Neutron-Alpha Scattering in the 20-MeV Range

Abstract

Angular distributions of neutrons scattered by $\alpha$ particles were measured from 16- to 26-MeV neutron energy by observing the recoiling $\alpha$ particles in a high-pressure gas scintillator. Above 22 MeV, disintegration particles from the ${\mathrm{He}}^4(n, d)\mathrm{T}$ reaction were observed, also. Data were taken in 25-keV steps over the ${\mathrm{He}}^5$ state at 16.7-MeV excitation energy (22.15-MeV neutron energy) and over the ${\mathrm{He}}^4(n, np)\mathrm{T}$, ${\mathrm{He}}^4(n, n^{\prime }){\mathrm{He}}^{4*}$, and ${\mathrm{He}}^4(n, 2n){\mathrm{He}}^3$ 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 $n-\alpha$ 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 ${\frac{3}{2}}^{+}$ assignment for the 16.7-MeV level. The measured yield curve for the ${\mathrm{He}}^4(n, d)\mathrm{T}$ reaction agrees with that computed from the cross section for the inverse reaction.