s . in s - de t ] 2 4 M ay 2 00 7 The Outer Tracker Detector of the HERA - B Experiment . Part III : Operation and Performance HERA - B Outer

In this paper we describe the operation and performance of the HERA-B Outer Tracker, a 112 674 channel system of planar drift tube layers. The performance of the HERA-B Outer Tracker system fullfilled all requirements for stable and efficient operation in a hadronic environment, thus confirming the adequacy of the honeycomb drift tube technology and of the front-end readout system. The detector was stably operated with a gas gain of 3 · 10 in an Ar/CF4/CO2 (65:30:5) gas mixture, yielding a good efficiency for triggering and track reconstruction, larger than 95% for tracks with momenta above 5GeV/c. The hit resolution of the drift cells was 300 to 320 μm and the relative momentum resolution can be described as: σ(p)/p [%] = (1.61±0.02)+(0.0051±0.0006) ·p [GeV/c]. At the end of the HERA-B running no aging effects in the Outer Tracker cells were observed.