KJ3055-Chapter 8: X-RAY SPECTROMETRY 
Total Internal Reflection X-ray Fluorescence Spectrometry (TXRF)


What is Total Internal Reflection ?

The following approach refers to light, but it applies to x-ray as well because both of them are particular forms of electromagnetic radiation.
Let’s first remember what reflection and refraction mean. When a light beam arises to the interface between two media, reflection occurs, i.e. a part of the incident beam is sent back such that incidence and reflection angle are equal.
A part of the light beam goes forward but the propagation deviates from the initial direction due to refraction (fig 1). So, refraction denotes the change of the light beam direction when crossing the interface between two media with different refraction indexes (n). 


 

Light refraction. n is lower in the upper medium is lower.

Refraction is governed by Snell’s law, which shows the relationship between characteristic angles and refraction indexes, as follows:

It is clear that, if the angle increases, the angle increases as well and, for a particular value (denoted), the refracted beam propagates along the interface direction, i.e. = 90o (full line in Fig. 2). From the Snell’s law we have therefore:
 

So,is a constant for a specific couple of media. This particular angle is called the critical angle for total internal reflection. If > ,  >90o and the light beam cannot escape to the second medium, but fully returns to the first one. The optical phenomenon occurring in this case is called total internal reflection. Note the difference between simple reflection and total internal reflection. In the first case the incidence and reflection angles are equal and independent of refraction indexes (Figs.1 and 3). In the second case propagation angles are not equal and depend on refraction indexes according to Snell’s law (Fig. 2). 

 



Total internal reflection.

Fig. 3. Normal (specular) reflection.

Total Internal Reflection in X-ray fluorimetry

In the case of TXRF, the sample is prepared as a very thin solid layer on a flat support. The excitation x-ray beam is directed to the sample at a grater angle compared with.Total internal reflection occurs at the support-sample layer interface. Chemical elements in the sample are excited and emit specific x-ray lines. However, no x-ray emission by the support occurs (because the excitation light beam does not penetrate into the support). In addition, no x-ray scattering occurs due to the flat shape of the support surface. Consequently, the background radiation arising to the detector is very low and this allows a very low detection limit to be achieved. 

F.G. Banica, 2009-03-18