Gamma rays are a form of electromagnetic radiation similar to X rays. Gamma rays have a higher energy and a shorter wavelength than X rays do. However, the dividing line between these two forms of radiation is not clearly defined. Scientists typically apply the term gamma ray to electromagnetic radiation with energies above several hundred thousand electronvolts. One electronvolt is the amount of energy gained by an electron as it moves freely between two points with a potential difference of 1 volt (see Volt).
In 1900, Paul Villard, a scientist working in Paris at the same time as Marie and Pierre Curie, discovered gamma rays through studies of radiation emitted (given off) by nuclei of atoms. Uranium and other radioactive elements emit alpha particles or beta particles from their nuclei when they transform into new elements. An instant later, these nuclei may give off gamma rays.
A nucleus may also emit a gamma ray alone in an isomeric transition. In this transition, the emission of the ray does not follow a change in the composition of the nucleus. Rather, the nucleus merely loses a certain amount of energy. See Radiation.
Astronomical objects such as pulsars, supernovae, galaxies, and the sun also produce gamma rays. The highest-energy gamma rays ever detected come from a cloud of gas and dust known as the Crab Nebula. These rays have an energy of about 1 trillion electronvolts.
The most energetic event ever detected was a burst of gamma rays that occurred in a distant galaxy. In 1997, two orbiting telescopes detected this burst in a galaxy that is about 12 billion light-years from the earth. A light-year is the distance that light travels in a vacuum in a year. This distance equals about 5.88 trillion miles (9.46 trillion kilometers). The burst lasted for about 40 seconds. It released hundreds of times more energy than is released in a supernova explosion. This gamma-ray burst is the highest-energy event known—other than the big bang, the explosion that began the universe. Astronomers do not know what caused the burst.
Gamma rays lose energy when they pass through matter. They lose this energy by interacting with electrons or the nuclei of atoms. The electrons absorb energy from the gamma rays, then leave their orbits. This process, called ionization, changes electrically neutral atoms into electrically charged atoms. Electrons are negatively charged, and so the atoms become positively charged.
A high-energy gamma ray in the vicinity of a nucleus can change into matter by producing an electron pair. One member of this pair is an electron. The other is a positively charged particle known as a positron. A positron is an antiparticle of an electron. The two particles differ only in their electric charge.
When the opposite process occurs—that is, when an electron and a positron collide—both particles are destroyed. In this annihilation process, two gamma rays are almost always formed.
Small amounts of gamma rays bombard our bodies constantly, primarily from naturally occurring radioactive materials in rocks and soil. Some of these materials enter our bodies in the air we breathe, the food we eat, and the water we drink. Gamma rays passing through the body produce ionization in tissue. This process can harm the body’s cells. However, gamma rays can also be of benefit. They are helpful in destroying some types of cancers and tumors. They are also used to inspect metal for flaws and to preserve foods.
