Lifetime of the 78-keV Level ofP32

A mechanical-recoil method has been used to measure the lifetime of the 78-keV first excited state of ${}_{}{}^{32}{}_{}{}^{}\mathrm{P}$. This state was populated near threshold by the ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}({}_{}{}^4{}_{}{}^{}\mathrm{He},p){}_{}{}^{32}{}_{}{}^{}\mathrm{P}$ reaction, using an enriched ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ ${\mathrm{O}}_2$ target on the down-beam side of a thin copper backing, so that the ${}_{}{}^{32}{}_{}{}^{}\mathrm{P}$ nuclei recoiled into vacuum. The 78-keV $\gamma$ rays were detected, after passing the edge of a movable absorber, by a narrow rectangular Ge(Li) detector (2×2×20 mm). From the exponential curve of count rate vs absorber displacement, a value of $\tau =402\mathrm{}\pm 13$ psec is found for this level. A similar measurement, using the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}({}_{}{}^4{}_{}{}^{}\mathrm{He},n){}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$ reaction, gave an upper limit of $\tau \le 26\pm 6$ psec for the 74-keV $M1\mathrm{}$ transition from the 657-keV excited state of ${}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$.