Neutron powder diffractometer beam line with a toroidally bent asymmetric crystal monochromator: Monte Carlo simulation studies

Monte Carlo simulation studies are carried out for beam lines for neutron powder diffraction. Programs are developed for studying the propagation of neutrons starting from reactor end of beam tube and progressing through various optical elements—collimators, slits, monochromator crystal, sample and finally detector. The simulation results are seen to be in fair agreement for an existing beam line using a plane crystal monochromator on Dhruva reactor, BARC, Mumbai. The programs are then used to design a beam line using a toroidally bent monochromator crystal and open collimator geometry. The simulation results show that a perfect bent crystal monochromator based diffractometer provides better resolution and higher intensity as compared to that based on a plane crystal monochromator. A complete optical design of the beam line is worked out taking into consideration vertical and horizontal radii of curvature for the asymmetrically cut monochromator crystal, sample size and detector resolution function.