P. M. Walker

MULTI-QUASIPARTICLE POTENTIAL-ENERGY SURFACES

Abstract Diabatic-blocking calculations, with inclusion of γ -deformation, are performed for the first time. Significant configuration dependence is found for multi-quasiparticle potential-energy surfaces. The calculations of quasiparticle energies require a renormalised pairing strength which includes shape and blocking effects in reproducing experimental odd-even mass differences. The detailed excitation energies known in 178 W are well reproduced. For the 182 Os K π =25 + isomer, the calculation shows | γ |≈10° and the obtained quadrupole moment agrees with the measured value.

Decay of high-spin isomers in Os nuclei by barrier penetration

Abstract A decay scheme for the 130±20 ns high-spin isomer in 182 Os has been established. The excitation energy of the isomer is 7049±1 keV and it has I π = 25 (+) . A 2.4% decay directly to the yrast 24 + level at 5988 keV is observed. In 184 Os a 20±5 ns isomer is observed at 2366±1 keV excitation energy with I π = 10 + . Again, direct transitions into the yrast 8 + and 10 + levels are observed. Contrary to previous speculations, there is no compelling evidence for stable triaxial shapes in the structure of the levels through which the isomers decay. The abnormally short half-lives observed, as well as the unusual decay patterns, are best understood in terms of a γ-soft nuclear potential. Motion in the γ-direction allows the isomer to decay via barrier penetration from an axially symmetric prolate shape with the angular momentum along the nuclear symmetry axis (deformation aligned state) via oblate shapes to another prolate shape with the angular momentum perpendicular to the nuclear symmetry axis (rotation-aligned state).

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.

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.

Anomalous four and five quasiparticle isomer decays in 174Hf and 175Hf

Abstract High-spin isomeric decays in 174 Hf and 175 Hf are found to populate a large number of different quasiparticle structures. Many weak γ-ray transitions are revealed that violate the usual K -selection rules. Analysis of transition hindrance factors shows the importance of K -mixing in the populated levels. The effects of both chance near-degeneracies and Coriolis coupling are separately identified. The role of high- K components in Fermi-aligned “tilted” bands is emphasized. The 4-quasiparticle excitation energies are compared with the sum of the constituent 2-quasiparticle energies.

Study of high-spin states in 181, 182Os

Abstract High-spin states in the nuclei 181, 182Os have been populated in the 150 Nd( 36 S ,x n ) reactions and studied with the ESSA30 array. The nucleus 181Os has also been studied at the NBI tandem accelerator using the 167Er(18O,4n) reaction. The previously known bands in both nuclei have been extended to higher spins and two new side bands have been found in 181Os. In the latter nucleus the ground state has been established to have I π = 1 2 − . The extraction of the ratios of reduced transition probabilities B(M1) B(E2) from branching and E2 M1 mixing ratios permitted configuration assignments for most of the bands in both nuclei. The analysis has been carried out within the semiclassical vector model for M1 radiation. The positive-parity yrare sequences in 182Os and the band based on the I π = K π = 23 2 − state in 181Os have been interpreted as t-bands arising from a rotation about a tilted axis. The alignment behaviour and the crossing frequencies are for most of the bands consistent with predictions of the cranked shell model.

Pairing reduction and rotational motion in multi-quasiparticle states

Abstract Calculations of pairing energies in multi-quasiparticle states using the Lipkin-Nogami prescription predict a discrete reduction in pairing with a geometric dependence on seniority. An abrupt transition from superfluid to normal motion is therefore not expected, even with a large number of orbits blocked. Recent experimental results on rotational bands associated with multi-quasiparticle intrinsic states allow comparisons to be made for a subset of orbitals as a function of seniority. Approximate agreement is obtained between the observed moments-of-inertia and calculated values supporting the view that for the highest seniority states identified so far, the pairing persists at nearly half of the full value.

Structural Evolution in the Neutron-Rich Nuclei ^{106}Zr and ^{108}Zr

The low-lying states in ¹⁰⁶Zr and ¹⁰⁸Zr have been investigated by means of β-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹⁰⁸Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64. The deformed ground state of ¹⁰⁸Zr indicates that a spherical subshell gap predicted at N = 70 is not large enough to change the ground state of ¹⁰⁸Zr to the spherical shape. The possibility of a tetrahedral shape isomer in ¹⁰⁸Zr is also discussed.

OPENING UP THE A 180 K-ISOMER LANDSCAPE : INELASTIC EXCITATION OF NEW MULTI-QUASIPARTICLE YRAST TRAPS

Abstract New high-spin isomers in 175 Lu, 181 Ta and 186 W target nuclei have been strongly excited by pulsed 238 U beams. The use of inelastic excitation opens up an uncharted region in the neutron-rich K-isomer landscape. First results establish the key role of multi-quasiparticle configurations in the high-angular-momentum structure.

High-K structures and triaxiality in 186Os

Abstract The high-spin structure of the stable nucleus 186Os has been studied, using a radioactive 14C beam. New rotational bands built on multi-quasiparticle states with Kπ = 5−, 7−, 9−, 10+ and 15+ are observed and the bandhead energies are compared to predictions of blocked BCS calculations. The first crossing of the ground-state band at I = 14 h , involving a high-K t-band structure, is observed and explained using a two-band mixing model. The structure at higher angular momentum is dominated by intrinsic states, that exhibit a dramatic loss of isomerism. Potential-energy-surface calculations, with Lipkin-Nogami pairing, show these configurations to be triaxial, accounting for the breakdown in K conservation. The relation between the K projection and the total angular momentum is investigated for these non-axial states.