J.C. Lisle

Measurements on prototype Ge and BGO detectors for the Eurogam array

Abstract This paper presents the results of tests, both with sources and in-beam experiments, on prototype Ge and BGO detectors of the Eurogam array. Timing and energy resolution data are reported along with results for various methods of ballistic deficit correction. The first measurements with the new “shared suppression” technique are presented.

Lifetimes of high spin rotational states

Abstract Lifetimes of 8 + , 10 + and 12 + states in fourteen rotational and quasi-rotational doubly even nuclei have been measured with a Doppler broadened lineshape technique. In addition, the lifetime of the 14 + state in 164 Dy has been determined. The B (E2) values have been extracted and compared with rotational model predictions. All the 8 + → 6 + and 10 + → 8 + transitions and most of the 12 + → 10 + transitions are well described by the model. However, the 12 + → 10 + transitions in 162 Dy, 164 Er and 170 Yb are found to be retarded significantly. It is Suggested that the hindrances may be related to the known backbending properties of two of these nuclei.

Study of 170,171,173Ta isotopes at high spinṡ

Abstract High-spin data are presented for 170 Ta and 171 Ta for the first time, and the known level scheme of 173 Ta is extended to higher spins. Anomalies found in the crossing frequencies for the alignment of a pair of neutrons are considered. At high spin a large and constant value of J (2) , with J (2) > J (1) , is found for two 171 Ta bands and the implications of this are discussed. The π i 13/2 [660]1/2 + , α = + 1/2 orbital is identified as it crosses the rotational band built on the [402]5/2 + , α = + 1/2 orbital in 171 Ta. Magnetic transition probabilities obtained from measured branching ratios show the effects of the aligning pair of neutrons and the spectator orbital. The data are interpreted using cranked shell-model calculations and a semiclassical vector-coupling scheme developed by Donau and Frauendorf.

Single-neutron states at high spins in ytterbium nuclei: Evidence for the quenching of static neutron pair correlations

Abstract The decay schemes of 167,168,169 Yb are established to the largest angular momenta yet known in stably-deformed rare-earth nuclei using the 124 Sn( 48 Ca,3–5n) reaction. The systematics of the resulting spectrum of single-neutron states in these and neighbouring Yb isotopes at large and small angular momenta are discussed with regard to that expected in the presence and absence of static neutron-pair correlations. This comparison, together with cranking-model and “gauge-space” analyses, provides evidence for the effective disappearance of static neutron-pair correlations for the odd- N isotopes at ħω ⩾ 0.38 MeV. A composite empirical spectrum of single-neutron states is constructed for the “unpaired” regime and is compared with cranking calculations.

High-spin spectroscopic study of 167Lu

Abstract Three decay sequences associated with the 7 2 + [404], 9 2 − [514] and 1 2 − [541] Nilsson states of 167 Lu have been established to high spin using the 123 Sb( 48 Ca,4n) 167 Lu reaction. The quasineutron and quasiproton band crossings, the energy signature splittings, and the B (M1)/ B (E2) values are discussed and compared with similar quantities for the lighter lutetium isotopes. Configuration-and isotopic-dependent deformations, reflected in such comparisons, indicate that 167 Lu has a larger and more stable prolate deformation than the lighter isotopes and that the 1 2 − [541] configuration has a larger quadrupole deformation than the 9 2 − [514] configuration. The shift of the AB neutron band crossing occurring in the 1 2 − [541] sequences to a larger rotational frequency cannot be understood in terms only of deformation differences. The configuration and neutron-number dependent shapes are also discussed in a “gauge”-space analysis.

Configuration dependent pairing from band crossing frequencies

Abstract Data are presented on the crossing frequencies of odd-parity rotational bands in 161 Er. The crossing frequencies are shown to be correlated with the sign and magnitude of the quadrupole moments of the valence neutron orbits in odd- N rare earth nuclei. This can be understood in terms of configuration-dependent “blocking” of pairing-energy contributions requiring the inclusion of quadrupole terms in the pairing-correlation energy.

Using high-fold data from the new generation of γ-ray detector arrays

Abstract The new generation of high-efficiency gamma-ray spectrometers allows the collection of high-fold gamma-ray coincidence data. Some problems associated with the sorting and subsequent analysis of this data are discussed. A method is presented to allow the creation of statistically correct spectra when selection criteria are placed on gamma-ray energies.

Excited bands in the doubly-magic superdeformed 152Dy nucleus: evidence for the first N = 7 proton hyper-intruder orbital

Abstract Five excited superdeformed bands have been observed in the doubly closed-shell superdeformed nucleus 152 Dy. Three of the new bands are interpreted in terms of single neutron excitations across the N = 86 shell gap. One of these bands appears to partly decay into the yrast superdeformed band. Another band involves a single proton excitation across the Z = 66 shell gap into the N = 7 hyper-intruder orbital. This is the first evidence of the influence of this deformation driving orbital on superdeformed structures in any mass region.

Angular momentum dependence of giant dipole vibrations in hot dysprosium nuclei

Abstract The spin dependence of the giant dipole resonance built on excited states in 156 Dy has been studied with a new technique using Ge-BGO spectrometers. The observed structure of the GDR and the measured γ-ray anisotropies are consistent with a change in nuclear shape from prolate to oblate with increasing spin. No evidence is found for centroid shifts nor for superdeformed shapes at high temperature.

Electromagnetic properties in 165Lu

Abstract Relative electromagnetic transition rates in 165Lu94 have been measured to high spin ( I≤ 63 2 ) for the two signature components of a band built on a π h 11 2 shell-model state. M1 transitions are observed through two band crossings. A DSAM measurement indicates that full collectivity persists for the π h 11 2 configuration up to I ≤ 53 2 and that for higher values of I a loss of collectivity could explain the observed increased values of B(M1, I → I−1)/B(E2, → I−2). The effects of alignment, shape changes and rotationally induced deoccupation of antialigned configurations are discussed. In the measured values of the ratio of transition quadrupole moments Q1/Q2 the influence of the vi 13 2 alignment is observed for the first time. The data are discussed within the context of triaxial nuclear deformation. Comparisons are made with the nuclei 161,163Lu.