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

The measurement of Ra and the identification of Ra-bearing minerals are important for studying the behavior of radium in the environment. Various instruments for measuring Ra 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 Ra, 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 Ra when the U 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 Ra, the energy spectrum of the U series was studied with modeling software. It appears possible to apply a thresholding on the energy spectrum to discriminate the Ra from the first alpha emitters of the U decay chain (i.e. U, U and Th, 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 U, as expected. The Ra was identified and localized, and different regions of interest were also analyzed with SEM-EDS elements cartography. This revealed Ra-rich barite (BaSO₄) phases measured at 3 ppm on average and containing no uranium; and uranium in siderite (FeCO), showing a strong Ra deficit compared with secular equilibrium. Spectroscopic autoradiography opens up possibilities for the analysis of heterogeneous geological samples containing natural alpha emitters such as U and Ra: the Ra can be localized and quantified at ultra-trace content, and the method developed can also identify newly (young) uranium phases by measuring U/Ra activity disequilibrium.