In-situ measurements of 137Cs in soil by unfolding method.

An improved in-situ spectrometry measurement of 137Cs concentration in soil is introduced. The method uses the information contained in the pulse spectrum in order to forego the need for soil sampling. The approach is based on the unfolding of responses of a collimated and uncollimated HPGe detector to primary 0.662 MeV photons and to photons scattered forward in the soil. The calibration of the in situ equipment has been performed by Monte Carlo calculations and by experiments. For unfolding of experimental detector responses the code SAND II has been found reliable and capable of calculating distribution of 137Cs in soil profile with adequate accuracy for environmental monitoring purposes. The analysis of the spectra indicates that 137Cs concentration in soil 10 y after Chernobyl accident would be measurable using a middle HPGe detector (20-30% relative efficiency) and a counting time on the order of 1 h. Even with smaller detectors, 137Cs concentrations of 5 kBq m(-2) are measurable, and the depth distribution of 137Cs activities above 10 kBq m(-2) in the soil can be estimated by the presented method when a counting time on the order of 3 h is used.