Tham khảo tài liệu 'modern developments in x-ray and neutron optics episode 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 62 J. Saroun and J. Kulda is represented by a rectangular circular or elliptical area at the interface between moderator and neutron channel with an associated neutron flux distribution. In RESTRAX the neutron flux is described either analytically as a Maxwellian distribution or more accurately by a lookup table. In the latter case a one-dimensional table with wavelength distribution is combined with two-dimensional tables describing correlations between angular and spatial coordinates. Such a table can be easily created by postprocessing of moderator simulation data which results in a much more realistic model compared to the analytical description and allows for simulations of neutron fluxes on absolute scale. Diffractive Optics Simulation of neutron transport through crystals in RESTRAX is based on a random-walk algorithm which solves intensity-transfer Darwin equations 8 numerically in principle for any shape of the crystal block. Details of the algorithm are described in 14 . It is based on the assumption of dominant effect of the mosaic structure on the rocking curve width where mosaic blocks are treated as perfect crystal domains. However the random walk is not followed through individual mosaic blocks which would be an extremely slow process in some cases. Instead the crystal is characterized by the scattering cross section per unit volume ơ s which depends on the misorientation angle of a mosaic block as o- e Qn-1ff n where n is the width of the misorientation probability distribution g x and Q stands for the kinematical reflectivity. The diffraction vector depends on the misorientation angle and in the case of gradient crystals also on the position in the crystal which can be expressed as G r Go VG r G e Y where the second term describes a uniform deformation gradient and the third one the angular misorientation of a mosaic block parallel e and perpendicular y to the scattering plane defined by G0 and incident beam directions. For a neutron
