Energy Dependence of X-ray Beam Size Produced by Polycapillary X-ray Optics

In this work we studied the dependence on X-ray energy of the diameter of an X-ray beam that was focused by polycapillary optics. We used the method of detecting characteristic X-ray fluorescence from a knife-edge sample which demonstrated an inverse exponential relation between the beam size and energy. This study was conducted with both PIXL (Planetary Instrument for X-ray Lithochemistry) flight optics and JPL in-lab breadboard optics in order to compare the effects of various polycapillary optic systems. The experimental results were also compared with Monte-Carlo simulations. This beam size-energy relation is important for accurately estimating the per-element interrogation volume of a sample which directly benefits PIXL in addition to other micro-focused XRF systems.