Theory of Modulation Effects in Resonant Nuclear Disorientation Experiments

An analysis is made of modulation effects in "resonant nuclear disorientation" experiments on radioactive nuclei which have been polarized by hyperfine interaction at low temperatures in ferromagnetic host metals. The inhomogeneously broadened nuclear magnetic resonance is detected through the destruction of the $\gamma$ radiation anisotropy by a resonant-frequency-modulated rf field. It is shown that to a good approximation the observed line shape is determined solely by the modulation amplitude and the distribution of hyperfine magnetic fields; for large rf fields, the signal amplitude depends separately upon the modulation amplitude and frequency. For finite rf field intensities, the signal amplitude also depends upon a parameter $k$ which is closely related to the saturation behavior which would be observed in the absence of inhomogeneous broadening. At high modulation frequencies, the dependence of the signal amplitude upon $k$ is easily calculated, and this should permit experimental determinations of $k$ values. Independent determinations should also be possible from studies of the time rate of destruction of the $\gamma$ radiation anisotropy immediately after applying a resonant rf field.