Establishing the ground-state spin of Kr71

Nuclei in the vicinity of the $N=Z$ line provide many sensitive probes of isospin symmetry. One example concerns the character and sequence of low-lying states of the $T=1/2$ mirror pair $^{71}\mathrm{Kr}$ and $^{71}\mathrm{Br}$ which has been under debate for several decades. In this paper we report a new measurement of the absolute $\ensuremath{\beta}$-branching to ground and excited states which, taken with our precise lifetime of ${T}_{1/2}=94.9(4)\phantom{\rule{3.33333pt}{0ex}}\text{ms}$, gives a superallowed ground state--to--ground state $\mathrm{log}(ft$) value of 3.64(4). This is only consistent with both $^{71}\mathrm{Br}$ and $^{71}\mathrm{Kr}$ having the same spin and parity, ${J}^{\ensuremath{\pi}}=5/{2}^{\ensuremath{-}}$, as expected from mirror symmetry. The $\ensuremath{\beta}$-delayed proton emission to the first-excited state in $^{70}\mathrm{Se}$ was observed for the first time which also strongly supports this assignment.