An X-ray ray tracing simulation code for mono- and polycapillaries: Description, advances and application

Abstract Polycapillary optics, consisting of bundles of narrow hollow glass channels, are regularly used in the field of (micro-)X-ray fluorescence (XRF) spectroscopy to focus X-rays down to a microscopic spot while increasing the flux density of the beam on the sample. Polycapillaries guide X-ray photons through multiple total reflection events, similar to how light is guided within optic fibers. Although the use of polycapillaries in XRF spectroscopy allows for fairly straightforward qualitative elemental analysis, fundamental parameter (FP) based quantification remains difficult due to the energy dependent photon transmission efficiency, focal size, acceptance, etc. of these optics. In order to predict the polycapillary and input beam parameter dependent beam forming properties, a multithreaded Monte Carlo based polycapillary X-ray ray tracing code is presented. Apart from supporting photon ray tracing in ‘ideal’ straight, conical and ellipsoidal shaped polycapillary optics, it also allows for the simulation of photon propagation through arbitrarily shaped optics to account for small deviations from the ideal shape, as is often the case in real world examples. The current code allows for the simulation of so-called ‘leak events’, where the probability of a photon traveling through a capillary wall is taken into account, and also includes support for photon beam polarization effects. The simulated results show good agreement with experimental results obtained at the BM26A beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The (poly)capillary X-ray ray tracing simulation code, called ‘polycap’, developed in the C language and with bindings for Python, is released under the GPLv3 license. The code is expected to assist in the quantification of (poly)capillary based X-ray fluorescence spectroscopy and may yield additional insight into the manufacturing and development of polycapillary optics.