Cooled, large mosaic pyrolytic graphite as thermal neutron filters

Neutron cross sections have been measured for oriented pyrolytic graphite with 80 mosaic at room temperature and with 150 mosaic at both room and liquid nitrogen temperatures. These data show that such large mosaic pyrolytic graphite can be used effectively as filters of thermal neutron beams, and that cooling improves their effectiveness. It is possible to select an optimum thickness and mosaic which transmits neutrons at the first-order (lambda) wavelength and rejects neutrons at the second (lambda/2) and higher-order wavelengths. A table is presented of the range of first-order wavelengths over which the filters can be effective. (orig.)