Solving the puzzles of the decay of the heaviest known proton-emitting nucleus 185Bi

Two long-standing puzzles in the decay of 185Bi, the heaviest known proton-emitting nucleus are revisited. These are the non-observation of the 9/2− state, which is the ground state of all heavier odd-A Bi isotopes, and the hindered nature of proton and α decays of its presumed 60-μs 1/2+ ground state. The 185Bi nucleus has now been studied with the 95Mo(93Nb,3n) reaction in complementary experiments using the Fragment Mass Analyzer (FMA) and the Argonne Gas-Filled Analyzer (AGFA) at Argonne National Laboratory’s ATLAS facility. The experiments have established the existence of two states in 185Bi; the short-lived, T1/2 = 2.8+2.3−1.0 μs, proton- and α-decaying ground state, and a 58(2)-μs γ-decaying isomer, the half-life of which was previously attributed to the ground state. The reassignment of the ground-state lifetime results in a proton-decay spectroscopic factor close to unity, and represents the only known example of a ground-state proton decay to a daughter nucleus (184Pb) with a major shell closure. The data also demonstrate that the ordering of low- and high-spin states in 185Bi is reversed relative to the heavier odd-A Bi isotopes, with the intruder-based 1/2+ configuration becoming the ground, similar to the lightest At nuclides.