Spectra of the gamma rays and internal conversion electrons emitted in the decay of 12-day ${\mathrm{Ir}}^{190}$ have been studied in detail. Internal conversion spectra were obtained with the aid of double-focusing and intermediate-image beta spectrometers, and a permanent-magnet spectrograph. Gamma-ray spectra were obtained by means of photoelectric conversion, employing a double-focusing spectrometer, and by scintillation techniques.

These measurements of the energies and relative intensities of gamma rays and internal conversion electrons give internal conversion coefficients for a number of transitions. These data, coupled with co-incidence studies of gamma rays, support the level scheme reported by Nielsen et al. for ${\mathrm{Os}}^{190}$ and establish new, odd-parity levels at 1384, 1568, and 1876 kev. An upper limit of 2×${10}^{-5\mathrm{}}$ is set on positron branching in the decay of ${\mathrm{Ir}}^{190}$. Relative electron capture transition probabilities for the decay of ${\mathrm{Ir}}^{190}$ to ${\mathrm{Os}}^{190}$ and ratios of reduced transition probabilities for electromagnetic transitions from a number of levels of ${\mathrm{Os}}^{190}$ follow from these results. They are compared with the predictions of the strong-coupling model and the asymmetric rotor model. The half-life of ${\mathrm{Ir}}^{190}$ was found to be 12.3±0.4 days. Even-$A$ iridium-osmium total disintegration energies are found to rise substantially above the values predicted by semiempirical mass formulas, suggesting a possible effect of the change in the nuclear deformation in this region.

• Received 13 May 1960

DOI:https://doi.org/10.1103/PhysRev.119.1953