Thin polyimide and silicon nitride windows for gas proportional counters

Proportional counters are widely used as X-ray detectors both on the ground and in space. Previous proportional counter designs utilized entrance windows made of stretched polypropylene which have relatively large leak rates for the gases used in the detector. Thin polyimide windows have been fabricated at MIT with low leak rates (3 x 10 E-9 mbar 1/s) and useful X-ray transmission properties down to 0.6 keV energy photons. We report on 1 micrometers thick windows made of silicon-rich LPCVD silicon nitride which have been made with leak rates below our detectable limit. The window fabrication process and test apparatus will be described. Data will be presented on leak tests and mechanical strength of thin windows fabricated from polyimide and silicon nitride. These windows could allow fabrication of a sealed proportional counter, thus eliminating the need for a gas replenishing system in space-based X-ray detectors.