Gamma-emitters survey im soils of the province of Buenos Aires-Argentina

M.L. MONTES; R.C. MERCADER; M.A. TAYLOR; J. RUNCO; P.A. IMBELLONE; P.C. RIVAS; J. DESIMONI

Mérida

Congreso; 4Th International Congress on Energy and Environment Engineering and management; 2011

Universidad de Extremadura

GAMMA EMITTER SURVEYS IN SOIL OF THE PROVINCE OF BUENOS AIRES-ARGENTINA M. L. Montes1, R. C. Mercader1, M. A. Taylor1, J. Runco1, P. Imbellone2, P. C. Rivas3 and J. Desimoni1 1Departamento de Física, Facultad de Ciencias Exactas, UNLP, IFLP – CONICET, Argentina. desimoni@fisica.unlp.edu.ar 2Instituto de Geomorfología y Suelos, UNLP, Argentina. 3 Facultad de Ciencias Agrarias y Forestales, UNLP, IFLP – CONICET, Argentina. 1. Introduction - Because of the lack of systematic information about the natural and anthropogenic activities baselines, the controversies concerning the nuclide transport in soil and the reactivation of the nuclear program in Argentina [1, 2], we have started a long term project to contribute to the gamma emitter activity nuclide determination in soils of the Province of Buenos Aires. This is an important agricultural and industrial area where nuclear power plants and research nuclear reactors are located, adding to the natural inventory small amounts of activities of anthropogenic nuclides. Despite the long period of nuclear of activity in the region, no systematic study has been carried out until now to establish the actual conditions. We present here, activities depth profiles down to 50 cm from the surface of the natural chains (238U, and 232Th, 40K), and 137Cs of unperturbed soil of the neighbourhood of La Plata city. Mössbauer Effect, pH, texture and organic carbon content measurements have been added. 2. Experimental - The gamma spectra were taken inside an EG&G Ortec low-background chamber in the range of 30 keV to 1.8 MeV, using a GMX10 gamma EG&G Ortec detector with a standard electronic chain and 8192 channels multichannel. Mössbauer spectra were recorded at room temperature in a 512 channels conventional constant acceleration spectrometer with a 57CoRh source. The pH, texture and organic carbon measurements were performed using the international protocols. 3. Results and Discussion - The 238U and 232Th chains were detected, as well as the presence of 40K. 232Th and 238U activity chains vary in depth and space, i.e., soils with low hematite content and absence of magnetite display slightly higher activities, probably related to the geological origin of the soils. The main contribution to the exposure dose arises from the 40K activity. The variation in the top layer activities gives rise to higher exposure dose in the coastal region. The only anthropogenic detected nuclide was 137Cs, arising from the fallout of the Southern Hemisphere weapon tests. Differences in depth distributions were detected; three soils follow a convective-diffusive process [3] with fitted parameters in agreement with worldwide results [4]. The deviation from this model observed in the depth profile of the La Plata River coastal area could be explained considering the high clay content, the flat relief and the water dynamics of the region. No correlation was observed between iron compounds and 137Cs profiles. 4. Conclusions - The results reported are the first ones to establish baseline activity for the naturally and anthropogenic occurring gamma emitter radionuclides in unperturbed soils around La Plata city. The 40K activity was correlated with Fe3+ iron species, probably related to pedogenic and edaphic properties of the soils, and/or to the presence of roots. The determined diffusion coefficient and convection velocity of 137Cs in soils are in the range of South American and worldwide determinations. Finally, putting together the gamma and Mössbauer results, it is tempting to conclude that there is not an apparent correlation between the 137Cs profile and the relative fraction of Fe3O4, as well as with other iron species. 5. References [1] J. M. De Vido: Discurso, Reactivación de la industria nuclear en Argentina (2006). [2] D. F Carlos Rey, www.pagina12.com.ar (2007). [3] P. Bossen, G. Kirchner, J. of Environ. Radioactivity 73, (2004) p.127. [4] IAEA Technical Report Series nº 42. Handbook of Parameter values for the Prediction of Radionuclide Transfer in Terrestrial and Freshwater Environments. (2010).