Mean Life of the 265-keV Excited State inAs75

Using an ultracentrifuge as the means of imparting velocities of up to 11.5×${10}^4$ cm/sec to a ${\mathrm{Se}}^{75}$ source, a resonance transmission experiment involving the 265-keV excited state in ${\mathrm{As}}^{75}$ has been carried out. If a branching ratio $\frac{{\Gamma }_0}{\Gamma }$ of 0.96 is assumed for this state, the transmission vs velocity data lead to a mean life ${\tau }_{\mathrm{level}}=(1.57\mathrm{}\pm 0.10)\times {10}^{-11\mathrm{}}$ sec. This value agrees with the result of an earlier self-absorption study, but differs considerably from the mean lives deduced from resonance scattering experiments.