Neutron-Alpha Scattering in the 20-MeV Range

Angular distributions of neutrons scattered by α particles were measured from 16- to 26-MeV neutron energy by observing the recoiling α particles in a high-pressure gas scintillator. Above 22 MeV, disintegration particles from the He⁴(n, d)T reaction were observed, also. Data were taken in 25-keV steps over the He⁵ state at 16.7-MeV excitation energy (22.15-MeV neutron energy) and over the He⁴(n, np)T, He⁴(n, n^')He4*, and He⁴(n, 2n)He³ thresholds. Total cross sections of helium were measured at 26 energies from 20- to 29-MeV neutron energy in a good geometry transmission experiment. Angular distributions at neutron energies 24 MeV were fitted with expansions of Legendre polynomials. Polynomials of order 0 through 4 provided a satisfactory fit. Below 22 MeV the angular distributions agree with calculations based on n-α phase shifts suggested by Perkins. Above 22 MeV the agreement is not as good. The total cross sections agree only qualitatively with these calculations. The total cross section and angular-distribution data are consistent with a 3/2⁺ assignment for the 16.7-MeV level. The measured yield curve for the He⁴(n, d)T reaction agrees with that computed from the cross section for the inverse reaction.