Heat of Mixing and Ground-State Energy of LiquidHe3-He4Mixtures

The heat evolved when ${\mathrm{He}}^3$ is added to liquid ${\mathrm{He}}^3$-${\mathrm{He}}^4$ mixtures at the saturated vapor pressure has been measured for temperatures in the region of 0.05° K. The starting concentrations varied between zero and six atomic percent of ${\mathrm{He}}^3$. Since the variation of the energy with temperature (the specific heat) is known, the experiments give the ground-state energy and the ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ chemical potentials at 0° K as a function of concentration. The measurements were made in a calorimeter connected by a wire of high thermal resistance to a dilution refrigerator operating at ∼0.02° K. The ${\mathrm{He}}^3$ was added through a long tube containing thermal anchors connected to the refrigerator.