Technique for in situ measurement of Cs-137 deposit in soil under clean protected layer

The method to determine the Cs-137 soil deposit in situ in case of contamination protected by a clean soil layer is described. The method is based on measurements of the count rates in three energy ranges by the Collimated Spectrum Sensitive Detector and their processing. The thickness of analyzed soil layer is up to 40 g/cm². This value corresponds to three mean free path (mfp, 1 mfp≈λ) of Cs-137 radiation in soil. The thickness of uncontaminated soil layer covering the contaminated one may achieve 2λ. The minimum detectable soil deposit is about 20 kBq/m². The deposit determination accuracy is 20% and the estimation of contaminated soil layer thickness slightly depends on the depth distribution of contamination. The measurement results are presented and compared with data of soil sampling. The developed technique is very effective for mapping of contaminated territories. The Cs-137 contamination maps of areas in Belarus and Chelyabinsk region of Russia are presented. The measurements of the Techa river bank contamination detected a presence of secondary contamination transference. This method allows one to forecast the change of dose rate after territory decontamination and determine an amount of work and its efficiency