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${}_{\mathrm{R}}$, 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, 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 - 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 (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${}_{\mathrm{R}}$ variation adequately. The paired superfluid system of nucleons in these nuclei, and their excitations, present properties of general physics interest. The new-found systematic behaviour of g${}_{\mathrm{R}}$ 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.

Comments: 5 pages, 2 figures, published in Physics Letters B726, 675 (2013)