A RF Preamplifier Design For Low Temperature NMR Experiments.*

It was reported by several research groups that the recently discovered possible supersolid state of solid ${ }^4$He has strong correlations with the presence of ${ }^3$He impurities and other defects. These observations aroused new interest in the NMR study of solid ${ }^4$He doped with ${ }^3$He. However, the extremely low concentration of ${ }^3$He (less than 100 ppm) and the long nuclear spin lattice relaxation times in the interested regime lead to a challenging task to achieve detectable NMR signals. We present a new design for a low temperature preamplifier working with a crossed-coil NMR probe. This preamplifier reduces the noise temperature to 1 K at a sample temperature as low as 250 mK when it is thermally anchored to the mixing chamber of a dilution refrigerator. With an adapted design, we are able to cool down the sample to about 10 mK and keep the noise temperature well below 4 K.