ИСТИНА

Some of the high-energy phenomena in atmosphere appear as fast (<1s) or slow (~1h) increases of gamma-ray flux probably connected with natural radioactivity or particle acceleration in atmospheric discharges. The study of both kinds is important for safety. Several experiments for the study of long-term and short-term flux and spectral variations of gamma-ray background were provided at sea level and in mountains. Scintillator detectors of moderately large size (from 50 to 100 mm) with energy resolution up to ~7% at 662 keV were used. The data were recorded in event-by-event mode with ~15 µs time resolution thus permitting detection of the terrestrial gamma-ray bursts at the moment of lightning during thunderstorms as well as slow variations on hour time scale. Measurements were done on the ground at Moscow region, and at mountain altitude in Armenia at Aragatz station. A number of flux increases (so-called Thunderstorm Ground Enhancements, TGE) were detected. Measured low-energy gamma-ray spectra usually contain a set of lines that can be interpreted as radiation of Rn-222 daughter isotopes. The increases of Rn-222 radiation reaching several tens of percent were detected during rainfalls with thunderstorm, as well, as during rainy weather without thunderstorms. The dynamics of the background changes can be described by the presence of solved Rn-222 in the rain water. High energy (>3 MeV) radiation sometimes appearing during thunderstorms can be caused by bremsstrahlung from electrons accelerated in thunderclouds. Such events with duration of several minutes and less were detected in mountains. Time and spectral characteristics of these events in 80-7000 keV energy range will be presented.