Observation of the proton emitter $^{57}_{116}$La$_{59}$

The quantum tunneling and emission of a single constituent nucleon provide a uniquely sensitive probe of nuclear structure at the limits of stability. For odd-odd proton emitters, the decay energy and partial half-life can reveal neutron-proton (np) correlations, including a theoretically predicted isoscalar np pairing mode whose experimental signature remains elusive. We report proton emission from the lanthanum isotope 116La, 23 neutrons away from the only stable La isotope (139La). 116La nuclei were produced in the 58Ni(64Zn, p5n)116La fusion-evaporation reaction and identified via proton radioactivity using the MARA mass separator and focal-plane silicon detectors. The measured proton energy Ep = 718 ± 9 keV and half-life T1/2 = 50 ± 22 ms, compared with Universal Decay Law calculations, indicate emission with orbital angular momentum l = 2 and an enhanced emission probability relative to the neighboring odd-even isotope 117La, despite a lower Q value. We propose this enhancement as a possible signature of strong neutron-proton pair correlations in this exotic neutron-deficient system. Isomeric γ decays from 116La and 117La are also reported.