Abstract

Lifetimes of low-lying excited states in the vi_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(E2; 21/2⁺ → 17/2⁺) /B(E2; 17/2⁺ → 13/2⁺) in ¹⁶⁹Os, similar to the “anomalously” low values of the ratio B(E2; 4⁺₁ → 2⁺₁ )/B(E2; 2⁺₁ → 0⁺_gs) previously observed in several transitional rare-earth nuclides with even numbers of neutrons and protons, including the neighboring ^168,170Os. Furthermore, the evolution of B(E2; 21/2⁺→ 17/2⁺)/B(E2; 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.