VERITAS触发系统
The VERITAS Trigger System
摘要：
VERITAS伽马射线天文台位于亚利桑那州南部，由四台直径12米的切伦科夫成像望远镜组成，每台望远镜都有一台499像素的光电倍增管相机。该仪器旨在探测能量超过100 GeV的天体物理伽马射线。在VERITAS能量范围的低端，夜空背景光的波动和来自宇宙射线簇射的单个μ子构成了重要的背景。VERITAS采用三层触发系统来降低这些背景事件的发生率：作用于单个像素级的初始触发器，作用于相对定时和像素级的模式触发器，作用于单个望远镜相机内的像素级触发器的相对定时和分布的模式触发器，以及阵列级触发器，其需要在多个望远镜中同时观测空气簇射事件。这个最终的符合要求大大降低了背景事件的发生率，特别是那些由单个μ子引起的事件。本文讨论了VERITAS触发系统各个层次的实现，并对它们的联合性能进行了表征。
英文摘要：
The VERITAS gamma-ray observatory, situated in southern Arizona, is an array of four 12m diameter imaging Cherenkov telescopes, each with a 499-pixel photomultiplier-tube camera. The instrument is designed to detect astrophysical gamma rays at energies above 100 GeV. At the low end of the VERITAS energy range, fluctuations in the night sky background light and single muons from cosmic-ray showers constitute significant backgrounds. VERITAS employs a three-tier trigger system to reduce the rate of these background events: an initial trigger which acts at the single pixel level, a pattern trigger which acts on the relative timing and pixel level, a pattern trigger which acts on the relative timing and distribution of pixel-level triggers within a single telescope camera, and an array-level trigger which requires simultaneous observation of an air-shower event in multiple telescopes. This final coincidence requirement significantly reduces the rate of background events, particularly those due to single muons. In this paper, the implementation of all levels of the VERITAS trigger system is discussed and their joint performance is characterized.
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