Properties of helium and gas thermometry

In this study, dielectric-constant gas thermometry (DCGT) measurements of the thermodynamic temperature in the range between 2.4 K and 26 Kare presented. The DCGT method as a primary thermometer is based on a measurement of the relative dielectric constant and the pressure of agas. This requires the knowledge ofthe polarizability of the gas on the necessary level of uncertainty, which is achievable at present only for helium via theoretical calculations. A comparison of the high-precision measurements of the thermodynamic temperature with literature data shows that the method offers the potential for the determination of the Boltzmann constant with an uncertainty that is sufficiently small for a new definition of the base unit Kelvin. For this purpose, the measurement ofpressure, capacitance changes and temperature on an unprecedented low level of uncertainty is required. The main physical emphasis of this work is placed on the study of the particle interaction between helium atoms. In this context, the two particle interaction, expressed in the second virial coefficient, is of special interest. A comparison between the latest theoretical calculations and the experimental data of this work points the progress of theory and experiment. Special attention is given to the measurements in the lowest temperature range. Already in the gas phase, new, fundamental differences between the isotopes 3He and 4He are observed. These are discussed and interpreted in the context of different theories. Finally a detailed treatment ofthe different uncertainty components corroborates the presented measurements.