US EPA

7335138 

Journal Article 

Characterization of Uranium Ore Samples by HPGe Gamma-Ray Spectroscopy 

Marchais, T; Perot, B; Carasco, C; Ma, JLuc; Allinei, PGuy; Toubon, H; Goupillou, R; Collot, J 

2020 

Yes 

IEEE Transactions on Nuclear Science
ISSN: 0018-9499 

67 

4 

654-661 

10.1109/TNS.2020.2966824 

WOS:000538154200016 

Gamma logging for uranium exploration is currently based on total counting with the Geiger-Muller gas detectors or NaI(Tl) scintillators. However, the total count rate interpretation in terms of uranium concentration may be impaired in case of roll fronts when the radioactive equilibrium of the natural U-238 radioactive chain is modified by the differential leaching of uranium and its daughter radioisotopes radium, radon, and so on. Indeed, in the case of secular equilibrium, more than 95% of gamma rays emitted by uranium ores come from Pb-214 and Bi-214 isotopes that are in the back end of the U-238 decay chain. These two isotopes might produce an intense gamma signal even when uranium is not present, or with a much smaller activity, in the ore. Therefore, gamma spectroscopy measurements of core samples are performed on the surface with HPGe detectors to directly characterize uranium activity from the 1001-keV gamma ray of (234)mPa that is at the beginning of the U-238 chain. However, due to the low intensity of this gamma ray, acquisitions of several hours are needed. In view to characterize uranium concentration within a few minutes, we propose a method using both the 92 keV gamma rays of Th-234 and U-235 and the 98.4-keV uranium X-ray. This last is due to uranium self-induced fluorescence caused by gamma rays of the uranium chain, mainly Pb-214 and Bi-214. The gamma rays of Pb-214 and Bi-214 undergo scatterings in the sample leading to photons with energy in the 100-keV range that favor photoelectric interactions and, thus, fluorescence. The comparison of the uranium activity obtained with the 92- and 98.4-keV lines allows detecting a uranium heterogeneity in the ore. Indeed, in the case of uranium nugget, the 92-keV peak leads to underestimated uranium concentration due to the gamma self-absorption, but on the contrary, the 98.4-keV peak leads to an overestimation because of increased fluorescence. In order to test this new approach, tens of uranium ore samples have been measured with a handheld HPGe FALCON 5000 detector. 

Gamma-ray spectroscopy; HPGe detector; Monte Carlo simulation; self-induced X-ray fluorescence; uranium mining