A Monte Carlo 3D ray-tracing model for the simulation of X-ray powder diffraction and fluorescence is presented. The model is primarily intended as a tool to aid the development of a compact instrument for in-situ mineralogical and chemical analyses of planetary surfaces. A reflection geometry is assumed, along with fixed-position CCD detectors in order to avoid moving parts. In the model, X-rays are produced either by an X-ray tube or a radioactive source. The X-rays interact sequentially with a user-determined series of model elements which can include apertures, micropore collimators, söller slits and powder samples. Model samples are assumed to be ideal powders of any mineral or mixture of minerals for which the crystal structures are available. X-ray propagation ceases at the CCD detector(s), at which accurate quantum efficiency and energy redistribution effects are calculated. Given the appropriate characteristics, further source and detector types could easily be added. All model elements are treated as infinitely-thin surfaces which may be flat or curved (spherical or cylindrical).

The utility of this model lies particularly in the flexibility of the geometrical arrangement of the various elements, and the quantitative accuracy of simulations. This accuracy is illustrated in figure 1, which shows the comparison of experimental and modelled diffraction of Mn-Kα x-rays from a barite (BaSO₄) pressed-powder sample.

 In addition, a simplified 2D model has been developed in order to determine the optimum placement of detectors with respect to 2θ resolution. The results of both models will be presented together with further comparisons with experimental data.

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1. Laboratory X-ray diffraction and fluorescence tests of the Martian analogue: The Theo’s flow rock from Ontario, Canada