Lifetime measurements of excited states in Os: Persistence of anomalous ( 2) Ratios in Transitional Rare Earth Nuclei in the Presence of a Decoupled Valence Neutron

Lifetimes of low-lying excited states in the $\nu {i}_{13/{2}^{+}}$ bands of the neutron-deficient osmium isotopes $^{169,171,173}$Os 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 $B\left(E2;21/{2}^{+}\to 17/{2}^{+}\right)/B\left(E2;17/{2}^{+}\to 13/{2}^{+}\right)$ in $^{169}$Os, similar to the "anomalously" low values of the ratio $B\left(E2;{4}_{1}^{+}\to {2}_{1}^{+}\right)/B\left(E2;{2}_{1}^{+}\to {0}_{\mathrm{gs}}^{+}\right)$ previously observed in several transitional rare-earth nuclides with even numbers of neutrons and protons, including the neighbouring $^{168,170}$Os. Furthermore, the evolution of $B\left(E2;21/{2}^{+}\to 17/{2}^{+}\right)/B\left(E2;17/{2}^{+}\to 13/{2}^{+}\right)$ with increasing neutron number in the odd-mass isotopic chain $^{169,171,173}$Os 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.