A low-mass drift chamber system for the HADES-spectrometer

A new high resolution (Delta M/M < 1%) and high acceptance (45%) di-electro n spectrometer (HADES) has been designed to investigate in-medium propertie s of hadrons. For tracking of all charged particles tin particular with suf ficient resolution for electrons) a system of 24 low-mass drift chambers (H elium based counting gas and Aluminum field and cathode wires), arranged in four tracking planes, is used. Design aspects of the chambers are reported . Results of performance optimization using various prototype detectors are discussed, including results of an ageing test. Stable operation in the hi gh-multiplicity environment of heavy ion collisions, and a spatial resoluti on of 70 mu m(sigma) over 80% of a cell have been demonstrated in two beam experiments.