Polarization ratio for X-ray diffraction by perfect unicrystals with finite thickness

The polarization ratio is expressed by |cos 2θ|ⁿ, where 2θ is the angle between the incident and the diffracted rays. For a perfect uniblock single crystal, n is usually taken to be equal to 1. For an ideally imperfect crystal, n is taken to be 2. Any other value of n is attributed to the extent of mosaicity. Numerical computation of the polarization ratio for a perfect nonabsorbing single crystal of infinite lateral extension but finite thickness shows that n, instead of having the constant value of 1, may have any value between 1 and 2, depending on the thickness of the slab, the structure factor of the reflecting plane, the volume of the unit cell and the angle of scattering. The numerical computations are based on Zachariasen's theory of diffraction of X-rays by perfect crystals [Zachariasen (1945). Theory of X-ray Diffraction by Crystals. New York: John Wiley].