In situ gamma-spectrometry several years after deposition of radiocesium. Part I. Approximation of depth distributions by the Lorentz function.

Several years after the deposition of fallout-radiocesium, the maximal activity of this radionuclide will not remain at the soil surface but be found rather in deeper layers. In order to estimate the total radiocesium contamination of a large area and the resulting gamma-dose rate by in-situ spectrometry, it is necessary to approximate the vertical distribution of this radionuclide by an analytical function. Observations at ten undisturbed grassland soils and Bavaria, Germany, show that the resulting depth distributions can be approximated closely by a three-parameter Lorentz function. This function characterises the observed distributions in all three critical sections, i.e. the surface layer, the distribution around the maximal concentration, and the tail at greater depth. It is also shown that the observed total activity per unit area of the soil due to 137Cs agrees very well with the corresponding value obtained from the integrated Lorentz function. The two coefficients of the Lorentz function, which characterise the location (depth) and width of the maximum in the activity distribution, are shown to be correlated. In part II of this study, it will be shown how the parameters of the Lorentz function can also be obtained by in-situ gamma-ray spectrometry. As a result, it is possible to use in-situ gamma-ray spectrometry to obtain the total 137Cs activity per unit area also for sites where the vertical distribution of this radionuclide in the soil is no longer exponential.