Development of a XRF Detection System for MOX Samples

The design of the detection system for nondestructive analysis of MOX pellets by the accurate measurement of the Pu/U ratio in trace concentrations (few %) by means of XRF spectrometry is presented. The parameters of multiple Peltier-cooled compact CdTe detectors are optimized through systematic experimental characterization, leading to a FWHM resolution better than 1 keV at 122 keV (3.2 μs shaping time) stable for days of operation. Given the small separation between the peaks to be quantified (~9 keV) and the low concentration of Pu, accurate spectrum analysis is mandatory. Acquired spectra are processed by an algorithm that subtracts the background and fits the peaks with a combination of Gaussian and exponential tail functions, required to model the slow collection of holes in these detectors. The lack of calibrated Pu/U samples has been addressed by realizing mixtures of Bi/Pb powders (with similar spectral separation) and by creating a virtual spectrum by measuring the background-subtracted U spectrum which is then scaled, shifted and added back to the total spectrum to emulate the Pu spectrum in the 1-10% range of concentrations. Experimental results obtained with the former samples allowed the assessment of repeatability (relative error within ±1.7%) for 10 measurements lasting 5 minutes (with a count rate of 20 kcps giving a total of ~900 kcounts), while the latter “virtual” spectra enabled the validation of the relative accuracy of whole analytical instrument within ±0.6%. Both values are well within the design specifications (±2% with a measurement time of max. 10 minutes). Thanks to a reliable net area calculation algorithm, the geometry and materials (filters, collimators, etc...) of the final instrument design have been optimized as well.