Transition quadrupole moments of high-spin states in 172W and their implications for the interpretation of band crossings in the light isotopes of W to Pt

Lifetimes of states in 172W have been measured by the Doppler-shift recoil-distance method using the reaction 124Sn(52Cr, 4n)172W at a bombarding energy of 225 MeV. The data were collected in the γγ-coincidence mode rather than the customary singles mode in order to reduce the complexities of the γ-ray spectra and to avoid some of the problems associated with side feeding to excited states. At low spin (I ≤ 8ℎ) the experimental transition quadrupole moments, Qt, are all near a constant value of 7.0 e.b. For states in the yrast band with I ≥ 10ℎ, the experimental Qt values are all near a constant value of 6.8 e.b. This is the trend predicted by cranked Hartree-Fock-Bogoliubov (HFB) calculations and by the calculated systematics of triaxial shape-driving forces which originate from the aligned i13/2 neutron orbitals around N=98. From the extensive total routhian surface (TRS) calculations in this mass region, it now seems clear that the low-lying band crossings in the light W to Pt isotopes are due to the alignment of various neutron i13/2 quasiparticle pairs and not proton h9/2 pairs. The Qt values of several states in the negative-parity bands are significantly smaller than those in the yrast band. Finally, a number of interband reduced E1 transition probabilities are extracted from the data. (orig.)