Recoil Broadening of Secondary Transitions in Neutron-Capture Gamma-Ray Cascades

Recoil Doppler broadening has been observed in secondary $\gamma$-ray transitions (4-9 MeV) in cascades following thermal-neutron capture in ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$. An increase in linewidth (typically 0.5-1.5 keV) is measured by use of a Ge(Li) pair spectrometer. An ambiguity in the ${\gamma }_1-{\gamma }_2$ angular correlation $W\left(\theta \right)$ exists in most cases and introduces an uncertainty in the calculated linewidths, but measured widths generally fall within the range of the calculations. A comparison of results in ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ from a gas sample and from a Melamine sample shows an attenuation of the effect for the 5271- and 5299-keV levels, and estimates of their lifetimes are consistent with known values. For ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ levels showing no attenuation, the estimated lower limits on the lifetimes are also consistent with previous measurements; the lifetime for the 8313-keV level is an improvement over existing information. A more accurate value is also deduced for the lifetime of the 6741-keV level in ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ from a comparison of the calculated and the observed broadening. It is also shown that in certain cases the Doppler broadening could provide information about the spin of the capturing state through the dependence of $W\left(\theta \right)$ on $J_c$.