Lifetime measurements of excited states in 169,171,173Os: Persistence of anomalous B(E2) ratios in transitional rare earth nuclei in the presence of a decoupled i13/2 valence neutron

Abstract Lifetimes of low-lying excited states in the ν i 13 / 2 + bands of the neutron-deficient osmium isotopes 169,171,173Os have been measured for the first time using the recoil-distance Doppler shift and recoil-isomer tagging techniques. An unusually low value is observed for the ratio B ( E 2 ; 21 / 2 + → 17 / 2 + ) / B ( E 2 ; 17 / 2 + → 13 / 2 + ) in 169Os, similar to the “anomalously” low values of the ratio B ( E 2 ; 4 1 + → 2 1 + ) / B ( E 2 ; 2 1 + → 0 gs + ) previously observed in several transitional rare-earth nuclides with even numbers of neutrons and protons, including the neighbouring 168,170Os. Furthermore, the evolution of B ( E 2 ; 21 / 2 + → 17 / 2 + ) / B ( E 2 ; 17 / 2 + → 13 / 2 + ) with increasing neutron number in the odd-mass isotopic chain 169,171,173Os is observed to follow the same trend as observed previously in the even-even Os isotopes. These findings indicate that the possible quantum phase transition from a seniority conserving structure to a collective regime as a function of neutron number suggested for the even-even systems is maintained in these odd-mass osmium nuclei, with the odd valence neutron merely acting as a “spectator”. As for the even-even nuclei, the phenomenon is highly unexpected for nuclei that are not situated near closed shells.