We have analyzed the data on the primary radiative transitions following slow neutron capture by ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$. We have made estimates of direct capture from optical potential models with physically realistic parameters. These parameters were varied to allow for the effects of a local $s$-wave level on the initial state. We find that the model estimates are in reasonable agreement with the measured cross sections of the 6.810- and 0.854-MeV transitions in ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$, the 1.262-MeV transition in ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, and, possibly, the 8.174-MeV transition in ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, indicating a major role played by a direct capture mechanism. In all these cases, the estimate of direct capture is greatly affected and reduced by large cancellation effects in the integrand of the radial dipole matrix element. The strengths of the 3.443-MeV transitions in ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$ and, to a lesser extent, the 4.945-MeV transition in ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, cannot be explained by our optical model formulations of direct capture, but these strengths are consistent with extrapolations to these light nuclides of generally accepted formulations of compound nucleus capture.

• Received 9 June 1986

DOI:https://doi.org/10.1103/PhysRevC.35.26