Abstract
Gamma logging for uranium exploration are currently based on total counting with Geiger Müller gas detectors or NaI (TI) 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 238U radioactive chain is modified by differential leaching of uranium and its daughter radioisotopes of thorium, radium, radon, etc. Indeed, in case of secular equilibrium, more than 95 % of gamma rays emitted by uranium ores come from 214Pb and 214Bi isotopes, which are in the back-end of 238U chain. Consequently, these last 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 in surface with high-resolution hyper-pure germanium HPGe detectors to directly characterize uranium activity from the 1001 keV gamma ray of 234mPa, which is in the beginning of 238U chain. However, due to the low intensity of this gamma ray, i.e. 0.84 %, acquisitions of several hours are needed. In view to characterize uranium concentration within a few minutes, we propose here a method using both the 92 keV gamma ray of 234Th and the 98.4 keV uranium X-ray. This last is due to uranium self-induced fluorescence caused by gamma radiations of 214Pb and 214Bi, which create a significant Compton scattering continuum acting as a fluorescence source and resulting in the emission of uranium fluorescence X-rays. The comparison of the uranium activity obtained with the 92 keV and 98.4 keV lines allows detecting a uranium heterogeneity in the ore. Indeed, in case of uranium nugget, the 92 keV line leads to underestimated uranium concentration due to gamma self-absorption, but on the contrary the 98.4 keV line leads to an overestimation because of increased fluorescence. In order to test this new approach, several tens of uranium ore samples have been measured with a handheld HPGe FALCON 5000 detector.
Highlights
Current gamma measurements used to characterize the uranium content of ore samples are mainly based on two estimators
The method is studied by MCNP6 Monte Carlo calculation by the Nuclear Measurement Laboratory of CEA, DEN, Cadarache, and validated with experimental data obtained by ORANO Mining during a measurement campaign intended to characterize uranium content and imbalance of the samples [2]
We have shown the possibility to measure the uranium concentration of uranium ore samples by gamma-ray spectroscopy using the 92 keV and 98 keV lines, which lead to a large measurement time gain compared to the reference 1001 keV peak
Summary
Current gamma measurements used to characterize the uranium content of ore samples are mainly based on two estimators. The first one is the total gamma count rate, which is approximately 90 % due to gamma radiations of 214Pb and 214Bi daughters (see further Fig. 1), in the back-end of 238U chain. It allows short acquisition times but it is subject to potential imbalances in this decay chain. The two rays at 92 keV and 98 keV are systematically present with a high intensity on uranium ore spectra (see example of a small concentration sample in Fig. 1), which makes it possible to envisage a significant measurement time reduction with respect to 1001 keV line
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