Nuclear magnetic moments (along with electronic magnetic moments) precess (spin) when placed in an external magnetic field, B. The frequency at which they precess, called the Lamor precessional frequency, wL, is directly proportional to the magnetic field.
When the magnetic moment μ is lined up with the field as closely as quantum physics permits, the potential energy of the dipole moment in the field has its minimum value, Emin. When μ is as antiparallel as possible, the potential energy has its maximum value, Emax. The figure below shows the two energy states for a nucleus with a spin of ½.
A nucleus with spin ½ can occupy one of two energy states when placed in an external field. The lower energy state Emincorresponds to the case where the spin is aligned with the field as much as possible according to quantum mechanics, and the higher energy state Emax corresponds to the case where the spin is opposite the field as much as possible.
The transitions between these two spin states can observed through a technique termed to as nuclear magnetic resonance (NMR). A constant magnetic field, B, is introduced to align magnetic moments, along with a second, weak, oscillating magnetic field oriented perpendicular to B. When the frequency of the oscillating field is adjusted to match the Larmor precessional frequency, torque acting on the precessing moments causes them to “flip” between the two spin states. These transitions result in a net absorption of energy by the spin system, an absorption that can be detected electronically.
The absorbed energy is supplied by the generator producing the oscillating magnetic field.
Nuclear Magnetic Resonance (NMR) and a related technique called electron spin resonance are very essential methods for studying nuclear and atomic structures and how those systems interact with their environs.
Magnetic resonance imaging (MRI) technique is actually based on nuclear magnetic resonance.
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