He3melting-curve thermometry at millikelvin temperatures

A pressure-versus-temperature calibration of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ melting curve is given for 1<T<250 mK. The calibration is based on the data of Halperin et al. and Greywall and Busch and is consistent with the revised National Bureau of Standards temperature scale. On the new scale, $T_A$=2.708 mK. With the use of the transition line between normal and superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ [i.e., $T_c$(P)] as a basis for intercomparison, the melting-curve scale is found to be proportional to the magnetic temperature scales of Paulson et al. and Haavasoja et al. Included is a description of the ${\mathrm{PrNi}}_5$ nuclear demagnetization refrigerator which was used to cool the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ samples to less than 0.3 mK.