Synchrotron Radiation

Type of physical science: Condensed matter physics

Field of study: Surfaces

Synchrotron radiation is the electromagnetic radiation emitted by charged particles when their paths are bent from a straight line. It is a source of x-rays and ultraviolet light, which are used for microcharacterization of materials.

Overview

Synchrotron radiation is emitted by a charged particle when the path of that particle is bent from a straight line. James Clerk Maxwell, a nineteenth-century English physicist, predicted that any charged particle experiencing accelerated motion would emit electromagnetic radiation.

Electromagnetic radiation carries energy and momentum away from the charged particle that produced it and propagates outward. If the particle accelerates along a straight path, such as electrons moving in a linear antenna wire, the electromagnetic waves will propagate outward in a dipole radiation pattern. This principle is used for radio transmission. When a charged particle is forced to move in a circular path, it experiences a continuous acceleration, called centripetal acceleration, which is directed toward the center of the circle. Because it is constantly accelerating, the particle will radiate electromagnetic energy continuously as it moves around this circular path. If the speed of the particle is low, the radiation is emitted into a fan-shaped pattern that is tangential to the circle of motion of the particle. If the particle approaches the speed of light, however, the emission pattern is distorted by relativistic effects and the radiation is concentrated into a cone facing forward along the instantaneous direction of the particle motion.

The frequency of the emitted radiation also varies with the speed of the particle. At low velocities, most of the radiation is concentrated at the orbital frequency, although a small amount of energy is emitted at frequencies that are integer multiples of the orbital frequency. As the particle velocity increases, more energy goes into these higher-frequency or shorter-wavelength harmonics. Thus, with increasing particle speed, the peak of the emitted radiation moves from the infrared to the visible ultraviolet and then to the x-ray region of the electromagnetic spectrum. As the speed of the particle approaches the speed of light, the orbital frequency decreases rapidly, and the harmonics are so closely spaced that the frequency distribution can be considered to be continuous.

In 1912, G. A. Schott described the properties of the electrons orbiting the nucleus of an atom and established the basic features of synchrotron radiation. A complete analytical treatment of synchrotron radiation was developed by Julian Seymour Schwinger in 1945. Schwinger calculated the rate at which energy would be emitted, the detailed frequency spectrum, and polarization properties of the radiation. Using circular particle accelerators such as cyclotrons and synchrotrons, scientists attempted to detect synchrotron radiation. In these devices, particles are confined to circular or near-circular orbits by magnets that bend their paths. As the particle path is bent, the centripetal acceleration gives rise to synchrotron radiation. In 1946, John P. Blewett detected synchrotron radiation for the first time, in the form of emissions from the bending magnets of the General Electric Research Laboratories' electron accelerator. Synchrotron radiation was observed visually a year later by Glen Elder, also using the General Electric accelerator. In 1948, Elder verified Schwinger's predictions of the polarization and frequency spectrum of the emitted radiation.

Initially, synchrotron radiation was regarded as a hindrance by particle physicists, who were interested in obtaining a beam of maximum energy from their accelerators. Synchrotron radiation was the mechanism by which particles being accelerated in these circular particle accelerators radiated away some of the energy being added by the machine. In 1956, however, Diron Tomboulian and Paul Hartmen demonstrated that the synchrotron radiation from the Cornell 320-megaelectronvolt electron synchrotron could be used as an intense source of ultraviolet light. By the early 1960s, researchers around the world had become interested in the use of synchrotron radiation as a source of ultraviolet and, from the higher-energy accelerators, x-ray photons. Synchrotron radiation from existing particle accelerators was made available to these researchers, and plans were made to convert some older electron synchrotrons that had outlived their usefulness as particle accelerators into sources of synchrotron radiation.

Synchrotron radiation has several unique properties that make it superior to other sources of ultraviolet rays or x-rays for many experiments. It is emitted in a continuous spectrum from the infrared into the x-ray energy range; it is highly polarized, with its electric field vector lying in the plane of the electron orbit, allowing primary synchrotron radiation to be distinguished by its polarization from radiation emitted by interaction with a sample; it is emitted in discrete bursts, since the electrons orbiting in the synchrotron are clustered; it is highly collimated in the vertical plane, making the beam quite intense; and it is emitted in a high-vacuum storage ring, so that it can be provided to an apparatus requiring a high vacuum. Each of these characteristics has proved useful in the applications of synchrotron radiation to the investigation of the physical properties of materials, including their geometric and electronic structures, elemental compositions, and optical characteristics.

As the demand for access to synchrotron radiation increased and the researchers recognized that many of the characteristics of the radiation could be tailored to particular experiments, the first accelerators and storage rings designed to be synchrotron-radiation sources were constructed. In the United States, the major national facility is the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory on Long Island. The NSLS includes two electron storage rings, one intermediate-energy ring optimized to emit ultraviolet radiation and one high-energy ring optimized to emit x-rays.

Although the first source of synchrotron radiation was the bending magnets that confined electrons to circular paths in accelerators and storage rings, the need for even higher-energy x-rays led to the use of other magnets, called wigglers and undulators. These wiggler and undulator magnets are inserted in the straight, magnetic-field-free regions of storage rings to force rapid deviations in the path of the electron beam. The acceleration produced by wigglers and undulators is much greater than that from bending magnets, creating intense pulses of x-rays of higher energy than can be achieved with the bending magnets.

Applications

The availability of intense beams of ultraviolet light and x-rays from synchrotron radiation opened up a wide variety of applications in many disciplines, including materials science, physics, biology, geology, planetary science, and medicine. Experiments that had previously been difficult or impossible because of limited source intensity quickly became common.

In the late 1960s, synchrotron-radiation sources were employed to study the interaction of atoms, molecules, and solids with ultraviolet and x-rays. X-rays are absorbed by matter primarily via the photoelectric effect, in which a bound electron is ejected from an atom when hit by an incident x-ray. In the Bohr model of the atom, electrons orbit the nucleus in a series of permissible energy levels, the lowest of which is called the K-shell, followed by the L-shell, and so on. Once an electron is ejected from a shell through the photoelectric effect, the atom is in an unstable state, having a vacancy in a low-energy shell and being charged as a result of the electron deficiency. The low-energy vacancy is filled by an electron from a higher-energy state that drops down to the vacant lower-energy state. In the process, the electron must emit energy in the form of a photon. The energy of this photon corresponds to the energy difference between the two shells. This emission process is called fluorescence.

The emission of fluorescence photons, generally in the x-ray energy range, makes possible the determination of the elemental composition of a sample. Since the energy difference between the shells varies from element to element, the elemental composition of a sample can be determined by measuring the number and energy of the fluorescence x-rays from that sample. This analysis technique, known as x-ray fluorescence, uses an incoming beam of electrons to knock out the bound electrons in the sample. The incident electrons decelerate in the sample and emit x-rays of their own, a process called bremsstrahlung.

This background of x-rays from the decelerating electrons limits the sensitivity of this technique to elements present at the 0.1 percent level. If an x-ray beam is substituted for the incident electron beam, the background radiation is eliminated, since the x-ray beam is not charged. Using the x-ray beams available from synchrotron-radiation sources, researchers have detected trace elements present at a concentration of ten parts per billion in particles as small as ten billionths of a gram.

The photoelectric emission process also provides a tool to determine the structure of molecules using extended x-ray absorption fine structure (EXAFS), a type of x-ray absorption spectroscopy. Originally demonstrated in the 1930s, EXAFS came into practical use only with the development of intense x-ray sources. As the energy of an incident x-ray beam increases, a sharp increase in the absorption of x-rays by the sample is seen at the energy required to eject electrons from each shell. There is no structure in the absorption for single atoms such as krypton gas; when samples of diatomic molecules such as bromine gas are studied, however, sinusoidal oscillations in the quantity of x-rays absorbed are observed near the ejection energies. Studying the detailed shape of the fine structure of the absorption as a function of energy provides information about how one atom is bound to another. When applied to solids, EXAFS can provide information on the distance between atoms in the structure.

Synchrotron radiation is used in the fabrication of integrated circuits. For many years, these circuits were fabricated by optical microlithography using ultraviolet light. The ultimate size, and thus speed, of integrated circuits fabricated by this technique was limited by the wavelength of the ultraviolet radiation. Using x-rays from synchrotron sources, industrial researchers were able to fabricate much smaller integrated circuits. X-ray lithography using synchrotron radiation is favored over other techniques from both a technical and an economic point of view.

Synchrotron-radiation sources have been used in a variety of microscopy experiments, in which an x-ray beam is substituted for the optical beam in a traditional microscope. Because the wavelength of an x-ray is much smaller than that of visible light, an x-ray microscope can have much greater resolution than an optical microscope. When combined with either x-ray absorption or fluorescence techniques, an x-ray microscope makes it possible to map the distribution of a particular element or elements in the sample. Although the resolution of the synchrotron x-ray microscope is comparable to that of a transmission electron microscope, the synchrotron x-ray microscope can analyze samples in air, whereas the transmission electron microscope sample chamber must be held under high vacuum, which causes many biological specimens to dehydrate and alter in form. Synchrotron radiation can also provide the intense, collimated source of monochromatic, single-energy x-rays required for x-ray diffraction and scattering experiments, which are used to identify the geometrical structure and atomic spacing of crystalline samples.

In the mid-1990s, synchrotron radiation began to be used in medical contexts as a source of x-rays for phase-contrast imaging. Regular x-ray imaging works by registering variations in x-ray attenuation due to the density, thickness, and composition of the subject. However, due to the danger of subjecting living tissue to high x-ray doses, the contrast may not be sufficient to register small but significant variations. Phase-contrast imaging addresses this problem by measuring not only attenuation but also the phase shift of the x-rays, which for most materials is a far greater change, often as much as one thousand times larger. There are various phase-contrast imaging techniques available, with three of the most popular being propagation-based imaging, in which the sample is directly irradiated and the resulting change in wave intensity is measured by a detector placed some distance behind the sample; analyzer-based imaging, in which the radiation is diffracted by a crystal called a monochromator before being directed at the sample, then filtered through an analyzer crystal prior to reaching the detector; and grating-based Talbot interferometry, which uses two diffraction gratings between the sample and the detector to generate contrast.

Context

The radiation emitted by charged particles moving in a circular path is called synchrotron radiation, since it initially was studied in association with particle accelerators such as synchrotrons. In these machines, strong magnets are used to bend the path of high-energy particles into circular paths. As the particles pass through each bending magnet, they experience an acceleration and consequently emit radiation. The synchrotron was designed to add energy to the particles in order to replace the energy that was radiated away. Synchrotron radiation imposes an upper limit on the energy to which particles can be accelerated in any given particle accelerator, since an equilibrium is reached between the rate at which energy can be added by the accelerator and the rate at which it is radiated away by synchrotron radiation.

By the early 1960s, it was recognized that the unique properties of synchrotron radiation made it an ideal source of intense ultraviolet and x-ray photons. Synchrotron radiation was employed in a wide variety of experiments for the determination of the electronic and geometrical structures, chemical compositions, and optical properties of materials. The small beam size made synchrotron radiation particularly suitable for experiments with microscopic samples.

The first synchrotron-radiation facilities were parasitic, in that they were grafted onto particle accelerators constructed principally to produce beams of high-energy electrons for particle-physics experiments. The rapid rise in applications for the synchrotron light resulted in the design and construction of facilities optimized for the output of synchrotron radiation. By 1986, dedicated synchrotron-radiation facilities had been constructed in China, Great Britain, France, Germany, Japan, Sweden, the United States, and the Soviet Union.

Principal terms

ELECTROMAGNETIC RADIATION: a propagating wave consisting of an electric field and a perpendicular magnetic field, such as light or radio waves

ORBITAL FREQUENCY: the number of revolutions completed in a unit of time

PHOTON: the basic quantum of electromagnetic radiation; has zero rest mass and an energy proportional to its frequency

ULTRAVIOLET LIGHT: electromagnetic radiation in the wavelength range from 4 x 10^-7 meters to 2 x 10^-8 meters, occurring between visible light and x-rays

X-RAYS: electromagnetic radiation in the wavelength range between 2 x 10^-8 meters and 4 x 10^-12 meters

Bibliography

Bienenstock, Arthur, and Herman Winick. "Synchrotron Radiation Research: An Overview." Physics Today 36.6 (1983): 11–18. Print. A review of how synchrotron radiation is generated and its uses as a research tool. Readable by nonspecialists and extensively annotated to reference original sources for the various research areas described.

Diemoz, P. C., A. Bravin, and P. Coan. "Theoretical Comparison of Three X-Ray Phase-Contrast Imaging Techniques: Propagation-Based Imaging, Analyzer-Based Imaging and Grating Interferometry." Optics Express 20.3 (2012): 2789–805. Print.

Owens, Alan. "Synchrotron Light Sources and Radiation Detector Metrology." Nuclear Instruments and Methods in Physics Research A 695 (2012): 1–12. Print.

Rowe, Ednor M., and John H. Weaver. "Synchrotron Radiation." Physics Teacher 15.5 (1977): 268–74. Print. An elementary discussion of the process of synchrotron radiation and a description of its uses. Well illustrated with photographs and diagrams of the Synchrotron Radiation Center at the University of Wisconsin.

Sparks, Cullie J., Jr. "Research with X-Rays." Physics Today 34.5 (1981): 40–49. Print. A well-illustrated description, suitable for general audiences, of the variety of experiments on the geometric and electronic structure of matter that have been performed using synchrotron radiation.

Willmott, Philip. An Introduction to Synchrotron Radiation: Techniques and Applications. Chichester: Wiley, 2011. Print.

Winick, Herman, and Sebastian Doniach. Synchrotron Radiation Research. New York: Plenum, 1980. Print. An extensive discussion of the techniques by which synchrotron radiation is generated and its applications to different areas of research.

Winick, Herman, et al. "Wiggler and Undulator Magnets." Physics Today 34.5 (1981): 50–63. Print. A detailed account of the design and capabilities of the wiggler and undulator magnets used to extend the spectral range and increase the brightness of synchrotron radiation sources.

Centrifugal/Centripetal and Coriolis Accelerations

Electron Emission from Surfaces

Radiation: Interactions with Matter

Optical Properties of Solids

Storage Rings and Colliders