A comprehensive study of the discharge formation mechanisms in Micro-Pattern Gaseous Detectors

在第一个资助期内，我们研究了GEM、厚GEM和Micromegas探测器在α粒子辐照下的固有稳定性极限。在不同CO2含量的Ar基和Ne基混合气体中进行测量，以研究气体对放电概率和临界电荷极限的影响。后者进行了评价，通过比较实验数据的Geant 4模拟框架内获得的结果。这些测量提供了GEM和THGEM之间的直接比较，并使我们能够评估几何参数（如孔尺寸、间距和（TH）GEM厚度）对结构稳定性的影响以及由此产生的临界电荷值，估计在（3-7）× 106电子的范围内。令人惊讶的是，尽管GEM和THGEM孔内存在明显的几何差异和不同的电场配置，但两种放大结构的结果彼此吻合得很好。我们观察到击穿极限强烈依赖于气体，并且混合物中较高量的猝灭剂不一定与较高的稳定性相关。Micromegas探测器获得的结果证实了观察到的气体依赖于放电稳定性。此外，我们观察到的放电概率缩放的线间距，这表明，Micromegas网格单元可以被视为一个独立的放大单元，类似于一个孔的GEM箔。这些研究的结果为进一步优化MPGD探测器，特别是多层堆叠提供了有价值的输入。此外，我们已经研究了（TH）的宝石涂层与不同的材料，使用放电光谱方法的稳定性。我们已经确定钼电极特别耐所谓的二次放电。到目前为止，这些结果还没有提供对MPGD中所有放电过程的完整理解，但它们为开发能够在不损坏的情况下维持极高电压设置的基于GEM的气体探测器开辟了新的前景。从这个意义上说，下文提出了迄今为止成功的方案的延续。我们计划研究具有新电极和MPGD混合堆叠的结构，用于未来的TPC，跟踪器或光电探测器，这些结构需要在极端HV设置下的高速率辐射环境中操作：允许高增益和/或低离子回流性能。

During the first funding period, we investigated the intrinsic stability limits of GEM, Thick GEM, and Micromegas detectors upon irradiation with alpha particles. The measurements were performed in Ar- and Ne- based mixtures with different CO2 content to study the influence of the gas on discharge probability and critical charge limits. The latter were evaluated by comparing the experimental data to results obtained within a Geant4 simulation framework. The measurements provided a direct comparison between GEMs and THGEMs and allowed us to evaluate the influence of geometrical parameters, such as hole size, pitch, and (TH)GEM thickness, on the stability of a structure and the resulting critical charge value, estimated to be within a range of (3-7)106 electrons. Surprisingly, the results for both amplification structures nicely agree with each other, in spite of the clear geometrical differences and different electric field configurations inside GEM and THGEM holes. We observe that the breakdown limit is strongly dependent on the gas, and that a higher amount of quencher in the mixture does not necessarily correlate with higher stability. The results obtained with Micromegas detectors confirmed the observed gas dependency on the discharge stability. In addition, we observed discharge probability scaling with the wire pitch which suggests that a Micromegas mesh cell can be treated as an independent amplification unit, similar to a hole in a GEM foil. The outcome of these studies provides valuable input for further optimization of MPGD detectors, multi-layer stacks in particular. In addition, we have investigated the stability of (TH)GEMs coated with different materials using discharge light spectroscopy methods. We have identified Molybdenum electrodes particularly resistant to so-called secondary discharges. The results did not so far provide a complete understanding of all discharge processes in MPGDs but they opened new perspectives for the development of GEM-based gaseous detectors capable of sustaining extreme high voltage settings without damage. In this sense, a continuation of the so far successful program is proposed hereafter. We plan to study structures with new electrodes and MPGD-hybrid stacks to be used in future TPCs, trackers, or photodetectors which require operation in high-rate radiation environment under extreme HV settings: allowing for high-gain and/or low ion backflow performance.