Absolute calibration of air fluorescence telescopes to detect the highest energy cosmic rays by a compact electron accelerator

位于美国犹他州沙漠的望远镜阵列，通过使用空气荧光望远镜（FD），观察到极高能量宇宙射线在空气中产生的广泛的空气阵雨。我们提出了一种非常独特的望远镜端到端绝对校准方法，该方法是通过观测射向空中的小型电子直线加速器（TA-LINAC）的电子束所引起的空气阵雨。其最大电子能量为40 MeV，输出功率为6.4 mW。我们的目标是在1%的精度内测量40 MeV的能量，在5%的精度内测量宽度为1μsec的160pC/脉冲的光束电流。柴田达信（研究员）和池田大介（研究生）与我们合作进行了这项研究。采用光束模拟的方法设计了TA-LINAC，研究了TA-LINAC的绝对标定方法。我们在高能加速器研究机构建立了TA-LINAC。它于2008年1月完工，我们成功地加速了光束。我们测量了决定束流能量的90度弯曲电磁铁的磁场值。精度在1%以内。我们测量了影响束流能量稳定性的磁场的稳定性。因此，我们可以测量39.75 MeV的电子能量，精度约为0.01%。我们用法拉第杯以3%的精度测量了光束电流。在束流运行过程中，我们使用磁芯监测器来监测束流电流。我们确认法拉第杯与核心监测器之间的线性关系，精度在5%以内。在确认性能后，我们将TA-LINAC送往美国。混凝土基座和辐射控制围栏的施工是在靠近一个FD站的TA-LINAC现场进行的。

The Telescope Array, located in the desert of Utah in USA, observes extensive air showers that extremely high energy cosmic rays generate in air by using air fluorescence telescopes (FD). We proposed a very unique method of end-to-end absolute calibration of telescopes by observing air shower induced by electron beams from a compact electron linear accelerator (TA-LINAC) shot in the air. It has the performance of maximum electron energy of 40 MeV and output power of 6.4 mW. We aimed at measuring the energy of40 MeV within 1% accuracy and the beam current of 160pC/pulse of 1μsec width within 5% accuracy. Tatsunobu Shibata (Researcher) and Daisuke Ikeda (Graduate student) cooperatec with us for this research.We designed TA-LINAC by using beam simulation, and investiaged the method of absolute calibration by using TA-LINAC. We constructed TA-LINAC in High Energy Accelerator Research Organization. It was completed in January, 2008, and we succeeded in accelerating beams. We measured the values of magnetic field of a 90-degree bending electromagnet that determines beam energy. within 1% accuracy. We measured the stability of magnetic field that affects the stability of beam energy. Thus we could measure electron energy of 39.75 MeV approximately within 0.01% accuracy.We measured beam current with an accuracy of 3% by a Faraday cup. During beam operation, we use a core monitor to monitor beam current. We confirmed linear relation between the Faraday cup and the core monitor within an accuracy of 5%. After confirming the performance, we sent TA-LINAC to USA. The construction of the concrete base and the fence of radiation control was performed at the site of TA-LINAC near one of the FD station.