Anomalous Scattering of Neutrons by Helium. (II)

Abstract

The anomalous scattering of neutrons by helium at neutron energies around 1 Mev has been investigated more closely. The backward scattering cross section shows a peak of 0.4 Mev half-width and indicates a doublet structure with a splitting of about 0.3 Mev. The absolute value of the ratio of the scattering cross sections of helium and hydrogen at 2.5-Mev neutron energy was redetermined and found to be considerably smaller than given in a previous investigation. This lowers the absolute values of the resonance scattering cross section given previously. The new values, however, are still consistent with the assumption that the resonance level is a $P$ level. The measurements are found to be in agreement with the dispersion theory. The present experiments do not fix the sign of the splitting, although there is some indication that the doublet is normal.