SQUID based LF-NMR measurements in an urban laboratory environment

Nuclear magnetic resonance measurements were performed in an urban laboratory environment using a second-order low-Tc superconducting quantum interference device (SQUID) gradiometer. A static-active combined compensation system, consisting of three pairs of 2x2 m2 orthogonal squared Helmholtz coils, a 3 axis fluxgate, and home-made feedback electronics were constructed, and the low frequency variation of the environment was reduced by more than one order of magnitude; And its application in LF-NMR measurement was also discussed. A simple but efficient pre-polarization (Bp) coil was fabricated, and Bp exceeding 100 mT was achieved. NMR signals of proton were studied in both time and frequency domain, during both midnight and daytime. In the case of daytime measurement, the authors suggest the use of selective average, which can partly overcome the disadvantage of fluctuations of the ambient magnetic field.