Band structures in 169Tm and the structures of Tm isotopes around N = 98

Abstract

The excited states of 169Tm have been studied via the 169Tm(32S,32S′)169Tm* reaction at the beam energy of 164MeV. The $\gamma$-rays were detected using the Indian National Gamma Array (INGA) setup, composed of 19 Compton-suppressed clover HPGe detectors. A new level scheme of 169Tm with 11 newly placed $ \gamma$-rays has been proposed. A band crossing in the $ \pi [541]1/2^{-}$ band and several interband E1 transitions between this and the $ \pi [411]1/2^{+}$ ground-state band have been observed for the first time in this nucleus. The role of the $N=98$ deformed shell gap has been discussed by comparing the band crossing parameters of the negative parity bands in Tm and other neighboring nuclei. The origin of the interband E1 transitions has been investigated in terms of coupling to octupole degrees of freedom. The shape evolution of the Tm isotopes around $ N=98$ have been studied in the projected and cranked shell model approaches, both of which predict a change in shape from an axial prolate to a triaxial one after band crossing in these nuclei. The new data and the calculations help to understand the unusual structural phenomena reported for the nuclei with $ N=98$.