X-ray fluorescence imaging system for fast mapping of pigment distributions in cultural heritage paintings

Abstract

Conventional X-ray fluorescence imaging technique uses a focused X-ray beam to scan through the sample and an X-ray detector with high energy resolution but no spatial resolution.The spatial resolution of the image is then determined by the size of the exciting beam, which can be obtained either from a synchrotron source or from an X-ray tube with a micro-capillary lens.Such a technique based on a pixel-by-pixel measurement is very slow and not suitable for imaging large area samples.The goal of this work is to develop a system capable of simultaneous imaging of large area samples by using a wide field uniform excitation X-ray beam and a position sensitive and energy dispersive detector. The development is driven by possible application of such a system to imaging of distributions of hidden pigments containing specific elements in cultural heritage paintings, which is of great interest for the cultural heritage research.The fluorescence radiation from the area of 10 × 10 cm2 is projected through a pinhole camera on the Gas Electron Multiplier detector of the same area.The detector is equipped with two sets of orthogonal readout strips.The strips are read out by the GEMROC Application Specific Integrated Circuits (ASIC)s, which deliver time and amplitude information for each hit.This ASIC architecture combined with a Field Programmable Gate Array (FPGA) based readout system allows us to reconstruct the position and the total energy of each detected photon for high count rates up to 5 × 106 cps.Energy resolution better than 20% FWHM for the 5.9 keV line and spatial resolution of 1 mm FWHM have been achieved for the prototype system.Although the energy resolution of the Gas Electron Multiplier (GEM) detector is, by principle, not competitive with that of specialised high energy resolution semiconductor detectors, it is sufficient for a number of applications.Compared to conventional micro-XRF techniques the developed system allows shortening of the measurement time by 2-3 orders of magnitude.