Neutron-diffraction study of deuterium gas in the critical region

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Abstract
The structure factors S(Q) for gaseous deuterium at densities and temperatures in the critical region have been measured for the first time using time-of-flight neutron diffraction at the ISIS pulsed neutron source. The data, when inverted to real space, determine the site-site radial distribution function of the nuclei in the sample. The experimental results are compared with a classical Monte Carlo simulation of a monoatomic Lennard-Jones fluid under equivalent conditions. After correcting for the effects of the molecular form factor in modifying the static structure factor, the simulations reproduce the qualitative trends of the data with density and temperature, with quite good quantitative agreement as well. No evidence is found for the enhanced ordering near the critical point which had previously been predicted from Raman scattering data.