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

Abstract

Relative yields of capture photons have been observed for four nuclei at angles of 55\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, and 125\ifmmode^\circ\else\textdegree\fi{} 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 ($p, \ensuremath{\gamma}$) reactions is due mainly to direct rather than collective capture amplitudes. Photons from $^{12}\mathrm{C}(n, {\ensuremath{\gamma}}_{0})^{13}\mathrm{C}$ peak backward, but this peaking cannot be straightforwardly accounted for in terms of the interference between the collective excitations dominant in this energy region.