The source produces the X-rays used for analysing samples with X-ray diffraction. Typically the source is an X-Ray tube. It consist of an evacuated ceramic or glass vessel that contains a tungsten filament as cathode which emits electrons, and an anode onto which these electrons are accelerated with a potential of several ten thousands of volts.
Several processes lead to the emission of electromagnetic radiation in the X-Ray range as the electrons hit the anode target.
First, the electrons are quickly decelerated by passing the nuclear cores. This gives the so-called Bremsstrahlung or white radiation effect.
Second, the accelerated electrons hit inner shell electrons from atoms of the target material, remove them and leave holes behind. These holes are quickly filled from higher level electrons of the same atom. On falling down to the lower energy level the atoms emit characteristic radiation, which corresponds to the energy levels of the anode material.
The most intense characteristic lines are the Kα1 and Kα2 radiation and the Kβ. The first two are usually used in diffraction experiments. All other types of radiation are unwanted by-products. The wavelengths of each of these characteristic lines are summarised in the table below for two of the most common anode elements.
Copper is highlighted in red because it is the most common anode material in Powder XRD. However, if a sample shows a fluorescence effect then the sample will simply absorb and re-radiate X-rays from the copper anode, leading to large amounts of noise in the output. This can be solved by using the cobalt anode, which provides less signal intensity but will also remove the noise caused by the fluorescence background.