Lifetime measurements of excited states in Os-169,Os-171,Os-173: Persistence of anomalous B(E2) ratios in transitional rare earth nuclei in the presence of a decoupled i(13/2) valence neutron

Lifetimes of low-lying excited states in the nu 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(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 neighbouring 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 eveneven 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. (C) 2021 The Author(s). Published by Elsevier B.V.