at 19 u00b0 k and the comparable power consumption of the higher temperature stage 13 would be approximately 100 watts pumping of permanent gases is provided by the cryopanel assembly 18 that is in thermal contact with the low temperature stage 13 the assembly 18 is enclosed by a radiation shield consisting of a baffle 15 composed of circular concentric chevron plates a polished cylinder 17 and a circular bottom plate 16 in thermal contact with the higher temperature stage 13 the baffle 15 is connected to the higher temperature stage 13 only by the plate 16 and cylinder 17 which establish a heat conductive path therebetween typically the cryosurface assembly 18 has fins the insides of which are coated with an absorption medium such as activated charcoal for the absorption of hydrogen a flange 19 is provided for connection to a roughing pump not shown which is only used for start up with a suction opening 20 of 500 mm diameter the weight of the mass to be cooled at the higher temperature of the stage 13 is approximately 20 kg and the mass cooled to the lower temperature of the stage 14 is approximately 4 kg because of the large masses involved a typical cool down time of about two hours is required to reach equilibrium temperatures during this time cooling power consumption is greater than thereafter obviously the cooling down time and power consumption could be reduced by reducing the mass being cooled this would be accomplished by reducing the lengths of the heat conductive bridges between the stages 13 and 14 respectively and the assemblies 15 and 18 the present invention significantly reduces the bridge between.