Abstract
Several multi-quasiparticle isomers have been observed for the first time in a gamma-spectroscopy experiment performed at the Australian National University Heavy Ion Accelerator Facility. A fusion-evaporation reaction using a 58 MeV, 9Be beam, incident on a 160Gd enriched target was used to populate nuclei in the vicinity of 164Er. Of specific interest was a known four quasiparticle (4QP) isomer in 164Er with an uncertain half-life. A measurement of the half-life, and the observation of high DeltaK branches from the isomer was anticipated to extend our understanding of 4QP isomer decays outside of the A≈180 isomer group, establishing whether the models derived for this region are applicable at A=164. A pulsed beam provided ideal conditions for the half-life measurement of all gamma-decaying levels observed using the CAESAR array, which consists of nine HPGe detectors and two LEPS detectors. Twelve different isotopes with observable gamma-ray transitions were detected. Of these, five are observed to possess new isomers: 161Dy, 162Dy, 163Ho 162Er and 165Er. Half-life measurements and structural assignments for the new isomers are presented in this work. The 162Dy isomer, a 2QP structure decaying by a highly forbidden E2 transition, could be compared with other 2QP isomers that share the same decay characteristics. The 162Dy isomer agrees well with systematics correlating the reduced hindrance with the product of the valence neutron and proton numbers (NpNn) over an extended N, Z range. Small deviations from NpNn dependence are analysed for the 2QP isomer decays, and interpreted as arising from a weak dependence on the isomer excitation energy relative to the yrast line. A precise measurement of the 4QP, 164Er isomer's half-life is made, and DeltaK=12 branches are found decaying to the ground state band. The reduced hindrance of these branches could be compared with a plethora of other 4QP and 5QP isomers that share the same decay characteristics. The 164Er isomer agrees well with systematics correlating the reduced hindrance with the isomer excitation energy relative to the yrast line, a model that previous to this experiment had only been applied to the A≈180 isomer group. An analysis yielding an estimate for the K value of the s-band (Ks) in 164Er is performed. A negative is obtained for 164Er, necessitating a re-evaluation of the assumption of equality in the quadrupole moment of the s-band and ground state bands; a remarkable determination in a well-deformed nucleus. A larger quadrupole moment for the s-band in 164Er is calculated to reproduce positivity in the K value, and the theoretical interpretation of this increased deformation allows for the relative magnitude of s-band quadrupole moments and K values in other nuclides in the N=82→126 shell to be predicted.