Gamma-ray spectroscopy of 127Ba at moderate spins

The near-yrast spectroscopy of 127Ba has been examined in detail up to spin 39/2 with the reaction 117Sn(13C, 3n) at 60 MeV incident energy. Gamma-ray spectroscopy was performed with the 8π spectrometer, an instrument comprising 20 Compton-suppressed HPGe detectors and 71 BGO ball elements. Evidence is presented for the identification of single-neutron quasiparticle bands corresponding to Nilsson orbitals [532]5/2, [523]7/2, [411]1/2 and [402]5/2. The properties of the [523]7/2 band suggest that the nucleus has a triaxial shape when that orbital is occupied. We draw attention to: (i) The signature splitting; (ii) staggering of B(E2) ratios; (iii) the mean Q1/Q2 ratio; and (iv) the comparison of the staggering in the B(M1)/B(E2) ratios with the signature splitting. All these factors point to a γ-deformation in the neighbourhood of γ∝-20deg. Based on the large B(M1)/B(E2) ratios, the observed three-quasiparticle bands involve proton excitation. Of these bands, two have clear-cut structural assignments νh11/2 π(h11/2)2 and νd5/2 π(h11/2)2. However, the third band presented difficulties and no assignment was consistent with all the data. The shift in crossing frequency for the π(h11/2)2 with a νh11/2 spectator is very large and corresponds to a shift in the routhian of about 220 keV. This we interpret to be due to a negative γ-deformation for bands involving νh11/2. The opposite shift of ∝300 keV for the same crossing with a νd5/2 spectator cannot be explained as a shape polarization effect. (orig.)