Construction of Radioactive Concrete Pads for Calibration of Ground and Airborne Gamma-Ray Spectrometers

Transportable calibration pads are portable reference sources used for the calibration of gamma-ray spectrometers. Eight radioactive concrete pads were built in Brazil, following the model adopted by the Canadian Geological Survey having the dimensions of 1 m x 1 m x 0.3 meter. Two of the pads were designed to measure background radiation. Three pads were enriched with one of the three main natural radioactive elements ( ⁴⁰ K, ²³⁸ U and ²³² Th) with traces of the other two elements. The remaining three pads contain varying amounts of these radioactive elements. To assess the homogeneity of the gamma radiation flux emitted from the concrete surfaces, two different analytical approaches were employed. The first approach involved measuring the radiation flux intensity over a regularly spaced grid on each concrete pad. The second approach aimed to evaluate the spatial distribution of gamma radiation intensity by fitting a low-degree polynomial surface to the radiation flux values obtained from the grid measurements. The elemental concentrations of the pads were determined using gamma-ray spectrometry, based on the activity measurements obtained from a high-resolution gamma-ray spectrometer. Finally, the Compton scattering calibration constants for a NaI(Tl) portable gamma-ray spectrometer were determined, yielding values consistent with those reported for this type of instrument using the eight radioactive concrete pads.