research papers 354 doi:10.1107/S1600577515000223 J. Synchrotron Rad. (2015). 22, 354–365 Journal of Synchrotron Radiation ISSN 1600-5775 Received 16 September 2014 Accepted 7 January 2015 # 2015 International Union of Crystallography Validation of a Geant4 model of the X-ray fluorescence microprobe at the Australian Synchrotron Matthew Richard Dimmock, a * Martin Daly de Jonge, b Daryl Lloyd Howard, b Simon Alexander James, b Robin Kirkham, c David Maurice Paganin, d David John Paterson, b Gary Ruben, c,b Chris Gregory Ryan c and Jeremy Michael Cooney Brown d a Department of Medical Imaging and Radiation Sciences, Monash University, Clayton, VIC 3800, Australia, b Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia, c CSIRO, Clayton, VIC 3168, Australia, and d School of Physics and Astronomy, Monash University, VIC 3800, Australia. *E-mail: matthew.dimmock@monash.edu A Geant4 Monte Carlo simulation of the X-ray fluorescence microprobe (XFM) end-station at the Australian Synchrotron has been developed. The simulation is required for optimization of the scan configuration and reconstruction algorithms. As part of the simulation process, a Gaussian beam model was developed. Experimental validation of this simulation has tested the efficacy for use of the low-energy physics models in Geant4 for this synchrotron-based technique. The observed spectral distributions calculated in the 384 pixel Maia detector, positioned in the standard back-scatter configuration, were compared with those obtained from experiments performed at three incident X-ray beam energies: 18.5, 11.0 and 6.8 keV. The reduced -squared ( 2 red ) was calculated for the scatter and fluorescence regions of the spectra and demonstrates that the simulations successfully reproduce the scatter distributions. Discrepancies were shown to occur in the multiple-scatter tail of the Compton continuum. The model was shown to be particularly sensitive to the impurities present in the beryllium window of the Maia detector and their concentrations were optimized to improve the  2 red parameterization in the low-energy fluorescence regions of the spectra. Keywords: X-ray fluorescence; Gaussian beam; Monte Carlo; simulation; Geant4; Maia. 1. Introduction The X-ray fluorescence microprobe (XFM) beamline at the Australian Synchrotron enables quantitative elemental mapping of a wide range of samples (Lombi et al., 2011; McColl et al., 2012; Howard et al., 2012). The XFM elemental mapping technique primarily involves raster scanning of a sample through a focused X-ray beam at a certain incident monochromatic photon energy and collecting fluorescence spectra at each position in the scan. These spectra can be analysed efficiently to produce maps (typically two-dimen- sional) of elemental concentration. The use of a rotation stage (de Jonge et al. , 2010) or focusing polycapillary (Donner et al., 2012) enables the position dimensionality of the data stack to be increased from two to three. As the size of the parameter space increases and the possibility of volumetric segmentation (de Jonge & Vogt, 2010) is realised, optimization of the scan geometry and reconstruction algorithms becomes increasingly important. In order to help facilitate scan optimization, a model of the XFM end-station has been developed and validated against experimental data. The model utilizes version 10.0.p01 of the GEometry ANd Tracking (Agostinelli et al., 2003; Allison et al., 2006) Monte Carlo software (Geant4) to propagate X-rays from the exit of the focusing optics, through the sample and onto the detectors. The measured beam- profile is that of a Gaussian beam. In order that the char- acteristics of the beam (waist and focal depth) were accounted for, a Hermite–Gaussian beam was incorporated into the simulation. The resulting Geant4 output data were processed to account for the detector resolution effects before being compared with experimental data. Validation of the model includes data collected at three incident beam energies: E 0 = 18.5, 11.0 and 6.8keV. The spectra were split into fluor-