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Solid-state nuclear magnetic resonance magnetometry at low temperature with application to antimatter gravity experiments by ALPHA Evetts, Nathan


The Einstein Equivalence Principle (EEP) has never been directly examined with an antimatter test body. To address this, ALPHA is planning to measure the Earth’s gravitational field using antihydrogen atoms as test masses. The experiment calls for the careful release of antiatoms from a magnetic trap and requires precise characterization of the magnetic fields that are used. This thesis considers the need for magnetic field characterization from the perspective of Nuclear Magnetic Resonance (NMR) as a magnetic sensing technique. A variety of theories from statistics and condensed matter physics are assembled into a framework for understanding and predicting the performance of solid-state NMR magnetometers. This framework is applied to two types of NMR sensors that were developed for ALPHA’s gravity experiments. Data acquired while commissioning the apparatus provide experimental confirmation of the theoretical precision predicted for each sensor type. A room temperature sensor based on ¹H NMR in natural rubber exhibits a precision of about 270 nT at 1 T. A cryogenic sensor based on ²⁷Al NMR in microparticulate aluminium displays a precision of about 34 µT at 25 K and 1 T. These NMR data are then used to estimate the experimental constraint that should result from upcoming measurements of the gravitational field (g¯) experienced by antihydrogen atoms in the lab. Improvements to the expected constraint on g¯ are sought through advances in state-of-the-art solid-state NMR magnetometry. A broad survey of candidate materials has been conducted in search of NMR samples that promise better precision at low temperature. Yttrium-bismuth is proposed as a sample with the potential to improve sensor precision by a factor of ∼7 over the use of aluminium. Further gains in performance are predicted through the introduction of broadband (or “delay line”) probes. A brief review and assessment of these probes is presented.

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