Nuclear Induction Signals for Long Relaxation Times

Equations of motion, previously derived by Bloch (1946), for the nuclear magnetic moment in a nuclear induction experiment are applied to the steady state reached when the strong magnetic field Hz is swept sinusoidally about its resonance value. Different approximations for the shapes and sizes of nuclear induction signals are derived for various limiting cases, depending (a) whether the passage of Hz is ` adiabatic ' or ` extremely non-adiabatic ' (very weak radio-frequency field) and (b) whether the relaxation times are shorter or longer than the period of the sweep of Hz. It is found that quite large signals may be induced by even a very weak radio-frequency field if the relaxation times are very long.