ATTENUATION EFFECTS ON THE KERMA RATES IN AIR AFTER CESIUM DEPOSITIONS ON GRASSLANDS

Since the reactor accident of Chernobyl, cesium depth profiles and nuc lide-specific kerma rates in air have been determined for various gras sland sites in south Bavaria and in Ukraine. The sites are described b y soil characteristics, annual precipitation, distance from release po int, mode of deposition, and activity per unit area. The effects of su rface roughness and migration of cesium into the soil on the kerma rat e in air over grasslands was determined by two methods. The kerma rate s in air obtained by the evaluations of in situ gamma-ray spectrometry results and of measured activity distributions in the soil showed onl y negligible differences for the observation period of 6 years after d eposition. For the sites in Ukraine the kerma rate in air per activity per unit area was found to be systematically 40% higher than in Bavar ia. The results from Bavaria on the attenuation of the kerma rate and a data set, including experiences from the weapons test fallout, are a nalytically approximated as a function of time up to 25 years after de position.