Progress Toward Development of Low-Temperature Microwave Refractive Index Gas Thermometry at NRC

Abstract

Progress toward the development of a low-temperature microwave refractive index gas thermometry implementation for primary thermometry at NRC is reported. A prototype quasi-spherical copper resonator has been integrated into a cryogenic system with a 5 K base temperature, and preliminary microwave measurements in vacuum have been completed to characterize the resonator between 5 K and 297 K. The dependence of experimental results on spectral fitting background terms, 1st- and 2nd-order shape corrections, and waveguide corrections has also been explored. The current NRC results agree with previous room-temperature measurements on the same resonator at NIST, and indicate no significant change in resonator shape between room temperature and low temperature. The temperature dependences of the resonator electrical conductivity and linear thermal expansion coefficient, as obtained from the microwave resonances, agree with published literature values for oxygen-free high-conductivity copper measured using other techniques.