What is XRF and How Does XRF Work?
Bowman Analytics X-ray products utilize X-ray Fluorescence technology for material thickness and composition analysis. X-ray was discovered by German physicist Wilhelm Rontgen in 1895. He called the unknown light source that caused his film to expose “X-ray”. He published his discovery with an X-ray image of his hand. Now it’s known that X-ray is a form of electromagnetic radiation, and that its frequency is higher than ultraviolet but lower than gamma rays. Most X-rays have a wavelength of from 0.01 to 10 nanometers, as shown in figure 1, with frequency arranged from low to high.
1845 – 1923
Wilhelm Conrad Roentgen was a German physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range known as X-rays or Roentgen rays, an achievement that earned him the first Nobel Prize in Physics in 1901.
X-ray can also be defined as a particle (Photon), and is described using an energy unit eV. Energy unit and wavelength unit are interchangeable. Therefore, X-ray is both a wave and a particle. This is an important concept in understanding some properties of X-rays. X-rays can be generated by Bremsstrahlung, which is the deflection of an electron or another charged particle. Inside an X-ray tube electrons are accelerated to the target material. Upon impact the kinetic energy of the electrons is transferred into X-rays and heat. The process of X-ray generation is very inefficient. The majority of the electron energy is converted to heat instead of X-rays.
It’s interesting to note that the wave properties cannot explain the photoelectric effect. Albert Einstein and Max Planck proposed that light did not behave like a wave, but rather like discrete “packets” with a specific energy content. Years later, American chemist Gilbert Lewis named the light packets photons. But people remained skeptical about Einstein’s theory until 1923, when American physicist Arthur Compton discovered X-ray scattering. He bombarded graphite with X-rays and found that the scattering X-ray has less energy. The phenomenon was named Compton scattering, which can only be explained by the Einstein-Planck theory. During collision with an electron, a particle like an X-ray transfers part of it’s momentum to the electron, and as a result the X-ray is deflected in a different direction and emitted with less energy and a different wavelength.
While the Einstein-Planck theory works to explain Compton scatter, there is one problem. To possess momentum a photon must have mass, because the definition of momentum in classical physics is mass times velocity. But a photon has no mass. The answer to that came from Einstein. He postulated that in the fundamental sense energy and mass are equivalent and interchangeable, and formulated his concept into the famous relationship, E=MC2. Years later Einstein received the Nobel physics prize for his photoelectric theory.
X-ray fluorescence is related to photoelectric interaction. When photoelectric interaction happens, an electron is knocked out from its orbit and leaves a vacancy. Electrons from higher energy orbits can jump down to fill the vacancy. The energy difference between the two orbits is released as fluorescence X-rays, i.e. secondary X-rays. Fluorescence X-ray from each element has a signature energy, and is called characteristic X-ray.
1845 – 1923
Henry Gwyn Jeffreys Moseley known as Harry Moseley was an English physicist. Moseley’s outstanding contribution to the science of physics was the justification from physical laws of the previous empirical and chemical concept of the atomic number.
What is XRF Used For?
Bowman Analytics X-ray unit utilizes the X-ray fluorescence principle to measure thickness and composition. The sample is bombard with X-rays from an X-ray tube, and generates fluorescence X-rays. By measuring the energy of the fluorescence X-rays we can tell what elements are present in the sample, and by counting the photons of the fluorescence X-rays we can determine the thickness of the sample and the composition of each element.
X-ray is invisible to the human eye. A device called an X-ray detector is needed to convert X-rays into a signal that it can be measured and processed, so that we can extract information from the X-rays. The detector, in conjunction with electronics, converts X-rays into an electrical signal (digital pulse processor). There are three main criteria for detector properties; energy resolution, detection efficiency, and robustness. Energy resolution is the ability of separating two photons with a small energy difference. Detection efficiency means the efficiency of recording x-rays. All Bowman Analytics X-ray units use the latest technology solid state detector called a Silicon PIN Detector or a Silicon Drifted Detector.
Once XRF is collected from a sample, software is used to convert X-ray intensity to thickness or composition. The software consists of two parts; spectrum process and quantitative analysis. The function of spectrum processing software is to reliably extract X-ray intensities from a spectrum. This involves tasks such as energy calibration, spectrum stabilizing, peak identification, dead-time correction, sum peak correction, escape peak correction, overlap correction, and background removal.
Quantitative analysis software calculates thickness and composition from XRF intensities. Due to the matrix effect, there is no simple relationship between intensities and thickness/composition. The matrix effect is an inter-element or inter-layer effect. The fluorescence X-rays from one element could either be absorbed by or enhanced by other elements in the sample. Therefore, the relationship of composition/thickness to fluorescence X-ray intensities of one element depends on other elements that exist in the material. It depends on what those elements are, and how much of each of those elements is present.
There are two ways to perform quantitative analysis; the empirical method (Emp) and the FP method. Empirical methods, such as interference coefficients method, alpha coefficients method, etc., approximate the matrix effects with a polynomial function. The method requires that multiple standards within a narrow range be used in a calibration. The advantage is that this method does not require sophisticated calculation, and is easy to understand and implement.
The FP method corrects for matrix effect through a theoretical calculation. The calculation is based on laws of physics and fundamental physics parameters. In theory, FP does not require any calibrations and can work over a large range. However in reality, a calibration is still needed to minimize the errors in physical parameters and measurement system uncertainties, The algorithm for FP was published in the 1970s, and differences between various FP software systems are not significant. An FP cal is more complicated than an empirical cal and requires more computing power, which is readily available nowadays.
Bowman Analytics employs both methods (Emp and FP) on the Bowman Xray System software platform.