Shell structure underlying the evolution of quadrupole collectivity inS38andS40probed by transient-fieldg-factor measurements on fast radioactive beams

Abstract

The shell structure underlying shape changes in neutron-rich nuclei between $N=20$ and $N=28$ has been investigated by a novel application of the transient field technique to measure the first-excited state $g$ factors in $^{38}\mathrm{S}$ and $^{40}\mathrm{S}$ produced as fast radioactive beams. Details of the new methodology are presented. In both $^{38}\mathrm{S}$ and $^{40}\mathrm{S}$ there is a fine balance between the proton and neutron contributions to the magnetic moments. Shell-model calculations that describe the level schemes and quadrupole properties of these nuclei also give a satisfactory explanation of the $g$ factors. In $^{38}\mathrm{S}$ the $g$ factor is extremely sensitive to the occupation of the neutron ${p}_{3/2}$ orbit above the $N=28$ shell gap as occupation of this orbit strongly affects the proton configuration. The $g$ factor of deformed $^{40}\mathrm{S}$ does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.