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

Abstract

For the first time, a wide range of collective magnetic g-factors $g_R$, obtained from a novel analysis of experimental data for multi-quasi-particle configurations in high-K isomers, is shown to exhibit a striking systematic variation with the relative number of proton and neutron quasi-particles, $N_{\mathrm{p}}-N_{\mathrm{n}}$. Using the principle of additivity, the quasi-particle contribution to magnetism in high-K isomers of Lu–Re, $Z=71\text{–}75$, has been estimated. Based on these estimates, band-structure branching ratio data are used to explore the behavior of the collective contribution as the number and proton/neutron nature ($N_{\mathrm{p}}$, $N_{\mathrm{n}}$), 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 $g_R$ variation adequately. The paired superfluid system of nucleons in these nuclei, and their excitations, present properties of general physics interest. The new-found systematic behavior of $g_R$ in multi-quasi-particle excitations of this unique system, showing variation from close to zero for multi-neutron 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.