Thermal Boundary Resistance between Solids and Helium below 1°K

Results are reported from measurements of the thermal boundary resistance between liquid ${\mathrm{He}}^3$ and copper or epoxy resin, and between solid ${\mathrm{He}}^3$ or liquid ${\mathrm{He}}^4$ and copper. Measurements were made in the range 0.05 to 0.9°K and at pressures up to 395 psi. No quantitative agreement has been found with theory. Above 0.11°K, the thermal boundary resistance of liquid ${\mathrm{He}}^3$ and liquid ${\mathrm{He}}^4$ both have a temperature dependence stronger than $T^{-3\mathrm{}}$. Below 0.11°K the thermal boundary resistance of ${\mathrm{He}}^3$ varies as $T^{-3\mathrm{}}$ over the full pressure range in the liquid. In the case of ${\mathrm{He}}^4$ at low pressure, however, the stronger temperature dependence continues to the lowest temperature measured (0.075°K). It is not clear whether the qualitative difference in the behavior of liquid ${\mathrm{He}}^3$ and liquid ${\mathrm{He}}^4$ is associated with zero sound in liquid ${\mathrm{He}}^3$.