What Is Synchrotron Radiation?

Synchrotron radiation is electromagnetic radiation emitted when charged particles, typically electrons, are accelerated radially as they travel along a curved path at relativistic velocities. The emission follows directly from Maxwell's equations: any accelerating charge radiates, and centripetal acceleration in a magnetic bending field causes intense, highly collimated radiation spanning a broad spectral range from the infrared through the visible and ultraviolet into the hard X-ray region. Discovered inadvertently in 1947 at the General Electric Research Laboratory in Schenectady, New York, synchrotron radiation was initially regarded as an energy loss nuisance in particle physics accelerators. It was subsequently recognized as an exceptionally powerful scientific tool, and dedicated light sources were built starting in the 1960s to exploit its unique properties. The Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC National Accelerator Laboratory is one of the longest-operating dedicated synchrotron facilities, providing beams to researchers across chemistry, materials science, and biology.

The radiation is characterized by high intensity (orders of magnitude brighter than laboratory X-ray tubes), broad tunability, natural collimation, high polarization, and a pulsed time structure derived from the bunched electron beam. These properties collectively enable experiments that cannot be performed with conventional laboratory sources.

Synchrotron Light Sources

A modern synchrotron light source consists of an electron gun, a linear accelerator to boost electrons to injection energy, a booster ring that accelerates them to full energy, and a storage ring in which the electrons circulate for hours, continuously emitting radiation. Dipole bending magnets along the ring produce broadband radiation, while specialized insertion devices called wigglers and undulators, which force the beam through alternating magnetic fields, generate far more intense and spectrally structured beams. Undulators in particular produce quasi-monochromatic radiation at harmonics of a fundamental wavelength determined by the undulator period and the electron beam energy, with peak spectral brightness many orders of magnitude above bending-magnet radiation. The Brookhaven National Synchrotron Light Source II (NSLS-II) is among the world's highest-brightness storage ring sources, with an electron beam emittance below 1 nanometer-radian.

X-ray Properties and Beamlines

X-rays constitute the most widely used portion of the synchrotron spectrum for scientific experiments. Hard X-rays (photon energies roughly 5 keV to 100 keV) penetrate bulk matter and are suited to diffraction, tomography, and fluorescence imaging. Soft X-rays (roughly 100 eV to 5 keV) are surface-sensitive and drive spectroscopic techniques such as X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), which probe the local chemical environment around specific atomic species. Each beamline at a synchrotron facility is optimized for a particular photon energy range and set of techniques, with monochromators, mirrors, and focusing optics tailored to the experiment. The SLAC National Accelerator Laboratory's explanation of synchrotron light sources describes how these beamlines are designed and the range of science they enable.

Biomedical and Scientific Applications

Synchrotron X-ray crystallography has determined the three-dimensional structures of tens of thousands of proteins, viruses, and enzyme complexes, including the ribosomes and many drug targets central to modern pharmacology. The anti-influenza drug oseltamivir (Tamiflu) and a large fraction of other structure-based drug candidates were developed using protein crystal structures solved at synchrotron beamlines. Beyond structural biology, synchrotron radiation supports materials characterization, environmental science, archaeology, and paleontology.

Applications

Synchrotron radiation has applications across a wide range of disciplines, including:

• Protein and virus structure determination for drug discovery
• Non-destructive imaging and tomography of materials, fossils, and cultural artifacts
• Semiconductor and thin-film characterization in microelectronics research
• Environmental monitoring through trace-element speciation in soils and water
• Medical imaging research including phase-contrast and fluorescence X-ray techniques