Development of Gamma Spectroscopic Tools for Uranium Ore Samples and Borehole Exploration

Measuring uranium ore samples with high-resolution gamma spectrometers requires long acquisition times and expensive high-purity germanium (HPGe) detectors that are difficult to operate in the field (crystal cooling at very low temperature). In this work, we present a spectrometer based on a 1.5"×1.5" LaBr 3 (Ce) scintillator able to measure, in a few minutes, the uranium concentration and the possible disequilibrium of the uranium chain. The method is based on the total counts in two energy bands of the gamma spectrum, of which one at low energy that is sensitive to the top of the ²³⁸ U chain (including ²³⁴ Th gamma rays near 92 keV and the 98.4 keV X-ray due to uranium self-induced fluorescence). The second band, around the 609 keV peak of ²¹⁴ Bi (bottom of the chain), is insensitive to uranium and is used for normalization. We will also present the development of a borehole logging probe based on a LaBr 3 (Ce) detector and two energy-band approaches, first with a narrower low-energy band focused on the 98.4 keV X-ray and the same normalization with the 609 keV peak. Their ratio is almost insensitive to uranium grade, owing to competitive effects of self-fluorescence and self-absorption, but it gives the radioactive disequilibrium through a linear correlation. In the second approach, the low-energy band is sensitive to the attenuation of the Compton continuum due to uranium, divided by the counts in an energy band encompassing the 1001 keV gamma ray of ^234m Pa. The ratio is not sensitive to the disequilibrium, but is linear with uranium concentration. Finally, it is also possible to measure the net area of the 1001 keV gamma ray with the LaBr 3 (Ce) probe, at high uranium grade. Among these three approaches, the most efficient one is chosen depending on uranium grade and radioactive disequilibrium. The 1001 keV net area is also used at low grade with a longer acquisition time to self-calibrate the probe in real boreholes.