Studies of Decay Schemes in the Osmium-Iridium Region. I. IsomersOs190m(10 min)andOs189m(5.7 hr)

While the decay-scheme of the even-even isomer ${\mathrm{Os}}^{190m}\left(10\mathrm{} min\right)$ has some similarity with that of the "rotational" isomer ${\mathrm{Hf}}^{180m}$, it differs from the latter in that ${\mathrm{Os}}^{190}$ lies in the transition region between the nuclei with rotational and those with near-harmonic level schemes. It was previously believed that the lifetime determining transition in ${\mathrm{Os}}^{190m}$ was a 620-kev transition followed by three lower energy gamma rays. We find, however, that the isomeric transition is a previously overlooked 38.4-kev $M2\mathrm{}$ transition with an ${\mathrm{}\left|M\right|\mathrm{}}^2=2.6\mathrm{}\times {10}^{-9\mathrm{}}$ ("$K$ forbidden"). It is followed by four successive $\gamma$ rays. Internal conversion electron studies by means of an intermediate image spectrometer yielded $\gamma$-ray energies of 187±1 kev, 359±2 kev, 500±3 kev, and 614±3 kev. All four transitions are of electric quadrupole character, suggesting that the levels populated are 8+, 6+, 4+, 2+, 0+ (ground state). Delayed coincidence measurements by A. W. Sunyar show that the two lowest energy transitions of 359 and 187 kev take place between the levels 4+→2+ and 2+→0+ respectively and it may be expected that also the two higher energy $\gamma$ rays follow each other in the order of decreasing energy. However, in contrast to the level spacings in true rotational nuclei, the level energies are far from being proportional to $I(I+1\mathrm{})\mathrm{}$, and cannot even be represented with the help of a correction term $\sim I^2{\mathrm{}(I+1\mathrm{})\mathrm{}}^2$.