Abstract: The eutectics Co-C, Pt-C, and Re-C, with phase transition temperatures of 1597 K, 2011 K, and 2747 K, respectively, are presently investigated for their suitability to serve as reference points for dissemination of T (and T 90) within the context of the "Mise en pratique of the definition of the Kelvin" (MeP-K) at high temperature. Temperatures are to be measured by means of radiation thermometry of cavity radiators embedded in the associated eutectic. This paper deals with the determination of the cavity spectral effective emissivities, which are influenced by the reflective properties of the graphite constituting the cavity on the one hand, and by the temperature distribution within the cavity and over the radiation-shield structure in front of the cavity, on the other. We have begun a comprehensive effort to determine the cavity effective spectral emissivities at 405 nm and 650 nm. The overall program involves diverse measurements on representative graphite samples, furnace-temperature profile measurements, calculations of temperature distributions, and, finally, based upon this information, calculation of effective emissivity dependencies for cavity radiators. Here we report the current status of the study, including cavity temperature distributions and Monte Carlo modeling results, associated with appropriate steps of the envisaged overall project. For the time being, the modeling assumes current estimates of the graphite emissivity and the bi-directional reflectance distribution function (BRDF), which will be updated as experimental data become available.

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