R A Bark

Shape coexistence in very neutron-deficient Pt isotopes

Yrast bands in 176Pt(N=98) and 178Pt have been identified. The level scheme of 176Pt changes from a quasi-vibrational pattern to that of a well deformed rotor, at very low spins, a behaviour similar to that attributed to shape coexistence in the light Hg isotopes. Analysis of The 176-188Pt yrast bands supports a shape coexistence interpretation, the excitation energy of the intruder state behaving as predicted by Wood, (1981).

Configuration-dependent deformations in 171Re

Abstract The level scheme of 17175Re96, has been studied using (heavy ion, xnyp) reactions. Rotational bands associated with the one-quasiproton Nilsson configurations 5 2 + [402] , 1 2 + [411] and 9 2 − [514] and the “cross-shell” orbitals from the h 9 2 and i 13 2 protons (nominally 1 2 0 − [541] and 1 2 + [660] ) have been identified. Less extensive results for 173Re have also been obtained. Differing (configuration dependent) deformations are required to explain the frequencies and alignment gains in the neutron band crossings. The relative differences are consistent with predicted deformation changes in the “deformation-driving” h 9 2 and i 13 2 (proton) orbitals. Signature splitting in 9 2 −[514] and 5 2 + [402] bands at low spin suggests some γ-deformation. Competing in-band and out-of-band E2 decays in the region of the “real” crossing between the 1 2 + [660] and 5 2 + [402] bands are explained through particle-rotor band-mixing calculations with the ad hoc inclusion of ΔN = 2 mixing. Limited agreement between the observed 1-quasiparticle energies and predicted values underlines the limitation of currently accepted nuclear potentials in this region. Small alignment gains in the 5 2 + [402] and 1 2 + [411] bands, before the AB neutron alignment can be related to the low-spin anomaly in 172Os and explained using three-band mixing. The absence of a similar effect in the 9 2 − [514] band is discussed.

Band crossings in 170Os

Abstract Excited states in the neutron-deficient nucleus 170 Os were identified up to spin (24 + ) in the yrast band and to spin (23 − ) in the lowest negative-parity band. Deformation systematics implied by the 2 + state energies for the very light osmium isotopes are compared with theory. Band-crossing frequencies, alignments and alignment gains are compared with cranked shell-model calculations. Deformation changes are required to obtain detailed agreement. A three-band mixing approach is invoked to explain the low-spin yrast anomaly in 172 Os and to reproduce the yrast band in 170 Os. The excitation energy of the postulated “intruder” band in 170 Os and 172 Os is deduced.

Properties of 152− states in 215Ra and 217Th; evaluation of the 152− to 92+ E3 strength in N = 127 isotones

Abstract The lifetime of the yrast 15 2 − state in 215 Ra was measured using pulsed beams and γ-ray and electron techniques. Transition multipolarities were established from measured conversion coefficients. The B (E3) of the 15 2 − → 9 2 + transition is found to be considerably larger than previously reported. A candidate for the corresponding transition in 217 Th was also observed. The E3 strength of the 15 2 − → 9 2 + transition in the N = 127 isotones is evaluated in the light of these and other recent results. Interpretation in the framework of particle-octupole vibration coupling requires a systematic lowering of the core 3 − vibration as proton pairs are added to 208 Pb.

Shape coexistence or particle alignment in the light osmium isotopes 171Os, 172Os and 173Os

Abstract Decay schemes of the nuclei 171 Os, 172 Os and 173 Os were studied using the reactions 144 Sm( 30 Si,3n) 171 Os, 144 Sm( 31 P, p2n) 172 Os and 149 Sm( 28 Si, 4n) 173 Os. The 5 − 2 [523] and i 13 2 bands were identified in the odd nuclei. Low-spin properties of these bands are analyzed using the particle-rotor model, while band-crossing frequencies and alignments are examined with the aid of the cranked shell model. A low-frequency band crossing was found in the 5 − 2 [523] band of 173 Os. At higher frequencies, the measured B( M 1) B( E 2) transition rates and signature splittings in the 5 − 2 [523] bands of both nuclei are consistent with a neutron alignment. In the i 13 2 band of 173 Os, an upbend, in the unfavoured sequence, possibly due to the AD crossing, occurs at a lower frequency than a similar upbend, presumably from the BC crossing, in the favoured sequence. These crossings were also analyzed with phenomenological three-band-mixing calculations.

Low-frequency band crossing in 171Re: a deformed intruder interpretation

The decay sequence based on the 5/2+(402) proton orbital has been identified in 171Re up to spin 41/2. An anomalous upbend in the alignment is observed below the expected backbend, a phenomenon similar to that observed in the yrast sequence of 172Os. The properties of the 5/2+(402) band can be understood in terms of two crossings, one at low spin with a band based on a deformed intruder configuration, the second at higher spin with a band composed of aligned i13/2 neutrons.

High-spin states and intrinsic structure in 174Os and 175Os: Alignments and strong interaction

Abstract High-spin states have been identified in 174Os and 175Os using (HI, x>n) reactions and gamma-ray techniques. Rotational bands built on low-lying intrinsic states in 175Os have been observed for the first time. The intrinsic states are identified as a mixed i 13 2 neutron state (nominally 7 2 + [633] ) and the 1 2 − [521] and 5 2 − [512] Nilsson configurations. Differences in the alignment of the two signatures of the decoupled band built on the 1 2 − [521] state and the comparison between the signature splitting of routhians and if B(M1)/B(E2) values in the i 13 2 band suggest softness towards gamma deformation. Crossing transitions between the negative-parity bands at intermediate spin have been observed and the interaction strength derived from the branching ratios. A comparison of the properties of bands in 174Os and 175Os shows some expected similarities, but also surprising differences which point towards the need for a more comprehensive treatment in strong interaction cases. Depending on the choice of reference the alignment in the i l3 2 band might show degenerate BC and AD crossings at a relatively low frequency. This may be consistent with the alignment observed in the negative parity side bands in 174Os.

A shape coexistence interpretation of the phenomenology of alignment curves in Os and Re isotopes

A model of shape coexistence, based on an intruder pi (h92/)2 excitation, previously applied to explain low-frequency upbending phenomena in 171Re and 172Os, is extended to heavier Os and Re isotopes. A phenomenological three-band mixing analysis of the yrast bands of even Os nuclei and the 5/2+(402) and 9/2-(514) bands of the neighbouring Re isotopes is used to deduce the excitation energies of the proposed intruder configuration as a function of neutron number. These are shown to be qualitatively consistent with an (h92/)2 excitation. The implications of this model for other nuclei are discussed.

Enhanced E3 transitions and mixed configurations for core excited isomers in 210At and 211At

Abstract The lifetime and branching ratio of the 19 + isomer in 210 At have been measured. Its enhanced E3 decay and g -factor, and those of the related 39 2 − isomer in 211 At are compared with the results of a semi-empirical shell model calculation which includes couplings to the 3 − octupole vibration, resulting in mixed configurations. Lifetimes were also obtained for the 15 − isomer in 210 At, and the 29 2 + isomer in 209 At.

Spectroscopy of high-spin states of 206Po

Abstract The yrast and near-yrast energy levels of 206 Po have been investigated to over 9 MeV excitation and up to spins of 24. The measurements consisted of γγ-coincidence data, internal-conversion-electron spectra, time spectra of γ-rays relative to a pulsed beam, excitation functions and γ-ray angular distributions. Two new isomers, with lifetimes in the 1 ns range, were found. The observed level structure is compared with the predictions of empirical shell-model calculations in which 206 Po is regarded as a 208 Pb core with two valence protons and four valence neutron holes. The agreement is generally satisfactory for the observed odd-parity levels and for even parity levels with J > 12; those with J = 6−12 are better accounted for by weak coupling of two valence protons to a 204 Pb core in its 0 + 1 and 4 + 1 states.