DOE OSTI2011
A physical picture for coherent emission of {gamma} rays by a nuclear array excited with synchrotron radiation is given. The particular case of a pure nuclear Bragg reflection from the {sup 57}FeBO{sub 3} crystal is analyzed. During free de-excitation of the nuclei, a nuclear exciton polariton is developing inside the crystal and generates at the exit of the crystal a coherent {gamma}-ray beam. In the crystal the nuclear levels of {sup 57}Fe are split because of a combined magnetic and electric interaction. The rich picture of {gamma}-ray interference is described, which involves geometrical space, energy, and spin domains. In the vicinity of the Neel temperature of the crystal the magnetic splitting of nuclear levels nearly collapses. These conditions lead to drastic changes in the angular, energy, and temporal properties of the emitted radiation. The emission angular function, which in the approximation of a plane incident wave represents the emission intensity for different angular settings of the crystal near Bragg angle, strongly broadens and transforms to a double-hump structure with a central dip between the peaks. The energy and temporal distributions of the emitted radiation crucially depend upon the crystal angular setting. Beyond the central dip, the energy distribution of nuclear scattering acquires a complicated form with several satellites at various energies. In contrast, at the exact angular position of the central dip, the energy spectrum exhibits a single line shape with the line width close to the natural width of the nuclear resonance. The obtained results constitute the theoretical basis for the understanding and for the further elaboration of the {sup 57}Fe synchrotron Moessbauer source--the device that provides a collimated beam of intense and polarized {gamma} radiation in an energy bandwidth of nanoelectronvolts, the nuclear resonance natural level width.
74 ATOMIC AND MOLECULAR PHYSICS↗