Synchrotron Radiation Fundamentals

What is synchrotron radiation (SR)?

Synchrotron radiation is the electromagnetic radiation emitted by relativistic charged particles as they travel along a curved trajectory under the influence of an electromagnetic field.

From the definition, it can be seen that the generation of synchrotron radiation or synchrotron light requires the fulfillment of the following three conditions:

(1) There must be charged particles, which may not necessarily be electrons but could also be other charged particles;

(2) The particles must be "relativistic," meaning that they have high energy and travel at speeds close to the speed of light;

(3) The motion of the particles must have an angle with respect to the electromagnetic field.

Meeting these three requirements allows for the generation of synchrotron radiation. It is a form of "radiation," but why is it "synchrotron"? That's because the earliest "artificial" synchrotron radiation was observed on a "synchrotron accelerator" (in the United States in 1947), and later the name stuck. According to the definition of synchrotron radiation, we can speculate that cosmic rays contain a significant amount of "natural" synchrotron radiation.

Figure 1: Schematic diagram of electron deflection in a magnetic field producing synchrotron radiation

Why do we need synchrotron radiation?

1. Synchrotron radiation offers high intensity and brightness. The concept of brightness in a strict sense involves considerations of radial divergence and can be quite complex. Here, we can simply understand brightness as the number of photons per unit area and unit time. Typically, the brightness of a synchrotron radiation source is about 6-10 orders of magnitude higher than that produced by an X-ray tube. In general, the higher the brightness of a light source, the better its signal-to-noise ratio.

Figure 2:Comparison of brightness among some synchrotron light sources in the United States.

Generally, the typical brightness of the beam for XAFS experiments is on the order of 10^14-10^15, with an energy of approximately 10 KeV.

2. Synchrotron radiation provides a continuous tunable spectrum for XAFS experiments, obtaining the absorption coefficient as a function of photon energy. Therefore, during the spectrum acquisition process, it is necessary for the incident photon energy of the absorption spectrum to be continuously tunable over a large energy range. Only synchrotron radiation can maintain high intensity over a wide energy range. In contrast, in laboratory settings, only the brightest X-rays are typically used for experiments. For example, in XRD, Cu targets are commonly used, with characteristic spectral lines at wavelengths: Kα1 (8265.6 eV, 1.54056 Å). For XPS, the X-ray energies corresponding to dual anodes are Mg target Kα1 at 9.8903 Å (1256.3 eV) and Al at 8.34 Å (1486.6 eV). It is often challenging to achieve the required brightness for other X-ray energies in an X-ray tube for experiments.

3. Other characteristics of synchrotron radiation include high collimation, polarization (synchrotron radiation is polarized light), pulse time structure (due to synchrotron radiation typically being generated by electron bunches), and cleanliness (synchrotron radiation is generated under ultra-high vacuum conditions).

4. Combining the unique characteristics of synchrotron radiation, researchers have developed many practical experimental methods (especially X-ray techniques) and have conducted extensive research in various fields using these methods. Currently, typical synchrotron radiation characterization techniques include X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), X-ray analysis of biological macromolecular structures, X-ray absorption fine structure (XAFS), X-ray magnetic circular dichroism (XMCD) technique, X-ray fluorescence analysis (XRF), X-ray imaging techniques, vacuum ultraviolet photoionization mass spectrometry, photoemission techniques, angle-resolved photoelectron spectroscopy (ARPES), high-pressure material structure analysis techniques, and synchrotron radiation micro/nano fabrication techniques (LIGA), among others.

Figure 3: Characterization techniques based on synchrotron radiation

Generations and Basic Construction of Synchrotron Radiation Facilities

Devices capable of generating synchrotron radiation are called synchrotron radiation facilities or synchrotron light sources. Since the discovery of synchrotron radiation, theoretical research has been conducted, and synchrotron radiation facilities have been designed and built to obtain high-quality and stable synchrotron light. By the 1970s, synchrotron light sources gradually began to be put into practical use. Things are constantly changing and developing, undergoing upgrades and replacements, and synchrotron light sources also have their own "generations."

Up to now, synchrotron light sources can be divided into the following four generations:

The first generation consists of multipurpose sources primarily used for high-energy physics experiments, which can be storage rings or synchrotron accelerators. Examples include the CHESS light source at Cornell University in the United States and the BSRF synchrotron radiation facility in Beijing. BSRF relies on the Beijing Electron-Positron Collider and operates in a dedicated synchrotron radiation mode part of the time. In this mode, its overall performance roughly reaches the level of second-generation light sources.

The second generation comprises dedicated synchrotron light sources, typically utilizing bending magnets to generate synchrotron radiation. These are electron storage rings with usually lower energies. Examples include the NSLS light source at Brookhaven National Laboratory in the United States (800MeV), the LNLS light source at the Brazilian Synchrotron Light Laboratory (1.37GeV), and the NSRL light source at the National Synchrotron Radiation Laboratory in Hefei, China (800MeV). NSRL is suitable for research in the soft X-ray and vacuum ultraviolet bands and can be extended to longer wavelengths in the infrared and far-infrared bands.

The third generation also consists of dedicated synchrotron light sources, with the main difference from the second generation being higher energy. Examples include the APS light source at Argonne National Laboratory in the United States (7GeV), the ALS light source at Lawrence Berkeley National Laboratory (1.9GeV), the ESRF facility in Europe (6GeV), BESSY II in Germany (1.7GeV), Diamond in the UK (3GeV), SOLEIL in France, SPring-8 in Japan (8GeV), and the SSRF facility in Shanghai (3.5GeV). With the completion of SSRF, mainland China has three synchrotron light sources in simultaneous operation, resulting in a more reasonable layout. Currently, third-generation sources are the mainstream synchrotron light sources worldwide.

The fourth generation is considered to be free-electron laser (FEL) light sources. X-ray FELs not only produce unparalleled high-brightness radiation but also have fully transverse coherence and operate in pulsed mode. Representative FEL light sources include LCLS in the United States and Euro XFEL in Germany. China has also recently proposed plans to build soft XFEL and hard XFEL facilities.

Figure 4: The Development of Synchrotron Radiation Facilities

The construction and maintenance of synchrotron light sources are complex engineering endeavors, involving a large number of components. Taking the typical third-generation light source, the APS, as an example, let's briefly introduce the basic composition of the facility:

Linear Accelerator: Provides initial velocity to electrons.

Booster: Further accelerates the electrons.

Storage Ring: Generates synchrotron radiation after electron injection. Various insertion devices, such as wigglers (the strongest magnetic field and highest total radiation power insertion component) and undulators (characterized by high brightness), are installed in the storage ring.

Beamlines and Experimental Stations: Several beamlines are led out from ports, with experimental stations located at the end of each beamline.

The development of synchrotron radiation theory and experimental techniques has greatly facilitated advancements in various disciplines such as physics, chemistry, biology, materials science, and environmental science. Many significant technological breakthroughs have been achieved on the interdisciplinary platform provided by synchrotron radiation facilities. For instance, researchers like V. Ramakrishnan from the UK, T. Steitz from the US, and A. Yonath from Israel successfully mapped the 3D positions of ribosome atoms using synchrotron radiation X-ray protein crystallography methods, leading to their Nobel Prize in Chemistry in 2009 for their work on the structure and function of ribosomes.

Japanese scientists utilized techniques such as synchrotron radiation inelastic scattering at the Spring-8 facility to study the structure of water and ice, resolving longstanding debates about the properties of water. Synchrotron radiation also plays a crucial role in cosmological research. For example, scientists analyzed the internal structure and mineral composition of comet particles using synchrotron infrared microspectroscopy at the SOLEIL facility, providing clearer insights into the mysteries of the solar system.

In Japan, researchers analyzed the three-dimensional structure and properties of asteroid dust samples returned by the Hayabusa probe using techniques like synchrotron radiation X-ray microtomography (CT), yielding information about the evolution of asteroids. In China, academicians Chen Zhu and Saijuan Chen from Shanghai Ruijin Hospital utilized synchrotron radiation X-ray absorption spectroscopy (XAS) to study the mechanism of As2O3 in leukemia treatment, providing theoretical and practical bases for leukemia treatment.

Academicians Yigong Shi and Ning Yan from Tsinghua University made a series of advancements in research on cell apoptosis and transporter proteins using synchrotron radiation X-ray crystallography and structural analysis of biomolecules. Synchrotron radiation is playing an increasingly important role in life science research. It can be said that synchrotron light sources act as the "lamp of progress" in advancing human technology.