States built on the 9/2(+) isomers in Y-91,Y-93
The isotopes Y-91,Y-93 are located close to Zr-90, a nucleus that has a neutron shell closure (N = 50) and a proton subshell closure (Z = 40), and close to the stability line. However, relatively little is known about excitations above their 9/2(+) isomers. In the present work the isotopes Y-91,Y-93 have been studied in the fission of compound systems formed in two heavy-ion-induced reactions using the Gammasphere array for gamma-ray spectroscopy. States with excitation energies up to 6.9 and 7.1MeV above the previously known 9/2(+) isomers of Y-91,Y-93 were established, respectively. The coupling of a proton-hole occupying the g(9/2) orbital to the yrast states in the core nuclei of Sr-90 and Zr-92 can account for the first excited states of Y-91. A comparison with the first excited states in the neighboring N = 52, 54 isotopes is also considered. The experimental results are compared to predictions from previously reported shell-model calculations. The present results enhance previous knowledge on Y-91,Y-93 from fusion-evaporation reactions and multinucleon transfer reactions, and highlight the roles of these complementary reactions in probing moderate spin excitations in this mass region.