Microsecond isomers in Tl-187 and Pb-188

Lifetime measurements of states in nuclei with A=187 and 188 have been perf ormed, using reactions between Gd-155 and Ar-36 and following the transport of evaporation residues to the focal plane of a gas-filled recoil separato r. In a separate experiment using the Tb-159(S-32,4n) reaction the gamma-de cay of isomeric levels in Tl-187 has been studied using delayed gamma-gamma coincidence measurements. From observation of their subsequent gamma decay , the mean lifetimes were measured to be 1000+/-55 ns and 1600+/-100 ns. Al though it was not possible to characterize the isomers completely, they are proposed as candidates for one-proton, two-neutron excitations. In the cou rse of this study, the decay of an isomer in Pb-188 was also observed and i ts lifetime was measured as 1150+/-30 ns, and limits of 20-600 ns were plac ed on the meanlife of an isomer conjectured in Pb-187.