Observation of energetic radiation associated with a thunderstorm activity at the top of Mt. Fuji

Torii, Tatsuo  ; Sugita, Takeshi*; Tanabe, Sachiko*; Kimura, Yoshihisa*; Kamogawa, Masashi*; Yajima, Kazuaki*; Yasuda, Hiroshi*

Fluctuations of energetic radiation that were seemed to be caused by a summer thunderstorm activity were observed at the top of Mt. Fuji. The largest of such fluctuations was gradual and lasted for about 20 minutes, and was found to be high-energy rays having a continuous energy spectrum up to 10 MeV or more. As for the feature of these fluctuations, it seems naturally that such fluctuations are caused by the bremsstrahlung photons generated by the runaway electrons produced continuously with an intense electric field in the thundercloud rather than originated in the process of lightning discharge. The 5-inch NaI detector used in the observation was so sensitive to photons that it could not have differentiated electrons from photons even if high-energy electrons had been incident upon the detector. When conducting the analysis of energy spectrum, sufficient convergence was achieved in the calculation even in the case where only the sensitivity to photons was considered.