Gamma background increase during precipitation: its origin and energy balance

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   Thunderstorm effect on a muon flux is a well known phenomenon and was
   observed at many stations. Nevertheless, we were the first to find gamma
   background increase in the near ground layer of the atmosphere associated
   with precipitation and observed during all seasons in the Arctic region.
   This paper presents the results of new experiments on the improved
   detection system for gamma radiation in Apatity. The system consists of
   three units measuring radiation coming from the atmosphere: a small and a
   large scintillation detectors with NaI(Tl)crystals and a charged particle
   detector on the basis of Geiger-Muller counter. Measurements of gamma
   radiation differential spectrum in the range of 0.2-5 MeV are continuously
   carried out by the large scintillation detector and a multichannel
   pulse-height analyzer. During precipitation (during snowfall in winter or
   rain in summer) the increase of gamma radiation up to 50% is observed. The
   increases correspond to differential spectrum change of gamma radiation in
   the range up to 3 MeV. There is no increase in the charged component at
   the same time. The results of a series of experiments show, that in the
   observed gamma background variations, their initial source is muons,
   generated in the atmosphere from cosmic rays and having high
   penetrability. Here the muons themselves do not participate in
   Bremsstrahlung due to their big mass. During precipitation muons get
   additional energy when passing through cloud electric fields. Via muon
   decay additional energy is partially transferred to electrons, than via
   Bremsstrahlung it is converted to electromagnetic form. Energy balance was
   calculated from the measured additional radiation flux: during gamma
   background increase the flux increases by 23 keV.cm.squ.s. Such energy
   flux will be provided when field intensity in a cloud is about 3.6 kV/m.
   This estimation is in a good agreement with the numerous measurements of
   the electric field in clouds: 2-5 kV/m.