Thermodynamic temperature of the triple point of xenon measured by refractive index gas thermometry

Measurements of the refractive indices of helium and argon using a quasi-spherical microwave resonator and upgraded experimental protocol are reported at the temperatures of the triple points of water and xenon. The results at the triple point of water are used to determine the compressibility of the resonating cavity, which is bounded by the copper shell of the resonator. This experimentally-determined compressibility is consistent with the literature value for copper, but with much smaller measurement uncertainty. The measured compressibility is extrapolated to the triple point of xenon, and combined with the refractive index results at that temperature to determine the thermodynamic accuracy of the International Temperature Scale of 1990 (ITS-90): (T - T90) = (-6.9 +/- 1.7) mK at ITS-90 temperature T-90 = 161.405 96 K, corresponding to a xenon triple point thermodynamic temperature of T = (161.3991 +/- 0.0017) K. The experimental compressibility is further extrapolated to the triple points of argon, oxygen and neon, and used to re-analyze earlier refractive index gas thermometry measurements made using the same resonator, yielding updated values with reduced uncertainties: (T - T-90) = (-4.1 +/- 1.6) mK at T-90 = 83.8058 K, (T - T-90) = (-2.0 +/- 0.8) mK at T-90 = 54.3584 K, and (T - T-90) = (-0.61 +/- 0.49) mK at T-90 = 24.5561 K. The ITS-90 thermodynamic accuracy results of the present refractive index gas thermometry study agree with those previously reported by acoustic gas thermometry and dielectric constant gas thermometry.