Positionsensitive proportional counters

Proportional counters can be constructed to provide the approximate location of the incident x ray, or more specifically, the position of the photoelectric conversion in the gas. This is indicated by the location on the anode of the deposited cloud of charge. This location can be determined by making the anode wire resistive and measuring the charge reaching each end of the anode as shown in Fig. 2b. Quartz wires with carbon coatings may be used for this purpose. The relative amounts of charge collected at the two ends of the anode indicate the location of the deposited charge. If the charge is deposited 1/4 of the way along the wire, the ratio of the detected voltage pulses will be inversely proportional to the wire lengths (or resistances) to the two ends, namely 3:1.

This technique can provide the location of the incident x ray along one dimension to better than a millimeter in counters of size about 60 mm. Techniques that make use of many parallel (and/or orthogonal) anode wires can yield two-dimensional positional information. Such detectors are used in x-ray astronomy and also in high-energy particle physics experiments. Imaging proportional counters are known for their high detection efficiency, but their spatial (positional) resolution (<1 mm) is only modest.

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