In most of the XRD instrument Cu is used and in most of the XPS instrument Al/Mg is used?
Is there any specific reason behind this??
Dear Arindam,
besides the thermal conductivity it is the wave length/x-ray photon energy aspect of the k-alpha radiation emerging from those targets.
For XPS you need some quite low energies/ large wave lengths whereas for XRD you need some higher energies in order to get deeper into the material and gather informations from beneath the surface region. Wave length of Cu -K-alpha is just good compromise to have the most interesting XRD peaks in a suitable 2theta range for most of the materials.
K-alpha yield with respect to bremskontinuum background is a further aspect to have the atomic number as low as possible for the specific application.
The material to be used as should withstand for high temperature without melting as heat is developed when electrons hit the target. These materials have high melting point
secondly it is easier to get monochromatic X-ray line by using these materials
The choice of anode depends on the material for XRD analysis. routine analysis is normally employed with Cu radiation. The anode must be metallic and with high melting point. Copper gives the shortest wavelength above 1 Å . However if the sample contains Cr, Mn, Fe or Co you may be better off with Cobalt or Chromium radiation.
Mg and Al provide dominant resonance in the X-Ray spectrum and its a monochromatic X-Ray source that provides good resolution of photoelectric peaks in XPS spectrum.
The choice of anode affects background level, peak intensity, number of peaks in the investigated range and resolution of peaks.The change in background level is mainly due to XRF and peak intensity is due to change in the penetration depth. Resolution is also dependent on the radiation used. Higher wavelength radiations (such as Cr) increases the distance between the peaks so overlaps can be avoided, but if the material is heavily deformed broadening will also increase. So, it might be necessary to find an optimal point in between those.
The reason why it is common to have Cu tubes all around, most people use diffractometers not more thanr simple powder diffraction. Cu is a good compromise for powder diffraction of many compounds. Moreover, XRD equipment are widely located in central laboratories where different people use the equipment for many different things. It is usually desired to change the tube for a few experiments (and a simple user may not be authorized to do it). Another reason of Cu tube is that it is easier too cool anode since it is highly conductive, so it can operate at relatively high voltages (increase the intensity) and tube lifetime is usually better than some other anodes using the same cooling.
So, there is no simple rule, you should decide on the radiation depending on the material and the application.
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