The effect of gravity on the resolution for time-of-flight specular neutron reflectivity

The effect of gravity on neutron scattering is negligible if a thermal spectrum of up to 10 Å is used. Modern cold sources produce spectra with an ample quantity of cold neutrons. Gravity may play a crucial role for the cold part of the spectrum in neutron scattering experiments demanding high angle resolution. This mainly concerns the reflectometry method where a small deviation in the angle distribution may lead to visible effects. We present a theoretical model that takes into account the effect of gravity on the resolution function as well as the flux distribution for the specular neutron reflectometry. Theoretically calculated reflectivity curves convoluted with the model resolution function were compared with Monte-Carlo simulations, which imitated real measurements. The good agreement between the calculated and simulated curves allows one to apply the theoretical model to treat the real experimental data.