Active Cavity Absolute Radiometer Based on High-Temperature Superconductors for MBIR Calibrations

To implement the detector-based radiometric scale in the Medium Background Infrared (MBIR) facility at the National Institute of Standards and Technology (NIST), an electrical substitution cavity radiometer that can be operated just above liquid nitrogen temperature has been developed. 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 mK at 89 K for a receiver having an open-loop thermal time constant of about 75 seconds. For a flux level of 1 mW to 10 mW, the rms noise floor over a measurement cycle time is below 20 nW. This was 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.

The MBIR ACR is an active-cavity-type radiometer. The cavity receiver is conical in design, with a 1 cm diameter and a 30°-apex angle. The inside surface is painted with Z302 specular black paint. There is a wire-wound heater, and a superconductor temperature sensor, which has a critical temperature of 90 K, attached to the cavity. The sensor leads, heater leads and the cavity lip are attached to a cold shell surrounding the cavity that acts as a heat sink for the wires. The entrance to the cavity is fitted with a baffle tube, limiting aperture, and shutter. The ACR is mounted on the actively cooled table inside the MBIR chamber and will be operated near 90 K. The ACR is used to provide absolute measurement of the broadband radiance of large-area 300 K cryogenic black-body sources, absolute calibrations for sensors and radiometers, including the TXR.

References

Active cavity absolute radiometer based on high-Tc superconductors,
J.P. Rice, S.R. Lorentz, R.U. Datla, L.R. Vale, D.A. Rudman, M.L.C. Sing and D. Robbes,
Metrologia 35, 289-293 (1998).

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