Synchrotron Light

Synchrotron Light: The Radiation that Went from the Exceptional to the Commonplace

Adapted from the article “Luz síncrotron: a radiação que foi do extraordinário ao cotidiano” by  Harry Westfahl Junior, (Laboratório Nacional de Luz Síncrotron, Brazil), published in Ciência Hoje, originally in Portuguese:  https://cienciahoje.org.br/artigo/luz-sincrotron-a-radiacao-que-foi-do-extraordinario-ao-cotidiano/

Since it was first observed nearly a thousand years ago as a result of the explosion of a dying star in the sky, synchrotron light—radiation emitted by ultra-fast, electrically charged particles—has evolved from a curious and striking phenomenon to one of the most useful scientific tools for studying matter in its minutest details. In Brazil, the Sirius accelerator—a source of synchrotron light—is among the most advanced in the world. Today, applications of this radiation permeate various areas of scientific knowledge.

Imagine observing the sky in the year 1054, as usual. Then – you notice a new star, shining nearly as bright as a full moon. This 'invited star' became visible during the day for almost a month, and at night for nearly two years.

The cosmic phenomenon responsible for this sight, as splendid as it was terrifying, was the collapse of a star about ten times the mass of the Sun, giving rise to what we know today as the Crab Nebula.

The light from this explosion (technically called a supernova)—observed from various points on the planet and documented by Chinese astronomers—represents the first historical record of what we now call synchrotron light.

Synchrotron light is electromagnetic radiation emitted by relativistic charged particles, that is, those traveling at speeds close to the speed of light in a vacuum (300,000 km/s). This radiation is emitted every time these particles (for example, protons and electrons) are deflected from their paths.

These particles can be deflected from their trajectory when they experience forces transverse to their movement—imagine these forces as a side gust of wind while you run.

Synchrotron radiation has been known since the 19th century, thanks to the equations proposed by the British James Clerk Maxwell (1831-1879) and considered jewels of theoretical physics to this day.

According to Maxwell's equations, the explanation for synchrotron light is as follows: transverse forces cause disturbances in the electric field of relativistic charged particles. And these disturbances propagate—as oscillations of the electric and magnetic fields of the particles (i.e., in the form of electromagnetic radiation)—both through the vacuum and through materials.

From the Slow to the Frenetic

Electromagnetic radiation is a general term for oscillations of electric and magnetic fields that travel at the speed of light. It comes in various 'types'—many of which, as we will see, cannot be seen by our eyes.

Electromagnetic radiation is classified according to its frequency (number of oscillations per second). This can vary greatly: from the slower 'waves' (such as radio waves) to those that oscillate 'frenetically,' like gamma rays, emitted by certain radioactive elements.

Between these two extremes are: microwaves, which heat up our last-minute meals and connect our cellphones; infrared ('heat'); and visible light, the light we see, from calm red to vibrant violet.

As we increase the frequency, we enter the area of ultraviolet radiation—invisible to us but harmful to the skin—leading up to X-rays, used in radiography and medical tomography.

A Bolder Idea

Today, we know that, in its last breath, that dying star released fast (relativistic) particles and intense magnetic fields into the cosmos. These phenomena resulted in the celestial spectacle witnessed in 1054.

In the last century, we went beyond understanding the phenomenon of synchrotron light. We did something truly extraordinary: we invented devices capable of accelerating particles to speeds close to that of light. These machines are called particle accelerators.

We started with linear accelerators, in which particles are accelerated along a straight and long 'track' (tube) by oscillating electric fields concentrated in metal cavities along the straight path.

Soon, it was realized that it would be possible to aim for much higher energies, where particles could reach speeds increasingly closer to that of light—this latter being an insurmountable limit of nature for any object with mass.

Next, we ventured into circular accelerators, called cyclotrons, proposed at the end of the 1920s. In these devices, a constant magnetic field keeps the particles in circular motion, giving them a bit more momentum with each pass, aided by electric fields that push them in the direction they are moving (longitudinal acceleration) and synchronized with their spin.

Thus, these particles pass through the same accelerator structure multiple times, describing a spiral that expands as their energy increases.

The invention of cyclotrons was a milestone that transformed the physics of accelerators. However, this advancement was surpassed by even bolder ideas that culminated in the development of synchrotron accelerators.

In cyclotrons, as the particles are accelerated to increasingly higher energies, synchronization with the longitudinal electric fields breaks down. This occurs due to a phenomenon predicted by the theory of relativity by the German-born physicist Albert Einstein (1879-1955): the closer a particle gets to the speed of light, the heavier it becomes. In cyclotrons, this delays the rhythm of their return and causes them to lose sync with longitudinal acceleration.

In synchrotrons, this limitation was overcome through a mechanism in which the different energies of the particles are constantly auto-synchronized with the longitudinal acceleration: particles with higher energy receive less acceleration, while those with lower energy receive more acceleration—this mechanism acts like a spring that seeks to balance the energy of each particle around an average value.

Additionally, as the average energy of the particles increases, the magnetic field is adjusted to keep them on a constant radius trajectory, preventing them from expanding into a spiral, as happens in cyclotrons.

With this invention, it was possible to build gigantic accelerators, achieving ever more impressive energy levels.

Universal bulbs

These marvels of modern technology are not limited to simply accelerating particles. For example, in collider synchrotrons, physicists are capable of recreating, through the collision of particles at very high energies, conditions similar but on a minuscule scale to those of the Big Bang, the process associated with the creation of the universe.

A prime example of this is the Large Hadron Collider—better known by its acronym, LHC—the most powerful synchrotron collider currently. This machine, located at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, creates beams of protons accelerated to extremely high energies.

The collisions between these protons result in a firework of new particles. Observing the results of these collisions allows testing the most astounding theories about the essence of matter.

Protons have a positive electric charge. Therefore, when accelerated to speeds close to the speed of light and forced to alter their trajectories, they emit synchrotron light.

It is worth making a comparison here. The light emitted by the supernova of 1054 has a similar origin: charged particles were deflected from their paths due to super intense magnetic fields, both generated in the explosion.

In the case of collider synchrotrons, like the LHC, all the energy of the particles should be focused on the collisions. Therefore, the emission of synchrotron light in these machines is an undesired side effect.

However, if the interest is to generate this type of radiation, then, it would be logical to design accelerators with a specific purpose: to maximize its emission. These machines are named: synchrotron storage rings.

Instead of protons, these devices accelerate electrons, which are about two thousand times ‘lighter’ than protons—therefore, easier to accelerate, emitting significantly more synchrotron light than protons.

Although storage rings and collider synchrotrons share the same technological base, they handle synchrotron light in different ways: in colliders, the radiation is an unwanted byproduct; in rings, collisions are avoided, and synchrotron light is the sought-after treasure.

The synchrotron storage rings (or simply, synchrotrons, for our purposes here) are like ‘universal lamps’ of high intensity: they illuminate with various types of light (from infrared to gamma rays) and with a concentrated power that would make even the Sun envious.

The Dance of the Electrons

The light produced by a synchrotron is sent to devices that physicists call beamlines—which, due to their size and complexity, resemble more of telescopes.

In the beamlines, the radiation is initially filtered based on its ‘colors’ (frequencies)—technically, it is called monochromatic. Then, the synchrotron light is focused on samples of materials that need to be studied.

This interaction generates three-dimensional (3D) images of very high resolution—here, we are exploring the fact that this radiation propagates through the material.

The secret to the contrast of these images lies in how the synchrotron light makes the electrons in the materials ‘dance’. These particles absorb the radiation's energy and, as they ‘dance’, return this energy in a huge variety of phenomena that can be directly monitored.

In the beamlines, this subatomic dance is captured by sensors ‘tuned’ to detect specific ‘colors’ (frequencies) emitted by these electrons. With this, we can extract a multitude of information about how these particles are distributed in the materials and, from there, infer where the properties of the studied sample come from.

Each ‘color’ we capture on the detectors tells us a different story about the electrons and their roles in the materials. In the realm of infrared (heat), we spy on the atoms of the sample as if they were stretching and shrinking their chemical bonds. This type of dance reveals the types of chemical bonds that intertwine the atoms and in what quantity they manifest.

When we delve into the range of ultraviolet and low-energy X-rays, we start to snoop on the ‘jumps’ that electrons, upon absorbing this type of radiation, make from a less energetic orbit to a more energetic one. With this, we learn about other electrons in the ‘neighborhood’ and the patterns of how this electronic population moves.

Yet, synchrotron light in the range of X-rays or even more energetic can penetrate deeply into the material—sometimes, because of the energy it carries, this radiation even manages to knock electrons off their paths, generating a cascade of secondary radiation emission phenomena.

These phenomena—foundations of so-called X-ray spectroscopies—behave like an ‘echo’ that allows us to ‘hear’ the deepest stories that materials have to tell us.

All this allows us to see inside the materials with revealing transparency—a true tomography of the studied material, only millions of times more detailed and with many more ‘colors’.

Moreover, when X-rays pass through materials with a well-organized atomic structure (crystalline), a typical optical phenomenon occurs (interference), which, for physicists, acts as an 'atomic ruler,' that is, something capable of measuring the distance between the atoms of the material and, consequently, locating them with extraordinary precision.

Across the World – and in Brazil

At this moment, there are just over 30 synchrotron light sources in operation worldwide. In the Southern Hemisphere, Australia and Brazil have these machines—with Latin America proudly being a unique case.

Almost all these facilities operate administratively in a very similar manner: they are multi-user laboratories open for research by local scientific communities. They also coincide in their scientific operation: the storage ring simultaneously sends synchrotron light to various beamlines, 24 hours a day.

At these beamlines, researchers from various geographic regions bring their samples for analysis and test not only hypotheses about how materials function at a microscopic level but also investigate their potential applications in physics, engineering, biology, materials science, archaeology, paleontology, chemistry, environmental sciences, etc.

Synchrotrons are convergence environments for so many scientific disciplines that they resemble the Olympic events of science, playing a vital role in the global scientific infrastructure.

Brazil, having ventured on this journey since the 1980s, entered this exclusive club, emerging as one of its most prominent members. With Sirius, our state-of-the-art 'jewel' financed with federal resources, we are at the forefront of this exploration.

Our team of researchers can be compared to explorers of unknown frontiers, revealing not only what materials are but also what they could become.

As Fine as a Hair

As with any technological advance, synchrotrons progress through generations—the Sirius currently belongs to the most advanced of them. One characteristic that makes it part of this elite technological group has to do with the diameter of the beam of relativistic electrons generated by these machines, as this is crucial for obtaining high-definition images of the matter.

A beam with a wider diameter produces 'blurred' images, which creates difficulties in precisely establishing the details of the sample's structure. In contrast, a finer beam allows electrons in materials to react with greater 'clarity'—or, as physicists say, more coherent synchrotron light generates sharper images.

With each new generation of synchrotrons, we enhance this precision, reducing the size of the beam, on average, by nearly tenfold, improving the resolution of our images and the speed of capturing the radiation returned by the sample.

Currently, in the fourth generation, we have achieved a beam narrower than a strand of hair—thus, in the order of thousandths of a centimeter. This allows us to observe matter with a level of detail never seen before.

Globally, there are only three fourth-generation storage rings. Sirius, at the National Center for Research in Energy and Materials in Campinas (SP), Brazil, is one of them, alongside EBS-ESRF (acronym for Extremely Brilliant Source at the European Synchrotron Radiation Facility) in Grenoble, France, and MAX-IV in Lund, Sweden.

With more projects currently under development worldwide, the competition promises to become even fiercer.

Recipe for a Synchrotron

To assemble a fourth-generation synchrotron, we start with the basics: the building. It needs to be as large as a football stadium, meticulously planned and protected against any vibration.

Around the circular ring of the accelerator, there are thousands of magnets, each custom-made to guide electrons through tubes with a vacuum so perfect it rivals the lunar void.

The mechanical components? They require the precision of a watchmaker and the agility of a microchip manufacturing machine. All this is orchestrated by advanced electronic circuits, responsible for steps ranging from energizing the electrons to capturing the 'response' of the samples in the form of digital data.

And, at the end of this incredible chain, supercomputers run algorithms that translate this symphony of 'colors' into three-dimensional images of materials—so detailed they leave even professionals in the field astonished.

At Sirius, we saw most of this technology developed and manufactured in Brazil, with the participation of leading national companies. And what secrets do these engineering wonders reveal?

In the field of health, the X-ray beams generated by synchrotrons around the world were decisive in deciphering the map of the atomic structure of the proteins of SARS-CoV-2 (the COVID-19 virus), marking a monumental leap towards the development of vaccines.

The data from these studies continue to enrich the Protein Data Bank, a vast global repository of protein structures that has been essential for the invention of new drugs and the development of revolutionary molecules (enzymes) for biotechnology.

In terms of the environment, the three-dimensional chemical images generated by synchrotrons reveal, for example, the traffic of nutrients between the soil and plants. Paleontology has been transformed by the ability of these machines to perform X-ray tomography of the interior of fossils without altering them—in this case, the synchrotrons act like a time machine that preserves while revealing.

In the management of natural resources, high-resolution tomographic images support the technology of so-called digital rocks, which open new perspectives for modeling the extraction of oil—especially that hidden in the tiny pores of pre-salt rocks. This method allows us to spy on the phenomena occurring in the pore dimension of samples maintained under temperature and pressure conditions similar to those found in reservoirs.

And, when it comes to renewable energies, synchrotrons have helped in the search for more efficient solar panels and longer-lasting batteries, based on the analysis of the 'heart' of the materials and their physicochemical processes.

By combining multiple experimental techniques, across different beamlines, synchrotrons allow hypotheses about less understood microscopic mechanisms of light-to-energy conversion to be tested.

Finally, almost a thousand years have passed since the first documentation of the dazzling synchrotron light in the sky to the point where humanity has mastered it through science and technology—this light is here to stay and illuminate our tireless quest for knowledge.

Synchrotrons have become a vital part of the global scientific infrastructure in the research of synthetic and natural materials. And our Sirius shines today as the newest and one of the brightest stars in this constellation.

Driven by curiosity and urgency to solve the world's challenges, this process has transformed the complex into the extraordinary and the extraordinary into the everyday, allowing us to probe the most intimate secrets of matter.