The X-ray tube is the heart of an X-ray diffractometer. Inside, the tungsten filament is heated, and electrons on the hot filament surface have enoughy energy to leave the filament surface. The power supply puts a high voltage between the filament (high negative potential) and the target metal (at ground potential). Electrons that from the filament are accelerated by the electric field between the filament and the target, and strike the target with an energy equal to the tube voltage. Focusing plates within the tube (not shown here) cause the electron beam to hit the target along a thin line about 0.4 mm by 10 mm.
When energetic electrons strike the target, they knock electrons from the inner electron shells of the target atoms. Electrons in the outer shells "fall down" to fill the vacant inner shell, loosing energy by emitting X-ray photons of characteristic energy and wavelength. The X-rays radiate in all directions, but some pass through four thin beryllium windows. TOXIC; DON'T TOUCH THE BERYLLIUM WINDOWS! Because the electron beam strikes the target along a line, if you could look through either of the two beryllium windows that are parallel to the line focus you would see X-rays shining from a line. These are called "line focus" windows and are used in most powder diffractometry, such as this instrument does. If you could look through the other two windows that are perpendicular to the line focus you would see X-rays emitted from what appears to be a point, which is the foreshortened perspective view of the line. These are the "point focus" windows that are used for powder cameras and single crystal cameras.
The tube fits into a housing above the high voltage generator box. In this housing, one beryllium window is left uncovered so X-rays can shine onto your sample. Cobalt X-rays are produced in three wavelengths: Co Kα1 and Co Kα2 have similar wavelengths and will not be discussed separately here, and Co Kβ, which has a somewhat shorter wavelength and so higher energy. This is a nuisance, because having Kα and Kβ X-rays will double the number of diffraction peaks and so complicate your pattern. Kβ X-rays are therefore removed with a filter. During normal operation the X-rays on our instrument are filtered through thin iron foil that strongly absorbs Co Kβ X-rays but is nearly transparent to Kα X-rays. The following four figures explain how this works.
Co has a K-shell excitation energy at 7.587 kV (1.608Å), the energy needed to remove a K-shell electron from the atom. As other electrons loose energy and fill the K-shell vacancy, X-rays are emitted. These X-rays have characteristic wavelengths of 1.621Å (Kβ) and 1.790Å (combined Kα1 and Kα2). With no filter the Kβ peak reaches your sample.
The Cu K excitation energy at 8.841 kV corresponds to a wavelength of 1.380Å. Because the Cu K excitation energy is higher than the energy of either Co X-ray lines, the Co radiation is only weakly absorbed by copper and both peaks are reduced slightly.
The Mn K excitation energy at 6.435 kV corresponds to 1.896Å, is lower in energy than either of the Co X-ray lines. Both Co lines therefore have sufficient energy to excite Mn K-shell electrons, and therefore are both strongly absorbed. The result is that both Co X-ray peaks are greatly reduced in size after passing through Mn foil, and a resulting diffraction pattern has no peaks at all.
Fe has a K excitation energy at 6.999 kV, or 1.743Å. Because this energy is between the CoKα and Kβ peaks, the higher energy Kβ X-rays excite the iron in the filter and are strongly absorbed. The Co Kα X-rays are not energetic enough to excite Fe, so they are only weakly absorbed. The result is a modest reduction in the size of the Co Kα line, and an almost complete elimination of the Kβ line. This results in a much simpler X-ray diffraction pattern, so an Fe filter is usually used with Co X-ray tubes.
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