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Choice of X-ray Target

The wavelength, λ, of the characteristic line giving rise to a particular transition is given by Moseley's Law:

1 / λ = c (Z - σ)2
where c and σ are constants, and Z is the atomic number of the metal used for the anode. From this equation it can seen that as the atomic number of the target increases, then the wavelength of the characteristic radiation decreases.

Since the target has to be metallic (so that it conducts electrons) and has to have a reasonably high melting point (40 kV at 30 mA generates 1.2kW of heat), this limits the choice of anode material to chromium (Cr), iron (Fe), cobalt (Co), copper (Cu), molybdenum (Mo), and a few other less commonly used materials for X-ray powder diffraction. The table below shows the Kα radiation for each element:

Anode Cr Fe Co Cu Mo Ag
Kα (Å) 2.29 1.94 1.79 1.54 0.71 0.56

Note the big gap in the values between Cu and Mo and the fact that only discreet wavelengths are available in the laboratory. Extensive lists of characteristic lines are given in the International Tables for Crystallography published by the IUCr.

In many cases, a choice of anode simply doesn't exist, either due to cost or more likely due to the loss of diffractometer time which occurs each time the tube is changed. Copper anodes are by far the most common (as shown above left) since copper gives the shortest wavelength above 1 Å. The wavelengths provided by, say, molybdenum and silver are normally too short for most powder diffraction work in the laboratory. Short wavelengths both scatter weakly and contract the diffraction pattern towards low Bragg angles with consequent loss of d spacing accuracy and resolution.

However, for certain research studies copper anodes are definitely not the best choice as illustrated by the data that follows. The picture of the coin shows a modern (1998) English "copper" penny from which the powder diffraction below (in orange) was measured using a copper anode:

The powder diffraction data are of an excellent quality. However, a powder diffraction experiment on the penny with the top surface removed yields the following data with a very high background and very little signal as shown below:
The high background is due to an effect called fluorescence. It results from the fact that iron atoms fluoresce in the X-rays produced by a copper target, and that modern (post-1980?) English pennies are now simply copper-coated iron coins. The greatest fluorescence for copper X-rays is actually from cobalt samples, but iron and, to a much smaller extent, manganese also fluoresce significantly. The practical solution to this problem is to use a cobalt or iron anode, and these should be used whenever regular analyses of iron containing samples, e.g. steel metals or iron corrosion products, are required. For the occasional analysis, an alternative solution is to use a post-sample graphite monochromator (as mentioned later).

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© Copyright 1997-2006.  Birkbeck College, University of London. Author(s): Jeremy Karl Cockcroft