Long-lived isomers in medium-heavy and heavy deformed nuclei

Abstract

The situation of long-lived (t 1 2 ≳ 1 s) isomers in deformed nuclei is critically examined both from experimental and theoretical viewpoints. A sequence of isomers with half-lives of the order of seconds and decaying through low-energy ( E < 300 keV) gamma rays to the ground states is identified in odd-mass nuclei. The phenomenon is widely occurring in doubly-odd deformed nuclei. For the six rare earth elements with 65 ⩽ Z ⩽ 75 and N ⩾ 91, multiple isomer sets are already known in 25 nuclides and predicted in 10 other isotopes. However, in 16 known cases the characterisation is uncertain. For the six transuranium elements, isomer pairs are known in 12 cases and are predicted in 12 other isotopes. Out of the 12 known pairs, only one isomer decays by gamma emission and only three have confirmed energy ordering. Guidelines for their proper characterisation and suggestions for identification of undiscovered isomers are presented.