I. Introduction
The Orano Mining prospectors perform measurements of uranium ore samples during exploration to quantify the total amount of uranium potentially present within the mine. During the exploitation phase, uranium assessment allows monitoring the amount of uranium extracted from the mine. Total gamma counting is widely used in borehole probes [1], complemented by high-resolution HPGe gamma spectrometry on samples [2]. The total gamma count rate is dominated by gamma radiations of 214Pb and 214Bi (see Fig. 1) in the back end of the 238U decay chain [1]. It allows short acquisition times, but it is subject to potential U/Ra imbalances in the decay chain due to differential leaching in roll front deposits [2]. The reference gamma ray used in the HPGe spectroscopy is the 1001-keV radiation of 234 mPa, which is at the beginning of 238U [3], [4] chain and, consequently, is not impacted by a potential disequilibrium of the decay chain. Moreover, the sample mineralogy has little impact on attenuation above approximately 200 keV. However, the 1001-keV gamma-ray intensity is very small (0.847% [5]), and its detection with sufficient statistical precision may require acquisition times of several hours for low-grade uranium ore samples.
Experimental spectrum of a 479-g ore sample with a density of 1.46 cm. The acquisition time is 7503 s, and the uranium concentration of this sample is 419 ppmU according to the measurement realized in Section IV.