Anomalous 𝐵_4/2 ratio in the yrast band of ¹⁶⁷Os

Several instances of anomalously low values of the 𝐵⁡(𝐸⁢2;4⁺ ₁→2⁺ ₁) 𝐵⁡(𝐸⁢2;2⁺ ₁→0⁺ _gs) ratio (𝐵_4/2)<1 have previously been observed in several nuclei among the highly neutron deficient W-Os-Pt isotopes with even numbers of neutrons and protons. The relatively large number of cases suggests a striking new phenomenon that currently has no theoretical explanation. A clue to its origin might be achieved by comparing the 𝐵_4/2 ratios in even-even nuclei with the corresponding ratios of transition rates in neighboring odd-mass nuclei. However, until now only one case ( ¹⁶⁹ Os ) has been observed with an unusually low corresponding 𝐵_4/2 ratio, approximately 1⁢𝜎 below 1. Electromagnetic transition rates between excited states in the extremely neutron deficient nuclide ¹⁶⁷ Os
were measured using 15 single-crystal germanium detectors from the jurogam 3 array coupled to the RITU gas-filled separator. The excited states of interest were populated via the ⁹² Mo ⁢( ⁷⁸ Kr ,2⁢pn) fusion-evaporation reaction and studied by selecting the rare reaction channel via recoil-𝛼-decay tagging. The lifetimes of low-lying states were measured using the recoil distance Doppler shift method with the APPA plunger device. From the measured lifetimes in the rotational-like band structure assigned to the 𝜈⁢𝑖_13/2 yrast intruder configuration, the 𝐵⁡(𝐸⁢2;17/2⁺→13/2⁺) and 𝐵⁡(𝐸⁢2;21/2⁺→17/2⁺) values were deduced, resulting in the ratio 𝐵⁡(𝐸⁢2;21/2⁺→17/2⁺) 𝐵⁡(𝐸⁢2;17/2⁺→13/2⁺) =0.49⁢(10). The results were compared with previous measurements in the Os isotopic chain and theoretical mean-field calculations, implying that the odd neutron valence particle has limited, if any, influence on the 𝐵_4/2 values for this particular single-particle configuration.