Identification of Prompt Proton Emission in N=Z-1 ^{61}Ga: Isospin Symmetry at the Limit of Nuclear Binding

Excited states in the proton drip line nucleus ^{61}Ga were populated via the fusion-evaporation reaction ^{24}Mg(^{40}Ca,p2n)^{61}Ga. The experimental setup at Argonne National Laboratory comprised a novel combination of the Gammasphere array with two CD-shaped double-sided Si-strip detectors inside the Microball CsI(Tl) charged-particle detection array, as well as the Neutron-Shell liquid scintillators and the Fragment Mass Analyzer. Owing to the setup's unprecedented in-beam proton spectroscopy and tracking capabilities, a coincidence between a 957.6(5)-keV γ ray and a 1.876(24)-MeV proton line was observed, which identifies the quasibound proton πg_{9/2} single-particle state in ^{61}Ga at E_{x}=2150(34)  keV. This probes isospin symmetry at the limit of nuclear binding by providing a unique challenge for the shell-model interpretation of mirror nuclei beyond doubly magic ^{56}Ni.