X-ray Diffraction
Secondary optics
The secondary optics receive the diffracted X-rays from the sample. The first component is a slit called the anti-scatter slit, which reduces the height divergence but also reduces diffusely scattered X-rays that are due to amorphous or air scattering. This is followed by a set of soller slits which prevent axially diverged X-rays entering the detector. In most instruments a monochromator is put between the soller slits and the detector. The monochromator(s) can be fitted either to the incident beam, or to the diffracted beam. Sometimes monochromators are fitted to both sides. There are three common methods of monochromating the beam. The first is to add a filter which removes most of the X-rays with energies below the Bragg edge. The second method used in some newer diffractometers is an energy sensitive detector that only “counts” those X-ray energies within a certain energy range. The third method is to use a large single crystal (usually graphite or silicon) to carry out a second diffraction process on the beam coming from the sample. The second diffraction event ensures that only the wavelength of interest meets the Bragg condition, and only this is directed to the detector. All other wavelengths are excluded.