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、Thick 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基混合物中进行了测量，研究了气体对放电概率和临界电荷极限的影响。后者通过将实验数据与在Geant4模拟框架内获得的结果进行比较来评估。这些测量提供了GEMs和THGEMs之间的直接比较，并使我们能够评估几何参数（如空穴尺寸、螺距和（TH）GEM厚度）对结构稳定性和由此产生的临界电荷值的影响，估计在（3-7）106个电子的范围内。令人惊讶的是，两种放大结构的结果非常一致，尽管有明显的几何差异和不同的电场结构在GEM和THGEM孔内。我们观察到，击穿极限强烈地依赖于气体，并且混合物中较高的淬灭剂量不一定与较高的稳定性相关。用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，我们观察到放电概率随导线间距的变化而变化，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GEM箔上的孔。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，我们还利用放电光谱学方法研究了不同材料包覆的（TH）GEMs的稳定性。我们已经确定钼电极特别抵抗所谓的二次放电。到目前为止，研究结果并没有完全了解mpgd中的所有放电过程，但它们为开发基于gem的气体探测器开辟了新的视角，这些探测器能够在不损坏的情况下维持极高的电压设置。从这个意义上说，今后将提出一个迄今为止成功的计划的延续。我们计划研究具有新电极和mpgd -混合堆栈的结构，用于未来的tpc，跟踪器或光电探测器，这些设备需要在极端高压设置下的高速率辐射环境中运行：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.