R. Holzmann

Octupole correlation effects near Z=56, N=88

Abstract Partial decay schemes for the very neutron-rich nuclei 146, 148, 150Ce have been determined by the study of γ-γ coincidences in 252Cf fission fragments. Similar behavior to that seen in 144, 146Ba, 148Nd and 150Sm, and interpreted in terms of strong octupole correlations, has been observed in yrast levels of 146Ce but not in 148Ce and 150Ce. This is in agreement with the predictions of cranked mean field calculations.

A prescription for the removal of Compton-scattered gamma rays from gamma-ray spectra

Abstract A method for the interpolation between and extrapolation from measured detector response functions is described. This technique enables the removal of the Compton-scattered background (i.e. “stripping” or “unfolding”) of γ-ray spectra quickly, easily and accurately. Examples are presented, for both source and in-beam spectra, taken with Ge detectors. Some potential pitfalls in the accurate unfolding of spectra are discussed, and approximate remedies suggested.

Lifetime measurements of terminating and collective high-spin states in 155DY and 156DY

Abstract Lifetime measurements for the isotopes 155 Dy and 156 Dy have been extended up to spins around 40ħ by the Doppler shift attenuation method in conjunction with γ-γ-coincidence measurements. Results have been obtained for the yrast band in 156 Dy and for three bands in 155 Dy. Strong retardations of the E2-strengt were observed for intruding terminating sequences, for which specific single particle configurations are suggested.

Level structure of 148Gd up to I=44☆

Abstract The level scheme of 148Gd has been extended to I=44 by use of a Compton-suppressed Ge spectrometer array. Up to I=38 the observed level spectrum consists of spherical and oblate states of aligned-particle type. A change in structure along the yrast line is indicated at higher spins, where fast E2 transitions suggest the onset of collectivity.

Lifetime measurements in 184Pt and the shape coexistence picture

Abstract Lifetimes for levels in the yrast band of 184 Pt have been measured up to spin 16 + using the recoil distance technique. The B (E2) values exhibit a marked increase in going from spin 2 to 10, consistent with a proposal that two bands of different deformations are mixing at low spin. This provides further support for shape coexistence occuring at low excitation energies in this region.

Evolution of nuclear structure with increasing spin and internal excitation energy in 152Dy

Abstract The total γ ray spectrum emitted by 152 Dy has been measured in two different reactions and decomposed into its constituent parts. From the measured decay times, multiplicities, multipolarities and spectral shapes, the average decay path has been reconstructed. The yrast single-particle structures have been shown to give way to highly collective bands at internal excitations energies > 1.5 MeV. A model, which takes into account the competition between statistical and collective decay at high spin and temperature, has been used to fit all features of the data, yielding Q t =7.0 +2.5 −1.5 e b for the collective bands.

Discrete and continuum Gamma-ray studies of 147Gd and 148Gd

Abstract Discrete and continuum γ-ray studies of the N = 83, 84 nuclei 147 Gd and 148 Gd have been performed using ( 36 S, 5n) and ( 36 S, 4n) reactions on 116 Cd. The level schemes of both nuclei have been extended to excitation energies above 18.5 MeV, with assigned spins up to 83 2 in 147 Gd and up to 46 in 148 Gd. Evidence for a change in character along the yrast line from single particle to collective was found at the highest spins in 148 Gd, but not in 147 Gd. For 148 Gd a study of γγ correlation matrices revealed a ridge-valley structure with a ridge spacing indicative of superdeformation. No evidence for a similar structure in 147 Gd was found.

Evidence for superdeformation in 148Gd

Abstract γ-γ transition energy correlation measurements were performed in 148Gd using Compton-suppressed Ge detectors. A broad first ridge was observed for 1.00 I (2) =78 h ℏ 2 MeV − is consistent with superdeformation in 148Gd. The results can be explained by cranked Strutinsky calculations.

Average spins of primary fission fragments

Abstract The average spins at which final fragments are formed after fission following fusion of 19 F with 197 Au have been measured directly from spectroscopic studies of discrete γ-ray lines. The final fragment spins can be translated into average spins at which primary fragments are formed by adding changes due to statistical γ-ray emission and post-fission neutron emission. The average primary fragment spins show marked variations with fragment mass. Possible causes for the structure are discussed within the framework of the statistical scission model of heavy-ion induced fission.

A new method for measuring the neutron-induced background in BGO Compton-suppressed Ge detectors applied to in-beam γ-ray studies

Abstract A method for measuring the response of intrinsic Ge detectors equipped with bismuth germanate (BGO) anti-Compton shields to evaporation neutrons is presented.