Elastic Scattering of Fast Neutrons byHe4

Angular distributions of neutrons scattered by helium have been measured at five neutron energies between 2.6 and 14 Mev. A large proportional counter filled with helium and krypton was employed, and the distribution in energy of alpha particles recoiling from collimated neutrons observed. All five distributions are similar, and are characterized by strong scattering of the neutrons in the forward direction, a minimum for neutrons scattered near 120° in the center-of-mass system, and a smaller peak for back-scattering. The location of the minimum moves forward and the ratio of differential cross sections $\frac{{\sigma }_{\mathrm{back}}}{{\sigma }_{min}}$ increases smoothly with increasing neutron energy over the range studied. The observed angular distributions are in good agreement with the distributions predicted from analysis of $p$-${\mathrm{He}}^4$ scattering, and support the large splitting of the $P$ states of ${\mathrm{He}}^5$, and the very broad width of the $P_{\frac{1}{2}}$ state. A summary of $n$-${\mathrm{He}}^4$ phase shifts and recent total cross-section measurements is given.