Operation of an X-ray transition-edge sensor cooled by tunnel junction refrigerators

We demonstrate successful cooling of an X-ray transition-edge sensor (TES) using solid-state refrigerators based on normal-metal/insulator/superconductor (NIS) tunnel junctions. Above the TES transition temperature (T (c) ), we use Johnson-noise thermometry to measure the cooling performance. In this regime, the NIS refrigerators cool the TES from 300 mK to 220 mK and from 220 mK to T (c) =160 mK. Below T (c) , we also observe cooling and demonstrate that the operation of the NIS refrigerators introduces no detectable noise into the TES readout.