Be-7 and Pb-210 deposition measured in rainfall in the city of São Paulo - Brazil

The short-lived cosmogenic isotope Be-7 (T1/2 = 53.3 d) and the natural daughter product of Ra-222, Pb-210 (T1/2 = 22.3 y) have been widely used as tracer soil erosion, transport processes in watershed and chronometers in the environment. These isotopes have also been utilized to determine the aerosol residence time as well as removal rates of aerosols. The concentrations of these radionuclides were determined in samples of rainfall during the period of April 2011 to July 2013 at Instituto de Pesquisas Energéticas e Nucleares - IPEN's campus located in the city of São Paulo, São Paulo, Brazil. The sampling site is approximately 10 km west from downtown São Paulo (23o32'S - 46o37'W at 760 m above sea level). Climate in the area is temperate tropical with dry period in winter and rainy in summer with the annual rainfall ranged from 443 mm to 2081 mm. The annual average temperature is 19.1oC, showing minimum and maximum of 15.3oC and 24.9oC, respectively. Be-7 was measured by non-destructive gamma-ray spectrometry and Pb-210 was measured by beta gross counting in a gas flow proportional detector, after radiochemistry procedure. For gamma-ray spectrometry a coaxial Be-layer HPGe detector with 25% relative efficiency, 2.09 keV resolution at 1.33 MeV and associated electronic devices were used, with live counting time varying from 100,000 s to 300,000 s. The spectra were acquired by multichannel analyzer Ethernim and, for the analysis, WinnerGamma software was used. The obtained results for both radionuclides in all samples show that they present a similar behavior with zones of analogous latitude, but in Northern hemisphere, in spite of São Paulo city being situated in low latitudes of Southern hemisphere. The concentrations displayed clearly seasonal variations with higher values in spring and summer time and with the amount of precipitation.