Seniority Isomers and Particle–hole Conjugation

Seniority is the number of nucleons not in pairs coupled to zero angular momentum. Conditions for the conservation of seniority are well known and are shown here to be related to particle–hole conjugation. The symmetry of seniority gives rise to selection rules in the γ decay of nuclei, which may lead to the formation of isomers. Examples of seniority isomers are discussed in the nickel and lead isotopes, and in the N=50 , 82 and 126 isotones. It is shown that a strong attractive quadrupole matrix element of the nucleon–nucleon interaction disfavours the existence of seniority isomers.