On the annual variation of photons originating from secondary cosmic ray components at sea level at Nagoya, Japan

To ascertain the variations of photons due to secondary cosmic rays near sea level in Japan, two kinds of scintillation detector of the same geometrical size but of different sensitivities to photons were used. One was a 3''φ spherical NaI (T1) scintillation detector, and the other a 3''φ plastic one. For both detectors, the difference of the counting rates due only to cosmic rays was regarded as the flux density of photons originating from cosmic rays. The flux density from the plastic detector showed a typically seasonal variation expected from flux densities of μ particles and electrons to be 6.8% lower in summer and 6.2% higher in winter. On the other hand, the cosmic ray photon flux density was 2.4% higher in summer and 4.5% lower in winter. The latter variation was elucidated based on Bremsstrahlung radiation caused by e± originating from decay of μ particles in the atmosphere near sea level. (author)