Free nuclear induction

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The study of nuclear magnetic resonance or nuclear induction, a recent field of research for which F. Bloch and E. M. Purcell have been awarded the Nobel Prize, has been carried out by a variety of techniques. The usual approach has been to observe the nuclear resonance of an ensemble of nuclear moments in a large static magnetic field as a function of a slow change in this field. Meanwhile, a small radio-frequency field is applied continuously to the nuclear sample in a direction perpendicular to the large field. An alternative method to this steady state or “slow passage” technique is one by which the radio-frequency energy is applied to the sample in the form of short, intense pulses, and nuclear signals are observed after the pulses are removed. The effects which result can be compared to the free vibration or “ringing” of a bell, a term often applied to the free harmonic oscillations of a shocked inductive-capacitive (LC) circuit. The circuit is first supplied with electrical energy from some source, and the supply of energy is suddenly removed. The LC circuit then remains for a time in the “excited state”, and the energy is gradually dissipated into heat, mostly in the circuit resistance. Similarly the atom or nucleus in the excited state can store energy for a time before it is completely dissipated, and in the case discussed here, the free oscillation or precession of an ensemble of nuclear spins in a large static field provides the ringing process.