Nuclear spin noise in NMR revisited

TL;DR

This paper presents a comprehensive theoretical model for nuclear spin-noise spectra in NMR, accounting for circuit effects, and validates it with experiments to explain various observed features and optimize conditions for pure in-phase Lorentzian signals.

Contribution

The paper introduces a detailed circuit-based model for nuclear spin-noise spectra in NMR, explaining experimental features and identifying conditions for optimal spin-noise signals.

Findings

Model reproduces experimental spectral features

Identifies conditions for pure in-phase Lorentzian signals

Validates predictions with extensive measurements

Abstract

The theoretical shapes of nuclear spin-noise spectra in NMR are derived by considering a receiver circuit with finite, preamplifier input impedance and a transmission line between the preamplifier and the probe. Using this model, it becomes possible to reproduce all observed experimental features: variation of the NMR resonance linewidth as a function of the transmission line phase, nuclear spin-noise signals appearing as a "bump" or as a "dip" superimposed on the average electronic noise level even for a spin system and probe at the same temperature, pure in-phase Lorentzian spin-noise signals exhibiting non-vanishing frequency shifts. Extensive comparison to experimental measurements validate the model predictions, and define the conditions for obtaining pure in-phase Lorentzian-shape nuclear spin noise with a vanishing frequency shift, in other words, the conditions for simultaneously obtaining the Spin-Noise and Frequency-Shift Tuning Optima.