X-ray Backscatter Modelling for Quantitative X-ray Fluorescence Microscopy Studies*

X-ray fluorescence (XRF) is a non-destructive method capable of detecting chemical elements (Z>10) in trace concentrations at the parts per million (ppm) level. Observed differences at the microscopic scale between normal and pathological tissues are linked to various human diseases such as cancer, Parkinson, or osteoporosis. The advent of commercial polycapillary x-ray lenses (PXLs) extended the realm of XRF microscopy from synchrotron to table-top units using x-ray tubes. In this study, a 50-kV x-ray tube integrated with a PXL unit irradiated for 5 minutes a polyester resin sample of 300 micrometers thickness. The sample was doped with arsenic (As) at the 12 ppm level to mimic the presence of a trace element in a soft tissue sample. A silicon detector was placed at the 137-degree backscattering angle to acquire the x-ray spectrum. Coherent and incoherent cross section calculations estimated the backscatter-to-incident ratio (BIR) as a function of the photon energy, experimental detection geometry, thickness, and bulk elemental composition (BEC). The fitting of the BIR model to the experimental data in the 15-25 keV photon energy range yielded the BEC and the thickness of the resin sample.