A.P. Byrne

High-spin structure of 190–194Hg and the cranked shell model

Abstract High-spin states in 190–194 Hg, populated in the 170 Er ( 24, 26 Mg, x n) and 184, 186 W( 13 C, x n) reactions, were investigated by in-beam γ-ray spectroscopic techniques. The level schemes in all five nuclei were extended to considerably higher spins and new band intersections were found. The band structure is interpreted within the framework of the cranked shell model. Remarkable agreement is obtained between the predictions of the model and the experimental data for these weakly oblate deformed nuclei. In 192, 194 Hg irregular sequences were found which could be examples of “ terminating bands”.

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.

Multi-quasiparticle and rotational structures in 179W: Fermi alignment, the ifK-selection rule and blocking

Abstract High-spin states in 179 W have been studied following the 170 Er(su13C, 4n) reaction. Rotational bands up to ifI 53 2 have been identified, based on 1-, 3-, 5- and 7-quasiparticle structures. Different alignment mechanisms compete in the generation of angular momentum at the yrast line. A Fermi-aligned if( i 13 2 ) 2 structure, coupled to high- K and to the if 7 2 − [514] orbital, forms the negative-parity yrast sequence above if I π = 31 2 − . This may be called a “t-band” since its description within the cranking model requires a “tilting” of the cranking axis. The anomalous decay of the if K π = 35 2 − 5-quasiparticle isomer is explained as arising from destructive interference of transition amplitudes coupling to the Fermi-aligned structure. Detailed analysis of the excitation energies of the multi-quasiparticle states indicates the quenching of both neutron and proton pair correlations, by comparison with blocking calculations.

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.

Spectroscopy of high-spin states in 211,212,213Fr

Abstract The level structures of 211 Fr, 212 Fr and 213 Fr have been observed to high spins, ~ 28 /kh, (and excitation energies ~ 8 MeV) using a variety of γ-ray spectroscopic techniques. The structure of these nuclides is discussed in terms of couplings of single-particle states through empirical shell-model calculations. Good agreement with experiment is obtained. In 212 Fr and 213 Fr core-excited configurations are required to explain the properties of the highest states. A number of long-lived states were observed in each nucleus some of which decay by enhanced E3 transitions. The E3 transition strengths are discussed.

g-Factors in 210Rn and octupole coupling of core-excited states in 210Rn, 211Rn and 212Rn

Abstract The g -factors of isomeric states in 210 Rn have been measured using the TDPAD technique. Semi-empirical shell-model calculations, with explicit inclusion of the couplings to the 3 − octupole vibration, are carried out for the core-excited states in 210 Rn and 211,212 Rn. The resulting mixed multi-particle configurations are used to explain simultaneously the g -factors and enhanced E3 transitions which connect several pairs of these states.

Low-spin non-yrast states and collective excitations in 174Os, 176Os, 178Os, 180Os, 182Os and 184Os

Abstract Excited states in the range of isotopes 174, 176, 178, 180, 182, 184 Os, populated in the β-decay of 174, 176, 178, 180, 182, 184 Ir parent activities have been identified using γ-ray singles and coincidence techniques, utilising the high efficiency of a Compton-suppressed array, and conversion electron techniques. Many new states at low excitation energies have been identified, complementing the level schemes previously established from in-beam studies. The new states include excited 0 + states in 174 Os, 176 Os 178 Os and 180 Os. A large body of data on decay properties, spins and parities, and relative E2 and E0 matrix elements has been obtained. The systematics of the quasi-β and quasi-γ bands is discussed. Some detailed analyses in terms of the schematic band-mixing model are presented, incorporating reproduction of the yrare states in the fits. In particular, the proposed interpretation of the anomalous low-frequency alignment gains in the yrast positive-parity states, as a consequence of mixing with a low-lying intruder band, is confronted. The B (E0)/ B (E2) ratios are over-estimated by this model although their form (spin and isotope dependence) is reproduced. Better agreement is obtained within the IBM, which uses however reduced effective charges to match experiment. Absolute measurements of the B (E0) and B (E2) values may be necessary to distinguish between the models and test conjectures of shape differences.

K-forbidden transitions from multi-quasiparticle states

Abstract Four-quasiparticle, K π = 12 + states in 172 Hf and 178 W, with half-lives less than 2 ns, are found to decay to their respective K π = 0 + ground-state bands, in competition with transitions to intermediate- K , two-quasiparticle structures. All decay transitions are weakly hindered. When taken together with other K -isomer decay rates, an overall trend of decreasing hindrance with increasing excitation energy is evident. Estimates based on density-of-states considerations approximately reproduce the trend of the data, but also indicate the need to include other K -mixing effects.

Contrasting high-spin yrast bands in 172Os and 174Os and an unexpected low-frequency anomaly in 172Os

Abstract The yrast bands of the neutron deficient isotopes 172 Os and 174 Os have been identified to spins of about 24. The yrast band in 174 Os shows no bandcrossing anomalies, confirming the shell effect observed in other N = 98 nuclei. In contrast, a strong backbend observed at a frequency of about 0.26 MeV in 172 Os is attributed to the s-band crossing. A weaker band-crossing is also observed at a lower frequency, about 0.24 MeV, in 172 Os. This unexpected anomaly may be due to either a deformation effect, or to a change in the s-band structure.

Rotational band on the 31 yr 16+ isomer in 178Hf

Abstract High-spin states in 178Hf have been identified using particle-γ-γ-time techniques and the incomplete fusion reaction, 176Yb(9Be,α3n)178Hf. The rotational band associated with the four-quasiparticle T 1 2 = 31 year, Kπ = 16+ isomer in 178Hf has been established. The gK−gR values obtained from the in-band decay properties confirm the configuration of the isomer as does its alignment which matches the sum of the alignments of the two Kπ = 8− two-quasiparticle bands that contain the components of the 16+ configuration. New information has also been obtained on the four-quasiparticle 14− band and on the two-quasiparticle 6+ and 8− bands.