A gamma particle is known by other names, such as X-ray, photons and light. Low-energy forms are called ultraviolet rays, infrared rays, even radio waves! A photon is one of the fundamental particles in nature and it plays an important role in being involved in the interactions between electrons. Photons are the most familiar particles in everyday existence. The light we see, the radiant heat we feel, microwaves to cook with, are all photons of different energies. A gamma is simply a name given to the most energetic of these particles.
Gammas are produced in abundance at SLAC when high-energy electrons impinge on any material and the electrons are subject to violent collisions. Photons that result from these collisions are formally called bremsstrahlung radiation, (bremsstrahlung is German for decelerated-radiation). These high-energy photons are loosely-called gammas by physicists although it is somewhat of a misnomer! Gamma particles are those that are given off by the spontaneous disintegration of a nucleus. The proper name for the high-energy photons created at SLAC would be either bremsstrahlung photons or X-rays. However, we are lucky that the word gamma is in common use. Otherwise, the EGS code would have been called something less attractive-sounding like EXS or EBS. More accurate maybe, but not nearly so nice.
Here's a picture of a Monte Carlo simulation of a high-energy electron producing several gammas in bremsstrahlung interactions.
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