An Overview of XRF Basics
1. Fundamental Principles
1.5 Tube-spectrum Scattering at the Sample Material
The purpose of X-ray fluorescence spectrometry is the qualitative and quantitative determination of the elements in a sample by measuring their characteristic radiation. As the sample is exposed to a beam of X-ray quanta from a tube, a proportion of these X-rays also reach the detector in the form of radiation background as a result of physical scattering processes. While the scattered Bremsstrahlung proportion generally produces a continuous background, the scattered characteristic radiation of the anode material contributes towards the line spectrum. Besides the lines of elements from the sample, the anode material's lines and the scattered Bremsspektrum usually appear as well as a background.
The intensity of the scattering depends on the composition of the sample: for samples that are mainly composed of light elements (light matrix), the proportion of scattered radiation is high. In samples composed mainly of heavy elements (heavy matrix), the scattered proportion is relatively low.
Background and characteristic scattering can be very effectively reduced by inserting a suitable absorption material between tube and sample.
There are two types of scattering whose physical scattering processes differ from each other and are referred to in literature as follows:
Rayleigh scattering = elastic scattering = classic scattering
Compton scattering = inelastic scattering
We will use these terms from now on and elaborate upon the effects of scattered characteristic radiation of the anode material.
Rayleigh scattering
The Rh quanta coming from the tube change their direction in the sample material without losing energy and can thus enter the detector and be measured. The peaks of the anode material (e.g. rhodium) appear in the line spectrum. If the element rhodium in the sample material is to be analysed using an Rh tube then the characteristic radiation coming from the tube must be absorbed by a primary beam filter before it reaches the sample.
Compton scattering
The Rh quanta coming from the tube strike the sample elements' electrons. In this process, some of a quantum's energy is transferred to an electron. The X-ray quantum therefore loses energy. The intensity of the quanta scattered by the Compton effect depends, among other factors, on the tube radiation's angle of incidence to the sample and on the take-off angle of the radiation in the spectrometer. As these angle settings are fixed in a spectrometer, a somewhat wider peak appears on the low-energy side of the appropriate Rh peak. These peaks are called "Compton peaks."