valve to the compressor the low temperature heat exchanger unit 25 forms the cold head of the refrigerator and generally includes a hollow manifold section 28 of u shaped construction and a heat exchanger generally referenced 30 an opening 29 passes through the unit and provides a flow path by which the helium gas flows between stages the heat exchanger itself is mounted in a recess 35 located on the first stage side of the manifold the exchanger consists of a series of stacked perforated plates 31 31 which are separated by interposed wire spacers 32 32 the spacers are typically formed in a g like configuration and are placed between the plates to support them in spaced apart parallel alignment within the recess an expanded circular opening 42 is formed in coaxial alignment with the recess that houses the heat exchanger and is adapted to receive a sleeve 35 therein in practice the sleeve is bottomed in the opening 42 directly over the heat exchanger and is joined to the manifold using any suitable means for creating a leak tight joint therebetween the opposite end of the sleeve is similarly joined to the bottom wall 40 of the first stage expansion chamber this end of the sleeve contains an enlarged hub section 36 that supports a radially disposed flange 37 the flange is secured in a.