Molecular contributions to the thermal neutron cross sections of O2, N2, and air

We provide an updated scattering library for the simulation of thermal neutron transport in air, including the effects of rotational and vibrational modes in N2 and O2 and neutron magnetic scattering in O2, showing their significance compared to smaller effects related to water humidity. The modeling is based on the Young-Koppel treatment of thermal neutron scattering from diatomic molecules, as well as ab initio simulations of the electronic density of O2. The theoretical predictions are benchmarked against experimental measurements of the total scattering cross section of air under monitored thermophysical conditions, in the neutron energy range between 0.6 meV and 10 keV. The updated scattering library is used to calculate excess neutron scattering in air, compared to traditional approaches, where only nuclear scattering from gases of free nuclei is implemented in Monte Carlo transport codes.