Recent aging studies for the ATLAS transition radiation tracker

The transition radiation tracker (TRT) is one of the three subsystems of the inner detector of the ATLAS experiment. It is designed to operate for 10 yr at the LHC, with integrated charges of ∼10 C/cm of wire and radiation doses of about 10 Mrad and 2×10¹⁴ neutrons/cm². These doses translate into unprecedented ionization currents and integrated charges for a large-scale gaseous detector. This paper describes studies leading to the adoption of a new ionization gas regime for the ATLAS TRT. In this new regime, the primary gas mixture is 70%Xe-27%CO₂-3%O₂. It is planned to occasionally flush and operate the TRT detector with an Ar-based ternary mixture, containing a small percentage of CF₄, to remove, if needed, silicon pollution from the anode wires. This procedure has been validated in realistic conditions and would require a few days of dedicated operation. This paper covers both performance and aging studies with the new TRT gas mixture.