High-spin states in 155Er

The high-spin decay scheme of Er-155, known previously, has been considerably extended by using the EUROGAM II gamma -ray array. The so-called quasi-isotropic gating data-analysis method., capable of providing spin and parity assignments with triple or quadruple coincidence data has been used. With such coincidence information, we have determined gamma -ray angular distributions, DCO ratios, and linear polarizations for many of the observed transitions. The level scheme is interpreted by using the particle-hole analysis based on the deformed Woods-Saxon potential and the monopole pairing interaction. The results indicate that at similar to 12.3 MeV excitation energy, all nine valence particles around the Gd-146(64)82 core exhaust the maximum spin of I= 79/2 (h) over bar. At higher spin, relatively high-energy transitions, of the order of similar to1.5 MeV, related to the breaking of the proton or neutron cores are observed. According to the calculations, this corresponds to an oblate-shape polarization of the nucleus with quadrupole deformation beta (2)similar to -0.12.