Establishing the ground-state spin of 71Kr

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 71Kr and 71Br which has been under debate for several decades. In this paper we report a new measurement of the absolute β-branching to ground and excited states which, taken with our precise lifetime of T1/2=94.9(4)ms, gives a superallowed ground state–to–ground state log(ft) value of 3.64(4). This is only consistent with both 71Br and 71Kr having the same spin and parity, Jπ=5/2−, as expected from mirror symmetry. The β-delayed proton emission to the first-excited state in 70Se was observed for the first time which also strongly supports this assignment.