SQUID Systems Adapted to Record Nuclear Magnetism in Low Magnetic Fields

We modified two SQUID measurement systems, originally developed and optimized for biomagnetic applications, with the aim to employ them for observing the precession of nuclear magnetism in low fields of a microtesla or less. To this end, coil systems for the polarization and for the observation of the nuclear magnetization were added to the set-up. In addition, SQUID readout electronics had to be fast enough to allow for short dead times after switching off the polarizing field. One system was a single channel gradiometer that is routinely used for measuring the relaxing magnetism of magnetic nanoparticles. The second system was a 304 SQUID system that was designed for multichannel vector-magnetocardiography and vector-magneto-encephalography. Both systems had in common that they operated in a magnetically shielded environment provided by multi-layer mu-metal walk-in rooms. With the single channel system we determined the natural proton NMR line width of benzene in the fast motional limit to be (112 plusmn 3) mHz. The accuracy of this value was improved by excluding experimentally the presence of line broadening by the inhomogeneity of the residual environmental field in the magnetically shielded room. To this end, the transversal relaxation of hyperpolarized ¹²⁹Xe gas was measured demonstrating that the spectrometer has a spectral resolution of about 6 mHz for ¹H. By using a 304 SQUID vector magnetometer system the precession of ¹H in multiple distilled water samples was recorded. From the spatially resolved data of the multi-SQUID system, positions and moments of the multiple samples were estimated.