To study optical properties of crystals at low temperatures different types of cryostats are used. If the crystal is fastened to a cold finger this eliminates the problem of optical access. For strong irradiation in luminescence experiments, however, it is necessary to immerse the crystal in the cooling bath. In this case windows must be installed to withstand thermal cycling and resist helium penetration into the vacuum. Several proposals have been made 1-4 but they still do not fit all demands especially regarding polarization measurements.
Three methods may be distinguished to overcome the problem of differential shrinkage of window material and metal during thermal cycling. In Figure l a the first method is sketched. The different expansion coefficients of glass and metal are compensated in a rather conventional way by an O-ring of teflon or indium. This kind of window seal is preferred if windows have to be removed repeatedly. There is one disadvantage: the mounting causes a strong mechanical stress and thus the window material shows birefringence.
Method 2 (Figure ) avoids any birefringence. A metal tube and the window are connected by a plastic foil (Hostaphan, 15 /am thick Kalle AG, 6202 which is wrapped