Characteristics of thin-film multilayer monochromator systems from ray-tracing calculations

A ray-tracing program was developed based on the Monte Carlo method to analyze the performance of a monochromating system based on thin-film multilayer monochromators. The monochromating assembly consisted of a collimator before the multilayer for limiting the dimensions of the beam incident on either one or two multilayer monochromators that reflect the neutron beam, and a final collimator after the multilayer to decrease the cross-section of the beam to match the sample, and also to reduce Δλ/λ of the emerging neutrons by limiting their divergence. The results show that, in general, the bandwidth of the beam reaching the spectrometer will be narrower than the value predicted by the differential form of Braggʹs law, and its dependence on the widths of the slits will also be significantly less. The large bandwidth of the reflected beam, that is normally obtained from these monochromators, does not necessarily imply a greater neutron flux. Imperfections, either natural or intentionally built-in during the deposition process, directly influence the intensity of the reflected beam. A perfect multilayer will not reflect a significant portion of the incident beam even though its bandwidth Δλ/λ may be large, and a multilayer with appropriate variations in d-spacings may be used to increase the intensity available for the experiments. In many situations, the use of a multilayer with smaller d-spacings will be advantageous if the reflectivity can still be maintained at a high value. For a two-multilayer system, the reflected intensity will be significantly less even if the second multilayer is exactly identical to the first and, for unidentical multilayers, there will be a further degradation of neutron flux. In both cases, the bandwidth Δλ/λ will remain essentially the same as for reflection from a single multilayer.