Hydrogen Deuteride for Cold Neutron Production: A Model for the Double Differential Cross Section

The present work deals with the modeling of the response to neutrons of heteronuclear diatomic liquids, with special interest in the case of hydrogen deuteride (HD), as a possible candidate for the moderation process required in the production of cold neutrons. Preliminary evaluations of the model giving the neutron double differential cross section of a heteronuclear vibrating rotor were performed in the recent past by using, as a first approximation, the ideal gas law for the center-of-mass translational dynamics. Here, the state-of-the-art methodology (based on the use of quantum simulations of the velocity autocorrelation function) for predicting the neutron response of moderately quantum fluids (like molecular hydrogen and deuterium at low temperatures) is applied to the heteronuclear form of this molecular liquid. The unavailability of the double differential cross section experimental data on liquid HD still compels us to test the calculations only at an integral level, i.e., against the only available measurements of the total neutron cross section of HD. Despite the well-tested and parameter-free computational approach, which includes proper consideration of the quantum effects, the present findings on HD indicate the evident need for more accurate measurements of its total cross section in extended ranges of incident energy, as well as of an experimental determination of the double differential cross section of this mild quantum liquid. For further applicative purposes, a very useful by-product of this study is the determination of the self diffusion coefficient D of the HD in the liquid phase.