4He(n,n)4He Analyzing Power in the 20 to 30 MeV Energy Range

Abstract

Scattering from 4He has been the most widely employed method for accurately measuring the polarization of nucleon beams. Below 18 MeV the analyzing power for the 4He(n,n)4He interaction is probably known to within about ± 0.03 at the forward angle minimum as well as at the backward angle maximum. This information has been obtained through energy-dependent phase-shift analyses of the available data which have gradually grown in quantity and quality. At higher energies, due to the complexity of the phase-shift analyses and the difficulty of the measurements, the understanding of the 4He(n,n)4He interaction is not as advanced. In order to provide additional information on the five-nucleon system and to calibrate the analyzing power near 119°(lab), measurements were made of Ay(θ) for neutron energies En from 20 to 30 MeV using neutron beams whose polarization values had been determined without any reliance on neutron polarization analyzers. Such neutron beams were obtained by utilizing the properties of the zero degree longitudinal polarization transfer coefficient K z′z of the T( \(\overrightarrow d ,\overrightarrow n \) )3He reaction as described in the contribution of Lisowski et al.1) Neutrons emitted at 0° from the T + d reaction initiated by a 75% longitudinally polarized deuteron beam were incident on a liquid 4He scintillator. Measurements of Ay(θ) were obtained for θ = 60° and 70° (lab) for En = 20.9 MeV and for 119° (lab) at five energies between 20 and 30 MeV, by detecting the scattered neutrons in coincidence with the 4He recoils. To perform the Ay(θ) determination the neutron-polarization axis was rotated to a transverse orientation by means of a transverse magnetic field between the tritium target and the 4He scatterer. The deuteron beam polarization was determined to within ± 0.01 by the quench-ratio method and Azz values were obtained from previous measurements.1)