A Calibration Study for Holdup Measurements

Uranium holdup is routinely measured using low-resolution NaI(Tl) gamma ray spectrometers, from which the mass of 235U is related to the count rate of the 186 keV spectral feature. This count rate is estimated according to a long-established procedure based on a crude two region of interest (ROI) method. When this approach is used, the 186 keV peak is not clearly separated from other 235U gamma ray peaks; the continuum estimate relies on the spectral shape being constant (or changing in a linear manner with enrichment and attenuation); and interferences cannot be corrected. In this work, a simple peak-fitting method is introduced to calculate the count rate for just the 186 keV 235U gamma ray peak, while separating out other nearby 235U energies, and it is compared with the ROI method currently in use. Experimental data were collected from a highly enriched 235U standard (approximately 10.9 g disc) using a 2.54 cm × 1.27 cm NaI(Tl) detector, with a 3.0 cm collimator and a thin Sn filter, at on-axis distances ranging from 10.5 to 60.5 cm. It is shown that the measured count rate obtained by peak fitting more accurately reflects the count rate in the 186 keV peak than the ROI method. The peak-fitting approach is more robust with regards to amplifier gain and resolution changes, and expected interferences can be included in the analysis library. Implementing this approach should improve the precision realized in current holdup measurement methods, while retaining straightforward and practical strategies, through the introduction of physics-based methods that lend themselves to more reliable uncertainty quantification.