B. Fabricius

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.

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.

Spectroscopy of 175Ir and 177Ir and deformation effects in odd iridium nuclei

Abstract Excited states in 175Ir and 177Ir have been identified using (heavy-ion, pxn) reactions and γ-ray spectroscopic techniques. Rotational bands based on intrinsic states arising from the odd-proton parents, h 9 2 , h 11 2 , i 13 2 and d 5 2 were established to high spin except for the 5 2 + [402] bands. Only the h 9 2 bands show sharp alignment gains compatible with alignment of ( i 13 2 ) 2 neutrons. The smooth apparent alignments of the i 13 2 proton bands can be attributed, at least qualitatively, to a much larger deformation. The complex alignment gains observed in the h 11 2 bands in these and other iridium isotopes in the range 173–181Ir are consistent with the effects of mixing with a deformed intruder (equivalently, a low-spin shape change). This conjecture is tested against the in-band decay properties using a three-band model. Examination of the band structure suggests a significant gamma deformation at very low spin, before the change to a more deformed configuration occurs.

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.

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.

Intrinsic states and collective structures in 181Ir

One- and three-quasiparticle states, and associated rotational bands have been identified in the odd-proton nucleus 181Ir. They were populated using the 169Tm(16O, 4n)181Ir reaction and established using a variety of time-correlated γ-ray and electron techniques. The 92−[514] and 52+[402] intrinsic states (from the h112 and d52 parents) were found to be metastable with mean of 193 and 430 ns, respectively. A third isomer, with Kπ = 232+ and a meanlife of 42 ns was identified and attributed to a one-quasiproton, two-quasineutron configuration, as were two other high-K band heads. The B(M1)B(E2) and gK − gR values extracted from the in-band decay properties of the observed rotational bands, based on these and other intrinsic states, were used to identify and test the proposed configurations. Interference effects were observed between states of the band assigned to the α = +12 signature sequence from the i132 configuration, with those from a second α = +12 sequence, and the states from one of the high-K bands (leading to complicated branchings in the decay scheme). The α = +12 sequence of the h92 proton band was extended to higher spins than known previously and its (unflavoured) α = −12 sequence identified. Both show alignments and alignment gains which are essentially well-behaved and consistent with a stable quadrupole deformation and small γ-asymmetry. In contrast, the h112 and d52 bands show complex alignment gains at low frequencies. These, and the in-band decay properties can be reproduced in a three-band model. However, essentially all bands, except for the h92 one-quasiparticle cases, exhibit an underlying alignment increase with frequency if reference parameters appropriate to the h92 band are used. Comparisons between the one-, two- and three-quasiparticle bands in which different orbitals are blocked, suggest the presence of alignment gains due to the h92 protons but conflicting evidence remains. Some of the alignment effects may be artefacts of larger deformations, or alternatively a reduction in pairing.

High-spin yrast isomer in 211Rn and 212Rn with enhanced E3 decays

Abstract New isomeric states with Jπ = 69/2+, τm = 13(1) ns in 211Rn and Jπ = 33−, πm = 7(1) ns in 212Rn have been identified. They decay by enhanced E3 transitions with strengths of 33(3) and 43(6) single particle units to the known 63/2− and 30+ isomers in 211Rn and 212Rn, respectively. The excitation energies and transition strengths agree well with predictions of the multiparticle, octupole-vibration coupled model.

Spectroscopy and shell model interpretation of high-spin states in the N = 126 nucleus 214Ra

Abstract Excited states in the N = 126 nucleus 214 Ra have been studied using γ-ray and electron spectroscopy following reactions of 12 C and 13 C on 206 Pb targets. Levels were identified to spins of ∼ 25 ħ and excitation energies of ∼ 7.8 MeV. Lifetimes and magnetic moments were measured for several levels, including a spin (25 − ) core-excited isomer at 6577.0 keV with τ = 184 ± 5 ns and g = 0.66 ± 0.01. The level scheme, lifetime and magnetic moment data are compared with, and discussed in terms of, empirical shell-model calculations and multiparticle octupole-coupled shell-model calculations. In general, the experimental data are well described by the empirical shell model.

Octupole coupling and proton-neutron interactions in 214Fr

Abstract Excited states in the odd-odd nucleus 214 Fr have been studied using γ-ray and electron spectroscopy following 208 Pb( 11 B, 5n) and 205 Tl( 13 C, 4n) reactions. Levels were identified to spins of around 36 ħ and excitation energies of ∼ 8.6 MeV. A number of isomeric states have been measured and g -factors obtained using the TDPAD method. At low spin, semi-empirical shell-model calculations appear to provide a good description of the states observed. An understanding of the structure of higher spin states in terms of the probable yrast configurations requires the addition of two units of spin at an energy of around 2 MeV. States with spins around 30 ħ are formed by core-excited configurations, with double core-excitation suggested for the highest states observed. The properties of many of the isomeric states can be understood in terms of multiparticle octupole coupling, with the properties of these states well reproduced by multiparticle octupole-coupled shell-model calculations.

Contrasting behaviour of proton h92 and h11/NU>2 bands in 175,177,179,181Ir interpreted in an intruder model

Abstract Rotational bands in the odd-proton isotopes 175Ir, 177Ir, 179Ir and 181Ir based on the h 9 2 , h 11/NU> 2 , i 13 2 and d 11/NU> 2 orbitals have been identified, the first three to high spin. In all cases the h 9 2 band shows backbending with a crossing frequency and alignment gain consistent with that expected from the neutron i 2 13 2 alignment. In contrast the h 11/NU> 2 bands (and the fragments of d 11/NU> 2 bands observed) show complex alignment curves. These alignments can be explained using a phenomenological three-band model which incorporates a deformed intruder band, analogous to that proposed in the very light Pt and Os nuclei. Mixed wave functions from the model reproduce the observed B(M1)/B(E2) ratios. The presence of the intruder band in the h 11/NU> 2 and d 11/NU> 2 configurations and its absence in the h 9 2 band is compatible with its proposed association with an ( h 2 9 2 ) 0 pair excitation.