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 need of absolute thermometry in a temperature range from below 20 mu K to above 1 mK. Thus, the sensitive range includes all phase transition temperatures 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 experiments were performed in magnetic fields of 50 to 200 mT and at temperatures down to 200 mu K using single vf-pulses. Special cave was taken for the orientation of the magnetic field with respect to crystal orientation which was obtained by in-situ pulsed NMR-experiments. It has been shown that in the investigated parameter range nonlinear effects in NMR due to high spin alignment had no significant influence. In pulsed NMR as well as in specific heat experiments it could be shown that spin-spin interactions are only weak in Ga and do not influence nuclear paramagnetism, at least at temperatures above 100 mu K. The advantages of using Ga instead other elements for population difference thermometry ave discussed.