and one side of the window an annular lead gasket having roughened surfaces is positioned adjacent to the other side of the window and a teflon gasket is positioned adjacent to the annular rim of the clamp flange a constant force means is provided for pushing the clamp flange toward the window and the sealing flange thereby producing a seal therebetween the window is preferably formed from alkali halide material application_number US 37234182 A description referring now to the drawings wherein like reference numerals designate identical or corresponding parts throughout the several views and more particularly to fig1 and 2 thereof a window seal structure 10 according to a preferred embodiment of the present invention is illustrated as including a continuous annular sealing flange member 12 which surrounds a cylindrical body 14 and which is permanently vacuum sealed thereto by joint 16 in a manner well known to those skilled in the art such as by welding the sealing flange member 12 is preferably made from 304 alloy stainless steel however other similar materials could also be utilized the sealing flange 12 includes a continuous annular rim 18 formed in the inner periphery of the flange 39 s upper surface 20 the annular rim 18 incorporates a raised continuous annular sealing knob 100 about its upper surface which will be discussed in greater detail below the annular sealing knob 100 is covered with a continuous annular lead gasket 22 which forms a seal between the sealing knob 100 and an alkali halide.