Active cavity absolute radiometer based on high-Tcsuperconductors

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Abstract
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 just above 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 µK at 89 K for a receiver having an open-loop thermal time constant of about 75 s. For a flux level of 1 µW to 10 µW, 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 black-body sources, and absolute measurement of the spectral radiance of laser-illuminated integrating spheres for improved spectral responsivity measurements of infrared transfer standard radiometers.