CONTAMINATION DEPTH OF 137CS IN CONCRETE STRUCTURES

Abstract

Abstract Uncertainty analysis for nondestructive estimation of contamination depth is presented. The contamination depth was determined using the peak-to-peak method as an in-situ measurement in which gamma spectra were measured by an HPGe detector. Since exponential activity distribution is a crucial assumption of this method, the distribution profile was confirmed by laboratory tests of core drill samples. The main parameter influencing uncertainty of contamination depth is uncertainty of relaxation length. The uncertainty is composed for statistical error represented by the ratio of net peak areas and systematic error given detection efficiency of measurement setup. Systematic relative error was evaluated to be 7.45%. Statistical relative error was evaluated to 9.97% for the proposed optimum net peak area. Variability of relaxation length was identified to be very low with mean value 2 mm with standard deviation 0.73 mm. For fixed relaxation length, it should be possible to estimate contamination depth by nonspectrometric devices.