Scattering of Fast Neutrons by Helium

Measurements have been made of the angular distribution of helium recoils for incident neutrons of energies from 0.6 to 1.6 Mev. The distribution curves also permit estimates of the total cross section in this energy range. The results confirm the existence of a cross-section peak of about 6.8×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$ around 1 Mev and indicate, under the assumption of $s$- and $p$-wave scattering only, that the peak is double. But preliminary attempts to fit the data to Bloch's detailed theory of $s$- and $p$-scattering with a split $p$-level have not been successful, and the sign of the postulated splitting is not established.