Synchrotron Radiation 101: How Light Sources Work and Their Applications

When a beam of electrons traveling close to light speed is bent away from a straight trajectory, it gives off a special kind of light is called synchrotron radiation.

Created by particle accelerators called synchrotrons, this kind of electromagnetic radiation has proved to be an incredible scientific tool for investigating matter, the universe, and so much more.

In this post, we’re exploring what synchrotron light is, how it’s produced, what it’s used for, and more.

 

How synchrotrons became light sources

First built in the 1940s, synchrotrons were not originally made to produce synchrotron light. As a type of circular particle accelerator, they were intended to study particle collisions and interactions. But it didn’t take scientists long to notice the synchrotron’s byproduct, an extremely bright light.

The first synchrotron to use the "racetrack" design with straight sections, a 300 MeV electron synchrotron at University of Michigan.

Called synchrotron radiation or synchrotron light, it can cover the full electromagnetic spectrum. It’s characterized “by high brightness—many orders of magnitude brighter than conventional sources—and [is highly polarized], tunable, collimated (consisting of almost parallel rays) and concentrated over a small area,” according to IOP.

More specifically, the power radiated from this beam is equal to(q2a2/c3), where a is the acceleration and q is the charge. This equation is called the Larmour formula, and it applies to radiation produced from both bending magnets with a circular trajectory and to undulators/wigglers, where the electrons oscillate back and forth.

(Natural sources of synchrotron radiation also exist in the universe. The Crab Nebula is one. But as you can imagine, it’s not useful for laboratory experiments. Though, it does tell us a lot about the plasma environment of space.)

It took more a decade after that first synchrotron was built, in 1956, for the first dedicated light source experiment to be carried out: an X-ray spectroscopy study by American scientists Diran Tomboulian and Paul Hartman. This happened at Cornell University’s accelerator when synchrotron light was directed off the accelerator ring towards an experimental station.

For more than 20 years, scientists utilized this synchrotron “byproduct” for their own work, while the machines themselves were primarily used for high-energy particle collisions. It was not until 1980 that the first dedicated light source facility was built in the United Kingdom at Daresbury. Today, there are dozens in use around the world.

Synchrotron Radiation Source (SRS), Daresbury Laboratory.

Light source setup

Particle accelerators are what create artificial synchrotron light, but we’re not going to cover those in detail here. (For the full rundown, see our previous post.) Instead, we will give a brief primer on some components unique to dedicated synchrotron radiation facilities.

EPSIM 3D/JF Santarelli, Synchrotron Soleil

Storage rings

These specialized facilities use storage rings to produce synchrotron radiation. Storage rings are exactly what they sound like: circular accelerators where electron beams can be stored and kept moving for many hours.

In fact, storage rings are synchrotrons. Unlike traditional ones that accelerate particles from low to high energies using radiofrequency (RF) cavities, however, these maintain beam energies and the RF cavities only replace energy lost during circulation.

 

Beamlines

As the electrons are circulating at the desired energy, they give off light. The direction this light moves depends on your frame of reference. In the laboratory frame, it travels in the same direction as the beam (with an opening angle of 1/ γ, where gamma is the relativistic time dilation factor); in the electron beam frame, it travels perpendicularly to the beam’s path.

No matter the frame, that light is allowed to escape the storage ring through ports leading to straight beamlines, which then end in experimental stations (pictured above). The available spectrum of this light depends on two things: (1) the beam’s energy and (2) the properties of the magnetic source (i.e. whether a bending magnet or an undulator or wiggler is being used). In practice, this means higher energy electrons allow for shorter wavelengths.

Typically, only a specific wavelength is desired for experimental use. In order to isolate it, a few mechanisms are used to condition the beam as it heads toward an experiment station.

One such tool is the monochromator. It “selects” a single wavelength of electromagnetic radiation with a narrow bandwidth.

For picking out X-rays, a crystal monochromator is used; for UV light, a grating monochromator is used. Slits control the physical width of the beam and the angular spread. Mirrors and lenses are used as focusing elements.

After the light is filtered and focused while traveling down the beamline, it strikes a sample.

 

Samples

“Sample” is a generic way of referring to the thing a scientist is researching using synchrotron light. It could be a crystal whose structure we wish to better understand. It could be some material or object that we want to image in high resolution. The possibilities are numerous, and we’ll explore some research topics further down.  

Using the light source, experimental samples are subjected to varying temperatures, pressures, etc. Detectors nearby record the data from the sample-light interaction and send it to computers for collection and analysis.

 

Types of synchrotron light experiments

What kind of experiments can be done using synchrotron light? Experiments themselves are too numerous to list, but the methods are a little easier to pin down by category. Here are three of the most common.

 

1.    Diffraction

This happens when synchrotron light is diffracted by the sample itself. Waves (in our case, light waves) are spread out as they pass an object or go through an aperture. After interacting with the sample, that light creates an image, called a diffraction pattern.

One can learn more about the sample’s nature by studying this image, e.g. an X-ray image.

 

2.    Spectroscopy

This happens when light is sent through a sample and measurements are taken on the other side to see which wavelengths are absorbed and/or emitted according to the sample’s characteristics.

Spectroscopy gives us a look into the sample’s electrical states or chemical bonds, in addition to its composition.

 

3.    Imaging

Using X-rays as our wavelength in this example, the light penetrates the sample and emerges on the other side, creating a contrast image of the sample’s interior. It’s similar to how doctors view a broken bone inside your arm with a hospital X-ray machine, except much more powerfully and with a much higher spatial resolution thanks to better focusing.

Industry and research applications

From investigating the structure of crystals and proteins to monitoring air pollution, synchrotron radiation is capable of shedding light on the molecular and atomic worlds. Here are a few ways both industry and research use it in practice.

 

Medical and pharmaceutical research

Perhaps one of the best-known applications of synchrotron light is in medical and pharmaceutical research. The high intensity of this light allows for the study of disease mechanisms, high-resolution imaging, and advances in microbiology and cancer radiation therapy.

 

Materials research

One of the advantages of synchrotron light for materials research is its high tunability. Particularly in the X-ray range, scientists can pick and choose exactly what kind of light they want to use in their experiment. This allows for high precision and time-dependent measurements that would be impossible under other circumstances.

Synchrotron radiation reflecting from a terbium crystal at the Daresbury Synchrotron Radiation Source, 1990

In short, synchrotron light sources “reveal the structure, chemical composition, electronic properties, and other features of specimens critical to materials science,” among other disciplines, according to Chemical & Engineering News.

 

Bio and environmental sciences

The same mechanisms that permit a look inside materials also allow researchers to examine macromolecules, proteins, and other structures. Crystallography—the science of crystal structures and properties—is an important application of synchrotron radiation.

But it’s in atmospheric research and clean combustion technologies where applications may be more visible to the public in the years to come.

For example, a 2018 study on auto-oxygenation using synchrotron radiation from Berkeley’s Advanced Light Source provided insight into atmosphere pollution because the chemistry between that and fuel combustion inside an engine is similar. The findings enabled more accurate fuel combustion simulations, but could, ultimately, help improve simulations predicting air pollution and global temperature, according to Phys.org.

 

Synchrotron radiation discoveries and successes

This is just scratching the surface of light source uses. Entire books have been dedicated to the subject. So instead of diving further into the research applications and theory, we’ll look at a couple far-reaching discoveries that apply to the masses. 

 

Giant magnetoresistance

One of the most far-reaching discoveries to come out of synchrotron radiation research is giant magnetoresistance.

This type of quantum mechanical process is observed as a big change in electrical resistance depending on the alignment of ferromagnetic layers in a structure. The main application of this phenomena is in magnetic field sensors, like those used in hard drives and other computer parts.

According to Ennen et al. in a 2016 paper, applications of this technology are “impressively broad, ranging from applications in the air- and space or automotive industry, non-destructive material testing, or the compass functionality in mobile phones to biomedical techniques, like biometric measurements of eyes and biosensors.”

Put simply, the modern technological world as we know it would not be possible without the discovery of giant magnetoresistance.

 

Tamiflu

Another wide-reaching discovery is the anti-flu drug Tamiflu. The structure of one of its active ingredients was determined using synchrotron radiation.

The drug is widely used today and was a go-to treatment for the 2005 H5N1 flu outbreak in Southeast Asia.

 

Synchrotron Nobel Prizes

There have been several Nobel Prizes awarded that depended on synchrotron radiation. One was the 2009 Nobel Prize in Chemistry given to Venkatraman Ramakrishnan, Thomas A. Steitz, and Ada E. Yonath for their work in figuring out the structure of the ribosome.

This research used, in part, X-ray crystallography, a method made possible by synchrotron light. “Venkatraman Ramakrishnan and other researchers were able to collaborate to map the structure of ribosomes, made up of hundreds of thousands of atoms,” according to the Nobel Prize website. “Among other applications, this has been important in the production of antibiotics.”

The Argonne National Laboratory synchrotron and the Stanford Synchrotron Radiation Lightsource helped win the 2012 Nobel Prize in Chemistry for “the structure and functioning of a protein complex on the surface of human cells, called a G-protein-coupled receptor, that receives signals from the cell’s environment and is a key target for drug development,” said Stanford National Accelerator Lab.

 

The future of synchrotron radiation: Free-electron lasers

First-generation synchrotrons were built in the mid-1900s and often couldn’t be iteratively upgraded. Second-generation machines, however, were constructed with the idea of continuous improvement. Many have been improved with successive add-ons over time, boosting their power and refining their capabilities.

Such piecemeal improvement can only take us so far. The third generation of these accelerators and beyond will likely involve entirely new technology and new construction.

The free-electron laser FELIX Radboud University, Netherlands.

One of the possibilities for third-generation machines is the use of free-electron lasers (FELs) as the beam source itself.

Regular lasers make light by jiggling electrons bound inside atoms. FELs make light by using magnets to agitate electrons that are unbound from atoms, a.k.a. “free” (hence the name).

The FELs use the same undulators as those in storage ring-based light sources, but thanks to an electron clumping effect called “micro-bunching,” the resulting synchrotron radiation is orders of magnitude more coherent and intense.

Another important feature is their ability to generate shorter—femtosecond, in this case—pulses with the same intensity in each peak that current synchrotron sources emit in one second. Such pulses can produce X-rays millions of times brighter than today’s most powerful light sources.

This kind of speed, precision, and power would allow researchers to probe matter in new ways.

While that might sound innocuous on its face, it is in fact remarkable because scientists will be able to scrutinize almost unimaginably small, complex structures. With next-generation synchrotron light sources, we have a microscope capable of peering into the very chemical and atomic processes that make up life and the world around us.