Chapter 3 Chrystal Structure in Materials AACrystal structur

Chapter 3: Chrystal Structure in Materials

AA_Crystal structures table are given below:

Solution

Target (anode)

The target material is a fixed component of the generator – generators with different target materials are available (eg, Mo, Rh, W), but these are not switchable within a single generator unit. A key part of the generator’s output is formed from the characteristic X-Ray Fluorescence of the target material. Since the fluorescence yield increases with atomic mass, increased output at a set voltage/current will be seen by using higher atomic mass target materials. In addition, the energies of the fluorescence lines will vary depending on the material used, and as a result different target materials can result in more or less efficient excitation of particular elements in the sample.

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

1 / = c (Z - )²

where c and 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 K radiation for each element: