been performed with the reactions 154Gd(35Cl, 4n) and 159Tb(32S, 4n). Positive γ-ray identification with the thallium isotopes was made via X-ray coincidences, and supported by mass selected γ-ray spectra, the latter obtained with the reactions 154Gd(36Ar, p4n) and 155Gd(36Ar, p3n). Rotational bands associated with both prolate and oblate shape were observed. The bandheads of the proposed oblate 13 2+[606] states were found to be isomeric, with meanlives of 12 ± 2 ns in 185Tl and 1.0 ± 0.2 ns in 187Tl. Prolate deformed i 13 2 bands were observed in both nuclei, while in 187Tl, bands due to h 9 2 and f 7 2 protons coupled to the prolate shape are also assigned. An h 9 2 band is tentatively assigned in 185Tl. The observation of these rotation-aligned bands at low excitation energy implies that the development of prolate deformed minima in the odd nuclei is not necessarily blocked by occupation of a single deformation-driving orbital. Equilibrium deformation calculations for intrinsic states in a range of thallium nuclei are presented. Experimental trends with mass number are reproduced, but absolute excitation energies, and energy differences between the prolate and oblate states are not, continuing the persistent discrepancy between theory and experiment in the mercury region. Theoretical calculations of intruder orbital occupation probabilities show a correlation between prolate deformation and h 9 2 and f 7 2 proton pair population, in particular of the 1 2-[541] orbital from the h 9 2 proton shell. They also show that blocking of the 1 2-[541] orbital significantly suppresses the prolate deformation. Implications for the structure of the prolate deformed mercury and thallium isotopes are considered, leading to the conclusion that the prolate mercury core nuclei consist of a mixture of low-Ω proton intruder excitations.