X-RAY-DIFFRACTION WITH A BRAGG ANGLE NEAR PI 2 AND ITS APPLICATIONS/

X-ray dynamical diffraction phenomena at a Bragg angle near pi/2 are s tudied. The X-ray transmissivity as well as the reflectivity from the (991) lattice plane of a silicon thin plate is observed. It agrees fai rly well with the diffraction pattern calculated on the basis of the D arwin approach. The possibility is discussed whether a set of two crys tal plates arranged face to face, in which the diffraction condition w ith a Bragg angle near pi/2 is satisfied, may be used as a very high r esolution monochromator.