n−He4Elastic Scattering near 20 MeV

The differential cross sections for the elastic scattering of neutrons by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ have been measured at incident neutron energies of 17.6, 20.9, and 23.7 MeV for lab angles between 20 and 140°. The cross sections were measured relative to the ${}_{}{}^1{}_{}{}^{}\mathrm{H}(n,n){}_{}{}^1{}_{}{}^{}\mathrm{H}$ cross sections by the neutron time-of-flight technique. The $0.75-M{}_{}{}^4{}_{}{}^{}\mathrm{He}$ scattering sample was a liquid contained in a thin-walled cryostat. Corrections were made for multiple scattering and attenuation. Phase-shift analyses were performed using both differential-cross-section and polarization data. The present phase shifts are joined to the well-established low-energy phase shifts giving a smooth energy dependence.