A proton is one of the building blocks of the atom. Protons, along with neutrons and much smaller electrons, make up the basic elements. When these microscopic particles are focused into a narrow ray and shot at extremely high speeds, it is called a proton beam. Proton beams are extremely useful things, both for experimental physicists and doctors.
How Proton Beams Are Created
Protons have a positive electrical charge. Things with opposite charges attract each other, while things with the same charge repel. This is the central principle in a particle accelerator--the machine used to make proton beams. The protons are accelerated through a tube by electromagnets. When a proton is behind the magnets, the magnet is switched to a negative charge to pull the proton towards it. As the proton speeds past the magnet, the charge is switched to positive, to push the proton away from it, speeding it up more. A whole row of protons in a row makes a proton beam.
A proton beam can go nearly the speed of light, but it takes a while to get the particles going. One way to do this is to use a large linear accelerator. Linear accelerators are massive things--up to 2 miles long.
Another way of doing this is with a circular accelerator. Circular accelerators, or cyclotrons, have a magnetic field designed to bend the path of the particle in a circle. A beam of protons in a cyclotron will spin around and around until they get enough speed. Then they will be shot out at their target.
Proton beams have many useful applications. They are often used in theoretical physics. Particle accelerators smash protons into other protons, as well as neutrons and other elementary particles. When the particles collide, the scientists measure the small particles thrown out from the collision. They try to discover things about gluons, quarks, and the other basic particles that make up protons.
Proton beams are also used in radiation therapy. The protons are carefully aimed and shot at tumors, where they damage their DNA and kill the cancer cells. This type of therapy does very little damage to the surrounding tissue. Unlike surgery, it does not require cutting, making it much safer and less harmful for certain kinds of tumors. In particular, proton beams have been very effective at treating certain types of eye cancer. Previously the eye had to be removed to get to the tumor, but now it can be targeted by the proton beam.