Solomon I. Woods, Stephen M. Carr, Adriaan C. Carter, Timothy M. Jung, Simon G. Kaplan, Raju V. Datla
The Low Background Infrared (LBIR) facility has developed and tested the components of a new detector for calibration of infrared powers down to 1 pW, with 0.1% uncertainty. Calibration of such low powers could be valuable for the quantitative study of weak astronomical sources in the infrared. The pW-ACR is an absolute cryogenic radiometer (ACR) employing a high resolution transition edge sensor (TES) thermometer, ultra-weak thermal link and miniaturized receiver to achieve a noise level of around 1 fW at a temperature of 2 K. The novel thermometer employs the superconducting transition of a tin (Sn) core and has demonstrated nano-Kelvin capability. Using an applied magnetic field from an integrated solenoid to suppress the Sn transition temperature, the operating temperature of the thermometer can be tuned to any temperature below 3.6 K. The conical receiver is painted on the inside with infrared-absorbing paint and has a demonstrated absorptivity of 99.94% at 10.6 µm. The thermal link is made from thin-walled Kapton tube and has exhibited very low thermal conductance near 2x10-7 W/K. In tests with a heater mounted on the receiver, the receiver/thermal-link assembly demonstrated a thermal time constant of about 15 s. Based on these experimental results, it is estimated that an ACR containing these components can achieve noise levels below 1 fW, and the design of a radiometer merging the new thermometer, receiver and thermal link will be discussed.