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Nuclear magnetic precession measured with SQUIDs

100%

Burghoff M.

tom R. 53, nr 9 bis

17--20

We have developed a superconducting quantum interference devices (SQUID) system measuring nuclear magnetic precession at very low magnetic fields with a spectral resolution beyond the natural line width of liquids. A coil system applies a magnetic polarization field and a perpendicular static detection field to the sample. The detection system for the very weak magnetic field generated by the processing nuclei consists of LTc-SQUID sensors in a liquid helium cryostat. We measured the 'H precession signal of benzene, distilled water and chloroform samples at various detection fields. With our set-up. broad band detection of the nuclear magnetic resonance of the investigated liquids with a very high resolution is possible. Benzene showed the narrowest natural resonance line width of 0.12 Hz followed by chloroform of about 0.17 Hz. The observed line widths increased linearly with the detection field with a slope independent of the investigated sample. This increase is attributed to the inhomogeneity of the detection field, rather than to intrinsic properties of the investigated liquids, and thus describes the properties of our measurement system. Reliable low field NMR and MRI depend strongly on field homogeneity of the detection field coils and the residual field inside the agnetically shielded room. At such low fields, high demands on the absolute homogeneity of the static field correspond to very moderate requirements on its relative homogeneity 111. Reduction of these system parameters and its influence is the prerequisite for a good frequency and spatial resolution. Here, we investigated our SQUID measurement system and estimated all main system parameters at very low fields below a microtesla.