What is an accelerator?
The generic definition of an accelerator is something that makes something else move faster. A baseball bat is an accelerator because it makes a baseball go faster. In the world of physics though, 'accelerator' means something a little more specific. Our accelerators are a whole class of machines that accelerate atoms, or more often, pieces of atoms, to very high speeds. In particle physics we use machines to shoot sub-atomic particles into other atoms. This helps us learn about what goes on inside an atom.
Accelerators are used for much more than research and are much more common than most people think. Accelerators range in size from 5.3 miles in diameter (Large Electron Positron Collider, or LEP at CERN in Geneva, Switzerland) to ones that are small enough to fit in your hand. There are particle accelerators all around you, like in your house! Did you know that television sets have small electron accelerators? On the inside of the screen are little dots made of a phosphor. The accelerator shoots out a stream of electrons and steers the electrons with magnets onto the phosphor dots. This process lights the dots one at a time. Because the whole process is faster than what your eye can detect, your brain mixes the dots into a picture. X-ray machines, like the ones you might have seen at a dentist's office, are now typically about the size of a book. Inside, they too contain a tiny electron accelerator. This particular kind of accelerator shoots electrons towards a heavy metal target. The electrons hit the target and cause a stream of x-rays to come out. The x-rays then go through your teeth and expose the film inside your mouth. Accelerators are also used in medical and industrial operations. For example, the welded joints in pipes that are used in critical applications are often x-rayed to make sure there aren't any unsafe imperfections. To x-ray them, the joint is wrapped in x-ray film and an accelerator/x-ray gun is shone through the weld, which exposes the film. This process is much safer than old processes. In the past, a piece of radioactive material was placed inside the pipe to expose the film. Today, with an accelerator as the source of the x-rays, we can turn the accelerator off, stopping all radiation. This is also true in radiation treatment for cancer.
Accelerators used for research use different materials, depending on what is being studied. Here at Jefferson Lab we accelerate electrons just like your television set does, except ours is MUCH more powerful. At the LEP accelerator they use electrons, but they smash them into... sit down for this... anti-electrons called positrons! Yes, real anti-matter, which is really very Star Trekish! At Fermilab, the site of what is currently the world's most powerful accelerator, they collide protons into anti-protons (yes more anti-matter). At RHIC in Brookhaven, New York, they plan to accelerate gold atoms.
Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)