Electromagnetic transition probabilities in the doubly odd N = 91 nucleus 154Eu

Abstract

Nanosecond lifetimes of isomeric states in 154Eu excited in the (n, γ) reaction have been determined by the generalized centroid-shift method using a plastic scintillator and a planar thin-window germanium detector in delayed γγ-coincidence experiments. The following subnanosecond half-lives of excited states have been obtained for the first time: T 1 2 (203.8 keV) = 0.80 ± 0.10 ns, T 1 2 (239.3 keV) = 0.96 ± 0.15 ns and T 1 2 (281.7 keV) = 0.25 ± 0.10 ns . For eleven excited states, upper limits of the half-lives have been estimated, among them T 1 2 (99.9 keV) ≦ 2 ns in contradiction to a previous result. From these and earlier half-life data, absolute transition probabilities are deduced and compared with Weisskopf and Nilsson-plus-pairing-model estimates. The Nilsson model without extended configuration mixings turns out to be incapable of reproducing satisfactorily the experimental B(E1) strengths of allowed E1 transitions. By comparison of the experimental dipole transition rates between states of similar structure in 152Eu and 154Eu, stronger influence of configuration mixings in 154Eu is revealed. Several experimental transition moments of allowed El transitions deviate from the relevant systematics in strongly deformed rare-earth nuclei. It is concluded that even at low excitation energies and low angular momenta the N = 91 nucleus 154Eu behaves as a transitional nucleus similar to the N = 89 neighbour 152Eu.