NQR on gallium single crystals for absolute thermometry at very low temperatures

It is shown that NMR/NQR-thermometry on Ga single crystals can serve the ne ed of absolute thermometry in a temperature range from below 20 mu K to abo ve 1 mK. Thus, the sensitive range includes all phase transition temperatur es around I mK of solid and fluid He-3, a necessity for an extension of the He-3-melting-pressure temperature scale towards lower temperatures, The ex periments were performed in magnetic fields of 50 to 200 mT and at temperat ures down to 200 mu K using single vf-pulses. Special cave was taken for th e orientation of the magnetic field with respect to crystal orientation whi ch was obtained by in-situ pulsed NMR-experiments. It has been shown that i n the investigated parameter range nonlinear effects in NMR due to high spi n alignment had no significant influence. In pulsed NMR as well as in speci fic heat experiments it could be shown that spin-spin interactions are only weak in Ga and do not influence nuclear paramagnetism, at least at tempera tures above 100 mu K. The advantages of using Ga instead other elements for population difference thermometry ave discussed.