The relation of wavelength dispersion and scan mode in X-ray diffractometry with a monochromator

The contribution to the size of a Bragg reflection due to the interaction of the wavelength dispersion of a monochromator crystal, M, and of a specimen crystal, c, is considered from the two-dimensional Δω, Δ2θ viewpoint. Its variation with change in scan mode is demonstrated. For the ω/θ scan mode, the contribution in respect of Δ2θ [i.e. (Δλ/λ)tan θ_c] is independent of the monochromator used and is the same as for the non-monochromator case. For the ω/2θ scan mode, the corresponding contribution [(Δλ/λ)tan θ_M] is constant and so, for the conventional one-dimensional measurement procedure, a receiving aperture of constant width may be used to collect intensities over the full range of θ_c.