Lifetimes of the first excited 2⁺ states in ^176,178,180Os

By use of the pulsed-beam technique, the lifetimes of the first excited 2⁺ states in ^176,178Os were measured for the first time and the lifetime of the 2₁⁺ state in ¹⁸⁰Os was determined to a greater accuracy. In addition, for ¹⁷⁸Os, 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^π=7^- isomers in ¹⁸⁰Os were determined more accurately. Together with previously published data for the even-even osmium isotopes, the newly determined B(E2,2₁⁺→0₁⁺) transition strengths show a maximum value at the N=104 midshell. This maximum corresponds to the simple expectation of the N_πN_ν rule of the interacting boson approximation (IBA) but remains to be explained by microscopic models.