Advanced Absolute Radiometers Using Superconducting Transition Thermometers

In our Phase 1 work we were able to replicate and build on recent results at NIST with superconducting transition (SCT) thermometers, which suggested that dramatic improvements may be achieved in detectivity and range of application, of cryogenic electrical substitution radiometers. Cryo-ESR's are widely used as the most accurate absolute standards of radiative flux and irradiance. In Phase 2, we will focus on the additional research required to demonstrate that the advantages of SCT thermometry can be translated into: (a) LN2-cooled ESR's of comparable accuracy, but easier accessibility, than conventional, LHe-cooled instruments; (b) LHe-cooled ESR's capable of improved detectivity for detector calibrations in the aerospace industry; and (c) more stable pyrheliometers operating at 90 K for space-borne monitoring of radiations in solar total and ultraviolet irradiance. This research will include development of high-Tcsuperconducting heater leads having low thermal conductance, of an SCT thermometer based on thin metal films with Tc between 2-4 K, and characterization of a prototype LN2-cooled ESR through intercomparison with LHe-cooled ESR's at CRI, and calibration at the NIST HACR facility. CRI is the world's leading manufacturer of cryo-ESR's, and identifies total domestic and foreign markets of $10-15 M for this important new technology if the Phase 2 objectives in accuracy and system detectivity can be met.