A Particle Beam is a heavy ion-particle cannon mounted on starships as a medium-range anti-ship weapon. Particle streams produced in either ring or linear particle accelerators inside the ship are fed to a magnetic-lens system and directed at enemy vessels.
Most systems are capable of firing a variety of particle streams to produce different effects and levels of damage. Proton, Antiproton, electron, positron, and alpha particle beams are the easiest to create and can cause massive damage, but they are also easiest for Deflector Shields to block, because they are charged particles. More exotic and difficult beams to create such as neutron and antineutron beams tend to be less damaging, but can only be blocked by Gravity Drive shields. Gluon beams can be devastating when they hit deuterium storage tanks for fusion reactors, causing massive fusion explosions, but don't do much damage otherwise. Erich Von Shrakenberg and Xinjao O'Reilly used this to cripple a Jurvain battle fleet during the later stages of the Battle of Rios in 2243.
A ligher version of Partcile Cannons, known as Particle Phalanx, are used as a point-defense system agasint enemy projectiles and starfighters. Although much smaller, they can only fire antineutrons, and don't have nearly the range and power of true Particle Cannons.
Due to the size of the particle accelerators, Particle Cannons can only be mounted on ships the size of a light cruiser (about a kilometer long) or larger. The smaller, more compact Particle Phalanx, however, can be mounted ships as small as destoryers (about half a kilometer long).