A cryostat assembly includes a first thermal shield positioned outside a working volume and a second thermal shield positioned outside the first thermal shield. A first cryocooler has at least two cooling stages operating with He4, a first stage of the cryocooler being thermally coupled to the second thermal shield, and a second stage of the first cryocooler being thermally coupled to the first thermal shield. A second cryocooler operates with He3 and has at least one cooling stage, a part of the second cryocooler, warmer than the cooling stage of the second cryocooler, being thermally coupled to the second stage of the first cryocooler.