Normal deformed bands and possibility of TSD structures in the $N=92$ $^{166}$W nucleus
Résumé
Triaxial Strongly Deformed (TSD) band structures have been observed in several LuHf-Ta isotopes in the A∼165 region. The occurrence of TSD bands in Lu (Z=71) and Hf (Z=72) isotopes is not consistent with predicted proton and neutron shell gaps, probably due to inadequate knowledge of energies of high-j orbitals at large triaxiality. Based on calculated shell gaps, among the Lu isotopes, 165Lu94 and 168Lu97 are good candidates for TSD structures. However, the strongest TSD band in 163Lu92 is observed to be three times more strongly populated than the corresponding one in 165Lu94, and TSD structures have not yet been observed in 168Lu97. Further, no TSD bands have been observed in 166Hf94, which is located at the centre of the predicted TSD island. The above observations appear to indicate that, at large triaxiality, TSD structures may be favored at N=92 than at N=94. Calculations by R. Bengtsson indicate that TSD band in 166W, with a positive γ value should become yrast at spin, I∼44~. The aforesaid systematic calculations of TSD and Normal Deformed (ND) structures in Hf, Yb and W isotopes further suggest that the N=92 isotones in these three elements have higher excitation energies at normal deformation in the spin range of 30-50 ~ [1]. So, the favorable position of TSD bands in the N=92 isotones may be an outcome of the fact that the band at normal deformation goes up in energy for this neutron number. There were two previous measurements to study high-spin states in 166W. In the first measurement, a 28Si beam was used by Gerl et al. [2] and four HPGe detectors were used to record data. In the second measurement by Simpson et al. [3], the p2n reaction channel was used, and the deexciting γ rays were detected using the POLYTESSA array. The positive-parity yrast band was observed up to an excitation energy of 9 MeV, and spin, I = 30~, well below where possible TSD bands are expected to be yrast. Two other negativeparity rotational bands were also reported up to spin, I = 23~.