Radiocesium gamma dose rates in a Greek pine forest: measurements and Monte Carlo computations

摘要

The radiocesium distribution in an Aleppo pine (Pinus halepensis, Mill.) forest ecosystem located in the Kassandra peninsula 105 km south of Thessaloniki, Macedonia, Northern Greece, was extensively studied during the last seven years. The radiocesium distribution in the different parts of the ecosystem was measured. A total ~(137)Cs inventory of 150 MBq ha~(-1) mainly due to the Chernobyl accident was measured in all parts of the ecosystem. The major part of this inventory (88%) is still in the upper 10 cm of the soil (84%) and the forest floor (4%). A small fraction of the inventory, less than 1 %, is in the above-ground biomass. The ~(137)Cs distribution in the soil follows an exponential decrease with depth and seems to level off to a constant value. It is probable that the same constant value also observed previously for other Greek forest sites describes the radiocesium concentration due to weapons fallout and indeed the exponential decrease, the radiocesium distribution due to the Chernobyl accident. The absorbed gamma dose rate in air 1 m above soil due to radiocesium was determined inside the ecosystem by combination of Monte Carlo computations with the MCNP code and in situ gamma spectroscopy measurement. The gamma dose rate in air due to ~(137)Cs is about 9 nGy h~(-1) where 60% is due to the unscattered radiation (661.6 keV) and 40% due to the scattered radiation of the primary photons in the different parts of the ecosystem (mainly in the soil). The results obtained with the Monte Carlo simulations for the unscattered radiation were in very good agreement with the experimental values deduced by in-situ gamma spec-trometry measurement. From the combination of the Monte Carlo simulations and an in situ gamma spectrometry measurement, a conversion factor C = 0.82 (nGy h~(-1))/(kBq m~(-2)) was deduced.