1995 Fiscal Year Final Research Report Summary
Energy Calibration of Extremely High Energy Cosmic Rays
| Project/Area Number |
06402006
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| Research Category |
Grant-in-Aid for General Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Research Field |
素粒子・核・宇宙線
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| Research Institution | Institute for Cosmic Ray Research (ICRR) Univ.of Tokyo |
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Principal Investigator |
NAGANO Motohiko ICRR,Univ.of Tokyo Professor, 宇宙線研究所, 教授 (00013384)
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| Co-Investigator(Kenkyū-buntansha) |
ISHIKAWA Fumio ICRR,Univ.of Tokyo Assistant, 宇宙線研究所, 助手 (30092133)
OHNISHI Munehiro ICRR,Univ.of Tokyo Assistant, 宇宙線研究所, 助手 (10260514)
HAYASHIDA Naoaki ICRR,Univ.of Tokyo Assistant, 宇宙線研究所, 助手 (50114616)
TESHIMA Masahiro ICRR,Univ.of Tokyo Assoc.Prof., 宇宙線研究所, 助教授 (40197778)
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| Project Period (FY) |
1994 – 1995
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| Keywords | cosmic rays / extremely high energy / energy spectrum / extensive air shower / energy calibration |
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| Research Abstract |
It has been suggested that there might be a cutoff in the energy spectrum of primary cosmic rays around 10^<20>eV,if they are extragalactic origin, since those cosmic rays lose their energy during traveling in the intergalactic space as a result of their interaction with universal background radiation. Recently very energetic cosmic rays well beyond this predicted cutoff were observed by the Fly's Eye detector at USA and Akeno Giant Air Shower Array (AGASA) at Akeno. It is quite important to reevaluate their energies experimentally.
In case of Fly's Eye experiment an ambiguity in energy determination remains in the fluorescence yield in the atmosphere. Therefore we measured the air fluorescence yield as a function of pressure with electrons between 1.4 MeV and 1000 MeV by means of a ^<90>Srbeta source and an electron beam from electron synchrotron. Results indicate that the fluorescence yield is proportional to electron energy loss from 1.4 MeV to 1000 MeV.With the aid of the 1966 US Mid-latitude Standard Atmospheric Model, we determined the altitude dependence of fluorescence yied of an 80 MeV electron.
The re-evaluation of energy of all events in the Fly's Eye data using this new fluorescence yield has been done by taking into account the different bands with their corresponding temperature and pressure dependence and the incident electrons' energy. We found that the difference between the new energy and the old energy divided by the old energy is less than-1.3%. That is, the re-analysis did not change their spectrum results.
We compared with their energy spectrum and the spectrum determined by AGASA.The difference in energies is about 12% and is negligible compared with each experimental error. The present result supports that the highest energy events from Fly's Eye and AGASA experiments are really beyond the predicted 2.7K cutoff energy.
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