The neutron cross section of the hydrogen liquids: substantial improvements and perspectives

Abstract

The design of moderators and cold sources of neutrons is a key point in research-reactor physics, requiring extensive knowledge of the scattering properties of very important light molecular liquids such as methane, hydrogen and their deuterated counterparts. Inelastic scattering measurements constitute the basic source of such information but are difficult to perform, the more so when high accuracy is required, and additional experimental information is scarce. The need of data covering as large as possible portions of the kinematic Q-E plane thus pushes towards the use of computable models, validated by testing them, mainly, against integral quantities (either known from theory or measured) such as spectral moments and total cross section data. A few recent experiments demonstrated that, at least for the self contribution, which dominates in the incoherent scattering case of hydrogen, accurate calculations can be performed by means of quantum simulations of the velocity autocorrelation function. This method is shown here to be by far superior to the use of standard analytical models devised, although rather cleverly, for generic classical samples. The neutron dynamic structure factor (and consequently the well-known S({\alpha},{\beta}) of parahydrogen and deuterium, suitable for use in packages like NJOY, are given and shown to agree very well with total cross section measurements and expected quantum properties.