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 \mu \mathrms 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.