Penetration Depths

It is known that X-Rays will penetrate some way into a material. For XRF analysis, the important question is from what depth within the sample does the spectrum arise. Unfortunately this is not a simple question, as there are many factors involved.

The two main points to consider are (a) the depth of penetration of the primary X-Ray beam into the sample, and (b) the escape depth from which fluorescent X-Rays can be detected. Both of these are directly linked to the energy of the X-Rays - the higher the X-Ray energy the deeper the X-Ray penetrates. In general, it is fair to assume that X-Rays will penetrate a few micrometers down to several millimeters, depending on the sample matrix. At best fluorescent X-Rays will be detectable from a few millimeters within the sample, but in many situations this could be reduced to a few micrometers or less.

Penetration of the primary X-Rays

The primary X-Rays should be considered in two parts, both of which are effected by the voltage setting of the X-Ray generator.

First of all, the characteristic X-Rays from the anode target material are at a fixed energy. If the generator voltage is sufficient to excite multiple lines (eg, K and L) then both high energy (K) and low energy (L) X-Rays will be incident on the sample. Usually the K lines will be more intense, and so these will dominate in considerations about penetration. If however, the voltage is reduced to such an extent that the higher energy X-Rays are no longer excited, then the characteristic X-Rays will be low energy L lines only - as a result the expected penetration will be greatly reduced.

Secondly, the bremsstrahlung (or continuum) X-Rays must be considered. As their name suggests, these X-Rays have a continuous energy range (up to a maximum equal to the accelerating voltage of the generator. The continuum spectrum is most intense towards the higher energy cut off - by reducing the voltage it is possible to reduce this "average energy" of the continuum, and thus reduce penetration.

Escape of fluorescent X-Rays

The ability of fluorescent X-Rays to penetrate through and escape from the sample itself depends again on their energy, which directly relates to which elements are being detected. The lighter elements (eg, Na, Mg, Al, Si) have very low energy X-Rays, and thus will be difficult to detect even at relatively small depths within the sample. Heavier elements (eg, Cu, Ag, Au) have much more energetic X-Rays which will be able to pass through large distances within the sample.

Clearly the sample composition itself is also an important factor. The higher the concentration of heavier elements which absorb strongly, the more reduced the chance of X-Rays escaping from deep within the sample.

To summarize, heavy elements (ie, energetic fluorescent X-Rays) will be detectable relatively deep within a sample matrix primarily comprised of light elements (ie, low absorption). Light elements (ie, low energy fluorescent X-Rays) will be detectable only at the surface of a sample matrix comprised of heavy elements (ie, strong absorption).