The high-spin decay scheme of ${}^{155}\mathrm{Er},$ known previously, has been considerably extended by using the EUROGAM II γ-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 γ-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 $\sim 12.3$ MeV excitation energy, all nine valence particles around the ${}_{64}^{146}{\mathrm{Gd}}_{82}$ core exhaust the maximum spin of $I=79/2\mathrm{}ħ.$ At higher spin, relatively high-energy transitions, of the order of $\sim 1.5$ 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 ${\beta }_2\sim -\mathrm{0.12.}$

• Received 21 November 2000

DOI:https://doi.org/10.1103/PhysRevC.64.034313