2002, pp 577-586

Closed-Cycle Cooling of Infrared Detectors to 0.25 K for the Polatron

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We have integrated a 4 K mechanical cryocooler with a three stage 4 He/ 3 He/ 3 He sorption refrigerator to achieve cooling down to 0.25 K. The cryocooler consists of two Gifford-MacMahon stages which cool 4He to below its inversion temperature for use in a Joule-Thomson expansion stage. This in turn provides cooling below the critical temperature of 4He. The sorption refrigerator is specified to achieve a heatlift of 3 microwatts at 0.25 K for 12 hours. This cryogenic system will be used for cooling of the bolometric detectors and optics in the Polatron, a ground-based receiver for measurement of the polarisation of the cosmic microwave background radiation at a frequency of 90 GHz. Cooling of sensitive detectors by mechanical cryocoolers can lead to sensitivity degradation due to a combination of microphonic pickup, electromagnetic interference and thermal dissipative heating. We describe the design features incorporated at system level to mitigate these effects. The Polatron also serves as a testbed for technologies to be used on the High Frequency Instrument on the ESA/NASA Planck satellite to measure the temperature anisotropies and polarisation of the cosmic microwave background radiation.