X-ray Sources

Laboratory Sources

For medical, dental and routine industrial use the common source is the "X-ray tube". Electrons are emitted from a heated tungsten filament (cathode), are accelerated to 20 – 100 keV using a high voltage supply, and focused (electrostatically) onto a metal anode. The electrons lose energy as they interact with the anode, and some of this energy is emitted in the form of X-rays. The size of the source is typically a few millimeters. The X-rays emitted are commonly both "characteristic" X-rays (fluorescent X-rays from the target material) and bremsstrahlung. The former involves a few well-defined energies, while the latter provide a continuous spectrum.

Diagram of an X-ray tube

The X-rays are emitted in all directions, and are typically collimated to derive a useful "beam". Hence most of the X-rays go to waste. To get enough X-ray intensity one tries to increase the electron current hitting the anode. The current limit is set by the heating of the anode. For high power operations the anode must be cooled. Common anode materials are copper and silver (good thermal conductivity) or tungsten (very high melting point). To increase the power beyond the maximum a static target can handle, rotating anode X-ray generators have been developed.

For special applications, such as projection microscopy, microfocus tubes have been developed. By using a smaller electron filament and more careful focusing, the size of the X-ray source can be reduced to a few microns.

Synchrotron Light Sources

Electrons accelerated to very high energy (several GeV, or 10⁹ eV) radiate a copious amount of X-rays when passed through a magnet. Because these electrons are moving at a speed close to the velocity of light, the emitted X-rays are highly collimated in the direction of the electron's velocity. In the last few decades dozens of synchrotron light sources have been built at major national laboratories around the world. Electron beams are kept in near-circular orbits by a ring of "bending magnets". X-ray beams derived from these magnets have a continuous spectrum, and are particularly useful for spectroscopy.

Light sources are built with large gaps between magnets, and these "straight sections" host "undulators". Undulators literally force the electron beam to undulate slightly under the influence of some dozens of permanent magnets of alternating polarity. The radiation builds up from each undulator in the forward direction, giving rise to an even more highly collimated beam of X-rays, with directional characteristics not unlike a laser-pointer. The transverse dimensions of the electron beam determine the "source size", and are typically of order 10 x 300 microns. One lightsource provides beams to dozens of beamlines, each of which serve a different experiment operating simultaneously, typically 24/7. Access is by peer reviewed proposals, and beam time is free of charge, except for proprietary use.