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キーワード:
Cherenkov light; Imaging telescope; Gamma-rays; Cosmic ray electrons
要旨:
In this paper, we present the results of a comparative analysis of the Cherenkov light produced by electromagnetic air showers initiated by primary gamma-rays and electrons. It is shown that the lateral distribution functions of the Cherenkov light initiated by these showers are different. This fact is explained by the difference in the development of cascades from gamma-rays and electrons; electron-showers start to develop and produce light earlier than gamma-showers, but they attenuate rapidly. By propagating much farther into the atmosphere, the gamma-initiated shower carries a Cherenkov component much closer to the ground level than electron initiated showers do; this results in a more intense pool of light close to the shower axis. The difference in the altitudes of electron- and gamma-air showers maximums logarithmically decreases with the energy and is more significant for low energy showers with E ≤ 20 GeV. This can be important for low energy threshold Cherenkov telescopes (below 10 GeV), in which case the background events are due to diffuse cosmic ray electrons only. The minimum energy of primary electrons required for their reliable detection by an imaging atmospheric telescope will be higher than that for primary gamma-rays. For the reasonable value of the geomagnetic field strength of B ≥ 0.3 G and at the initial energies of E ≤ 20 GeV, the pulse width of the Cherenkov light generated by electron-showers is considerably larger than that from gamma-showers. At higher energies the shapes of Cherenkov pulse from showers induced by both primaries become almost similar.
