S.S. Anderssen

Non-yrast states and shape co-existence in light Pt isotopes

Abstract Low-lying states in the even-even light platinum isotopes 176Pt, 178Pt, 180Pt and 182Pt have been populated using β+ /EC decay from parent gold nuclei, created in (HI,xn) reactions. State energies, spins and parities and γ-ray branching ratios were determined using γ-ray and electron spectroscopy. Whereas non-yrast states were observed in 178Pt, 180Pt and 182Pt, none were seen in 176Pt. The excitation energies of the observed states are analysed in terms of a band-mixing model, yielding the moments of inertia of the unperturbed bands. Branching ratios and ground-state-band quadrupole moments are calculated and compared with experimental values. The results indicate that the two lowest-lying 0+ states in each of the light Pt isotopes are formed from the mixing of two intrinsic states of different deformation, and other low-lying states can be described as admixtures of rotational states built on these intrinsic states, and on γ-vibrational states.

Non-yrast states and shape co-existence in 172Os

Abstract Previous studies of 172 Os noted an anomaly in the behaviour of the moment of inertia of the yrast band at low spin. A phenomenological model of shape coexistence based on interacting rotational bands was proposed to explain this anomaly and this model predicted low-lying non-yrast states. In order to test these predictions, the β-decay of 172 Ir has been used to populate 172 Os. Excited states have been observed and classified into positive-parity “quasi-β” and “quasi-γ” bands and a negative-parity band. The energies of the quasi-β band states are seen to be in general agreement with the predictions of the phenomenological model and the model is refined to take into account the new data. The bands involved are determined to have significantly different moments of inertia.

Measurement of the g-factor of the yrast 10+ state in 110Cd

Abstract The gyromagnetic ratio of the yrast 10 + state in 110 Cd has been measured using the ion-implantation perturbed-angular-distribution (IMPAD) technique. Assuming previously published values for the lifetime of the state and the internal magnetic field strength of Cd in Gd, a g -factor of −0.09(3) was obtained. This is somewhat smaller than the value expected for a pure (h 11/2 ) 2 two-neutron configuration. The difference can be accounted for by relatively small admixtures of collective and/or proton-configuration components in the 10 + state wavefunction.

Systematic measurements of transient fields for W, Os and Pt ions traversing Fe

Transient magnetic fields were measured for W, Os and Pt ions traversing iron hosts with average velocities in the range from approximately 1.6v0 to 4.8v0 (v0=c/137, Bohr velocity). Transient fields for W and Os in Fe are consistent with behaviour found for lighter rare-earth ions and are about 20% stronger than those for Pt in Fe over the majority of the velocity range examined. A measurement was made to confirm that possible heavy-ion beam induced attenuations of the transient field are negligible for low-velocity Pt ions excited by Ni beams. Results are discussed in terms of both empirical and model-based parameterizations of the transient field strength.

Magnetic moments in the 1/2-[521] ground-state band of 171Yb and Coriolis-induced renormalization of rotational g-factors in odd-A rare-earth nuclei

Abstract Gyromagnetic ratios of the states with spins between 7/2− and 21/2− (inclusive) in the 1/2−[521] ground-state band of 171 Yb were measured by the transient-field technique following Coulomb excitation. Multipolarity mixing ratios were determined from measured particle-γ-ray angular correlations and total branching ratios were determined from intensity balances in a γ-γ coincidence measurement. The electromagnetic properties of the low-lying natural parity states in 171 Yb are compared with particle-rotor calculations and can be well described in the limit of a pure K=1/2 band. A comparison of measured g-factors in the ground-state rotational bands of 155 Gd, 169 Tm and 171 Yb with particle-rotor model calculations suggests that the renormalization of the rotational g-factor in the low-K, single-quasiparticle bands of odd rare-earth nuclei, compared with their even neighbours, is predominantly due to Coriolis interactions.

Measured magnetic moments of 21+ states in 190–198Pt and interacting boson model description of M1 systematics in the platinum isotopes

Abstract The g-factor of the 21+ state in 190Pt has been measured for the first time using the thin-foil transient-field technique with a target enriched (to 4.19%) in 190Pt. Precessions for the 21+ states of 190,192,194,196,198Pt were measured simultaneously. The Coulomb excitation cross section to the 21+ state in 190Pt was also measured relative to the 01+ → 21+ excitations in 194Pt and 196Pt. The near constancy of g(21+) for the even 190–198Pt isotopes is in marked contrast with the behaviour observed in neighbouring even 182–186W and 186–192Os nuclei and the predictions of the proton-neutron interacting boson model in the limit of F-spin symmetry. It is shown that the observed g-factor and M1 transition systematics in the Pt isotopes can be described by appropriate F-spin symmetry breaking, while maintaining satisfactory descriptions of the energy levels and E2 properties.

Transient field measurements of first-excited stateg-factors in188,190,192Os

Transient-field precessions were measured simultaneously for the 21+ states of188,190,192Os as ions of these nuclei traversed a thin polarized iron foil. In contrast with most earlier measurements, the relative gyromagnetic ratios deduced here are independent of the level lifetimes. Emphasis is placed upon determination of theg-factor of the first excited state in190Os, as disparities in measurements of the lifetimes of this level have led to uncertainties in itsg-factor.

IMP AC in-beam and out-of-beam;g-factors and pre-equilibrium effects following ion-implantation

An out-of-beam IMPAC technique has been developed to measure theg-factors of the 2l+ states in the neutron-deficient Pt isotopes. The hyperfme field for Pt in Fe in those measurements was close to the field obtained in spin-echo and radioactivity measurements, in contrast with in-beam IMPAC measurements for194,196,198Pt, where smaller field strengths were observed. These data, and static fields from in-beam IMPAC measurements for other ions in the Pt region, show a correlation with the lifetime of the probe state, consistent with the static field being absent for about 10 ps after the ion has come to rest. New and previously published IMPAC data for76Os,77Ir,78Pt and79Au in Fe hosts, are examined to assess whether such measurements may be sensitive to the thermal spike created at the end of the range of an implanted ion.

High-spin states in 183Hg and shape coexistence in the odd-mass mercury isotopes

Abstract High-spin states in 183Hg have been identified using the reaction 155Gd(32S,4n). Three prolate-deformed rotational bands associated with the 1 2 − [521] , 7 2 − [514] and mixed i 13 2 neutron orbitals are observed, while the existence of an oblate 13 2 + bandhead is inferred, implying co-existing prolate and oblate nuclear shapes. A two-band mixing model used to fit the state energies of the i 13 2 neutron bands in 183,185,187Hg gives parameter values which are consistent with the existence of two bands with different deformations. The B(E2) ratios of the intra- and inter-band transitions in these coexisting bands are also investigated. Many of the features can be reproduced but difficulties remain, for example the results are not consistent with the assumption of coexistence between simple prolate and oblate shapes, a problem noted previously for the even-mass isotopes. Systematics of the prolate-oblate energy differences show that the energy of the prolate well relative to the oblate well is ∼ 350 keV lower in the odd-mass isotopes than in the even-mass isotopes. Possible reasons for this are described. The nature of the first alignment in the prolate bands in the mercury isotopes is discussed within the cranked shell model.

Yrast four-quasi-particle states in 182W

Abstract High-spin states of the stable isotope 182W have been studied using the reactions 176Yb(13C, α3n)182W and 176Yb(9Be, 3n)182W at beam energies of 65 and 40 MeV, respectively. Three, possibly four, new intrinsic states at high spins have been observed corresponding to different high-K four-quasi-particle structures. The Kπ = 15+ and 17− levels have lifetimes of 78(15) and 25(10) ns, respectively. Rotational bands are observed built on the Kπ = 16+ and 17− yrast states. The coupling of Nilsson orbitals which give rise to high-K states at the yrast line and the K-forbiddenness of the K π = 15 + isomeric decay are discussed.