Energy Calibration of Extremely High Energy Cosmic Rays

有人认为，如果原始宇宙线是河外起源的，那么它们的能谱可能在10^ eV附近有一个截止点<20>，因为这些宇宙线在星系际空间中传播时会由于与宇宙背景辐射的相互作用而失去能量。最近，美国的蝇眼探测器和Akeno的Akeno Giant Air Shower Array（AGASA）观测到了远远超过这一预测截止值的高能宇宙射线。在蝇眼实验中，大气中的荧光产额在能量测定上仍有不确定性。因此，我们测量了空气荧光产率作为压力的函数，电子在1.4 MeV和1000 MeV之间，通过<90>Srbeta源和电子同步加速器的电子束。结果表明，在1.4 - 1000 MeV范围内，荧光产额与电子能量损失成正比。借助于1966年美国中纬度标准大气模式，我们测定了一个80 MeV电子的荧光产额的高度依赖性。使用这种新的荧光产额对蝇眼数据中的所有事件的能量进行评估，相应的温度和压力依赖性以及入射电子的能量。我们发现，新能源和旧能源之间的差异除以旧能源小于-1.3%。我们将它们的能谱与AGASA测定的能谱进行了比较，能量的差异约为12%，与各实验误差相比可以忽略不计。这一结果支持Fly's Eye和AGASA实验中的最高能量事件确实超出了2.7K截止能量的预言。

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.