NMR Techniques at Liquid Helium Temperatures

In this master’s thesis I describe our recently upgraded measurement techniques and devices based on the nuclear magnetic resonance (NMR) techniques used to investigate superfluid helium. At present, our research concentrates on the superfluid phase 3He-B. The most important part in our NMR circuit is a two-stage cryogenic MESFET preamplifier operating at 4 K. It is capacitively coupled to a high-Q (Q approximately 40000 in test conditions) LC-resonance circuit. The 3He-B sample is placed inside the superconducting pick-up coil, which is a part of the resonance circuit. High sensitivity and low noise levels are the requirements for the measurement circuitry. Our measurements have shown the signal-to-noise level to be around 8000 which is enough to detect small changes in the signal from the superfluid 3He-B sample. I also present the theories on which our measurements are based and analyze the NMR responses measured from the normal fluid and from the superfluid. The effect of rotation and vortices on the signal response is explained. We have also measured spin-wave spectra at different temperatures and rotation speeds. The data we have gathered shows the NMR response to be congruent with the theories and earlier measurements. All 3He measurements have been performed in a rotating cryostat, with which we can achieve sample temperatures below 500 μK. Overall, the measurement accuracy is enough to investigate the properties of 3He-B on approaching the zero temperature limit.