Several distinct kinds of magnetic resonance exist. In cyclotron resonance the magnetic field is adjusted so that the frequency of revolution of a charged particle around the field lines is exactly equal to the frequency of the radiation. This principle is used to produce beams of energetic particles in particle accelerators.
Other magnetic resonance phenomena depend on the fact that both the proton and electron exhibit intrinsic spin about their own axes and thus act like microscopic magnets. Electron paramagnetic resonance (EPR) arises from unpaired electron spins in liquids or solid crystals. Because of their own magnetism, the spins line up with the external magnetic field. For a given magnetic field the spins can be made to
flip to the opposite direction when they absorb radiation at a corresponding
resonant frequency. From the point of view of quantum mechanics, the spin flips can be considered as transitions between states that become separated in energy when the magnetic field is applied. The effect is related to the splitting of spectral lines when an atom is subjected to a magnetic field (see spectrum; Zeeman effect).
Nuclear magnetic resonance (NMR) is analogous to EPR; however NMR is produced by the much smaller magnetism associated with unpaired nuclear spins. The NMR resonant frequency (usually that of protons in complex molecules) is slightly shifted by interactions with nearby atoms in the sample, thus providing information about the chemical structure of organic molecules and other materials. NMR is now extensively employed in medicine, although the use of the word
nuclear is avoided, the preferred name being magnetic resonance imaging (MRI). The technique provides high-quality cross-sectional images of internal organs and structures. Paul Lauterbur, an American physicist, and Peter Mansfield, a British physicist, shared the 2003 Nobel Prize in Physiology or Medicine for pioneering contributions that later led to the application of magnetic resonance in medical imaging.
Magnetic resonance can also occur without an external magnetic field from interactions of the electron and nuclear spins; such resonance produces the fine and hyperfine structure of atomic spectra.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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