145 Ba β − decay: Excited states and half-lives in neutron-rich 145 La

Abstract

Background: Neutron-rich nuclei in the A ≈ 140 – 160 mass region provide valuable information on nuclear structure such as quadrupole- and octupole-shape coexistence and the evolution of the collectivity. These nuclei have also a nuclear engineering interest because they contribute to the total decay heat after a fission burst. The information concerning 145 La is very limited. Purpose: The study of low-spin states in 145 La will provide a more detailed level scheme and enable the determination of the half-lives of the excited states. Methods: Low-spin excited states in 145 La have been investigated from the 145 Ba β − decay. The 145 Ba nuclei were directly produced by photofission in the ALTO facility or obtained from the β − decay of 145 Cs also produced by photofission. Gamma spectroscopy and fast-timing techniques were used. Results: A new level scheme was proposed including 67 excited levels up to about 3 MeV and 164 transitions. Half-lives in the few-nanosecond range were measured for the first excited states. Configurations for levels up to ≈ 600 keV were discussed. Conclusions: The available information on the low-spin states of 145 La has been modified and considerably extended. The analysis of the properties of the first excited states, such as excitation energies, decay modes, log ft values, reduced transition probabilities, and Weisskopf hindrance factors, has enabled the identification of the first members of the bands corresponding to the g 7 / 2 , d 5 / 2 , and h 11 / 2 proton configurations.