Distribution of fallout 137Cs in soils from Biscay (Spain) and the associated collective dose commitment

137Cs is an important contributor to the external irradiation dose received by world population as a consequence of the injections of radionuclides into the biosphere. The activity of 137Cs injected into the stratosphere in the atmospheric nuclear weapons tests performed between 1945 and 1980 amounts to 960 PBq. The amount of radionuclide deposited onto the ground at a given location is a function of i) rainfall and ii) surface characteristics. After atmospheric nuclear weapons tests had finished in 1980, the accident occurred in Chernobyl injected into the biosphere a new amount of 137Cs whose magnitude was estimated as 85 PBq. This activity was deposited onto the ground over a large region of Europe with a complicated deposition pattern. The amount of radioactive material from both sources that has been deposited on the Biscay region, located in the north of Spain, was unknown. Hence a program was developed with the aim of quantifying such deposition and so allow the evaluation of radiation doses received by population as well as possible future depositions of radioactivity. To determine such deposition a sampling program based on a sampling grid with a 15-km mesh was designed and applied. Seventeen sampling points were selected. At each sampling point, six 50 cm deep soil cores were collected along a straight line with 50 cm spacing between them. To measure the contents of 137Cs, samples were placed into 500 cm3 Marinelli beakers and analysed by gamma-ray spectroscopy with a p-type HPGe coaxial detector. From the results obtained it was observed that such deposition was entirely due to nuclear weapons testing. The contents of 137Cs were in the range 764 - 5880 Bq/m2 . A relationship between activity and rainfall was investigated, the correlation is similar to those reported by other authors. Finally, the dose commitment to the population due to internal and external irradiation by the deposited radionuclide has been calculated and a value of 575 Gy/person is obtained. (author)