Lifetime of the 78-keV Level ofP32

Abstract

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 $\ensuremath{\gamma}$ rays were detected, after passing the edge of a movable absorber, by a narrow rectangular Ge(Li) detector (2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}20 mm). From the exponential curve of count rate vs absorber displacement, a value of $\ensuremath{\tau}=402\ifmmode\pm\else\textpm\fi{}13$ 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 $\ensuremath{\tau}\ensuremath{\le}26\ifmmode\pm\else\textpm\fi{}6$ psec for the 74-keV $M1$ transition from the 657-keV excited state of $^{22}\mathrm{Na}$.