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Background

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Nuclei of isotopes which possess an odd number of protons, an odd number of neutrons, or both, exhibit mechanical spin phenomena which are associated with angular momentum. This angular momentum is characterized by a nuclear spin quantum number, I such that,

I = ¹/₂n, where n is an integer 0,1,2,3...etc.

Those nuclei for which I = 0 do not possess spin angular momentum and do not exhibit magnetic resonance phenomena. The nuclei of ¹²C and ¹⁶O fall into this category. Nuclei for which I = ¹/₂ include ¹H, ¹⁹F, ¹³C, ³¹P and ¹⁵N, while ²H and ¹⁴N have I = 1.

Since atomic nuclei are associated with charge, a spinning nucleus generates a small electric current and has a finite magnetic field associated with it. The magnetic dipole, µ, of the nucleus varies with each element.

When a spinning nucleus is placed in a magnetic field, the nuclear magnet experiences a torque which tends to align it with the external field. For a nucleus with a spin of ¹/₂, there are two allowed orientations of the nucleus; parallel to the field (low energy) and against the field (high energy). Since the parallel orientation is lower in energy, this state is slightly more populated than the anti-parallel, high energy state. (Figure 1)

The amount of electromagnetic radiation necessary for resonance depends on both the strength of the external magnetic field and on the characteristics of the nucleus being examined. The nucleus of the proton, placed in 14,100 gauss field, undergoes resonance when irradiated with radiation in the 60 MHz range (microwave radiation); higher magnetic fields, such as those common in superconducting magnets, require higher energy radiation and give a correspondingly higher resolution.