By use of the pulsed-beam technique, the lifetimes of the first excited $2^{+}$ states in ${}^{176,178}\mathrm{Os}$ were measured for the first time and the lifetime of the $2_1^{+}$ state in ${}^{180}\mathrm{Os}$ was determined to a greater accuracy. In addition, for ${}^{178}\mathrm{Os}$, a recoil-distance Doppler-shift experiment and an experiment to measure the nuclear deorientation effect that is due to the hyperfine interactions were also performed. The results obtained from this measurement are consistent with the lifetime value extracted by means of the pulsed-beam experiment. As well, the lifetimes of two $I^{\pi }=7$${}^{-}$ isomers in ${}^{180}\mathrm{Os}$ were determined more accurately. Together with previously published data for the even-even osmium isotopes, the newly determined $B(E2,2_1^{+}\to 0_1^{+})$ transition strengths show a maximum value at the $N=104$ midshell. This maximum corresponds to the simple expectation of the $N_{\pi }{N}_{\nu }$ rule of the interacting boson approximation (IBA) but remains to be explained by microscopic models.

• Received 24 January 2005

DOI:https://doi.org/10.1103/PhysRevC.72.034306