h
n photons from the X-ray source are produced by electron bombardment on an anticathode. Let's use an aluminium anticathode and let's suppose that this anticathode is slightly oxidised. Oxygen atoms of this anticathode will be excited by the electron bombardment exactly in the same way as aluminium atoms. Aluminium hn photons will be supplemented by those from oxygen. The specimen is then excited by two sets of photons and it will in response generate two superimposed spectra. The final spectrum then contains ghost peaks which must be identified so that they are not mixed up with other energy levels or chemical contributions. Oxygen is usually not the only contamination source in an X-ray source. X-ray sources are frequently twin anode sources, as a consequence there may be a cross contamination between anodes.
L'oxygène n'est pas la seule source de pollution sur une source RX. Ces
sources sont souvent à doubles anodes "twin anode" et les anodes
peuvent se polluer entre elles.
The following table summarises the main contamination types that may be encountered
| Contamination |
Al |
Mg |
| Mg |
+233 |
-- |
| Al |
-- |
-233 |
| O |
+961.7 |
+728.7 |
| Cu |
+556.9 |
+323.9 |
In the case of oxygen contamination on a Mg source, each spectrum line will be duplicated at a +961.7 eV distance. The ghost line intensities will be proportional to the contamination intensity. Using a second source is a very valuable method for ghost line identification.
"Ghost" peaks