- What are X-rays?
- How does a Synchrotron produce X-rays?
- Structure of a synchrotron
- Elemental analysis
- Atomic structure analysis
X-Rays form part of the electromagnetic spectrum, that is, the range of radiation which also includes visible light. Electromagnetic radiation is composed of of electric and magnetic waves oscillating at right angles to each other. X-rays have a wavelength between 10-9 and 10-12m.
X-rays are basically just another form of light but of a much smaller wavelength than the visible light you can see. Just the same as visible light therefore, X-rays can be thought of as waves or particles (see wave-particle duality). These particles are essentially packets of light, called photons, the brighter the light the more packets you see in a given time.
How is a Synchrotron used to produce X-rays?
A synchrotron is a particle accelerator in the form of a closed ring. Particle accelerators were originally built by physicists to perform experiments on the structure of atoms and the identification of subatomic particles. It was soon realised that a consequence of accelerating charged particles around corners was that they gave off X-ray light of very high energy. Soon other scientists were piggybacking on these accelerators in order to use these X-rays, until eventually facilities were built solely to make use of the X-rays and other electromagnetic radiation produced. Synchrotrons are designed to produce this radiation.
The structure of a synchrotron
There are 4 major components to any synchrotron facility. The injection system, the booster ring, the storage ring and the beamline.
Injection system – Electrons are produced in an electron gun. A cathode is heated up until it reaches a temperature at which electrons are released from its surface, this is called thermionic emission. These electrons are then accelerated by a linear accelerator and passed into the booster ring.
Booster ring – The booster ring is essentially made of two straight sections connected by curves to form an elongated oval. The electrons are accelerated in the straight sections using radio waves to boost their energy, and then bent around the curves using bending magnets. Once the electrons are up to a sufficient energy they are released into the storage ring.
Storage ring – The storage ring looks superficially like a circle but is actually a polygon, containing many short straight sections joined together at an angle. The electrons are accelerated around the corners using large bending magnets and boosted occasionally by radio waves. By bending the electrons around corners and accelerating them, X-rays are produced at a tangent to the ring.
How does accelerating electrons create X-rays? – When charged particles (such as electrons) are in uniform motion in a vacuum they give off electric field lines at a constant rate. If the particle is accelerated this causes it to in effect catch up with some of its previously emitted field lines and creates a distortion in the field. The distortion is electromagnetic radiation and travels away from the particle at the speed of light.
Beamlines – The beamlines are where the energy that comes off the storage ring is used for experiments. First it travels through the optics hutch where mirors and other optical equipment are used to select the wavelength required and focus the beam. The energy then travels into the experimental hutch where the beam interacts with a mounted sample.
The whole system is held under vacuum to stop the electrons losing energy by bouncing off air molecules. The video below is from the Diamond Light Source and shows the production of X-rays in a synchrotron. The red dot represents the electrons and the white lines are the X-rays.
How do we use X-rays to determine the elements present in our sample?
The sample is placed in front of an X-ray beam which interacts with the surface atoms of the sample. If the incoming X-ray photon is of a sufficiently high energy it can displace an electron held in the innermost atomic shell, which is lost to the atmosphere. This is known as the photoelectric effect (wikipedia link). The loss of an electron leaves the atom in an unstable state, so in order to become stable again an electron from an outer shell drops down to fill the gap. The electrons held in shells further away from the nucleus have more energy than those that are closer. Therefore in order to drop down into the inner shell, the outer shell electron must lose some energy. This energy is released from the atom in the form of another X-ray or X-ray photons. These X-ray photons are of energies that are characteristic of each element, thus allowing us to identify which elements are present in a sample, a phenomenon known as X-ray fluorescence (wikipedia link). Fluorescence is simply the release of light by something that has absorbed electromagnetic radiation.
By scanning the image through the beam in a series of lines we can build up a map of where the different elements occur in the sample.
How do we use X-rays to understand more about the atomic structure of elements?
To understand this aspect of the elements in our sample we analyse one element at a time. The energy of the X-rays directed at a sample is varied over a certain range in order to see how much of each energy the atoms of certain elements absorb. From that we get an absorbance spectrum similar to the one below. There are 2 main parts to the spectrum that tell us different things, XANES and EXAFS.
XANES (X-ray Absorption Near Edge Spectroscopy)
This portion of the spectrum tells us about what oxidation state the element is in. Each element has a stable state where it has the right number of electrons (negatively charged) to balance the charge from the protons (positively charged) in the nucleus. If an atom gains electrons on top of this stable state it becomes negatively charged, and if it loses electrons it becomes positively charged. The charge on the atom is known as its oxidation state, and it determines what other atoms it can bond to. The positive protons in the nucleus attract the negative electrons surrounding it. If the atom is negatively charged, i.e, has more electrons than normal, then the nucleus cannot hold them as strongly and it takes less energy to kick one out; however if it is positively charged, and therefore has fewer electrons than normal, then the nucleus is able to hold onto them tighter and it takes more energy to kick one out. It is the sudden absorbance of energy used to remove an electron from the atom that causes the large peak in the XANES region of the spectrum, it is known as the absorption edge.
EXAFS (Extended X-ray Absorption Fine Structure spectroscopy)
This part of the spectrum is generally more complicated and difficult to interpret. It is produced because the electron that is kicked out of the atom during the XANES part has relatively low kinetic energy, which means that it can be deflected by the other atoms surrounding it. It is by interperting the absorption of energy that results from the ejected atom bouncing around and off the adjoining atoms that we can learn what the original atom is bonded to and how long these bonds are. This is known as the atom’s coordination environment.
Image Electromagnetic spectrum by Inductiveload used under the terms of the Creative Commons Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license. Synchrotron image adapted from Synchrotron Soleil by EPSIM 3D/JF Santarelli
