Scattering of Slow Neutrons from Ammonia and Hydrogen Sulfide

Measurements of the scattering of slow neutrons from samples of gaseous ammonia and hydrogen sulfide have been made using the MTR phased-chopper velocity selector. The energies of incident neutrons were 0.025, 0.07, and 0.1 eV and the scattered neutrons were detected in counter banks in an angular range of 16° to 145°. The data obtained have been converted to reduced partial differential cross sections and have been compared with theoretical calculations based on scattering from a symmetric top molecule, from a spherical-top molecule and from a Krieger—Nelkin gas. The symmetric top gives the best fit to the data for both samples. This theory fits the ammonia data better than the hydrogen sulfide data; the deviation is at large energy transfers and is larger for hydrogen sulfide than for ammonia. The zeroth and first moments of energy transfer have been obtained from the data for the experimental range of momentum transfer. The zeroth moments for ammonia agree with the predicted values based on classical theory, but they are about 10% high for hydrogen sulfide. The experimental first moments for both molecules suggest an effective mass roughly 20% lower than the Sachs—Teller mass.