Three-Dimensional Diffraction Phenomenon at a 90-degree Bragg Reflection

The phenomenon of a Bragg reflection in the vicinity of the 90 degree Bragg angle has a number of fundamental and applied x-ray physics implications. For instance, the extremely high energy resolution that can be achieved [1, 2] in a back-scattered beam might allow one to perform highly monochromatic radiation experiments that cannot be implemented using far from 90 degree, Bragg reflection geometry. However, there have only a few reported experiments that were devoted to the measurement of a diffraction intensity profile at scattering angles close to a Bragg angle of 90 degrees. The major experimental difficulty associated with the direct observation and quantitative measurement of this intensity profile was to achieve a significant spatial separation of the incident and back-scattered beams. This experimental difficulty is obviously due to the lack of a suitable detector system that would allow the accurate measurement of the back-scattered beam intensity when it is very close to the incident beam.

This page presents a new experiment that was performed to observe a hitherto unreported three-dimensional diffraction effect of the reciprocal lattice point movement as a function of the tilt angle of the reflecting crystal in a plane perpendicular to the diffraction plane. To accomplish this experiment, a novel detector system was constructed to allow a direct measurement of the 90 degree Bragg reflection profile with very high (thousands of per cent) diffraction contrast. This improved detection method resulted in a dramatic visibility enhancement of the two peaks close to the Si(800) 90 degree Bragg reflection and it allowed the three-dimensional diffraction effect to be quantitatively analyzed (Nikulin, Davis and Cookson, Phys. Stat. Sol. (a), 1998).

[1] K. Kohra and T. Matsushita, Z. Naturforsch. Teil A27, 484 (1972).
[2] W. Graeff and G. Materlik, Nuclear Instruments and Methods 195, 97 (1982).

	The experimental set-up for observation of the three-dimensional diffraction effect at a 90 degree Bragg reflection performed at the Australian Synchrotron Research Program Facility (ANBF), beamline BL-20B, at the Photon Factory, KEK, Tsukuba, Japan. The experimental set-up is not to scale.
	The two-dimensional experimental intensity profile measured in the scintillator detector as a function of the angular deviation of the Si(800) crystal from the 90 degree Bragg position.
	The three-dimensional Ewald sphere construction showing the movement of the reciprocal lattice vector with the tilt of the crystal.
AFTER: Nikulin, Davis and Cookson, Phys. Stat. Sol. (a), 1998