Type-I Superconductor Pick-Up Coil In Superconducting Quantum Interference Device-Based Ultra-Low Field Nuclear Magnetic Resonance

In ultra-low field nuclear magnetic resonance (ULF-NMR) with strong prepolarization field (B-p), type-II superconducting pick-up coils may be vulnerable to flux pinning from the strong B-p. Pick-up coils made of NbTi, Nb, and Pb were evaluated in terms of acquired NMR signal quality. The type-II pick-up coils showed degraded signals above 61 mT maximum exposure, while the Pb pick-up coil exhibited no such degradation. Furthermore, a negative counter pulse following a strong B-p was shown to follow magnetic hysteresis loop to unpin the trapped flux in the type-II pick-up coil and restore the NMR signal. (C) 2014 AIP Publishing LLC.