Instrumentation for the Earth's Field NMR Technique

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Abstract
An instrument utilizing free precession in the earth's magnetic field has been developed for studying the field dependence of nuclear spin relaxation in liquids. Both 1H and 19F nuclei are accessible to measurement. The method requires that a current be switched on in the sample coil for a certain time, reduced quickly to some smaller value, and later reduced suddenly to zero. The resulting sample magnetization then precesses about the earth's magnetic field, inducing a voltage in the coil which is amplified and displayed on an oscilloscope screen. The sample coil, current switching circuit, signal detection circuit, and timing circuits are described. The instrument is capable of measuring spin‐lattice relaxation times (T1) of 30 msec and up in fields ranging from 2 to 500 G; the range of measurable spin‐spin relaxation times (T2) is 10 msec‐1 sec in the earth's field of 0.54 G. The signal‐to‐noise ratio for a 450 ml water sample is greater than 700:1 at a bandwidth of 20 Hz. Estimated precision of the relaxation time measurements is ±3%.

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