G.D. Dracoulis

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”.

Multi-quasiparticle states in the mass-180 region

Abstract Nuclei in the mass-180 region have many high-Ω single-particle levels close to the Fermi energy and are, therefore, prime candidates for high- K isomers. Since both neutron and proton level densities are rather low, one should expect blocking and particle-number fluctuations to be rather important. We have performed good-particle-number calculations and have shown that the simpler blocked BCS theory gives a good approximation to the multi-quasiparticle spectra if the pairing strength is chosen appropriately. This has allowed us to perform a systematic theoretical study of this mass region. Residual spin-spin interactions are shown to be essential in reproducing the energies and even the correct order of known states. Good agreement has been found for 175 Hf, 176 Hf and 177 Ta, where extensive data already exist. Predictions for new high- K states near the yrast line are made for these nuclei and for 178 W.

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.

Quadrupole collectivity and shapes of OsPt nuclei

Abstract E2 collective properties for the low-lying states in 186,188,190,192 Os and 194 Pt have been investigated experimentally by means of Coulomb excitation using 3.3–4.8 MeV/nucleon 40 Ca, 58 Ni, 136 Xe and 208 Pb beams. The deexcitation γ rays following Coulomb excitation were detected in coincidence with the scattered particles. Levels with excitation energies up to 3–4 MeV of the ground-state, γ and 4 + collective bands as well as excited 0 + states were populated in each nucleus studied. A semiclassical Coulomb-excitation least-squares search code GOSIA was used to extract E2 matrix elements from the measured γ-ray yields. For each nucleus studied, a unique and almost complete set of E2 matrix elements for the low-lying states has been determined, which includes both the magnitudes and signs of the transitional and diagonal matrix elements. The completeness of the set of measured E2 matrix elements makes it possible to determine the intrinsic quadrupole deformation for the low-lying states in these nuclei via a model-independent method. The results indicate clearly that the E2 properties for the low-lying states in these nuclei are correlated well using only the quadrupole collective degrees of freedom. The extracted E2 matrix elements are compared with the prediction of various collective models such as the asymmetric rigid rotor model, the γ-soft model of Leander, and the IBA-2 model. These particular models do not reproduce the data satisfactorily, however the general trends of the data are consistent with the descriptions of γ-soft type collective models through a prolate to oblate shape-transition region. That the enhanced B (E2) values between the I ,quasi- K π = 4,4 + state and members of the I ,quasi- K π = 2,2 + band are well reproduced by the γ-soft model is consistent with the interpretation of the I ,quasi- K π = 4,4 + state being a two-phonon γ-vibration excitation.

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.

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.