Using spectroscopic autoradiography of alpha particles for the quantitative mapping of ²²⁶Ra ultra-traces in geo-materials

The measurement of ²²⁶Ra and the identification of ²²⁶Ra-bearing minerals are important for studying the behavior of radium in the environment. Various instruments for measuring ²²⁶Ra are currently used: among the radiometric techniques that measure in bulk (no spatialization), there are gamma spectrometers and alpha spectrometers. Other instruments such as SEM-EDS can map the chemical elements thus providing information on the distribution of ²²⁶Ra, but are limited for ultra-trace analyses on natural geomaterials. Finally, autoradiography techniques can locate radioactivity, but are limited to the identification of the contribution of ²²⁶Ra when the ²³⁸U series is complete. This study focuses on spectroscopic autoradiography, a method for measuring both the energy of the alpha particle emissions and their positions on the autoradiograph. A gas detector based on a parallel ionization multiplier technology was used for this purpose. Alpha particle energy is dependent on the emitting radionuclides. In order to track the ²²⁶Ra, the energy spectrum of the ²³⁸U series was studied with modeling software. 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 was applied to a U-mill tailing sample prepared as a thin section. The sample was heterogeneous in terms of radioactivity and was not at secular equilibrium with ²³⁸U, as expected. The ²²⁶Ra was identified and localized, and different regions of interest were also analyzed with SEM-EDS elements cartography. This revealed ²²⁶Ra-rich barite (BaSO₄) phases measured at 3 ppm_Ra on average and containing no uranium; and uranium in siderite (FeCO₃), showing a strong ²²⁶Ra deficit compared with secular equilibrium. Spectroscopic autoradiography opens up possibilities for the analysis of heterogeneous geological samples containing natural alpha emitters such as ²³⁸U and ²²⁶Ra: the ²²⁶Ra can be localized and quantified at ultra-trace content, and the method developed can also identify newly (young) uranium phases by measuring ²³⁸U/²²⁶Ra activity disequilibrium.