The Charging-Up Phenomenon in Gas Electron Multiplier Detector

Fabio Sauli introduced Gas Electron Multiplier (GEM) technology in 1997, which has since evolved into one of the most versatile and widely adopted micro pattern gaseous detector (MPGD) technologies for tracking in High Energy Physics (HEP) experiments. Owing to its high rate-handling capability (∼MHz/mm2), good position resolution (∼100μm), and operational robustness in high radiation environments. GEM detectors have become an indispensable component of modern tracking systems. The heart of a GEM detector is a thin Kapton polyimide foil (∼50μm) clad with copper (∼5μm) on both sides and containing an array of regularly spaced holes (typically diameter of ∼70μm and pitch of ∼140μm) fabricated using photolithographic techniques. The presence of the dielectric substrate (Kapton) within the amplification region introduces a time dependent response when the detector is exposed to external irradiation, a phenomenon commonly referred to as the charging-up effect. This effect arises from the accumulation of charge on the insulating Kapton surfaces, leading to a gradual modification of the local electric field configuration inside the GEM holes and, consequently, a variation in the detector gain over time. The charging-up behaviour has been systematically investigated for triple GEM chamber prototypes using an Fe-55 radioactive source (5.9 keV X-rays). The characteristic charging-up time constant has been extracted, and its dependence on detector gain and irradiation rate has been examined. In addition, the uniformity of detector performance in terms of count rate, gain, and energy resolution has been studied both before and after the charging-up process. In this review article, the experimental setup, data acquisition methodology, and analysis procedures developed and carried out by our group are summarised. The key findings reported by other groups, relevant Monte Carlo simulation efforts, and future outlook for the charging-up investigation on GEM based detectors are also discussed in this article. The investigations and their outcomes reviewed here provide valuable insight into the charging-up dynamics of GEM detectors and their dependence on operational parameters.