Solving the Puzzles of the Decay of the Heaviest Known Proton-Emitting Nucleus Bi-185

Two long-standing puzzles in the decay of Bi-185, the heaviest known proton-emitting nucleus are revisited. These are the nonobservation of the 9/2(-) state, which is the ground state of all heavier odd-A Bi isotopes, and the hindered nature of proton and alpha decays of its presumed 60-mu s 1/2(+) ground state. The Bi-185 nucleus has now been studied with the Mo-95(Nb-93, 3n) reaction in complementary experiments using the Fragment Mass Analyzer and Argonne Gas-Filled Analyzer at Argonne National Laboratory's ATLAS facility. The experiments have established the existence of two states in Bi-185; the short-lived T-1/2 = 2.8(-1.0)(+2.3) mu s, proton- and alpha-decaying ground state, and a 58(2)-mu s gamma-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 (Pb-184) with a major shell closure. The data also demonstrate that the ordering of low- and high-spin states in Bi-185 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.