Inelastic Scattering of Slow Neutrons from Methane

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Abstract
Partial differential cross sections for the scattering of neutrons of 0.0150, 0.0252, 0.0706, 0.103, and 0.142 ev initial energy into angles of 16.3°, 26.0°, 36.4°, 47.6°, 59.5°, 72.1°, and 84.7° from samples of room temperature methane gas are presented. These are converted to scattering law and are compared to three calculations: (1) The ideal gas calculation gives limited agreement using a mass of 3.2, the average of the Sachs-Teller mass tensor; (2) the Krieger-Nelkin approximation which includes effects of zero-point vibrations is a better fit, but underestimates in the region of large momentum transfers; (3) a calculation using exact thermal orientation averages of the matrix elements gives the best fits. None of the calculations explain the results at small energy and momentum changes; this disagreement is attributed to the classical treatment of the rotational states in these calculations.