Introduction to Synchrotron Sciences

Synchrotron Radiation

Electromagnetic radiation is emitted when charged particles, usually electrons or positrons, moving close to the speed of light are forced to change direction under the application of a magnetic field: this is the basic operating principle of a synchrotron. The electromagnetic radiation is emitted in a cone in the forward direction, at a tangent to the orbit of the charged particles. Synchrotron light produced in this manner possesses a number of unique properties:

• High brightness: synchrotron light is extremely intense; it is hundreds of thousands of times more intense than that from conventional x-ray tubes. In addition synchrotron light is highly collimated.
• Wide energy spectrum: synchrotron light is emitted with energies ranging from infra-red light to hard, energetic, short wavelength x-rays.
• Tunable: through sophisticated monochromators and insertion devices it is possible to obtain an intense beam at any desired wavelength.
• Pulsed: electromagnetic pulses may be generated with a duration typically less than a nano-second (one billionth of a second).

To image or detect fine structure, the wavelength of the illuminating radiation must be of the same order as the structure. Synchrotron light is able to span the electromagnetic spectrum across almost all fields of scientific interest, from life-size imaging down to nanoscale, molecular and atomic length scales.

Synchrotron Design

In a synchrotron, electrons are generated by an electron gun and accelerated almost to the speed of light by a linear accelerator (LINAC) and a booster ring, and are then transferred into an outer storage ring. The electrons are forced to circulate in this ring by being deflected by bending magnets; they otherwise travel along straight evacuated segments. When the electrons are deflected they emit electromagnetic radiation, producing a beam of synchrotron light at the position of each bending magnet. These beams can be captured and used to perform a wide variety of experiments, the nature of which is determined by carefully selecting a narrow part of the beam's spectrum.

Synchrotron design has evolved rapidly over the past 20 years. In early designs, the principal sources of synchrotron light were the bending magnets. Although the intensity of this light is already very high, it can be further increased by several orders of magnitude by the inclusion of insertion devices in the straight sections of the storage ring. These devices consist of two rows of small magnets with alternating polarity perpendicular to the direction of motion of the electrons. This highly inhomogeneous magnetic field causes the electrons in the beam to deflect in a direction transverse to their motion. This sinusoidal motion causes more radiation to be emitted than is available from the deflecting action of the bending magnets alone.

There are two types of insertion devices. One is called a multipole wiggler (MPW), in which a cone of light is emitted at each turning point in the sinusoidal trajectory of the electrons through the inhomogeneous magnetic field. These cones interfere constructively with one another, with the final intensity increasing with the number of oscillations in the magnetic field along the transit of the electron beam. The beam appears as a broad beam of incoherent radiation in the direction of the straight section of the storage ring containing the insertion device.

The second type of device is called an undulator, which uses less powerful magnets to produce gentler undulations in the electron beam. In this case careful matching of the transverse motion to the transit speed of the electrons through the device can cause the amplitudes of the cones of light emitted in this manner to interfere constructively: the consequence of this is an enhancement in the intensity of perhaps a factor of 10000 for certain wavelengths. The optimal wavelengths can be changed by altering the gap between the component magnets, so that the synchrotron light is tunable to specific wavelengths. The beam produced by an undulator is narrow and coherent.

State of the art (third generation) synchrotrons aim to optimize the intensity of the emitted light that can be obtained from insertion devices. In particular, careful attention is given to the positioning of the straight sections that accommodate the insertion devices. In the Australian Synchrotron, it is planned to place the magnetic components of the undulators inside the vacuum of the storage ring. This will allow very close spacing of the magnets and extremely high beam intensities. For the wiggler devices, conventional magnets may be used, but they will be wound with superconducting wire, enabling extremely high intensities to be generated with very high specificity with respect to frequency (or equivalently, wavelength). The Australian Synchrotron will, consequently, be of an advanced third generation design comprising bending magnets, wigglers and undulators, enabling a wide range of experiments to be conducted in a single, national facility.

Synchrotron Sciences

The experiments are conducted by diverting a beam generated by one of these sources away from the storage ring and along a tube maintained under high vacuum called a beamline. The appropriate wavelength of radiation is selected from the spectrum that comprises the beam, and then further directed to an experimental end-station.

The experiments, measurements and processes that can be carried out fall into five general categories:

• Diffraction: for crystallography, including protein crystallography.
• Spectroscopy: for analysis of chemical composition and speciation in bulk materials and at surfaces, down to nanometre length scales.
• Imaging: from highly detailed imaging of small animals and, ultimately, humans, down to sub-micron dimensions using light from infra-red through to hard x-rays.
• Micromachining and lithography: manufacture of machines of micron-sized dimensions with exceptionally high depth to width aspect ratios
• Polarization: known as circular dichroism, or angle-resolved spectroscopy. Used for measuring the shape of complex molecules, especially proteins, and the properties of magnetic materials.