Aging of Gaseous Detectors: assembly materials and procedures

Aging of gaseous detectors is known as the degradation of their performance under the exposure to ionizing radiation. It is a complex phenomenon that depends on many parameters. Among others, aging depends on the gas mixture and may be enhanced by the presence of pollutants in the gas. The origin of the impurities is diverse and includes outgassing from assembly materials and the gas system components, and contamination of the detector during the assembly process. Systematic studies on this topic have been carried out. Methods used to ascertain the outgassing properties of materials are described and compared. Materials that might be used for assembling gaseous detectors and associated gas systems are catalogued according to their outgassing rate. Some factors affecting the aging rate in some fast gases are presented. Finally, a set of recommendations to build and operate gaseous detectors in high luminosity experiments is given. 1. Aging of gaseous detectors Gaseous detectors have been used for many decades in modern high energy physics experiments for particle detection and tracking. Soon after their development, it has been common to find problems associated with their long-term exposure to radiation, limiting their useful time. The observed phenomenology included the appearance of local and permanent damages detected as self-sustained discharges, excessive currents, gradual loss of energy resolution and decrease and non-uniformity of the gas gain. These observations led to the association of the problem to the presence of layers on the electrodes, possibly induced by pollution released by materials used in the gas system or chamber construction, or impurities in the gas itself. A branch of Chemistry called Plasma Chemistry studies processes with certain qualitative analogies to those that take place in the plasma surrounding the anodic wire in a gaseous detector, which might be considered as a dc plasma at 1 atmosphere. Plasma is a mixture, initially overall neutral, of positive and negative ions, neutral species, and chemically active free radicals; radicals can recombine back to the origin molecules or new ones. Polymerization is thus the process by which some monomer radicals associate in subsequent chain reactions to form a very large molecule, frequently of high molecular weight. Created polymers are solid, highly branched and cross-linked, have excellent adhesion to surfaces, are resistant to most chemicals and insoluble in most solvents. Deposits observed in aged ——— * [email protected] CERN, EP div., CH-1211 Geneva 23