Helium-Gas-Cooled Cryogenic Current Comparator Integrated with a Quantum Resistance Standard

Abstract

We report a cryogen-free cryogenic current comparator (CCC) system for precision resistance measurements. A helium-gas chamber was developed and installed on the 4 K stage of a cryogen-free dilution refrigerator equipped with a pulse-tube cryocooler. The CCC probe was housed in this chamber and was cooled through helium gas serving as a heat exchange medium. The metrological performance of this helium-gas-cooled CCC was evaluated through precision resistance-ratio measurements and found to be comparable to that obtained under liquid-helium cooling. A quantum Hall resistance (QHR) device was also integrated into the same refrigerator, enabling QHR/100~$Ω$ resistance-ratio measurements. The type-A uncertainty reached the 1~n$Ω$/$Ω$ level within an averaging time of 100~s, and the resulting resistance-ratio measurements agreed well with conventional liquid-helium-based measurements at the level of a few n$Ω$/$Ω$. The system provides this level of measurement performance while consuming less than 1~L of helium gas per thermal cycle.