Atomic Nuclei: Nuclear Relaxation Processes

When degenerate nuclear spin states align with or against an applied magnetic field as per Boltzmann's distribution, their magnetic moments precess around the z axis.

Recall that the excess population in the lower energy state results in a net magnetization, M, along the z axis, with no net contribution from the transverse components on the xy plane.

Upon excitation, with suitable radiofrequency radiation, the nuclei absorb energy.

The random distribution of magnetic moments becomes slightly phase coherent, causing the tipping of the net magnetization vector.

Continued excitation can equalize populations in the upper and lower spin states. The spin system saturates, and the absorption signal decreases.

The excited nuclear spins must now undergo relaxation and return to the equilibrium population distribution.

When the rate of relaxation is greater than or equal to the rate of excitation, the excess population is maintained, and a signal is observed.

During relaxation, coherence is lost and the net magnetization is restored to the equilibrium value.