Decay ofSe75and the Lifetime of the 280-kev State ofAs75

Abstract

The decay of 120-day ${\mathrm{Se}}^{75}$ has been investigated employing NaI(Tl) scintillation spectrometer and coincidence technique. The intensities of the electron capture transitions to the 403-, 572-, and 628-kev states of ${\mathrm{As}}^{75}$ are found to be in the ratio of 1:5.3×${10}^{-4\mathrm{}}$:2.4×${10}^{-4\mathrm{}}$. On the basis of $log\mathrm{ft}$ values of the $\beta$ and electron-capture transitions the spins and parities of the 477, 572, 628, and 814-kev states of ${\mathrm{As}}^{75}$ are assigned as ${\mathrm{½}}^{-}$, ${\frac{5}{2}}^{-}$, ${\mathrm{½}}^{-}$ and ${\frac{5}{2}}^{-}$ (or ${\frac{7}{2}}^{-}$), respectively. Employing the delayed coincidence method and a time-to-pulse-height converter, the mean lifetime of the 280-kev state is measured as 7.8×${10}^{-10\mathrm{}}$ sec. The partial $E2\mathrm{}$ lifetimes for the 265- and 280-kev levels are estimated to be 1.1×${10}^{-9\mathrm{}}$ and 5.2×${10}^{-9\mathrm{}}$ sec, respectively.