Quasi-particle and collective magnetism: Rotation, pairing and blocking in high-K isomers

For the first time, a wide range of collective magnetic g-factors gR, obtained from a novel analysis of experimental data for multi-quasiparticle configurations in high-K isomers, is shown to exhibit a striking systematic variation with the relative number of proton and neutron quasiparticles, Np Nn. Using the principle of additivity, the quasi-particle contribution to magnetism in high-K isomers of Lu Re, Z = 71 75, has been estimated. Based on these estimates, band-structure branching ratio data are used to explore the behaviour of the collective contribution as the number and proton/neutron nature (Np, Nn), of the quasi-particle excitations, change. Basic ideas of pairing, its quenching by quasi-particle excitation and the consequent changes to moment of inertia and collective magnetism are discussed. Existing model calculations do not reproduce the observed gR variation adequately. The paired superfluid system of nucleons in these nuclei, and their excitations, present properties of general physics interest. The new-found systematic behaviour of gR in multiquasi-particle excitations of this unique system, showing variation from close to zero for multineutron states to above 0.5 for multi-proton states, opens a fresh window on these effects and raises the important question of just which nucleons contribute to the ‘collective’ properties of these nuclei.