James Bond

The polarization of neutrons elastically scattered from ⁴He at laboratory angles of 20°, 40°, 60°, 80° , and 110° has been measured as continuous function of neutron energy between 1.5 and 6.0 MeV, using a double-scattering technique. The primary intense flux of unpolarized neutrons was generated at the Yale University Electron Linear Accelerator by the photodisintegration of natural lead. A fraction of these neutrons was elastically scattered at 50° (lab) from a graphite cylinder along a 27 m flight path. This polarized beam then impinged on a target of liquid helimn, and neutrons scattered by the helium were detected in an array of scintillation counters. The absolute polarization of the neutron beam was measured in a separate, true double-scattering experi­ment. Neutron energies were determined with good resolution using a nanosecond time-of-flight spectrometer. The generalized neutron spin precession method was used to reduce systematic uncertainties to negligible amounts. The polarization measurements were analyzed using a multilevel R-matrix reaction theory and, from this, neutron total and differential cross sections and polarizations were predicted. The predictions were compared with the results of other recent measure­ments and analyses and with the results of various theoretical models of the ⁵He system. A definitive, self-consistent set of phase shifts was obtained for the n-⁴He interaction in the energy region below 10 MeV.