Study of Even Gd Nuclei by Decay of Oriented Tb, With Analysis by Simple Boson Models

Abstract

We have studied low-spin states of 150,152,154,156Gd by nuclear orientation of β-decaying Tb in a Gd host. Especially by means of multipole mixing ratios, including E0/E2, we have checked and revised spin-parities and assignments to (quasi) rotational ground, β and γ bands. For 150Gd we propose the new interpretation 1207.2 keV (0β+), 1518.5 keV (2β+), 1700.1 keV (4β+), 1430.5 keV (2γ+), 1988.0 keV (3γ+), 2080.0 keV (4γ+). For 152,154,156Gd we find agreement with recent literature. Our comparison with theory includes available data on 158,160Gd and on states up to 10g+, 10β+, 7γ+. We review our previously proposed "projection model", which is basically of the Bohr-Mottelson geometrical type; its essential feature is angular-momentum projection from an intrinsic coherent phonon state. The algebraic boson model IBA-1 is also discussed. Both models are applied to energies and to E2 and E0 transition probabilities. Roughly viewed, the fits by both models are good for the energies, fair for the E2 transitions and poor for the E0 transitions. The four-parameter version of IBA-1 applied is unable to reproduce the nonadiabatic details of the bands, while even the two-parameter projection model does better. For 150Gd the simple boson models seem over-extended because of the proximity of doubly magic 14664Gd82. Description of the N = 88-90 shape transition requires discontinuous parameter behaviour in both models.