M. A. Bentley

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.

The observation of 84Mo

Abstract The 28 Si( 58 Ni, 2n) 84 Mo reaction has been studied at 195 MeV bombarding energy. The prompt gamma rays were detected in an array of twenty, escape-suppressed Ge detectors in coincidence with the recoiling nuclei, which were identified by A and Z with a recoil separator. A 443.8±0.3 keV gamma ray was observed from 84 Mo which is interpreted as the 2 + −0 + transition in this nucleus. This would indicate a quadrupole deformation e 2 ∼0.30. This result confirms that 76 Sr lie at the centre of the region of deformed nuclei with A ∼80.

Mirror nuclei at high spin in the f72 shell

New results for the Coulomb energy differences in the A=47 mirror nuclei 47Cr and 47V for yrast states up to spin 312ℏ are presented and shown to agree with earlier predictions made within a deformed, crancked shell model approach. The observed total difference in Coulomb displacement energies at low and high angular momenta is also explained within the spherical shell model. The first example of alignment in an N=Z nucleus, 48Cr, is reported.

Coulomb shifts and shape changes in the mass 70 region

The technique of recoil {beta} tagging has been developed which allows prompt {gamma} decays in nuclei from excited states to be correlated with electrons from their subsequent short-lived {beta} decay. This technique is ideal for studying nuclei very far from stability and improves in sensitivity for very short-lived decays and for high decay Q-values. The method has allowed excited states in {sup 78}Y to be observed for the first time, as well as an extension in the knowledge of T=1 states in {sup 74}Rb. From this new information it has been possible to compare Coulomb energy differences (CED) between T=1 states in {sup 70}Br/{sup 70}Se, {sup 74}Rb/{sup 74}Kr, and {sup 78}Y/{sup 78}Sr. The A=70 CED exhibit an anomalous behavior which is inconsistent with all other known CED. This behavior may be accounted for qualitatively in terms of small variations in the Coulomb energy arising from shape changes.

Band termination spectroscopy in 157Er

The level scheme of 157Er has been extended from a spin region where the nucleus behaves as a prolate rotor to a region where the spin is produced by the alignment of all or most of the available valence nucleons along the symmetry axis of a weakly deformed oblate shape. The level scheme was established at high spin using up to four-fold gamma -ray coincidences detected in the Eurogam spectrometer following the reaction 114Cd(48Ca,5n)157Er at a bombarding energy of 210 MeV. Particularly favoured states have been established at IK=69/2+, 81/2+, 71/2+, 77/2-, 87-/2 and 89-/2. Specific single-particle configurations are assigned to these special states by comparison with cranked Nilsson-Strutinsky calculations. These states are related to structures observed in the neighbouring nuclei 158Er and 157Ho. These data provide the spectrum of single-particle states for the lowest lying valence orbitals above the 146Gd closed core.

Spectroscopy of proton-rich 66^Se up to J^π = 6^+: isospin-breaking effect in the A = 66 isobaric triplet

Candidates for three excited states in the 66^Se have been identified using the recoil-{beta} tagging method together with a veto detector for charged-particle evaporation channels. These results allow a comparison of mirror and triplet energy differences between analogue states across the A = 66 triplet as a function of angular momentum. The extracted triplet energy differences follow the negative trend observed in the f_7/2 shell. Shell-model calculations indicate a continued need for an additional isospin non-conserving interaction in addition to the Coulomb isotensor part as a function of mass.

Study of the superdeformed band in 194Pb and 192Hg with EUROGAM

A subset of the Eurogam array has been used to investigate the decay out of the superdeformed (SD) band in 194Pb. Two new band members and a new decay pattern into the low lying states at normal deformation (ND) have been established for this band. In the level scheme at normal deformation, the linking transitions between the previously known floating ΔI = 1 sequence and the lower states have been established for the first time. In this talk we will also report on the preliminary results of the search for the linking transitions between the SD and ND states in 192Hg with the fully operational EUROGAM array.

Recoil-isomer tagging near the mass 140 proton drip line

This contribution discusses the recoil-isomer tagging technique and its application to study isomeric states near the mass 140 proton drip line. The recoiling nuclei from a fusion-evapn. reaction were implanted into a silicon detector at the focal plane of a gas-filled separator, where prompt and delayed g-ray transitions were correlated across isomeric states. New states were established upon a known 15 ms isomer in 142Tb and the feeding and decay of a new 0.5(2)ms isomeric state was established in 144Ho. This measurement represents the first observation of excited states in 144Ho. [on SciFinder (R)]