The Polarisation Factor in X-ray Reflection

Abstract

IN comparing the observed intensity of X-ray reflection from crystals with the theoretical formulae, and in particular in calculating the atomic scattering factor, F, from measurements of the absolute intensity of reflection, it has always been assumed that the incident X-ray beam is unpolarised. This has the effect of introducing a polarisation factor ½(1 + cos22θ), where θ is the glancing angle of reflection, into the expression for the intensity of reflection. Now experiments by Bishop (Phys. Rev., 28, 625 ; 1926) show that the characteristic radiation from a molybdenum Coolidge tube may be 15 per cent, polarised, and Kirkpatrick (Phys. Rev., 29, 632 ; 1927) points out that this must have introduced errors into the values of F calculated from experiment. He calculates for the polarisation factor where P is a measure of the degree of polarisation of the incident beam, being unity for an unpolarised beam, and I is the angle between the plane of reflection and the plane containing the incident beam and the electron stream in the X-ray tube.