We measured the cross section for ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}(n,\alpha ){}_{}{}^7{}_{}{}^{}\mathrm{Li}$ by reciprocity from the inverse reaction, using the $4\pi$ "graphite sphere" neutron detector. The branching ratio to the ground and first excited states of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ was measured using face-to-face surface-barrier silicon detectors. The resulting reaction yield, normalized ultimately to the thermal cross section, shows significant ($\gtrsim 2\%$) departure from $\frac{1}{v}$ in different directions at different energies for the two reaction channels: $\gtrsim 30$ keV (high) for ($n,{\alpha }_0$), $\gtrsim 100$ keV (low) for ($n,{\alpha }_1$), and $\gtrsim 170$ keV (low) for ($n,{\alpha }_0+{\alpha }_1$).

• Received 28 August 1967

DOI:https://doi.org/10.1103/PhysRev.165.1147