Intramolecular Coherent Scattering of Neutrons by CD4Gas

Abstract

Measurements on the scattering of neutrons by gaseous C${\mathrm{D}}_4$ at a temperature of 306°K have been taken on the Materials Testing Reactor (MTR) phased chopper velocity selector for a range of energy transfers between zero and 156 meV and momentum transfers between 1.0 ${\mathrm{\AA }}^{-1\mathrm{}}$ and 14 ${\mathrm{\AA }}^{-1\mathrm{}}$. Comparison of the data with theoretical calculations shows excellent quantitative agreement. The influence of intramolecular coherent scattering is most evident at small energy transfers. However, the intramolecular coherent scattering is also quite prominent when the energy transfer becomes comparable in magnitude to the energy of a vibrational state. At a temperature of 306°K, approximately 3% of the molecules are in excited vibrational states. In spite of the small percentage of vibrationally excited molecules in the gas, it is found that the contribution to the scattering by the vibrationally excited molecules becomes significant at an energy transfer of about 72 meV and dominates the scattering at larger energy transfers.