Probing the N = 14 subshell closure: g factor of the [formula omitted] state

B.P Mccormick,A.E Stuchbery,T Kibédi,G.J Lane,M.W Reed,T.K Eriksen,S.S Hota,B.Q Lee,N Palalani

doi:10.1016/j.physletb.2018.02.032

Abstract

The first-excited state g factor of 26Mg has been measured relative to the g factor of the Mg24(21+) state using the high-velocity transient-field technique, giving g=+0.86±0.10. This new measurement is in strong disagreement with the currently adopted value, but in agreement with the sd-shell model using the USDB interaction. The newly measured g factor, along with E(21+) and B(E2) systematics, signal the closure of the νd5/2 subshell at N=14. The possibility that precise g-factor measurements may indicate the onset of neutron pf admixtures in first-excited state even–even magnesium isotopes below 32Mg is discussed and the importance of precise excited-state g-factor measurements on sd shell nuclei with N≠Z to test shell-model wavefunctions is noted.

Highlights

The evolution of nuclear shell structure in exotic, radioactive neutron-rich nuclei is being studied intensively
As the G2 and G4 parameters are highly correlated for the available data, they were related through a single J = 1/2 electron-spin (Hlike) fraction parameter, as described in a previous study of highvelocity 24Mg ions [38], which used a methodology similar to that of the present measurement
We offer this suggestion because the measurements on 24Mg in iron reported by Speidel et al [27] seem to agree with other independent measurements, and correspond to expected Btf values for Ne, Mg and Si ions traversing iron at similar ion velocities [29,42]

Summary

Introduction

The evolution of nuclear shell structure in exotic, radioactive neutron-rich nuclei is being studied intensively. Speidel et al made a new measurement using the thin-foil transient-field method, which excludes the static field, and obtained g = +0.50(13), in agreement with Hartree– Fock calculations available at the time This result, which implies near equal contributions from protons and neutrons, is currently listed as the adopted value in Nuclear Data Sheets [26]. Modern shell model calculations and single-particle arguments contend that the N = 14 subshell closure should result in g(2+1 ) being much more heavily influenced by the proton contribution than the currently adopted measurement indicates Both Eberhardt et al and Speidel et al used (α, α ) reactions to excite and recoil 26Mg ions into an iron host. The 2+1 -state g factor of 26Mg was measured relative to a recent independent and precise measurement of g(2+1 ) in 24Mg [30]

Experiment

Results and analysis

Discussion