The performance and long term stability of an optically readout Time Projection Chamber with an electron amplification structure based on three Gas Electron Multipliers was studied. He/CF$_4$ based gas mixtures were used in two different proportions (60/40 and 70/30) in a CYGNO prototype with 7 litres sensitive volume. With electrical configurations providing very similar electron gains, an almost full detection efficiency in the whole detector volume was found with both mixtures, while a light yield about 20\% larger for the 60/40 was found. The electrostatic stability was tested by monitoring voltages and currents during 25 days. The detector worked in very stable and safe condition for the whole period. In the presence of less CF$_4$, a larger probability of unstable events was clearly detected.

中文翻译：

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具有 He/CF4 气体混合物的基于 GEM 的光学读出 TPC 的稳定性和检测性能

研究了具有基于三个气体电子倍增器的电子放大结构的光学读出时间投影室的性能和长期稳定性。在具有 7 升敏感体积的 CYGNO 原型中，以两种不同的比例（60/40 和 70/30）使用了基于 He/CF$_4$ 的气体混合物。通过提供非常相似的电子增益的电气配置，发现两种混合物在整个检测器体积中的检测效率几乎完全相同，而发现 60/40 的光产率高出约 20%。通过在 25 天内监测电压和电流来测试静电稳定性。探测器在整个过程中都处于非常稳定和安全的状态。在存在较少 CF$_4$ 的情况下，明显检测到较大概率的不稳定事件。