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A new approach is proposed in order to spatially localize and determine the equilibrium state of natural decay chains on hand-scale geological samples, thanks to a combination of three techniques: 1) Elementary chemical mapping by microprobe; 2) Alpha autoradiograph by gaseous detectors and 3) bulk alpha particle spectrometry. The quantitative nature of alpha autoradiograph and its comparison with U chemical maps allows to locate radioactive equilibrium state in four samples. This equilibrium state was confirmed by alpha spectrometry analysis. [ABSTRACT FROM AUTHOR]

Geoscience is a field of study where nuclear instruments find their place for conducting specific measurements of radioactivity, usually to improve the monitoring of natural processes using naturally-occurring radioactive tracers, but also for the nuclear industry linked to the mining sector. In geological samples, the location and identification of the minerals bearing radioactivity at the thin-section scale remains a major challenge as the detection limit of the usual elementary microprobe techniques is far higher than the concentration of most of the natural radioactive decay products. The location of each decay product, as in the case of uranium-238 series in a geomaterial is interesting for relating radionuclide concentrations to mineralogy. The present study aims to provide spectroscopic autoradiography analysis method for measuring the initial energy of alpha particles with a parallel ionization multiplier gaseous detector. This spectroscopic autoradiography method was successfully used to reproduce the alpha spectra from a 238U decay chain on a geological sample at the thin-section scale. The characteristics of this measurement are an energy spectrum resolution of 17.2% (FWHM) at 4647 keV and a spatial resolution of at least 50 μ m. Even if the efficiency of energy spectrum reconstruction is low (4.4%) compared to the efficiency of a simple autoradiograph (50%), this novel measurement approach offers the opportunity to select areas on an autoradiograph to perform an energy spectrum analysis within that area. This opens up possibilities for the detailed analysis of heterogeneous geological samples containing natural alpha emitters such as uranium-238 and radium-226. • A novel analysis approach of autoradiography imaging using a PIM detector. • The method enables the energy distribution measurement of alpha particle emissions. • Applications are possible for geosciences and biosciences. • The device combines a wide analysis surface area and a 50 μ m spatial resolution. [ABSTRACT FROM AUTHOR]