Reflectometry at a long pulse spallation source

The layout criteria for a reflectometer at a long pulse spallation source (LPSS) are discussed. It is shown how the most important instrument parameter, the wished resolution of the measured reflectivity profile, determines the choice of the wavelength and available wavelength band, the length of the instrument and the position of the disc choppers, which are necessary for applying the time-of-flight method. Using 4 disc choppers it is possible to measure the same spectrum with different resolutions without changing the beam geometry and the available wavelength band. All considerations are tested with Monte Carlo simulations, which were performed with a 2 ms pulse of a source working at 25 Hz.