Construction And Performance Of A Thin Cylindrical Multiwire Proportional Chamber With Cathode Pad Readout For The H1-Experiment

We discuss the construction of a thin (0.14% radiation length) cylindrical multiwire proportional chamber with cathode pad readout used in the first level vertex trigger of the H1-experiment at HERA (DESY). The chamber has an active length of 2190 mm, and anode plane radii of 157 and 166 mm with a 3 mm gap. The cathode consists of a graphite layer on a Kapton foil with a resistivity of 400 k-OMEGA/square. In order to have a two-dimensional readout the Al-layer on the back of the outer cathode of each chamber is segmented in 36 mm wide pads covering 45-degrees in azimuth. The performance of the chamber with various gas mixtures was studied. Typically a length of the high voltage plateau of 200 V and a time resolution of 75 ns base width has been achieved, sufficient to separate individual HERA bunch crossings. The behavior of the chamber at pad boundaries has been measured and the performance for particles crossing at angles down to curly theta = 16-degrees is described.