The development of high-efficiency cathode converters for a multiwire proportional chamber positron camera

A high-efficiency cathode converter for 511-keV photons has been developed for incorporation into a multiwire proportional chamber (MWPC) positron camera. The converter consists of a honeycomb pattern produced in a 1-mm-thick lead sheet to leave lead walls with a thickness of approximately 60 micron. The converter also serves as the cathode of an MWPC, the gap between the converter and the anode wire plane being 2.5 mm. This small gap results in a high secondary electron extraction efficiency without the need for additional drift voltages. Measurements of the efficiencies of a plane converter and of two types of structured converters in a single section MWPC are described and the efficiency is found to increase in proportion to the converter surface area. This result justifies the use of a simple theoretical model whereby an extrapolation to the efficiency of a detector consisting of a stack of 20 MWPC sections, each section having two converters, is made. The efficiency of this proposed system is calculated to be 17% for 511-keV photons.