Gas pixel chambers in the HERA-B experiment

A new type of wall-less gas wire chambers with a pad-type readout, called gas pixel chambers, has been designed, built and successfully operated in the HERA-B experiment. The pixel chambers were used as an inner part of a High-p_T system, a pretrigger on hadrons with high transverse momenta. The cell sizes in the chambers varied from 4×8mm² to 10.31×10.31mm². The chambers collected signals within 96nsec, the bunch separation in HERA. The chambers could operate in a hard radiation environment with particle fluxes up to 10⁶/cm²/s. The amount of material was only 0.9% of radiation length per chamber including support frames. The readout electronics was placed outside of the acceptance and the signals were brought there by very thin twisted pair cables with a wire diameter of 50μm. The system consisted of 12,000 channels. A similar type of chambers but with larger amount of material was used also in the inner part of the HERA-B muon system. Design of the chambers, performance and experience of operation are described.