Neutron transmission through pyrolytic graphite crystal II

The measured neutron transmissions through 6.7 mm thick pyroletic graphite (PG) crystal set at different take-off-angles with respect to the beam, as a function of wavelength, were compared with the calculated values using a general formula. An adapted version of the computer package graphite was developed in order to provide the required calculations in the neutron energy range from 0.1 MeV to 10 eV. An overall agreement was obtained between the formula fits and the measured data at different takeoff-angles. The major dips in transmission caused by various reflections were identified. From the shape of the dips due to 001 reflections, the mosaic spread of the used PG crystal has been determined within an accuracy of 0.12 degrees. A feasibility study is carried out on using two PG crystals to select from the reactor spectrum a neutron monochromatic beam with wavelengths longer than 0.3 nm and almost free from accompanying higher-order reflections. Calculation shows that 2 mm thick highly oriented PG (0.5 degrees FWHM on mosaic spread) crystal set at glancing angle 20.0 degrees reflects first-order monochromatic neutrons with 0.3 nm wavelengths. When 6.0 cm thick PG crystal (2 degrees FWHM on mosaic spread) set at 60.63 degrees take-off-angle is inserted on the way of the reflected neutrons, it transmits more than 70\% of the first-order neutrons while attenuating the high-order ones by more than 20 times. Similar results were obtained when the selected monochromatic neutrons had wavelengths longer than 0.3 nm. (C) 2010 Elsevier Ltd. All rights reserved.