Scattering of Neutrons by Polyethylene

The General Atomic neutron velocity selector has been used in conjunction with the electron Linac to produce monoenergetic neutrons in the range 0.167 to 0.499 eV. The scattering of neutrons at various angles between 30 and 150° by a thin specimen of crystalline polyethylene has been measured, and precise scattering cross sections σ(E₀, E, θ) have been determined. The experimental results are compared in detail with the theoretical work of Goldman, Parks, Koppel and Young, and McMurry. The detailed comparisons indicate that a more-or-less continuous realistic frequency distribution, or an appropriate collection of isolated oscillator levels, can be used as the basis of computing a reasonably satisfactory scattering cross section for polyethylene. It appears that the models of Goldman, Parks, and Koppel and Young all overemphasize energy transfers at ≈ 0.089 eV, and tend to underemphasize the largest transfers at ≈ 0.35 eV. The extrapolation technique of Egelstaff applied to the Scattering Law gives a frequency distribution that is similar in broad outline to that used by Parks. However, small significance can be attributed to this agreement because of the probable and large contributions of the multiphonon terms.