Ultralow-field NMR on room temperature samples using a low TC two-stage DC SQUID

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 100μl, 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 ∼ 4Hz). 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. Acknowledgements First of all, I would like to thank my supervisor Chris Lusher, whose hard work and effort permeates this thesis. In addition to many long and enlightening discussions about all aspects of this project, he has always taken the time to listen to any of my concerns, both work-related and personal, and I consider him to be not only a mentor but also a very good friend. The same goes for Andrew Casey, who provided me with financial support in the latter stages of my PhD, but who also gave me endless and invaluable moral support during the harder times. Furthermore, without his continued enthusiasm, expertise and great instincts when it comes to practical solutions, work on the Dewar could never have progressed as fast as it did. I am immensely grateful to my advisor John Saunders, without whom none of this would have been possible and who has been an important source of encouragement for my work. I would also like to thank Moreton Moore for being my moderator and always providing me with useful and positive feedback. I am indebted to Rainer Körber for putting up with me and my penchant for breaking sample cells during the first years of this project and especially for teaching me all of my experimental physics craft. Similarly, I want to thank Jan Nyéki for sharing his experimental know-how, but also for being a continued source of optimism in the face of adversity and for teaching me the value of the motto ’Never give up! ’. A huge thank you goes to all of the Physics workshop staff. A special mention is reserved for Dave Bosworth, without whose most useful advice and efficient machining skills, it would have been impossible to actually construct the Dewar. I am also grateful to Alan Betts, Leon Ellison and Howard Moore for among other things, sorting out all my electrical needs and keeping me safe. In addition, I would like to thank all of the support staff, in particular Andrea Hodgetts, Gill Green, Andy Alway, and who could forget Graham Cakebread, for helping me with all sorts of day-to-day issues. I also want to give a special thanks to Ian Murray and Francis Greenough for always being there if I needed help, advice or simply a chat. The daily life in lab would not have been the same without my contemporaries, with whom I have shared many laughs and equipment failures. First and foremost, I’d like to thank Aya Shibahara for our collaborative work and for being a dear friend. I can’t imagine being in lab without her nearby. I would also like to thank Lev Levitin for being truly unique, Rob Bennett for his cheerful enthusiasm, George Nichols for satisfying all my vacuum grease needs, Andrea Magee for having the best laugh ever and finally Giovanna Tancredi for bringing me closer to my Italian roots. I want to thank Quentin Pankhurst and especially Hugh Seton for agreeing to be my examiners. Finally, I would like to express my eternal gratitude to my family in Luxembourg, especially my grandparents, who had to make due without me for the entirety of my PhD. And last, but definitely not least, I am grateful for having had Ben Yager at my side through all of the toughest times and I truly cannot express in words how much his unwavering support has meant to me. I owe it all to you, babe!