How does XRF Work?

To understand how x-ray fluorescence works, a basic understanding of the structure of an atom is necessary. The nucleus of an atom is made up of both positively charged particles called protons and electrically neutral particles called neutrons. Orbiting around the nucleus are negatively charged electrons. Electrons can have different orbits, called shells, which are labeled sequentially starting with K, L, M, N, O, P, etc.

The electrons of the K shell are of the lowest energy; therefore, the bond to the nucleus is the greatest. The electrons of the L shell, M shell, etc. are of higher energy and are therefore not as tightly bound to the nucleus. When an outer shell electron jumps to an inner shell (e.g., an M or N shell electron jumps down to the L shell) less energy is required to maintain that lower energy orbit and thus the left over energy is emitted by the atom as a characteristic x-ray. These are the x-rays which are analysed by the detector within the XRF analyser.

Gamma rays or x-rays with sufficient energy can knock an atom’s electrons out of orbit. This primary exciting radiation is generated within the instrument by an x-ray source, either an x-ray tube or radiation emitted by the natural decay of a radioactive isotope. The source in the instrument is positioned in such a way as to allow the exciting x-rays to fluoresce the sample, but not enter the detector.

When an electron is ejected from its shell, the vacant shell is usually filled by an electron from another shell in a step-wise fashion. For example, when a K shell electron is emitted, an L shell electron jumps into its place and creates a subsequent vacancy in the L shell. Similarly, the L shell vacancy is filled by an M shell electron, with the simultaneous emission of the characteristic L x-ray of that element. This process continues to the outer shells in such a way the when K x-rays are generated, L, M, N (and so on) x-rays are also emitted. This cascading effort does not have to be initiated at the K shell. It can start at the L, M, or higher shells.

X-rays form part of the electromagnetic (EM) spectrum, and have similarities to other forms of EM radiation, such as infra-red and radio waves. Each element in the periodic table has a characteristic x-ray spectrum which is unique, rather like a fingerprint. These unique x-ray energies are measured with a high resolution Si PIN detector that identifies the energy of the incoming signal (which identifies the element), and counts the number of signals occurring at that particular energy (which defines the concentration of the element within the sample). Since each energy represents the presence of a specific element such as chromium (Cr), iron (Fe) or nickel (Ni), the specific element and its percentage concentration within the sample can be calculated by the instrument's computer.

Once the computer has the elemental composition, it may be enhanced to reference an onboard library to give specific information about the sample, such as alloy grades. The information may also be stored for future reference including downloading.