Section 6.2: X-ray Fluorescence Data
Identify the structure and general composition of an atom.
Understand the process of X-Ray Fluorescence (XRF) and how data can be represented in different graphs depending on the type of XRF technique.
De-excite / Electron Orbitals / Excite / Fluorescence / Photon / Planetary Model / Primary Peak / Quantum Model / Secondary Peak / X-ray Fluorescence / X-ray Line Scan / XRF Graph / XRF Line Scan Graph
See content or Module Glossary for definitions
What Does an Atom Look Like?
The Canadian Light Source (CLS) will be analyzing the tree core’s chemical composition through X-ray fluorescence (XRF). Before we can dive into XRF, we first need to understand how atoms are constructed. Scientists have debated for centuries how atoms are constructed, coming up with different models that would better explain atoms than the model before it (see Figure 6).
As the previous Figure indicates, scientists currently agree upon and work with the Quantum Model as it is believed to be the most accurate depiction of how an atom looks. For some applications, however, scientists will still use the Planetary Model to help visualize electron orbitals. Electron orbitals are paths or directions electrons move as they revolve around the nucleus (center of the atom composed of protons & neutrons). In the Planetary Model, the electron orbitals are depicted as circles. When we discuss the techniques used at the CLS, we will be using the Planetary Model.
What is X-ray Fluorescence?
X-ray fluorescence (XRF) is an analysis technique in science and one that the Research Team will be using on IDEAS with the TREE program. This technique starts by shining X-rays on a sample. X-rays are part of the electromagnetic spectrum and have relatively high energy (see Module 5 for more information). When X-rays come into contact with the atoms that make up the sample, three things can happen: the X-ray can pass through the sample (transmit), the X-ray can bounce off of the sample (scatter), or the X-ray can interact with the sample (absorb). XRF measures what happens when the X-ray interacts with the atoms in the sample.
Fluorescence happens when an X-ray interacts with the atoms in the sample (see Figure 8). The process of fluorescence with X-rays is as follows:
An X-ray from the beamline, labelled the incident X-ray, hits the sample.
The incident X-ray causes the electrons in the atom to gain energy and the electron either gets promoted to a higher energy level (orbitals further away from the center which is termed excited) or gets kicked out of the atom all together. This leaves a gap in that orbital.
An electron from a higher energy shell then loses energy, or de-excites, in order to drop down and fill the gap as atoms do not like to be in excited states. The energy that the electron loses is in the form of another X-ray, namely the fluoresced X-ray or photon.
The fluoresced X-ray detected by a detector in the IDEAS beamline which is able to count the number of X-rays that hits it and what energy those X-rays have. The fluoresced X-rays have different energies than the incident X-rays.
A computer counts the number of X-rays at each specific energy level detected and plots them on a graph, which is then interpreted by the Research Team, teachers, and students (explanation on how to interpret this data in the next sections).
With XRF, we are able to determine the absence or presence of elements in our sample because of the fluoresced X-rays. The fluoresced X-rays have very specific energies which are directly related to what element they come from. For example, the fluoresced X-rays of iron will have a very different energy than the fluoresced X-ray of lead.
Something else to note, is the process of electrons moving from different orbitals can happen from almost any orbital (not just ones above or below it; look back at Figure 7 and 8). For example, the electron that becomes excited might come from the innermost orbital or it might come from the second or third or so on. The same thing goes for the electron that falls down to fill the hole; it might come from any orbital that is a higher energy than the hole that is being filled. This means that each element will give off different but highly specific number of fluorescent X-rays at different energies which helps identify the element.
When we look at the spectra or data in the graph, each element has a unique fluorescence pattern much like fingerprints are unique for humans. Each element will have a primary peak (shown using iron as an example in Figure 9) which is the largest peak from the most likely electron transition and uses less energy to make this transition occur than the secondary peak. The secondary peak (Iron 2 in Figure 9) of an element is the next largest peak from the second most likely electron transition which will require a bit more energy to make this transition occur. The secondary peak is not always seen in the graph as it may be overlapped with another elemental peak and that is important to be aware of when interpreting data. However, the energy of the fluorescent X-rays for every element have already been calculated and is recorded in the XRF Data booklet. Researchers and students alike can identify the fluorescent X-rays from their sample to identify what elements are present in their sample.
What is an XRF Graph?
When data is returned to you, it will come in the form of an XRF graph, also called spectra (as shown in Figure 9). An XRF spectra is a plot of the intensity (the number of X-rays or photons) counted vs their energy. The height of the peaks is related to the number of the X-rays that hit our detector (called photon count) at that specific energy. Since we know the energies of the fluorescent X-rays from every known element, we are able to read these graphs by identifying the peaks as belonging to a specific element. XRF graphs will be used in TREE soil samples.
What is an XRF Line Scan?
For the cores, an XRF line scan is used. This scan takes an XRF measurement at specified increments along a straight line. For our use, we set the IDEAS beamline to take an XRF measurement at every tree ring along our entire tree core sample which the tree ring measurements have been provided by the MAD Lab. Since the XRF technique shows the absence or presence of elements in a sample, we can determine the elements present in each tree ring. We also know that each tree ring is formed in a single year. This means that we are able to see how the tree's chemistry changes with time.
With an XRF line scan graph, the data we see on the graph looks different. The XRF line scan graph is now plotted as the photon count of a single element vs the position along the tree core. Figure 10 shows a generic graph of XRF data using the line scan technique. As you can see, the different coloured lines indicate a different element. For your data, we have separated the elements into different graphs, and we have already changed the x-axis to display the years instead of physical position. Section 6.3 will cover more on how to read your specific data.
Additional Resources for Section 6.2
Graphing XRF & XANES: https://bit.ly/2LxE5ls
A webpage on the CLS Education pages of the CLS website that provides a basic overview of the process of XRF.
How Does it All Work? A Summary of the IDEAS Beamline at the CLS: https://bit.ly/35fprW8
A PowerPoint created by the Research Team that covers what the IDEAS beamline is and what it is capable of. For the advanced version, go to this link: https://bit.ly/2Hj7xJZ.