The reflectivity of a pyrolytic graphite monochromator

Abstract

It is shown that a pyrolytic graphite monochromator crystal scatters X-rays as a mosaic crystal in accordance with the Darwin formulism, both for reflecting power and integrated intensity, over a range in wavelength from 0.5 to 1.54 A. The scattering cannot be considered kinematic and, from estimates of the reflectivities parallel and perpendicular to the diffraction plane, polarization ratios are calculated which are in accord with published values. A simple description of the scattering process is given. The variation of polarization ratio with mosaic spread is discussed.

A diffracted-beam monochromator for long linear detectors in X-ray diffractometers with Bragg-Brentano parafocusing geometry

“…A larger effective mosaic spread reduces the reflectivity. 14 Further optimization of the setup is obtained by using a larger (than 8 mm) axial height slit. The effect of the size of the axial height slit has been evaluated using the LaB 6 {311}.…”

Monochromatic X-Ray Beams for NDT

“…The intensity of this diffracted beam is -3xl0 7 photonsjcm2 sec at a distance of 1m from the crystal. A graphite crystal is very efficient [2,3], reflecting approximately 50% of the photons which satisy Eq. 1.…”

Polarization ratios for repeatedly reflected X-ray beams

“…The polarization ratio was measured for the graphite crystal used by Lawrence (1982) using a simplification of the method used by Le Page, Gabe & Calvert (1979). The beam from an X-ray tube was diffracted by the crystal and then re-scattered by a uniform piece of transparent plastic, the intensities from the plastic in directions parallel to and perpendicular to the diffracting planes of the crystal being measured separately.…”

“…A comparison between the values of the polarization ratios for the two theories can be made in the case of identical graphite crystals [the same as the single sample used by Lawrence (1982) As can be seen, the kinematic approximation gives values of the polarization ratios which are too small and this will occur to a lesser or greater extent at all wavelengths and for all mosaic spreads. An increase in the mosaic spread would make the scattering more kinematic but would reduce the efficiency of the scattering.…”

“…Such a situation occurs 0108-7673/83/050753-03501.50 when the crystal is being used as a monochromator or a focusing device and, in these cases, as has been pointed out by Jennings (1981), extinction theory applies. Lawrence (1982) has shown that in the case of pyrolytic graphite, a commonly used monochromating material diffracting a large intensity from the (002) planes, the scattering can be described by the Darwin formulism. The reflectivity, R, in the symmetrical Bragg case, assuming no transmitted beam, is given by O" + ,///y --[((7 + t/y)2 __ 0.211/2 R= t7 tr is the reflectivity per unit length, ~, is the direction cosine of the incident and diffracted beams and g is the linear absorption coefficient.…”