Simple expression for effective neutron temperature due to rotational motions of cold-moderator molecules

An analysis is presented on the slowing-down of cold neutrons in moderators in which molecular or intramolecular rotations play a predominant role. Use is made of two kinds of approximation : (1) the classical approximation in which the rotational effects are taken into account in terms of an effective mass, and (2) the few-level approximation which becomes useful at the lowest limits of temperature. Further, the concept of effective mass is extended to include also the effects of a weak hindering barrier against rotation. The expressions derived for the effective cold neutron temperature in low temperature moderators are compared with experimental results obtained on typical materials - condensed methane and methyl compound, and satisfactory agreement is obtained. The analysis further reveals some particular characteristics inherent in the slowing-down of cold neutrons due to the rotational motions of molecules. Moreover, the present model is found valid for expressing the slopes in the 1/v-region presented by the scattering cross section of ammonium ions in low barrier materials. (auth.)