What is a Light Source?

The light sources which are members of lightsources.org are accelerators that produce exceptionally intense beams of X-rays, ultra-violet and infrared light, making possible both basic and applied research in fields ranging from physics to biology and technology, which are not possible with more conventional equipment.

‘Light’ refers to the breadth of the electromagnetic spectrum, which includes visible light, yet also includes light with wavelengths that we cannot see, such as: radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. These different types of light are used in everyday life, however. For example, airport scanners use X-rays to inspect the contents of your suitcase. Likewise, the right kind of light and the right equipment can help us see things in much finer detail than the human eye could possibly make out. This capability holds the key to answering some of the fundamental questions about the world around us, such as: what is our planet made from? What are the processes that sustain life? How can we conquer viruses?

Fig. 1 The electromagnetic spectrum spans the range from radio waves at long wavelengths to gamma rays at short wavelengths. (Courtesy: Advanced Light Source)

These questions can only be answered at the molecular level; at the level of atoms and electrons. Light sources provide a tool for answering these questions. They can be compared to a ‘super microscope’, by providing intensely bright forms of X-ray, infrared and ultraviolet light, which enables research on samples in the tiniest detail. Each range of light is suited to a particular job. To ‘see’ atoms, we need to use a form of light that has a much shorter wavelength than visible light. As a general rule, short-wavelength (hard) X-rays are most useful for probing atomic structure. Again as a general rule, long-wavelength (soft) X-rays and ultraviolet light are good choices for studying chemical reactions. Infrared is ideally suited to studying atomic vibrations in molecules and solids, and at its very long wavelength end (terahertz waves), it is also useful for certain types of electronic structure experiments. The identification of elements in samples is the province of X-rays.

This range of the electromagnetic spectrum is known as ‘synchrotron light’, as it is produced by a dedicated synchrotron machine. A synchrotron light source typically begins with an electron gun, containing a manmade material, to which an electrical and thermal current is applied. This results in electrons ‘lifting off’ and beginning their journey by being propelled down a linear accelerator (linac). They then enter a circular-shaped booster ring, where they are accelerated to relativistic speeds. Finally ,they enter another ring, often called a ‘storage ring’, where they circulate for hours. The electrons will travel in a straight line, so at points around the ring, special ‘bending’ magnets help them keep to their circular path. As the electrons circulate, powerful magnets keep them bunched together and focused.

Synchrotron light is produced when the electrons change direction around the ring. In synchrotrons, this happens when they are manipulated by bending magnets, or as they pass through insertion devices. At the points where the electrons change direction, they emit a fan of radiation (known as synchrotron light). This radiation branches off the storage ring, and enters laboratories, or ‘beamlines’. Here it is refined with devices such as monochromators and mirrors, before it is shone on the sample, enabling researchers to obtain detailed data about the sample’s structure and behaviour.

Free-electron lasers provide a complimentary source of light, which is produced differently.

A light-source is therefore, at its core, a set of particle accelerators which generate synchrotron light. Using these intense beams of light, scientists are able to carry out a variety of experimental techniques in a wide range of disciplines, from chemistry to energy, and cultural heritage to engineering.