Letter | Published:

Audio-frequency Nuclear-resonance Echoes

Nature volume 180, pages 13441345 (14 December 1957) | Download Citation



THE principal expense and source of difficulty in equipment for nuclear magnetic resonance is the magnet to provide the large magnetic field. It has been shown that a nuclear-resonance signal can be obtained using only the Earth's field1,2. This is the so-called free-precession experiment, in which the nuclear system is first polarized at right angles to the Earth's field by means of a simple current-carrying coil. After removal of this field, the nuclear magnetization precesses about the Earth's field (approximately 0.48 gauss) and induces an alternating voltage in the same coil at a frequency of about 2 kc./s., which may be observed after suitable amplification. The polarizing field serves the double purpose of producing a nuclear magnetization appropriate to a large field and hence a stronger signal to be observed in the smaller field, and also produces this polarization at right angles to the small field. The signal is attenuated by loss of phase-coherence of the precessing nuclei, due both to interactions between them with a time constant T2, and to inhomogeneity in the Earth's field (T2*). The method can be used for measuring the decay time, T2, which is a property of the material, by observing the decay of the precession signal, provided the inhomogeneity of the Earth's field is sufficiently small for T2* T2. In many cases, for example, in liquids such as water containing mobile protons, T2 may be several seconds and the corresponding field mhomogeneity must be less than 10 microgauss/cm. in order to measure T2. This implies that the experiment cannot be performed in the laboratory and usually requires the sample coil to be placed out of doors away from all buildings (we have found a brick wall to be unacceptably magnetic in this sense).

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    , Arch. Sci., 9, Report of 5th Colloque AMPERE, p. 218 (1956).

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  1. Department of Physics, Queen Mary College, Mile End Road, London, E.1. Sept. 4.

    • J. G. POWLES
    •  & D. CUTLER


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