Fine structure in the alpha-decay of the neutron-deficient 191Po and 191Bi isotopes

The alpha-decay properties of the neutron-deficient Po-191 and Bi-191 nuclei have been investigated at the RITU gas-filled separator using the 5n and p4n evaporation channels of the Ar-36+Dy-160-->Po-196* reaction. In contrast to previous studies two alpha-decaying isomeric states were observed in the decay of 191Po. Evidence was found for alpha-decay to excited levels in the Pb-187 daughter nucleus. The tentative interpretation of these levels could be a weak coupling of the odd neutron to an intruder 2p-2h deformed 0(+) state in the even-even core. The difference in the reduced alpha widths of the alpha transitions towards the ground state and excited state points to a deformed 13/2(+) isomeric state in Po-191. Branching ratios of b(alpha)(Pb-187m)=12(2)% and b(alpha)(Pb-187g)=7(2)% have been experimentally measured. For Bi-191 two new alpha-lines have been observed. A more precise half-life value of T-1/2 = 115(10)ms was determined for Bi-191m.