Fast switching coil system for sample premagnetization in an unshielded ultra-low-field nuclear magnetic resonance experiment

Abstract

We present a system developed to premagnetize liquid samples in an ultra-low-field Nuclear Magnetic Resonance (NMR) experiment. Liquid samples of a few milliliter are exposed to a magnetic field of about 70 mT, which is abruptely switched off, so to leave a transverse microtesla field where nuclei start precessing. An accurate characterization of the transients and intermediate field level enables a reliable operation of the detection system, which is based on an optical magnetometer.The ultra-low-field (ULF) regime in Nuclear Magnetic Resonance (NMR) experiments 1 corresponds to precession field intensities ranging around the microtesla level, where the nuclear precession is so slow to make the usual detection approaches -based on Faraday induction phenomenon-inadequate or unfeasible. Instead, non-inductive, highly sensitive detectors can be used, such as SQUIDs and optical atomic magnetometers 2 .Optical magnetometers are acknowledged for their practicality (they operate at room temperature) and robustness, making them eligible as high-performance sensors in on-field applications, such as in tomography in hostile environment 3 and material characterization 4 .Optical magnetometers designed to operate in a weak field, can tolerate much stronger (e.g. tesla level) ones, and recover immediately their operativity, as soon as the weak (e.g. microtesla) field is restored. In ultra-low-field NMR, this is the case when the sample is premagnetized in the same position where the nuclear precession is detected. Such in situ magnetization constitutes an approach complementary to remotedetection techniques, where the sample is premagnetized by a strong field in a region at some distance from the sensors and is subsequently shuttled to the weak field region, in the proximity of the sensor that detects the nuclear precession.This note describes an arrangement that makes possible to perform in situ magnetization in a setup previously developed to register NMR 5,6 and MRI 7,8 signals with a remotedetection scheme.