Time Dependence of Resonantly Filtered Gamma Rays from ${\mathrm{Fe}}^{57}$

The time dependence of gamma rays emitted by the 14.4-kev state of ${\mathrm{Fe}}^{57}$ has been studied by delayed-coincidence measurements between a 123-kev gamma ray preceding formation of the state and the 14.4-kev gamma ray from the state. When no filter was used, the number of gamma rays decreased exponentially with the known half-life of 0.1 μsec. When a foil of ${\mathrm{Fe}}^{57}$ (which was resonant to 14.4-kev radiation) was used as a filter, the number of gamma rays observed through the filter did not decrease exponentially. Instead, the filter absorbed almost none of the gamma rays first emitted by the 14.4-kev state; at later times the absorption increased. Data were taken with three different thicknesses of absorber and with emission and absorption peaks separated by 0 to 11 times the width of the resonance. The energy separation resulted from the Doppler shift associated with a constant velocity between source and absorber. These data were, for the most part, in good accord with the prediction of a theory based on a classical model for absorber and source. In particular, the results verified the theoretical prediction that at certain times the intensity of radiation observed would be greater with the filter than without it.