Geometry of Multiwire Proportional Counters

Figure 4.1 shows a schematic cross section of a multiwire proportional counter for X-ray astronomy. An anode grid with thin wires of the order of 10 |im and a wire pitch of 1-2 mm sandwiched between 2 cathode wire grids is the position sensing X-ray detector. A second anode grid below the X-ray detector operates as anticoincidence detector for background rejection. The wire grid assembly is mounted in a gas tight detector housing. X-rays entering the detector through the window interact predominantly by the photo effect with the detector gas in the absorption and drift region. Electrons generated by this interaction drift in the electrical field through the

window absorption and drift region position sensitive X-ray detector anticoincidence counter

Fig. 4.1 Multiwire proportional counter for X-ray astronomy

Fig. 4.1 Multiwire proportional counter for X-ray astronomy window absorption and drift region position sensitive X-ray detector anticoincidence counter cathode grid into the high field region of the anode grid, where they are amplified in an avalanche by a factor of about 104-105. The acceleration of electrons and ions in the electrical field generates a negative signal on the anode and positive signals on the upper and lower cathode of the proportional counter. The anode signal is normally used to determine the energy and the cathode signals to derive position of the event. Multiwire detectors have the same energy resolution capability like single wire detectors with one exception. The individual anode wires of the anode grid act as separate amplifiers and the amplification is very sensitive to the pitch tolerances of the anode grid. Pitch tolerances in the order of micrometer have to be realized to achieve a homogeneous gain in the order of several percent.

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