Yrast isomers in exoticN=81 nucleusYb151studied using a fragment mass analyzer

Abstract

Product recoils from the reaction Ru +255 MeV ' Ni -+ ' Hf* were analyzed using the Argonne Fragment Mass Analyzer and transported to a collector foil behind the focal plane, where decay of ps isomers could be studied under low background conditions. The recoils were dispersed in A /q as they traversed the position-sensitive focal-plane detector, and recoily-ray coincidences could thus be used to assign masses for specific y-ray cascades. By use of these methods, yrast isomers with half-lives of 2.6(7) ps and 20(1) ps have been firmly assigned to the exotic N =81 nucleus "'Yb, and the main features of the isomeric decays have been established. The yrast level structure of "'Yb and lighter N =81 isotones are compared.