HE-3 MELTING PRESSURE TEMPERATURE SCALE BELOW 25 MK

Using Co-60 gamma-ray anisotropy radiation as a primary thermometer, with a Pt NMR susceptibility secondary thermometer, we have made high-precision measurements of the He-3 melting pressure versus temperature from 500 mu K to 25 mK. Temperatures obtained for the fixed points on the melting curve are: the superfluid A transition T-A = 2.505 mK, the A-B transition T-AB = 1.948 mK, and the solid ordering temperature T-N = 0.934 mK. We provide a functional form for P(T), which, with the fixed points, constitutes a convenient temperature scale, based on a primary thermometer, usable to well below 1 mK.