Anatomy of a Bragg reflexion and an improved prescription for integrated intensity

Using a narrow aperture in front of a detector, the intensity distribution of a single Bragg reflexion has been determined experimentally in terms of two variables, the angular setting of the crystal and that of the detector aperture. The resultant two-dimensional distributions, corresponding to the main scan procedures, ω, ω/θ, ω/2θ, show in a direct pictorial manner the interaction of the X-ray source, the spectral composition and the mosaic (and fragment) composition of the crystal. The clarity of presentation contrasts with earlier studies where the various components were projected onto one dimension. The two-dimensional presentation allows a clearer appreciation of (a) the rôle of the various components of the experiment and (b) the significance and function of the various scan procedures, ω, ω/θ, ω/2θ, corresponding to σ = 0, 1, 2 respectively, σ being the 2θ:ω scan ratio. This study leads to a new improved prescription for the measurement of integrated intensity which is more compatible with the spatial distribution of the Bragg reflexion. It also points to the existence of an inbuilt systematic error source in the conventional prescription, only made evident as a result of the extension into the second dimension. The possibility of a different procedure for the measurement of integrated intensity - the 'slice' ω/2θ scan - is indicated. This scan has potential advantages, one being that it would allow a more precise specification of background, hence bypassing the problem of variable truncation and leading therefore to the establishment of improved accuracy in structure factors. This study should also prove of interest for the future application and exploitation of linear and two-dimensional counters in single-crystal studies.