THERMAL MODELING OF ABSOLUTE CRYOGENIC RADIOMETERS

This work consists of a detailed thermal modeling of two different rad iometers operated at cryogenic temperatures. Both employ a temperature sensor and an electrical-substitution technique to determine the abso lute radiant power entering the aperture of a receiver. Their sensing elements are different: One is a germanium resistance thermometer, and the other is a superconducting kinetic-inductance thermometer. The fi nite element method is used to predict the transient and steady-state temperature distribution in the receiver. The nonequivalence between t he radiant power and the electrical power due to the temperature gradi ent in the receiver is shown to be small and is minimized by placing t he thermometer near the thermal impedance. In the radiometer with a ge rmanium resistance thermometer, the random noise dominates the uncerta inty for small incident powers and limits the ultimate sensitivity. At high power levels, the measurement accuracy is limited by the uncerta inty of the absorptance of the cavity. Recommendations are given based on the modeling for future improvement of the dynamic response of bot h radiometers.