Monte Carlo simulations of an energy dispersive K X-ray fluorescence system

Monte Carlo simulation with the MCNP4B2 transport code was used to model an energy dispersive X-ray fluorescence (EDXRF) spectroscopy system. The model was based on an EDXRF system developed for quantification of mercury Kα X-rays in aqueous solution. Two HPGe detectors, a planar and a coaxial, were modeled. The model output imitated the pulse height distribution from the experimental detector. The model and the experimental absolute peak detection efficiency was compared and found to be in good agreement at low energy (40-122 keV) for the planar HPGe and at high energy (122-1408 keV) for the coaxial HPGe. The model of the coaxial HPGe was applied to quantification of mercury in solution. Direct comparison between the model and the experiment was problematic because of the discrepancy in the absolute detection efficiency at ∼70 keV, corresponding with the K X-ray energies. Comparison of the results of the model and experimental relative to their respective base case was within 10%. Model study indicated that system modification to lower the minimum detectable concentration (MDC) by as much as a factor of four was possible with several alternatives in source-sample-detector arrangement.