The ${\mathrm{Li}}^7(p, p^{\prime }){\mathrm{Li}}^{7*}$ reaction was used as the source of Doppler-broadened 478-kev gamma rays to determine the mean life of the first excited state of ${\mathrm{Li}}^7$ by the nuclear resonance fluorescence technique. Both scattering and self-absorption experiments using both LiF and Li as the scattering and absorbing materials were performed.

The resonance scattering gives a mean life of (1.09±0.07)×${10}^{-13\mathrm{}}$ sec after a 2% correction for slowing down of the ${\mathrm{Li}}^{7*}$ nuclei has been made. This correction follows from the measured stopping cross sections for lithium ions. An additional correction, for the effect of collisions in which the ${\mathrm{Li}}^{7*}$ nuclei lose a large fraction of their energy, is found difficult to estimate, although it is certainly not more than a few percent. For this reason, the scattering measurement is regarded as giving a lower limit for the mean life.

The self-absorption measurements give a mean life of (1.15±0.14)×${10}^{-13\mathrm{}}$ sec. Debye temperatures of 400°K and 700°K were used for Li and LiF, respectively. The error given, about twice the statistical error of the data, includes estimates of the effect of other experimental uncertainties.

• Received 15 December 1958

DOI:https://doi.org/10.1103/PhysRev.114.862