Fore-Aft Anisotropy in the Radiative Capture of 14-MeV Neutrons

Relative yields of capture photons have been observed for four nuclei at angles of 55°, 90°, and 125° in bombardments with 14-MeV neutrons. The yields from ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$, and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ show smaller fore-aft anisotropies than those observed in corresponding proton captures. This suggests that the forward peaking in ($p, \gamma$) reactions is due mainly to direct rather than collective capture amplitudes. Photons from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(n, {\gamma }_0){}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ peak backward, but this peaking cannot be straightforwardly accounted for in terms of the interference between the collective excitations dominant in this energy region.