Low-Resolution Gamma Spectrometry of Uranium Ores to Determine U Concentration and U/Rn Imbalance

The measurement of uranium ore samples by high-resolution gamma-ray spectroscopy requires long measurement time and expensive high-purity germanium detectors. In this work, we present the possibility to measure uranium concentration by low-resolution gamma-ray spectrometry with an easy-to-operate and cost-effective NaI(Tl) detector. This method is based on the analysis of two energy bands of the NaI gamma spectrum, which allows estimating a possible “U/Rn” imbalance between the top (
²³⁸
U and its daughters up to
²³⁰
Th) and the bottom (
²²⁶
Ra,
²²²
Rn and their daughters) of the
²³⁸
U decay chain in roll-front deposits. Indeed, in case of secular equilibrium, more than 95% of gamma rays emitted by uranium ores come from
²¹⁴
Pb and
²¹⁴
Bi isotopes, which are in the backend of the
²³⁸
U chain. Consequently, a disequilibrium in the chain might produce an overestimation of uranium concentration if U/Rn < 1, or an underestimation if U/Rn > 1. Therefore, the estimation of the disequilibrium between the beginning and the end of the chain represents a crucial issue in uranium mining and exploitation. Finally, the uranium concentration is derived from the U/Rn activity ratio and from the “Rn” activity of the backend chain that is measured by conventional gamma-ray spectroscopy with the 609-keV peak of
²¹⁴
Bi. The total uncertainty is less than 30% within a few minutes of acquisition, and we observe a very good agreement with inductively coupled plasma mass spectrometry (ICP-MS) analyses performed on real samples.