Gas leak through ultrathin radiation entrance windows

The permeation rates through an ultrathin radiation entrance window membrane have been measured for several common detector gas components, as well as for some atmospheric gases, by collecting the permeated gas into a clean volume equipped with an absolute pressure transducer. The dependence of the leak rate on the differential pressure (1.0 bar in most of the measurements) has also been discussed. Formulas with the permeation rate as a function of the molecular diameter or the molecular mass are presented. They give a rough estimate for the dependence of the permeation rate on the gas properties. The structure of the window membranes used in the measurements is presented.