Synchrotron light source

Source: Wikipedia, the free encyclopedia.
Synchrotron radiation reflecting from a terbium crystal at the Daresbury Synchrotron Radiation Source, 1990

A synchrotron light source is a source of

free electron lasers
. These supply the strong magnetic fields perpendicular to the beam that are needed to stimulate the high energy electrons to emit photons.

The major applications of synchrotron light are in

electronic structure to the micrometer and millimeter levels important in medical imaging. An example of a practical industrial application is the manufacturing of microstructures by the LIGA
process.

Synchrotron is one of the most expensive kinds of light source known, but it is practically the only viable luminous source of wide-band radiation in far infrared wavelength range for some applications, such as far-infrared absorption spectrometry.

Spectral brightness

The primary figure of merit used to compare different sources of synchrotron radiation has been referred to as the "brightness", the "brilliance", and the "spectral brightness", with the latter term being recommended as the best choice by the Working Group on Synchrotron Nomenclature.[2] Regardless of the name chosen, the term is a measure of the total flux of photons in a given six-dimensional phase space per unit bandwidth (BW).[3]

The spectral brightness is given by

where is the number of photons per second in the beam, and are the root mean square values for the size of the beam in the axes perpendicular to the beam direction, and are the RMS values for the beam solid angle in the x and y dimensions, and is the relative bandwidth, or spread in beam frequency around the central frequency.[4] The customary value for bandwidth is 0.1%.[2]

Spectral brightness has units of time−1⋅distance−2⋅angle−2⋅(% bandwidth)−1.

Properties of sources

Especially when artificially produced, synchrotron radiation is notable for its:

  • High brilliance, many orders of magnitude more than with X-rays produced in conventional X-ray tubes: 3rd-generation sources typically have a brilliance larger than 1018 photons·s−1·mm−2·mrad−2/(0.1%BW), where 0.1%BW denotes a bandwidth 10−3ω centered around the frequency ω.
  • High level of polarization (linear, elliptical or circular).
  • High collimation, i.e. small angular divergence of the beam.
  • Low emittance, i.e. the product of source cross-section and solid angle of emission is small.
  • Wide tunability in energy/wavelength by
    the megaelectronvolt range
    ).
  • Pulsed
    light emission (pulse durations at or below one nanosecond
    , or a billionth of a second)..

Synchrotron radiation from accelerators

Synchrotron radiation may occur in accelerators either as a nuisance, causing undesired energy loss in particle physics contexts, or as a deliberately produced radiation source for numerous laboratory applications. Electrons are accelerated to high speeds in several stages to achieve a final energy that is typically in the gigaelectronvolt range. The electrons are forced to travel in a closed path by strong magnetic fields. This is similar to a radio antenna, but with the difference that the relativistic speed changes the observed frequency due to the Doppler effect by a factor . Relativistic

Lorentz contraction
bumps the frequency by another factor of , thus multiplying the gigahertz frequency of the resonant cavity that accelerates the electrons into the X-ray range. Another dramatic effect of relativity is that the radiation pattern is distorted from the isotropic dipole pattern expected from non-relativistic theory into an extremely forward-pointing cone of radiation. This makes synchrotron radiation sources the most brilliant known sources of X-rays. The planar acceleration geometry makes the radiation linearly polarized when observed in the orbital plane, and circularly polarized when observed at a small angle to that plane.[citation needed]

The advantages of using synchrotron radiation for spectroscopy and diffraction have been realized by an ever-growing scientific community, beginning in the 1960s and 1970s. In the beginning, accelerators were built for particle physics, and synchrotron radiation was used in "parasitic mode" when bending magnet radiation had to be extracted by drilling extra holes in the beam pipes. The first storage ring commissioned as a synchrotron light source was Tantalus, at the Synchrotron Radiation Center, first operational in 1968.[5] As accelerator synchrotron radiation became more intense and its applications more promising, devices that enhanced the intensity of synchrotron radiation were built into existing rings. Third-generation synchrotron radiation sources were conceived and optimized from the outset to produce brilliant X-rays. Fourth-generation sources that will include different concepts for producing ultrabrilliant, pulsed time-structured X-rays for extremely demanding and also probably yet-to-be-conceived experiments are under consideration.[citation needed]

Bending electromagnets in accelerators were first used to generate this radiation, but to generate stronger radiation, other specialized devices – insertion devices – are sometimes employed. Current (third-generation) synchrotron radiation sources are typically reliant upon these insertion devices, where straight sections of the storage ring incorporate periodic magnetic structures (comprising many magnets in a pattern of alternating N and S poles – see diagram above) which force the electrons into a sinusoidal or helical path. Thus, instead of a single bend, many tens or hundreds of "wiggles" at precisely calculated positions add up or multiply the total intensity of the beam.[citation needed]

These devices are called wigglers or undulators. The main difference between an undulator and a wiggler is the intensity of their magnetic field and the amplitude of the deviation from the straight line path of the electrons.[citation needed]

There are openings in the storage ring to let the radiation exit and follow a beam line into the experimenters' vacuum chamber. A great number of such beamlines can emerge from modern third-generation synchrotron radiation sources.[citation needed]

Storage rings

The electrons may be extracted from the accelerator proper and stored in an ultrahigh vacuum auxiliary magnetic storage ring where they may circle a large number of times. The magnets in the ring also need to repeatedly recompress the beam against Coulomb (space charge) forces tending to disrupt the electron bunches. The change of direction is a form of acceleration and thus the electrons emit radiation at GeV energies.[citation needed]

Applications of synchrotron radiation

Beamlines

Soleil

At a synchrotron facility, electrons are usually accelerated by a synchrotron, and then injected into a storage ring, in which they circulate, producing synchrotron radiation, but without gaining further energy. The radiation is projected at a tangent to the electron storage ring and captured by beamlines. These beamlines may originate at bending magnets, which mark the corners of the storage ring; or insertion devices, which are located in the straight sections of the storage ring. The spectrum and energy of X-rays differ between the two types. The beamline includes X-ray optical devices which control the bandwidth, photon flux, beam dimensions, focus, and collimation of the rays. The optical devices include slits, attenuators, crystal monochromators, and mirrors. The mirrors may be bent into curves or toroidal shapes to focus the beam. A high photon flux in a small area is the most common requirement of a beamline. The design of the beamline will vary with the application. At the end of the beamline is the experimental end station, where samples are placed in the line of the radiation, and detectors are positioned to measure the resulting diffraction, scattering or secondary radiation.

Experimental techniques and usage

Synchrotron light is an ideal tool for many types of research in materials science, physics, and chemistry and is used by researchers from academic, industrial, and government laboratories. Several methods take advantage of the high intensity, tunable wavelength, collimation, and polarization of synchrotron radiation at beamlines which are designed for specific kinds of experiments. The high intensity and penetrating power of synchrotron X-rays enables experiments to be performed inside sample cells designed for specific environments. Samples may be heated, cooled, or exposed to gas, liquid, or high pressure environments. Experiments which use these environments are called in situ and allow the characterization of atomic- to nano-scale phenomena which are inaccessible to most other characterization tools. In operando measurements are designed to mimic the real working conditions of a material as closely as possible.[8]

Diffraction and scattering

thin films. The high resolution and intensity of the synchrotron beam enables the measurement of scattering from dilute phases or the analysis of residual stress. Materials can be studied at high pressure using diamond anvil cells to simulate extreme geologic environments or to create exotic forms of matter.[citation needed
]

Structure of a ribosome subunit solved at high resolution using synchrotron X-ray crystallography.[9]

proteins and other macromolecules (PX or MX) are routinely performed. Synchrotron-based crystallography experiments were integral to solving the structure of the ribosome;[9][10] this work earned the Nobel Prize in Chemistry in 2009
.

The size and shape of

small angle X-ray scattering (SAXS). Nano-sized features on surfaces are measured with a similar technique, grazing-incidence small angle X-ray scattering (GISAXS).[11] In this and other methods, surface sensitivity is achieved by placing the crystal surface at a small angle relative to the incident beam, which achieves total external reflection and minimizes the X-ray penetration into the material.[citation needed
]

The atomic- to nano-scale details of

Amorphous materials, including liquids and melts, as well as crystalline materials with local disorder, can be examined using X-ray pair distribution function analysis, which requires high energy X-ray scattering data.[14]

By tuning the beam energy through the absorption edge of a particular element of interest, the scattering from atoms of that element will be modified. These so-called resonant anomalous X-ray scattering methods can help to resolve scattering contributions from specific elements in the sample.[citation needed]

Other scattering techniques include

energy dispersive X-ray diffraction, resonant inelastic X-ray scattering, and magnetic scattering.[citation needed
]

Spectroscopy

amorphous materials[15] as well as sparse species such as impurities. A related technique, X-ray magnetic circular dichroism (XMCD), uses circularly polarized X-rays to measure the magnetic properties of an element.[citation needed
]

X-ray photoelectron spectroscopy (XPS) can be performed at beamlines equipped with a photoelectron analyzer. Traditional XPS is typically limited to probing the top few nanometers of a material under vacuum. However, the high intensity of synchrotron light enables XPS measurements of surfaces at near-ambient pressures of gas. Ambient pressure XPS (AP-XPS) can be used to measure chemical phenomena under simulated catalytic or liquid conditions.[16] Using high-energy photons yields high kinetic energy photoelectrons which have a much longer inelastic mean free path than those generated on a laboratory XPS instrument. The probing depth of synchrotron XPS can therefore be lengthened to several nanometers, allowing the study of buried interfaces. This method is referred to as high-energy X-ray photoemission spectroscopy (HAXPES).[17] Furthermore, the tunable nature of the synchrotron X-ray photon energies presents a wide range of depth sensitivity in the order of 2-50 nm.[18] This allows for probing of samples at greater depths and for non destructive depth-profiling experiments.

Material composition can be quantitatively analyzed using X-ray fluorescence (XRF). XRF detection is also used in several other techniques, such as XAS and XSW, in which it is necessary to measure the change in absorption of a particular element.[citation needed]

Other spectroscopy techniques include

soft X-ray emission spectroscopy, and nuclear resonance vibrational spectroscopy, which is related to Mössbauer spectroscopy.[citation needed
]

Imaging

X-ray nanoprobe beamline at the Advanced Photon Source

Synchrotron X-rays can be used for traditional

diffraction limit of visible light, but practically the smallest resolution so far achieved is about 30 nm.[19] Such nanoprobe sources are used for scanning transmission X-ray microscopy (STXM). Imaging can be combined with spectroscopy such as X-ray fluorescence or X-ray absorption spectroscopy in order to map a sample's chemical composition or oxidation state with sub-micron resolution.[20]

Other imaging techniques include coherent diffraction imaging.[citation needed]

Similar optics can be employed for

MEMS structures can use a synchrotron beam as part of the LIGA process.[citation needed
]

Compact synchrotron light sources

Because of the usefulness of tuneable

collimated coherent X-ray radiation, efforts have been made to make smaller more economical sources of the light produced by synchrotrons. The aim is to make such sources available within a research laboratory for cost and convenience reasons; at present, researchers have to travel to a facility to perform experiments. One method of making a compact light source is to use the energy shift from Compton scattering near-visible laser photons from electrons stored at relatively low energies of tens of megaelectronvolts (see for example the Compact Light Source (CLS)[21]). However, a relatively low cross-section of collision can be obtained in this manner, and the repetition rate of the lasers is limited to a few hertz rather than the megahertz repetition rates naturally arising in normal storage ring emission. Another method is to use plasma acceleration to reduce the distance required to accelerate electrons from rest to the energies required for UV or X-ray emission within magnetic devices.[citation needed
]

See also

References

  1. ^ Handbook on Synchrotron Radiation, Volume 1a, Ernst-Eckhard Koch, Ed., North Holland, 1983, reprinted at "Synchrotron Radiation Turns the Big Five-O Archived September 16, 2008, at the Wayback Machine
  2. ^
    PMID 15840926
    . Retrieved 8 April 2022.
  3. .
  4. .
  5. ^ E. M. Rowe and F. E. Mills, Tantalus I: A Dedicated Storage Ring Synchrotron Radiation Source, Particle Accelerators, Vol. 4 (1973); pages 211-227.
  6. .
  7. ^ The Physics of Electron Storage Rings: An Introduction by Matt Sands Archived 2015-05-11 at the Wayback Machine
  8. PMID 22432568
    .
  9. ^ .
  10. ^ The Royal Swedish Academy of Sciences, "The Nobel Prize in Chemistry 2009: Information for the Public", accessed 2016-06-20
  11. .
  12. .
  13. .
  14. ^ T. Egami, S.J.L. Billinge, "Underneath the Bragg Peaks: Structural Analysis of Complex Materials", Pergamon (2003)
  15. .
  16. .
  17. .
  18. .
  19. ^ Argonne National Laboratory Center for Nanoscale Materials, "X-Ray Microscopy Capabilities", accessed 2016-06-20
  20. PMID 20978688
    .
  21. ^ "Miniature synchrotron produces first light". Eurekalert.org. Retrieved 2009-10-19.

External links