Ultralow-field NMR on Room Temperature samples using a low TC Two-Stage DC SQUID

摘要

This thesis describes the development of low-field Nuclear Magnetic Resonance (NMR) systems based on Superconducting QUantum Interference Device (SQUID) detection for use on room temperature samples and presents initial test results using various liquid samples. The original proof of principle low-field SQUID NMR spectrometer consists of a cryogenic dipper probe designed for small liquid samples on the order of 100 μl, which is operated in a liquid-helium Dewar equipped with a simple μ-metal shield. The samples are kept at room temperature inside a vacuum cell placed in the centre of a compact assembly of superconducting NMR coils. The two-stage DC SQUID sensor has a coupled energy sensitivity of ∼ 50 h, where h is Planck’s constant, at 4.2K and is coupled to the receiver coil via a superconducting flux transformer, offering highly sensitive broadband and frequency-independent signal detection. The obstacle of small sample polarization in low magnetic fields is overcome by means of sample prepolarization. Using the low-field SQUID NMR dipper probe, proton signals from distilled water samples were observed down to 93 nT (corresponding to a Larmor frequency of ∼ 4 Hz). With the benefit of sample temperature control, two-component free induction decays were obtained from oil-water mixtures at temperatures between 275K and 300K. The dipper probe was also extensively used to measure proton NMR relaxation times T1 and T2 for aqueous solutions of coated magnetite (Fe3O4) and cobalt-ferrite (CoFe2O4) nanoparticles in micro-Tesla fields to gain knowledge on their effectiveness as contrast agents for Low-Field Magnetic Resonance Imaging (LF-MRI). Finally, preliminary work on the design of the follow-up SQUID NMR system is presented. It will allow for larger samples, which will be placed underneath a cryogenic low-noise Dewar, housing the SQUID sensor and receiver coil, in the centre of room temperature coils providing the static background field and polarization pulses. The whole set-up will be operated inside a two-layer mu-metal magnetically shielded enclosure that will screen out extraneous magnetic fields such as the Earth’s field. With the addition of gradient coils, such a system can be used for LF-MRI test experiments.