Active Cavity Radiometer Based on High-TC Superconductors
Joseph P. Rice, Raju V. Datla, Leila R. Vale, David A. Rudman, M L. Sing, D Robbes
To implement the detector-based radiometric scale in the new Medium Background infrared [MBIR] facility at the National Institute of Standards and Technology [NIST], we have developed an electrical substitution cavity radiometer that can be operated at liquid nitrogen temperature. This MBIR Active Cavity Radiometer [ACR] utilizes a temperature-controlled receiver cone and an independently temperature-controlled heat sink. Being a thermal-type detector, low noise and drift of the radiometer signal depends mainly on low-noise temperature control of the receiver and heat sink. Using high critical-temperature [Tc] superconducting thin film temperature sensors in the active control loops, we have achieved closed-loop temperature controllability of better than 10 K at 89 K for a receiver having an open-loop thermal time constant of about 75 seconds. For a flux level of 1 υW to 10 υW, the rms noise floor over a measurement cycle time is below 20 nW. This is the lowest noise level yet reported for a liquid nitrogen cooled electrical-substitution radiometer and it is the first demonstration of the use of high-Tc superconductors in such a radiometer. Potential uses for this ACR in the MBIR facility include absolute measurement of the broadband radiance of large-area 300 K cryogenic blackbody sources, and absolute measurement of the spectral radiance of laser-illuminated integrating spheres for improved relative spectral responsivity measurements of infrared transfer standard radiometers.