We have studied experimentally the emission of $\gamma$-ray photons by synchrotron-radiation-excited ${}^{57}$Fe nuclei in an iron borate crystal. The crystal was set in the vicinity of the Bragg angle for pure nuclear diffraction of the synchrotron radiation. The electronically forbidden but nuclear-allowed (333) reflection was employed to extract the nuclear scattering signal. The isolation of nuclear scattering was possible for two reasons. First, in contrast to the case of Rayleigh scattering, the phase in nuclear resonant scattering depends on the direction of the magnetic moment. Second, the iron atom spins in a ${}^{57}$FeBO${}_3$ crystal are antiferromagnetically ordered. Because they are flashlike excited by synchrotron radiation, the nuclei in the crystal afterwards emit their own recoil-free radiation within a narrow angular range. The angular dependence of the emission was measured with a highly collimated beam by setting the crystal in different angular positions near the Bragg angle. At room temperature the angular dependence of the emission intensity has a functional form with a single symmetric maximum. However, on approaching the Néel temperature, the angular dependence of the emission intensity is transformed dramatically in shape and width. The maximum broadens greatly, splits, and acquires a double-peak shape. The energy and time distributions of the emitted radiation appear to be strongly dependent on the angular position of the crystal relative to the incident beam within the angular range of the emission. The experimental measurements are fully consistent with theoretical predictions. The results obtained can be used in developing synchrotron-radiation-based techniques with neV resolution.

• Received 26 June 2012

DOI:https://doi.org/10.1103/PhysRevA.86.053808