YRAST ISOMERS IN EXOTIC N=81 NUCLEUS YB-151 STUDIED USING A FRAGMENT MASS ANALYZER

Product recoils from the reaction Ru-96 +255 MeV Ni-58 --> 154Hf were analyzed using the Argonne Fragment Mass Analyzer and transported to a collector foil behind the focal plane, where decay of mus isomers co uld be studied under low background conditions. The recoils were dispe rsed in A/q as they traversed the position-sensitive focal-plane detec tor, and recoil-gamma-ray coincidences could thus be used to assign ma sses for specific gamma-ray cascades. By use of these methods, yrast i somers with half-lives of 2.6(7) mus and 20(1) mus have been firmly as signed to the exotic N = 81 nucleus 151Yb, and the main features of th e isomeric decays have been established. The yrast level structure of 151Yb and lighter N = 81 isotones are compared.