Preliminary results of atmospheric deposition of 7Be by rain events, in central Argentina

JURI AYUB, J.; DI GREGORIO, D.E.; HUCK, H.; VELASCO, R. H.; RIZZOTTO, M.

Buzios, Brasil.

Simposio; 8th International Symposium on the Natural Radiation Environment; 2007

Beryllium-7 is a natural radionuclide that enters to the ecosystems through wet and dry depositions. Also, 7Be have numerous applications in studies of transport processes in atmospheric, terrestrial and aquatic systems. Atmospheric wet deposition of 7Be is being measured in central Argentina (San Luis province). Rain traps were installed and individual rain events have been collected. Rain traps were placed 1 m above the ground when rain began and removed when finishing the same one. The largest particles were removed by filtration. After exposure, rain collector was emptied and washed with distilled water. The filtrate was analyzed by gamma spectrometry. The gamma counting was undertaken using a 1.033 kg intrinsic high-purity natural germanium crystal built by Princeton Gamma-Tech. The cryostat was made from electroformed high-purity copper using ultralow-background technology. The detector gamma efficiency was determined using a water solution with known amounts of chemical compounds containing long-lived naturally occurring radioisotpes, 176Lu, 138La and 40K. All rain samples of around 400 ml, were counted during 1 day. The 7Be detection limit for the present measurements was as low as 0.2 Bq l-1. Until now 32 rain samples (from November 2006 to May 2007) have been collected and analysed. In this region rainfalls take place seasonally, existing a dry season (May to October) and a rainy season (November to April). Data from the latter season, show that events corresponding to low rainfall (<10 mm) are characterised by significantly higher activity concentrations (Bq l-1).  Whereas, events corresponding to high rainfall (>10 mm) show lower concentrations of 7Be. The initial activities of each individual event vary from 0.8 to 3.5 Bq l-1. At the same time, precipitations vary between 4 and 70 mm. The integrated activities by event of 7Be are linearly correlated with the magnitude of precipitation (r2 of 0.77; p<0.001). These preliminary results are in agreement with those obtained for other sites. Several authors have proposed a linear relationship between 7Be depositions with rain intensities. Rainfalls constitute the main input mechanism of 7Be from the atmosphere to the ecosystems. The monitoring of this process is of great importance at the time of using 7Be like an indicator of different events that happen in those ecosystems.