Magnetic dipole bands in Hg-190: first evidence of excitations across the Z=82 sub-shell in Hg nuclei

An experiment aimed at studying high-spin states in Hg-190 was performed wi th the Eurogam II array. The data have revealed the presence of cascades of magnetic dipole transitions with some unexpected properties. Unlike the M1 bands previously observed in the heavier tig isotopes, these structures ha ve extremely large B(M1)/B(E2) ratios. The observation of a third dipole ba nd with much lower B(M1)/B(E2) values in the same spin/excitation energy re gime suggests that the bands may represent configurations occurring in diff erent minima in the potential energy surface. Configuration-dependent Crank ed Nilsson-Strutinsky calculations predict the presence of a minimum in the nuclear potential energy surface at a deformation of epsilon approximate t o 0.2, gamma approximate to -90 degrees. occurring when a proton is excited across the Z = 82 shell-gap into an h(9/2) orbital. It is suggested that t he bands exhibiting anomalously large B(MI)/B(E2) ratios may be associated with this minimum. (C) 2001 Published by Elsevier Science B.V.