The imaging performance of multiwire proportional chambers
A simplified description is given of the operation of a multiwire proportional chamber (MWPC) in the soft X-ray imaging application. Expressions are developed to allow the calculation of the distribution of induced charge on the cathodes of an MWPC. With extensions to permit direct comparison, the calculations are subjected to detailed experimental verification. A generalised, approximate formulation of the distribution with one independent parameter is described. The prediction of cathode system position response using the theoretical distributions is demonstrated. The available MWPC position readout methods are reviewed, and where possible their differential non-linearity is measured experimentally. A new position-sensitive cathode of good linearity and spatial resolution is presented. The effect of the wires of an MWPC on its imaging performance is briefly considered. An attempt is made to assess the contribution to MWPC spatial resolution of the range of the electrons produced initially by an X-ray absorption event in argon-methane mixtures. In conclusion, the important causes of MWPC imaging imperfection are noted and classified.