Analytical determination of contamination depth of ¹³⁷ Cs point sources in reactor building walls

Abstract The presence of radioactive contamination in concrete structures poses a silent yet serious threat to both environmental safety and public health. Such contamination, if not properly localized, can lead to prolonged radiation exposure, difficulties in decommissioning, and unnecessary disposal of large amounts of low-level radioactive waste. This study presents a Monte Carlo simulation-based method to determine the type and exact position of a gamma-emitting contaminant located on or inside a concrete wall. Two collimated detectors are positioned on opposite sides of the wall, and the depth of the source is calculated using the ratio of photopeak counts measured simultaneously in a single simulation run. A proportional relationship between the count rates eliminates the need for prior knowledge of the wall materials attenuation coefficient. The simulated setup carefully models realistic experimental conditions, including detector geometry, wall composition, and source characteristics. It is demonstrated that the photopeak count is inversely proportional to the source distance from the wall surface. The simulation results show good agreement with experimental measurements, confirming the validity and reliability of the proposed non-destructive method for localized contamination assessment.