Nuclear Resonance and Magnetic Field Changes of 1 in 10⁶

Abstract

IT is well known¹ that when a nuclear magnetic resonance signal is obtained in a rapidly rising magnetic field, the nuclei continue processing after resonance is passed. Their precessional frequency increases with the magnetic field, and while doing so forms beats with the exciting radio-frequency. These beats appear as ‘wiggles’ after passing the main resonance. In a perfectly homogeneous field, these wiggles die away exponentially at a rate determined by the relaxation time T₂². T₂ is related to the natural line-width by the relation T₂ = 1/γΔH, where ΔH is the line-width and γ the gyromagnetic ratio of the nuclei being used. If the magnetic field is inhomogeneous by more than ΔH over the sample volume, say by ΔH, then the rate of decay of the wiggles is generally believed to be determined by the time, ∼1/γΔH. This has proved a limitation in the measurement of transverse relaxation times T₂ ≫ 10⁻³ sec., as for such measurements a magnetic field inhomogeneity of ≲ 0.003 gauss is required; and, hitherto, this requirement has not been satisfied. Torrey³ has proposed and used a method in which long relaxation times T₂ can be studied in the presence of an inhomogeneous magnetic field. The observation of his phenomenon depends on maintaining the magnetic field at resonance, and is quite unrelated to the ‘wiggles’ experiments.