Gamma-ray bursts of atmospheric origin in the MeV energy range

Downward gamma ray bursts of atmospheric origin in the MeV energy range were observed for the first time. The measurements were carried out on ground (at 2005 m above sea level) with a NaI(TI) 10 x 20diameter cm scintillation detector. Two kinds of events were observed: slow increases of the radiation below 3 MeV due to rain washout, and minute range bursts of gamma rays up to 10 MeV. The correspondence of these last events with perturbed weather conditions is here analyzed and the correlation with lightning activity is considered.