Study of terrestrial gamma-ray background in presence of variable radioactivity from rain water

International audienceA number of groups have reported significant reduction in the flux of low energy (0.1-3 MeV) gamma-rays in observations carried out during the past total solar eclipses. However, the contribution of the radon induced radioactivity to the overall gamma-ray background can become substantial, especially during episodes of rain. Depending upon the pattern of the rainfall radon induced gamma-ray background may vary significantly on time scales of ~10 min, making the interpretation of the data in terms of an extraterrestrial effect such as a total solar eclipse rather difficult. A reliable estimate of the low energy terrestrial gamma-ray (TGR) background is necessary before attempting to measure the possible contribution of any extraterrestrial phenomenon. The knowledge of the precise energies and branching ratios of radon and other radio-isotope induced gamma-rays was exploited to accurately reproduce the TGR background, even in the presence of a large and variable contribution from radon induced radioactivity from fresh rain water. The measurement of the TGR background has paved the way for studying the variation of the soft gamma-ray flux during the long duration total solar eclipse that occurred on 22 July 2009 in the middle of the Monsoon season in India