These gratings diffract the intercepted X-rays, changing their direction
by amounts that depend sensitively on the X-ray energy, much like a prism
separates light into its component colors. For a grating, this
effect is due to a regularly spaced pattern. For example, music CDs can act
as a diffraction grating: the grooves diffract the light, so that
different colors are seen reflected off the surface at different angles.
Two types of gratings are in the HETG, in order to distinguish between them, the HEG and MEG gratings are oriented at slightly different angles, so that the X-rays are diffracted in an "X" pattern on the detector.
|The HETG gratings are made of fine gold wires or bars with a regular spacing, or period : 0.2µm or 2000Å for the high energy gratings, and 0.4µm or 4000Å for the medium energy gratings. Since the size of the gold grating bars is smaller that a wavelength of visible light, special fabrication techniques are required to make them. The gratings take advantage of the fact that the gold bars are partially transparent to X-rays, so that the diffraction is more efficient, and more X-rays are captured in the high resolution spectrum. The HETG gratings are designed to cover an energy range of 0.4 to 10 keV.|