Uranium enrichment measurement using SrI2(Eu) scintillators and an adapted peak-ratio method in the <250 keV energy range

We present the results of preliminary research utilising the excellent energy resolution of europium-doped strontium iodide scintillators (SrI2[Eu]) to determine the uranium enrichment of light water reactor (LWR) nuclear fuel pellets [1]. A common technique for measuring uranium enrichment with low-resolution scintillators is the peak ratio method - comparing the relative magnitude of prominent emission peaks of 186 keV and 1001 keV from 235U and 238U, respectively. This presents a challenge as detailed detector efficiency calibration curves need to be collected to use the method, given the divergent attenuation coefficients at these disparate γ-ray energies [2].
The peak ratio method can be performed using the sub-250 keV emissions from uranium with highresolution semiconductor detectors and comparing X-ray emissions. However, we have shown that low-resolution SrI2[Eu] scintillators can achieve an energy resolution of <5% FWHM at 186 keV. This performance, examples shown in Figure 1, allows for the 163 and 205 keV 235U emissions to be resolved and excluded from analysis. Using this performance, it is possible to compare the magnitude of the 238U X-ray emissions and the pure 186 keV γ-rays from 235U from low-enriched sintered uranium dioxide samples, to calculate the enrichment.
Though this method is not as precise as the enrichment meter principle, it does allow rapid approximation of the enrichment of a uranium sample. Furthermore, the method is geometrically independent and does not require detector efficiency calibration. These benefits could render the method useful in nuclear fuel manufacturing contexts, where coarse enrichment checks of uranium powders and tens-of-thousands of pellets in a production cycle is needed for process quality assurance.

[1] A. J. Parker et al., “Enrichment measurement by passive gamma-ray spectrometry of uranium dioxide fuel pellets using a europium-doped, strontium iodide scintillator,” Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip., vol. 1062, no. 169191, 2024.
[2] D. Reilly and N. Ensslin, Passive Nondestructive Assay of Nuclear Materials. 1991.