Status of the outer tracker for the Hera-B experiment

The outer tracker system of the Hera-B experiment is composed of large size, small diameter drift chambers operated with an Ar–CF4–CO2 (65–30–5) gas mixture. The general layout and details of module construction are described. An intensive R&D program has been carried out to ascertain the long-term behaviour of honeycomb modules under irradiation; results of irradiated prototypes are presented. A set of operational conditions, gas mixture, clean assembly materials and adequate production techniques have been found which ensure proper operation of the outer tracker during several years in the harsh Hera-B environment.