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 beta-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, J pi = 5/2-, as expected from mirror symmetry. The beta-delayed proton emission to the first-excited state in 70Se was observed for the first time which also strongly supports this assignment.