principle of x-ray

Wavelength dispersive spectrometers differentiate X-rays based on their wavelength. They are mechanical systems consisting of two main components, an analyzing crystal and a detector. They work on the principle of X-ray diffraction. When the electron beam hits the sample, X-rays are produced. A portion of those X-rays will hit the analyzing crystal in each of the spectrometers. Those X-rays which have the right wavelength will diffract into the detector, where they will be counted. For the diffracted X-rays to enter the detector the spectrometer must be properly aligned. This means that the detector, analyzing crystal, and the spot on the sample where the X-rays are generated, must all be on the circumference of a circle, called the Rowland circle. Under these conditions only the X-rays with the wavelength of interest will be counted by the detector. To measure X-rays with a different wavelength, the analyzing crystal and detector must be rotated around the Rowland circle such that the incident angle of the crystal changes, while maintaining position on the Rowland circle. In practice, the crystal is moved directly away from the sample and rotated, and the detector is moved around on the circumference the circle. As a result, the center position of the Rowland circle changes, but its diameter stays the same. The position of the spectrometer can be given in terms of: 1) the wavelength of the X-rays it is collecting, 2) the diffraction or incident angle, or 3) the distance between the sample and the crystal (L-value).