Nuclear Magnetic Resonance and Spin Lattice Equilibrium

Abstract

BY measuring the absorption of radio-frequency energy by a substance in a magnetic field due to nuclear magnetic resonance^1,2, an estimate can be made of the order of magnitude of the time required for the establishment of thermal equilibrium between the spin system and the lattice. The absorption coefficient is proportional to the difference between the population of the magnetic energy states. The effect of the radio-frequency field is to tend to equalize the population of the states, so that in a strong radio-frequency field the absorption coefficient is less than in a weak field. This tendency to equalize the population of the states is opposed by the effect of the spin-lattice coupling, which tends to restore thermal equilibrium with the lattice. By finding the magnitude of radio-frequency field required to produce an appreciable reduction in the absorption coefficient, the relaxation time for transfer of energy from the spin system to the lattice can be calculated.