Tracing and quantification of ²²⁶Ra in tailings by spectroscopic autoradiography

The present study aims to provide spectroscopic autoradiography (SA) analysis method for measuring the initial energy of alpha particles with the BeaQuant gaseous detector. This method, that has been developed thanks to Geant4 modelling of the detector, focuses on alpha particles because 1- the RN of interest are alpha emitters, 2- the detection efficiency is better for alpha than for beta, 3- alpha particles have a discrete energy of emission compared to beta that have a continuous emission spectrum. This spectroscopic autoradiography method was successfully used to reproduce the alpha spectra from a uranium ore at secular equilibrium (the whole ²³⁸U decay chain) with a spatial resolution of at least 50 μm and an energy spectrum resolution of 17.2% (FWHM) at 4647 keV. Even if the efficiency of SA is low (4.4%) compared to the simple detection efficiency of autoradiography (50%), this measurement approach offers the opportunity to select areas of interest on an autoradiography to perform an energy spectrum analysis within that area. The alpha emitters from ²³⁸U decay chain present initial emission energy ranging from 4.2 MeV for ²³⁸U to 7.7 MeV for ²¹⁴Po. By studying more precisely the alpha energy spectrum and the half-life of each RN, it appears possible to apply a thresholding on the energy spectrum to discriminate the ²²⁶Ra from the first alpha emitters of the ²³⁸U decay chain (i.e. ²³⁸U, ²³⁴U and ²³⁰Th, all below 5 MeV). The developed method is applied to mine tailings prepared in the form of thin sections, and the resulting maps highlight different signals area: signal '1' with areas close to the secular equilibrium, signal '2' with areas presenting a deficit of the 3 first alpha emitters of the ²³⁸U decay chain (i.e. ²³⁸U, ²³⁴U and ²³⁰Th) and signal '3' for areas with a deficit of the 5 last alpha emitters (from ²²⁶Ra to the end of the decay chain). Some of these regions of interest have been observed/analyzed with SEM-EDS, allowing to highlight: -radium-rich barite (BaSO₄ with 5 ppm of ²²⁶Ra) in the signal '2' area. ²²⁶Ra is incorporated into the mineral structure through solid solution mechanism. -siderite (FeCO₃) containing uranium (up to 1% detected in EDS) in the signal '3'. Uranium in the siderite is not at secular equilibrium, that means this uranium has been remobilized during the ore treatment (for uranium recovery), and later trapped by surface mechanisms on siderite. If areas showing a secular equilibrium have been highlighted (that is not the case in the studied tailings), they should correspond to uranium bearing minerals inherited from ore and thus not impacted by ore treatment. Spectroscopic autoradiography opens up possibilities for the detailed analysis of heterogeneous geological samples containing natural alpha emitters such as uranium-238 and radium-226: The ²²⁶Ra can be localized and quantify at ultra-trace content; and the method devolved also can help to identify new formed young uranium phases by measuring ²³⁸U/²²⁶Ra ratio equilibrium state. These analyses will then be used to implement the reactive transport models build to study the mobility of ²³⁸U and ²²⁶Ra in the mine tailings and the surrounding