Partial conservation of seniority and nuclear isomerism

Accepted for publication in Physical Review LettersWe point out the possibility of the {\em partial} conservation of the seniority quantum number when most eigenstates are mixed in seniority but some remain pure. This situation occurs in nuclei for the $g_{9/2}$ and $h_{9/2}$ shells where it is at the origin of the existence of seniority isomers in the ruthenium and palladium isotopes. It also occurs for $f$ bosons