Decay Spectroscopy of Isomerically Pure Beams of 178Au and its Daughters

In many regions of the nuclear landscape, competition between spherical and deformed configurations leads to the phenomenon of shape coexistence due to the delicate balance between shell and collective effects. The region around the Z = 82 proton-shell closure and the N = 104 neutron midshell contains some of the clearest examples of this phenomenon and has thus been the focus of intense theoretical and experimental study in recent decades.

The results presented in this thesis form a contribution to this long-term programme through an in-depth decay spectroscopy study of 178Au (Z =79, N =99) and its daughters. In-source laser spectroscopy by Cubiss et al. [1] at the ISOLDE facility confirmed the existence of a previously predicted, strongly deformed isomer, 178Aum, coexisting with the more weakly deformed ground state, 178Aug. As part of Cubiss’ study, highly selective resonant laser ionisation allowed separate α-decay studies of 178Aug,m to be performed. In the present work, the same technique was used to produce isomerically pure beams of each of these two states for a detailed decay spectroscopy study of the α- and EC/β+-decay branches of 178Aug,m, and their daughters, at the ISOLDE Decay Station (IDS).

Separate fine-structure α-decay schemes for 178g,mAu →174g,mIr and 174g,mIr →170Re were produced, expanding substantially on those from earlier works. The first ever isomerically selective electron-capture (EC)/β+ decay studies of 178Aug,m →178Pt and 174Irg,m →174Os were also performed for this thesis. All together, these decay schemes include 70 new states, 11 new α decays and 119 new γ-ray transitions. The half lives of 178Aug,m were measured which gave values consistent with those determined by Cubiss but have smaller uncertainties. The multipolarities of γ-ray transitions observed following the α decay of 178Aug,m to 174Ir were determined, as were reduced α-decay widths and hindrance factors, HFα. The proportion of EC/β+ decay feeding to each state in 178Pt and 174Os was calculated, allowing log f t values for these decays to be experimentally determined for the first time. These were used in conjunction with HFα to propose spin-parity and configuration assignments for 178Aug,m and 174Irg,m and infer information on the shape of 178Pt. The E0-transition strengths for decays between the states in two strongly mixed rotational bands in 178Pt were determined and used to calculate a lower limit for the difference in the mean-square charge radii of the bandheads. Using mass measurements reported by Cubiss and α-decay Q values deduced in this work, the excitation energy of the states fed by the α decay of 178Aug,m and 174Irg,m were determined. While all α decays from 178Aug were assigned as feeding directly or indirectly to 174Irg (and 178Aum to 174Irm), it was found that 174Irg feeds to a long-lived, low-spin state 47(17) keV more energetic than the high-spin state fed by 174Irm. This indicates a possible inversion of state ordering in 170Os relative to 174Ir and 178Au.