Thermal conduction measurement of miniature coaxial cables between 0.3 and 4.5Â K for the wiring of superconducting detectors

Thermal conduction of thin coaxial cables made of 70-30 CuNi and stainless steel 304 with a PTFE insulator was measured in a temperature range between 0.3 and 4.5 K. Thermal conductivity of the 70-30 CuNi and PTFE was measured independently and their contribution to the thermal conduction of the coaxial cables was investigated. The thermal conductivity of the 70-30 CuNi differed from the literature by 25% at 0.36 K and 40% at 4.2 K, and the alloy exhibited a weak temperature dependence, which indicated the effects of mechanical treatment. It has been confirmed that the thermal conduction of the coaxial cables are low enough to keep a cold stage of 3He cryostats at a temperature below 0.3 K, even when one hundred cables are installed between 0.3 and 3 K for the read-out of superconducting tunnel junction arrays. The cables were installed in a cryogen-free 3He cryostat, and the operation below 0.3 K was successful.