[论文解读] Mechanism for reduction of the afterpulsing rate of PMTs

Kai Morita, Mitsunari Takahashi+8 more|arXiv (Cornell University)|Mar 3, 2026

Astrophysics and Cosmic Phenomena被引用 0

一句话总结

研究表明要减少PMT后脉冲需要光照与高压共同作用，表明被加速电子对残留气体的电离（尤其在后极板处）驱动气体耗竭并降低后脉冲率。该效应与集成阳极电流和PMT增益相关。

ABSTRACT

Photomultiplier tubes (PMTs) are used in Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect Cherenkov light produced by air showers induced by gamma rays in the atmosphere. The afterpulsing rate of the PMTs for the Large-Sized Telescopes (LSTs) of the Cherenkov Telescope Array Observatory (CTAO) was found to increase if they were kept unused in storage. In contrast, PMTs that had been operated in the first LST showed a slight decrease in the rate. This decrease could be explained by a reduction of residual gas caused by ion feedback, although the detailed mechanism remained unclear. In this study, to investigate factors responsible for the evolution in the afterpulsing rate, we operated several PMTs under different high voltage and light illumination conditions. We monitored their rate daily for three weeks to compare their evolution under different conditions. We found that the reduction of afterpulses require both illumination and high-voltage operation. Notably, the reduction strongly depends on the applied high voltage and is closely correlated with the integrated anode current. Therefore, we conclude that the reduction of residual gas is mainly caused by ionization occurring at later dynodes of the PMTs, and the ions are trapped by the dynodes. We also discuss a possible explanation of the reduction of afterpulsing rate by later dynodes.

研究动机与目标

• 研究为何存储时空余PMT的后脉冲增加，但在望远镜环境的长期运行后降低。
• 确定是否需要同时具备光照和高压才能降低后脉冲。
• 确定哪些PMT区域及电气条件驱动该降幅机制。

提出的方法

• 在约3周内对21只PMT在不同光照和HV条件下运行并每日监测后脉冲。
• 测量PMT增益以将电荷换算为光电子数并对后脉冲计数进行归一化。
• 进行受控HV测试（1100 V vs 1400 V）以研究HV对后脉冲的依赖并将其与集成电流相关联。
• 使用激光脉冲量化主脉冲电荷和来自10万条波形的后脉冲计数。
• 分析后脉冲率、集成阴极电流和集成阳极电流之间的关系。

实验结果

研究问题

• RQ1 后脉冲降低是否需要光照和HV操作两者？
• RQ2 降低是否主要由前部区域（光电阴极至第一极极）的电离还是由后部区域（后续极极至阳极）的电离引起？
• RQ3 后脉冲率如何随HV、增益和积分电荷变化？
• RQ4 电离导致气体耗竭并随之降低后脉冲的主导区域是哪一？

主要发现

• 只有在PMT被照明且施加HV时，后脉冲率的降低才发生。
• 在较高HV时降低幅度更大，并与集成阳极电流相关。
• 电离很可能发生在后部区域（后续极之间至阳极之间），离子被极极钉住在上而降低残留气体。
• 同样的HV依赖性出现在后脉冲和其降低上，表明同一电离过程驱动两者。
• 在前部区域产生的离子不会到达光电阴极，而是在极极表面被中和，表明前区域起源的后脉冲会受到后区域气体耗竭的影响。
• 降幅的大小与高能电子相关（在1100 V时大约100 eV，相比750 V时约60 eV），与氦气作为主要残留气体相一致。
• 观测到的后脉冲主要来自前部区域，而后部区域的扩散驱动的气体耗竭降低了该速率。