An X--Y encoder with fast centroid calculation for large multiwire proportional chambers

The system described here is part of a data acquisition system for macromolecular crystallography using a two-dimensional wire chamber with a spherical drift space. The readout consists of 2x128 cathode strips (4 mm width) which are connected to shaping amplifiers coupled to a fast high precision position encoder. Cathode amplitudes are converted digitally with flash analog-to-digital converters and valid cathode clusters give between 3 to 5 digital amplitudes. A specialized VME module calculates the center of gravity of the cluster. The entire system is constructed as a VME structure and is under program control from a VALET-plus VME microcomputer system (developed at the CERN) which allows fine tuning of the electronic system. Data acquisition rate is presently 200 kHz with less than 20% data loss. The useful sensitive area of the detector is a disk (diameter 486 mm). Local positional resolution is around 150 μm root-mean-square and long range positional reproducibility is better than 200 μm