Geiger--Müller tubes are arranged in rows of 9 tubes side by side and then another layer below at right-angles to form X and Y coordinates. When a Cosmic Ray (muon) passes through the brass plates and two layers tubes simultaneously (within a millisecond) the result is registered in one of 81 LEDs (or pixels) in the corresponding X+Y coordinate.
Radiation shielding (brass plates) is placed between the two layers to reduce the probability of false coincidence detections caused from terrestrial radiation in and around the individual Geiger-- Muller Tubes.
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