Environmental external radiation at some Bulgarian localities

Studies of the environmental radiation background are important from several points of view. First, they permit to estimate the exposure of humans to natural radiation background as a function of different geographical and geophysical parameters. Second, such studies can give also relevant information concerning the consequences of human activities on the presence of ionizing radiation and its sources in the environment. Our both institutions have started since 2000 year common studies in the field of environmental radiation background and its variation with the locality considered. First we have decided to compare the measuring methods used. They were compared on the territory of the Institute of Nuclear Research and Nuclear Energy of the Bulgarian Academy of Sciences (INRNE BAS), particularly in the surroundings of IRT 2000 research reactor. Further, they were compared also at the monitoring station on the territory of INRNE BAS. Finally, the studies were enlarged to the Moussala Observatory of the Institute at Rila Mountains. Several measuring instruments were used to characterize external environmental radiation exposure: Environmental radiation dose rate meter NB 3201 developed in the Czech Republic with a plastic scintillator with small NaI-TL crystal used to compensate the energy dependence to low energy photons; MDU-Liulin semiconductor spectrometer with Si-diode as the sensitive element able to characterize the radiation with both low and high LET; Thermoluminescent detectors (TLD) CaSO4:Dy, powder of this TLD material was filled to an Al dose. Results obtained are presented, analysed and discussed. A good agreement of the results obtained was observed, also when compared to the results of other experimental groups. (authors)