Low-spin structure of Br513586 and Kr503686 nuclei: The role of the g7/2 neutron orbital

Abstract

Low-spin excited levels in $_{35}^{86}\mathrm{Br}_{51}$ and $_{36}^{86}\mathrm{Kr}_{50}$, populated following ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay and the neutron-induced fission of $^{235}\mathrm{U}$, were measured using the Lohengrin fission-fragment separator and the EXILL array of Ge detectors at the PF1B cold-neutron facility of the Institute Laue-Langevin Grenoble. Improved populations of excited levels in $^{86}\mathrm{Br}$ remove inconsistencies existing in the literature on this nucleus. Directional-linear-polarization correlations, analyzed using newly developed formulas, as well as precise angular correlations allowed the unique ${1}^{\ensuremath{-}}$ and ${2}^{\ensuremath{-}}$ spin and parity assignments to the ground state of $^{86}\mathrm{Br}$ and the 4016.3-keV level in $^{86}\mathrm{Kr}$, respectively. Based on these results we propose that the Gamow-Teller ${\ensuremath{\beta}}^{\ensuremath{-}}$ decays of $^{86}\mathrm{Se}$ and $^{86}\mathrm{Br}$ involve the $\ensuremath{\nu}{g}_{7/2}\ensuremath{\rightarrow}\ensuremath{\pi}{g}_{9/2}$ transition in addition to the $\ensuremath{\nu}{p}_{3/2}\ensuremath{\rightarrow}\ensuremath{\pi}{p}_{3/2}$ transition proposed earlier. In $^{86}\mathrm{Kr}$ we have identified ${1}_{1}^{+}, {2}_{3}^{+}$, and ${3}_{1}^{+}$ levels, analogous to the mixed-symmetry states in $^{94}\mathrm{Mo}$, which in $^{86}\mathrm{Kr}$ are from proton excitations, only. Large-scale, shell-model calculations with refined interactions reproduce well excitations in $^{86}\mathrm{Br}$ and $^{86}\mathrm{Kr}$ and support our interpretations.