Gamma-ray spectroscopy of 127Ba at moderate spins

The near-yrast spectroscopy of 127Ba has been examined in detail up to spin 392 with the reaction 117Sn(13C, 3n) at 60 MeV incident energy. Gamma-ray spectroscopy was performed with the 8 π 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]52, [523]72, [411]12 and [402]52. The properties of the [523]72 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 Q1Q2 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∼−20°. 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 νh112 π(h112)2 and νd52 π(h112)2. However, the third band presented difficulties and no assignment was consistent with all the data. The shift in crossing frequency for the π(h112)2 with a νh112 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 νh112. The opposite shift of ∼300 keV for the same crossing with a νd52 spectator cannot be explained as a shape polarization effect.