Study of heat leaks to copper nuclear demagnetization stage

Heat leaks to a copper nuclear stage at ultra-low temperatures have been investigated. We have constructed a massive copper nuclear stage and performed its cooling with negligible amount of heat release sources such as plastics and epoxies. With this stage we have managed to cool a Pt-NMR thermometer down to 45 muK at the bottom of the stage. The whole heat leaks were found to be 1 nW at a final field of 10 mT on the 10th day after cooldown to 4.2 K and remain fairly constant during 2 months. The small and time-independent heat leaks enable us to obtain the lowest temperature rapidly and maintain temperatures below 100 muK for several weeks. (C) 2003 Elsevier Science B.V. All rights reserved.