Recent aging studies for the ATLAS transition radiation tracker

The transition radiation tracker (TRT) is part of the inner detector of the ATLAS experiment. It contains 52000 150 cm long straws in the barrel, and 320000 radial straws of 39 to 55 cm height in the two end-cap parts. After 10 years of operation the most exposed straws will accumulate a radiation dose of about 10 Mrad, and up to a neutron fluence of 2 × 10¹⁴ n/cm². These doses result in unprecedented ionization currents and integrated charge (∼10 C/cm of wire) for a large-scale gaseous detector. The original operating gas was Xe-CF₄-CO₂ 70-20-10, a reasonable compromise between conflicting requirements. The use of CF₄ permits, in addition to obtaining a fast drift time, to achieve lifetimes of a few tenths of one C/cm for the straws. In the present baseline, reviewed in 2002, the TRT will operate with a Xe-CO₂ mixture with a small percentage of oxygen that adds a significant safety margin to a simpler binary mixture. This mixture is however less robust in terms of aging, particularly it is very sensitive to silicon pollution, and special measures will be required to reach the necessary level of purity for the gas systems components. It has been found that a small addition of CF₄ to the gas in the high luminosity LHC environment is efficient to clean up aged wires. Periodic cleaning runs of the TRT tracker are foreseen, if needed.