The total neutron cross-section of an ortho para mixture of gaseous hydrogen at 75 K

From the data of a transmission experiment we have extracted the total neutron cross-section of a sample of gaseous hydrogen (T=75.03 K, p=84.8 bar, n=8.42 nm-3) with a thermodynamic equilibrium ortho-para content (48% ortho, 52% para). The experiment was carried out on the PEARL instrument operating at the ISIS pulsed neutron source. After an accurate data reduction, the neutron spectra have been analyzed in the framework of the Modified Young and Koppel (MYK) theory, which is a successful extension to interacting fluids of the original Young and Koppel model valid for a dilute gas of hydrogen molecules. The total cross-section calculated with MYK theory, whose unique unknown parameter--the mean kinetic energy of the molecular centre of mass--was obtained through an independent path integral Monte Carlo simulation, shows a satisfactory agreement with the experimental results.