The role of relaxation time
The nuclear spin system in a stable external magnetic field is subjected to two effects. One is that the magnetic field tries to position the magnetic moment of the atomic nucleus along the direction of the magnetic field, and the other is that the thermal motion of the molecule tries to prevent the nuclear magnetic moment from adjusting the position. Finally, the magnetic moment overlaps with the stable magnetic field and reaches a dynamic balance. At this time, the magnetization in the direction of the magnetic field is the largest, while the magnetization in the direction perpendicular to the magnetic field is zero on average. If the nuclear system is exposed to an electromagnetic field in a different direction, the magnetization will deviate from the original equilibrium position, generating a transverse magnetization perpendicular to the original magnetic field direction, and a longitudinal magnetization parallel to the original magnetic field will also decrease. When this electromagnetic field is removed, the unbalanced state of the nuclear system cannot be maintained, but must be restored to the equilibrium state. People call the process of recovery to equilibrium a relaxation process. The process that the nucleus recovers from the excited state to the equilibrium state is called the relaxation process. This process follows the law of exponential change, and its time constant is called the relaxation time.