IN-SITU GAMMA-SPECTROMETRY SEVERAL YEARS AFTER DEPOSITION OF RADIOCESIUM .1. 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 radi ocesium contamination of a large area and the resulting gamma-dose rat e by insitu spectrometry, it is necessary to approximate the vertical distribution of this radionuclide by an analytical function. Observati ons at ten undisturbed grassland soils in Bavaria, Germany, show that the resulting depth distributions can be approximated closely by a thr ee-parameter Lorentz function. This function characterises the observe d distributions in all three critical sections, i.e. the surface layer , the distribution around the maximal concentration, and the tail at g reater depth. It is also shown that the observed total activity per un it area of the soil due to Cs-137 agrees very well with the correspond ing value obtained from the integrated Lorentz function. The two coeff icients of the Lorentz function, which characterise the location (dept h) 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 par ameters 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 Cs-137 activity per unit area also f or sites where the vertical distribution of this radionuclide in the s oil is no longer exponential.