Anomalous Scattering of Neutrons by Helium

The ratio of the cross sections for elastic backward scattering of neutrons in helium and hydrogen has been measured as a function of the neutron energy. This ratio is practically constant between two- and six-Mev neutron energy and has a value of about 1.3. However, at one Mev, the ratio increases to about nine. This anomaly must be caused by resonance scattering occurring from the formation of an unstable compound nucleus ${\mathrm{He}}^5$. Theoretical considerations show that the level of ${\mathrm{He}}^5$ involved must be a $p$ level.