Low Radioactivity Cryostat System for a 20mK Particle Detector

A system for cooling detectors used in particle astrophysics is described. The volume accommodating particle detectors is 30 cm diameter by 30 cm high and attained a temperature of 8.5 +/- 0.5 mK. The need for low radioactive backgrounds has driven design, selection of materials and assembly techniques. Unusual features include a detector package cooled by conduction by tubes ~1m in length connected to the port of a commercial side access dilution refrigerator, swinging support system for detectors and heat shields, fabrication using electron beam welding, assembly techniques for thermally conducting bolted joints, flexible thermal conduction paths, and copper flange on copper gasket vacuum seals at LHe temperature. A first cool down of the system resulted in temperature distributions and thermal loads consistent with thermal modeling.