Determination of Uranium Isotopic Ratios by HRGS Using Various Efficiency Calibration Approaches

Abstract

The effect of various efficiency calibration approaches on the value and source of the HRGS measurement uncertainty of ²³⁴U/²³⁸U, ²³⁵U/²³⁸U, ²³⁴U/²³⁵U isotopic ratios for the purposes of technological control, nuclear forensics, and environmental monitoring has been studied. The Canberra Broad Energy Germanium detector BEGe3830 and five samples of uranium certified reference materials CRM 969 and CRM 146 with a content of ²³⁵U/U ranging from 0.7 to 20.0 wt. % have been used. To calculate the uranium isotope ratios, the acquired gamma spectra were processed using: commercial MGAU (LLNL), FRAM (LANL), ISOCS software (Canberra/Mirion Technologies), based on intrinsic and absolute efficiency calibration approaches. It has been found that maximum relative biases, for the ²³⁴U/²³⁸U and ²³⁴U/²³⁵U isotopic ratios determined using the MGAU \ FRAM \ ISOCS software, are ~ 25 % \ ~ 10 % \ ~ 10 %, and the random uncertainty is varied within ± [18-25 % \ 2-15 % \ <=3 %], respectively. In the case of ²³⁴U/²³⁸U isotopic ratio determination using the MGAU \ FRAM \ ISOCS software, maximum relative biases come to ~ 3 % \ ~ 4 % \ ~ 1 %, and the random uncertainty values decrease to ± [1 % \ 1 % \ 1 %], respectively. In the present paper, we propose a combined intrinsic efficiency calibration approach with the use of the polynomial functions for the analytical description of the relation ε_rel.i(E_i). In this approach maximum relative biases, in the determination of the ²³⁴U/²³⁸U and ²³⁴U/²³⁵U isotopic ratios is 2.7 % at a random uncertainty of <= 1 %,  and in the case of the ²³⁴U/²³⁸U ratio a maximum relative deviation is 0.5 % at a random uncertainty of <= 0.7 %.