Abstract
LC resonance magnetic sensors are widely used in low-field nuclear magnetic resonance (LF-NMR) and surface nuclear magnetic resonance (SNMR) due to their high sensitivity, low cost and simple design. In magnetically shielded rooms, LC resonance magnetic sensors can exhibit sensitivities at the fT/√Hz level in the kHz range. However, since the equivalent magnetic field noise of this type of sensor is greatly affected by the environment, weak signals are often submerged in practical applications, resulting in relatively low signal-to-noise ratios (SNRs). To determine why noise increases in unshielded environments, we analysed the noise levels of an LC resonance magnetic sensor (L ≠ 0) and a Hall sensor (L ≈ 0) in different environments. The experiments and simulations indicated that the superposed ringing of the LC resonance magnetic sensors led to the observed increase in white noise level caused by environmental interference. Nevertheless, ringing is an inherent characteristic of LC resonance magnetic sensors. It cannot be eliminated when environmental interference exists. In response to this problem, we proposed a method that uses matching resistors with various values to adjust the quality factor Q of the LC resonance magnetic sensor in different measurement environments to obtain the best sensitivity. The LF-NMR experiment in the laboratory showed that the SNR is improved significantly when the LC resonance magnetic sensor with the best sensitivity is selected for signal acquisition in the light of the test environment. (When the matching resistance is 10 kΩ, the SNR is 3.46 times that of 510 Ω). This study improves LC resonance magnetic sensors for nuclear magnetic resonance (NMR) detection in a variety of environments.
Highlights
Nuclear magnetic resonance (NMR) technology is widely used in many fields such as biology, chemistry, physics, materials and geophysics [1,2]
We compared the behaviours of magnetic sensors with L ≈ 0 and L 6= 0 in a magnetically shielded room and in an unshielded field environment to observe the changes in the noise levels of different sensors
The results showed that the noise behaviours of the LC resonance magnetic sensor with L 6= 0 differed in the various environments, whereas those of a Hall sensor with
Summary
Nuclear magnetic resonance (NMR) technology is widely used in many fields such as biology, chemistry, physics, materials and geophysics [1,2]. Compared with high-field (HF) NMR, NMR at low-field Bm (LF) with Larmor frequency f L in the kHz range offers some advantages [3]. I.e., surface nuclear magnetic resonance (SNMR) fields, the high homogeneity of the Earth’s magnetic field (50–60 μT) increases the amplitude and duration of the free induction decay (FID). The spin-lattice relaxation time T1 is more material dependent than in high field cases, resulting in improved T1-contrast imaging [5]. The disadvantage that accompanies the low-field is a weak signal. An effective method to improve the signal-to-noise ratios (SNRs) of LF-NMR measurements is to increase the sensitivity of pick up sensors.
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