The Outer Tracker of HERA–B

The Outer Tracker of the HERA–B experiment at DESY is a gaseous detector that provides tracking of charged particles over a large volume in a high-rate, hadronic environment. The radiation load at 40 MHz interaction rate is comparable to what will be encountered by large trackers in future LHC experiments. The Outer Tracker allows pattern recognition for event reconstruction, momentum measurement, and highly selective triggering on dileptons from J/ψ decays. Its wire–chamber modules comprise 110,000 honeycomb drift cells of up to 4.5m length operating with an Ar/CF4/CO2 gas mixture. The detector was fully installed in January 2000 and is currently operating in HERA-Bʹs first physics run. During detector development different types of severe aging effects were observed. The solutions to the aging problems, detector construction, and detector performance during the early commissioning phase are discussed.