7.8:
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.
7.8:
Atomic Nuclei: Nuclear Relaxation Processes
In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis, the precessing magnetic moments are randomly oriented around the z-axis. This 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 radiofrequency radiation, the nuclei absorb energy, and the excited spins acquire some coherence. It follows that Mx and My are no longer zero, Mz decreases, and the net magnetization, M, tips toward the y-axis. Upon continued excitation, the population difference between the spin states can decrease, along with the signal intensity. This is called saturation.
Eventually, the excited nuclear spins return to the equilibrium state through a process called relaxation. During relaxation, the xy coherence disappears, and the net magnetization is restored to the equilibrium value along the z-axis.