New high spin states and band termination in Y-83 and Zr-84

The gamma decay of high spin yrast states in Y-83 up to I-pi = 59/2(+) and 53/2(-) have been observed using the reaction Ni-58(Si-29,3p) at 110 MeV and the Gammasphere Early Implementation Array. The level scheme has been substantially extended due to the observations of several new transitions in all of the bands. A sequence of transitions feeding into the positive parity yrast band above I-pi = 47/2(+) seems to be consistent with a noncollective oblate structure expected at these high spins. A similar cascade is found in the data for Zr-84. A new forking of the favored negative parity band is found which may be due to neutron alignment polarizing the core to a different shape. This suggests that the ''isomeric'' band in Y-83, for which one more connecting transition was found, is of a similar nature to other high-K bands found in this region. Lifetime measurements in the unfavored negative parity band are consistent with cranking calculations which predict a nearly oblate shape with a deformation parameter beta(2) approximate to 0.2. A qualitative analysis of line shapes at very high spins suggests the persistence of collectivity in the yrast sequence to the highest excitations seen.