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Measurement of nuclear magnetic moments by a magnetic resonance spectrometer Collins, Thomas LeGear


A recording nuclear magnetic resonance spectrometer has been built and is in operation. The instrument is based upon a simple oscillating-detector of original design, for which the author presents a complete analysis of sensitivity and signal to noise ratio. This analysis is based upon the van der Pol oscillator and contains a theoretical estimate of the modulation noise in such an oscillator. Comparison with other methods shows the ultimate sensitivity of the oscillating detector to be the same as that obtainable by bridge methods, but the former is much more flexible. Although similar instruments are apparently in use, no details are given in the literature. Other features of the spectrometer are a 1200 lb. electromagnet and a lock-in detector of great stability. Some of the resonances recorded by the spectrometer are discussed. These are: (a) Cu⁶³ in the wire of the detector coil. (b) Br⁷⁹ and Br⁸¹ in NaBr and KBr solutions. (c) I¹²⁷ in Nal and KI solutions. (d) Sb¹²¹ in SbCl₅ and HSbCl₆. The copper signal provides a criterion of the sensitivity of the spectrometer. The measurement of the resonant frequency agrees with values reported by Knight for metallic copper. The resonance widths of the bromine isotopes are tenfold narrower than values given by Pound, an important discrepancy since it throws doubt upon our ability, at present, to calculate quadrupole moments from nuclear magnetic resonance. The iodine width in Nal agrees-with Pound's value. The antimony resonance in HSbCl₆ confirms work of Proctor (private communication). The resonant frequency in SbCl₅ is about .07% higher. This shift is an example of the chemical effect recently discovered for fluorine and nitrogen isotopes.

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