In situ gamma-ray spectrometry in forests: determination of kerma rate in air from 137Cs

A method is presented to determine the kerma rate in air from 137Cs due to Chernobyl fallout in forests. In situ gamma-ray spectra from several forest sites in Russia, in the Ukraine and in Southern Germany are evaluated with the aim of deducing the ratio of primary and forward scattered photons for 137Cs. With this ratio and the results of Monte-Carlo simulations of photon transport the contribution of scattered photons to the total kerma is assessed successfully. Scattered photons contribute between 42% and 50% to the total kerma rate from radiocesium, which is less than according values for grassland areas. The contribution of radiocesium to the total kerma rate varies between 40% and 90%, whereas radiocesium stored in the forest biomass contributes only a few percent. The mean mass depth of radiocesium ranges from 2.6 to 6.4 g cm-2 in the forest soils