Spectral deconvolution approach for Beryllium-7 detection: Evaluating soil redistribution with NaI(Tl) detectors

Abstract

Beryllium-7 is a cosmogenic radionuclide produced in the upper atmosphere through the spallation of cosmic rays and falls to Earth mainly by wet deposition. Given its short half-life (53 days) and strong adsorption to the clay present in the soil, 7Be inventory can be evaluated to obtain information about soil redistribution over periods of up to 6 months. By performing these measurements, one can infer the efficiency of soil management and conservation procedures. The isotope presence is measured using gamma-ray spectrometry to detect its decay emission, specifically a photon with an energy of 477.6 keV. Since other isotopes found in soil emit photons with similar energies, it is common to use High-Purity Germanium (HPGe) detectors due to their high energy resolution. However, these detectors are expensive and require cryogenic cooling during operation. An alternative is the Thallium-Activated Sodium Iodide (NaI(Tl)) detector, which is less costly and does not require cryogenic cooling, but has poorer resolution and cannot separate the emissions from 228Ac (463.0 keV), 7Be, and 208Tl (510.7 keV). This work aims to apply spectral deconvolution to quantify 7Be in soil samples collected from an agricultural mega-parcel situated on a hill slope and evaluate the soil redistribution experienced within. The activities of the interfering isotopes were quantified using other emissions, and the results were used to calculate the corresponding area in the desired region of the spectra. From this, 7Be activities were calculated and compared with the HPGe results to validate the deconvolution technique. The calculated activities for the three radioisotopes showed similar correspondence between the two detectors, indicating that the applied method is adequate for evaluating soil redistribution at lower costs. The calculated net soil deposition was 36.4 ± 6.4 kgm−2 for the NaI(Tl) detector and 26.4 ± 7.9 kgm−2 for the HPGe detector during the evaluated period. Furthermore, these results suggest that a similar approach can be applied to quantify other isotopes using NaI(Tl) detectors as an alternative to HPGe detectors.