Abstract
Previously unobserved isomeric states in 188Ta,190W, 192Re, 193Re, 195Os, 197Ir, 198Ir, 200Pt, 201Pt, 202Pt and 203Au, with half-lives ranging from 10 ns to 290 mus have been populated and studied using a fragmentation reaction in conjunction with a forward focussing spectrometer. In most cases, this provided the first ever spectroscopic data made available for the nucleus, and 200Pt presented the first new seniority 4 state, on the basis of gamma-gamma coincidences, following a fragmentation reaction. Half-lives have been measured and tentative level schemes have been drawn for each isomer, spins and parities being consistent with blocked BCS calculations, hindrance factors, systematics and the relative intensities of gamma-rays and X-rays (where possible). Isomeric ratios have been measured, values ranging from 1 % to 64 %. Potential Energy Surface calculations were performed in parallel to the blocked BCS calculations, in order to provide deformation parameters, excitation energies and quasiparticle configurations. Ground state (or lowest level) shape calculations reveal a change from axially symmetric, through triaxial, to spherical shapes across the data set, from 188Ta to 203Au, as 208Pb approached. Weisskopf hin-drance factors provide evidence for the erosion of the goodness of the K-quantum number, compatible with soft or axially symmetric shapes. The prolate-oblate phase transition region, with respect to tungsten, osmium and platinum, shows 195Os to be the pivotal nucleus in the osmium isotopic chain, with a calculated triaxial ground state. On comparison with the systematics of the region, results obtained for 190W show evidence for a Z = 74 sub-shell closure, analogous to that at Z = 64. Finally, new isotopes, 167Tb, 170Dy and 199Ir were discovered.