Half-life and internal conversion electron measurements in low-lying levels of ^{125,127}Ba

Abstract

Decay spectroscopy using on-line mass separators has the advantage of allowing the study of level properties in the low energy region, including band head information, since g transitions between low-spin states can be measured more intensively under lower background conditions than can those with in-beam spectroscopy measurements. Neutron-deficient nuclides of 125,127 Ba have been studied by means of in-beam spectroscopy, and the level structures for high-spin states were successfully interpreted within the framework of the IBFM model @1‐5#. Decay studies of these nuclides have rarely been reported. Therefore, the last evaluations in Refs. @6, 7# were mainly based on in-beam studies. In 1991, the possibility of static octupole deformation, similar to that of the A5145 and 225 regions proposed, was proposed for the A5130 region by Cottle @8#. The presence of static octupole deformation is represented by parity doublets, which have equal spins and opposite parity levels of rotational bands with alternating bands. The E1 transitions between parity doublets are characterized by a two to four orders of magnitude enhancement compared to those of more normal cases. The 127‐130 Ba isotopes were successively studied by in-beam conversion electron measurements to investigate for parity doublets, and no evidence of them was observed @9,10#. These studies, however, focused only on the conversion electron measurements; the transition probabilities were not measured. It is necessary to measure not only the conversion electrons, but also the half-lives of the excited states in order to explain this feature. The aim of this investigation was to study the level properties of 125,127 Ba in the low energy region, focusing on a search for E1 transitions, and to study their properties by measuring the half-lives of low-lying excited states. The half-lives and conversion electrons were measured for the first time through the decays of 125,127 La, by means of a delayed coincidence technique and a cooled Si~Li! detector, respectively. The expected enhanced E1 transitions were not observed. Level properties based on the Nilsson model are proposed.