2.7. Secondary Fluorescence

The absorption of an incident X-ray to produce another X-ray produces an effect termed secondary fluorescence. Secondary fluorescence produces an excess intensity as in the case of Ni and Fe above. Some elements undergo self-absorption: K-line X-rays may be absorbed by other atoms of same element to produce L-line X-rays. For a given element of interest, any X-rays of energy higher than E_c produced from other elements can cause fluorescence. The energy at which the mass absorption coefficient increases abruptly is the absorption edge and corresponds to E_c exactly.

As a further example, consider a material composed of 1/3 Fe, 1/3 Ni, and 1/3 Co. One would expect about 33.3% of the total X-rays observed to be produced by each element, but instead we see 40% Fe, 28% Ni, and 32% Co. The Fe in the material is fluoresced by Ni-Ka producing Fe-Ka X-rays and absorbing Ni-Ka.

Figure 2.7a. Plot of the X-ray absorption curves for Fe (black), Co (red), and Ni (blue), showing the Ka absorption edges. The locations of the Ka lines for these elements is also shown. Note than Ni-Ka x-rays have sufficient energy to fluoresce Fe-Ka x-rays.

 

Copyright 1997-2003, James H. Wittke

Last update: 01/18/2006 01:47 PM.