Exact resolution function for double-disk chopper neutron time-of-flight spectrometers: Application to reflectivity

The exact resolution function of the transfer vector for the HERMÈS reflectometer at the Laboratoire Léon Brillouin is calculated as an example of a neutron time-of-flight spectrometer with a double-disk chopper. The calculation accounts for the wavelength distribution of the incident beam, the tilt of the chopper axis, collimation and gravity, without an assumption of Gaussian distributions or the independence of these different contributions. A numerical implementation is provided. It is shown that data fitting using this exact resolution function allows much better results to be reached than with the usual approximation by a Gaussian profile.