Theory of Modulation Effects in Resonant-Nuclear-Disorientation Experiments. II

An analysis is made of modulation effects in resonant-disorientation experiments on radio-active nuclei which have been polarized by hyperfine interaction at low temperatures. The degree of destruction of the $\gamma$- and $\beta$-radiation anisotropy is calculated by assuming that, because of the inhomogeneous broadening, the modulation results in individual nuclear-spin packets experiencing a series of resonant passages. The effect of each passage is characterized by the adiabatic parameter. Between passages the spin-lattice relaxation is assumed to follow the theory of Gabriel. The methods apply for any spin; detailed results are given for spin-1 nuclei. This is the first quantum-mechanical treatment of the effect of a resonant-frequency-modulated rf field on the radiations from oriented nuclei. It is shown that if the rf field is sufficiently large complete destruction of the radiation anisotropy is expected. This has not been observed in experiments on ferromagnetic metals and suggests that in these the nuclear interaction is more complicated than that of the nuclear magnetic moments with the static magnetic field and the enhanced applied rf field. The results of the calculations should apply for nonmagnetically ordered samples and possibly for ferromagnets with weak hyper-fine interactions.