Thermal response characteristics of a SPRIGT primary thermometry system

This article describes the construction and thermal characterization of a cryogen-free cryostat designed for single-pressure refractive index gas thermometry (SPRIGT) of helium-4 in the range 5–25 K. The system is cooled using a G-M type two-stage pulse tube cryocooler. The temperature can be controlled with a standard uncertainty of 20 µK over the whole working range. The use of a thermal switch to attenuate the modulated heat flux arising from the periodic temperature oscillation of the cryocooler is described. From the temperature evolution over the whole system the thermal characteristics are estimated using a simple model.