Position sensing in a multiwire proportional chamber
Several aspects of X-ray position location in an imaging multiwire proportional chamber (MWPC) are examined in this work. Experimental confirmation is obtained of existing theoretical predictions which take into account the presence of avalanche angular localisation. An attempt is made to exploit the effects of the latter by means of a position-encoding RC transmission line, revealing that the variations in signal risetime caused by angular localisation are detrimental to linear operation of the line. Other results indicate that successful exploitation by different means is possible. A brief study of electron diffusion effects in MWPC's is made, giving some agreement with predictions, but also exposing an effect which degrades resolution and improves linearity in one sensing direction only (that perpendicular to the anode wires). This effect has yet to be explained. The series capacitance line is examined in the context of MWPC operation, and is found to offer superior spatial resolution (though slightly inferior linearity) to that obtainable with an RC line. The concept of the graded-density (GD) cathode is introduced, and an experimental study shows that the device can be fully competitive as the encoding cathode of a MWPC. Once again, the spatial resolution is superior to that obtained with an RC line, although some small local readout non-linearity mars the present performance. Finally, the cascading of two such electrodes to form a single "subdivided" GD cathode is studied. In this case the cathode behaves similarly to a series capacitance line, and the facility for partially compensating for the resultant nonlinearity, by adjustment of the density gradings, is demonstrated.