Full-field X-ray Fluorescence Imaging with Coded Apertures

Abstract

X-ray fluorescence (XRF) imaging is a widely used tool for the determination of the spatial distribution of elements in a sample. We developed and implemented a method for full-field XRF imaging based on coded apertures (CA) at the BAMline at BESSY II (HZB). This technique has its origins in astrophysics [1, 2], and is also used in nuclear medicine [3] or radiation detection, e.g., for nuclear decommissioning [4]. The principle of CA based XRF imaging has already been described [5, 6], and a coded aperture microscope for X-ray fluorescence full-field imaging has been designed and constructed recently at NSLS [7]. Detector Mask Fluorescent X-rays Incoming X-rays Sample Figure 1: The principle of X-ray fluorescence imaging with coded apertures. In short, the whole sample is irradiated by X-rays, causing fluorescence radiation, which falls through a mask with known hole pattern, called coded aperture, onto an area sensitive detector. As the recorded image consists of overlapping projections of the object, a reconstruction step is performed to retrieve the image information. Figure 1 shows a general scheme of the experiment. In this work, we present the first experiments on a test sample containing different elements. We performed measurements with a coded mask based on a modified uniformly redundant array [8] with an energy-dispersive 2D detector, the SLcam [9]. Different reconstruction methods were tested [10]. The best results were achieved with an iterative algorithm, and are presented here. We could obtain resolution that are better than predicted for a single pinhole with the same diameter as the pinhomes in the CA. Furthermore, we performed simulations to test the performance of the reconstruction, and to provide a basis for further investigations of the ideal parameters for near field CA imaging and for refinement of the algorithms. [1] E. Caroli, et al., Space Science Reviews, 45(3), 1987, 349-403. [2] G.K. Skinner, Applied Optics, 47, 2008, 2739-1749. [3] R. Accorsi, F. Gasparini, and R.C. Lanza, Nuclear Insturments and Methods in Physics Research A, 474, 2001, 273-284. [4] M.J. Cieślak, K.A.A. Gamage, and R. Glover, Radiation Measurements, 92, 2016, 59-71. [5] A. Haboub, et al., Rev Sci Instrum, 85(6), 2014, 063704. [6] Kulow, A., et al., J. Anal. At. Spectrom., 35, 2020, 347-356. [7] D.P. Siddons, et al., J. Synchrotron Radiat.., 2020, 35, 347-356. [8] S.R. Gottesman, and E.E. Fenimore, Applied Optics, 1989. 28(20), 4344-4352. [9] Scharf, O., et al., Anal Chem, 83(7), 2011, 2532-8. [10] Kulow, A., et al., J. Anal. At. Spectrom., 35, 2020, 1423-1434.