The lifetime of the 0.559-MeV $2^{+}$ level of ${\mathrm{Se}}^{76}$ has been measured using a nuclear resonance fluorescence technique. In this work a gaseous source of As${\mathrm{H}}_3$ was employed and the 0.559-MeV gamma rays emitted following the beta decay of ${\mathrm{As}}^{76}$ were scattered from a Se scatterer and from a comparison Zn scatterer. From the difference in the observed scattering, the average resonant scattering cross section for the emitted gamma ray line was obtained. With an estimate of the ratios of the nuclear matrix elements for the ${\mathrm{As}}^{76}$ beta decay, the fraction $\frac{N\left(E_{\mathrm{res}}\right)}{N}$ of the 0.559-MeV gamma rays having energies that differ by 0.5 eV or less from the resonant energy is computed, and the lifetime of the 0.559-MeV state is calculated. The result obtained is ${\tau }_{0.559}=\left(1.55_{-0.19\mathrm{}}^{}{}_{}{}^{+0.13\mathrm{}}\right)\times {10}^{-11\mathrm{}}$ sec. In a similar way, an estimate of the lower limit of the lifetime of the 1.216-MeV level of ${\mathrm{Se}}^{76}$ is found to be ${\tau }_{1.216}>3\mathrm{}\times {10}^{-12\mathrm{}}$ sec.

• Received 23 October 1962

DOI:https://doi.org/10.1103/PhysRev.129.2583