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The interest in using visible and near-infrared scintillation in Gas Electron Multipliers (GEMs) in experimental approaches based on optical readouts has grown recently. One gas mixture used for this purpose includes CF$_4$, which is known to cause etching in the interior walls of the GEM holes [1]. One question may arise: Where will this etched material be re-deposited? The question is essential for many reasons, but one of them concerns the longevity of the sensor, which is related to the formation of insulation layers on the electrodes with time and the occurrence of the Malter effect. In this work, we report our findings on GEM surface studies using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) technique on severely damaged GEM foils used with CF$_4$ mixtures. We found solid evidence for with deposition of sub-products of polyimide etching in the vicinity of the roles and possible correlation of parks. Complex molecular fragments from polyimide fragmentation deposit on top of the electrode in an apparent chemical reaction with a fluorine content of the gas. Comparisons with polyimide fragmentation by other means show the same molecular signature, indicating the origin. Conversely, pristine GEM samples showed the absence of the same molecular fragment, indicating that the etching occurred during its use.
[1] M. Alfonsi et al. “Studies of etching effects on triple-GEM detectors operated with CF$_4$ based gas mixtures”. Em: IEEE Transactions on Nuclear Science 52.6 (dez. de 2005), pp. 2872–2878. issn: 0018-9499. doi: 10.1109/TNS.2005.855640.
