Gamma Radiation
Environmental Protection Agency fact sheet
What are gamma rays?

A gamma ray is a packet of electromagnetic energy--a photon. Gamma photons are the most energetic photons in the electromagnetic spectrum. Gamma rays (gamma photons) are emitted from the nucleus of some unstable (radioactive) atoms.

What are the properties of gamma radiation?

Gamma radiation is very high-energy ionizing radiation. Gamma photons have about 10,000 times as much energy as the photons in the visible range of the electromagnetic spectrum.

Gamma photons have no mass and no electrical charge--they are pure electromagnetic energy.

Because of their high energy, gamma photons travel at the speed of light and can cover hundreds to thousands of meters in air before spending their energy. They can pass through many kinds of materials, including human tissue. Very dense materials, such as lead, are commonly used as shielding to slow or stop gamma photons.

Their wave lengths are so short that they must be measured in nanometers, billionths of a meter. They range from 3/100ths to 3/1,000ths of a nanometer.

What is the difference between gamma rays and x-rays?

Gamma rays and x-rays, like visible, infrared, and ultraviolet light, are part of the electromagnetic spectrum. While gamma rays and x-rays pose the same hazard, they differ in their origin. Gamma rays originate in the nucleus. X-rays originate in the electron fields surrounding the nucleus.

What conditions lead to gamma ray emission?

Gamma radiation emission occurs when the nucleus of a radioactive atom has too much energy. It often follows the emission of a beta particle. 

What happens during gamma ray emission?

Cesium-137 provides an example of radioactive decay by gamma radiation. Scientists think that a neutron transforms to a proton and a beta particle. The additional proton changes the atom to barium-137. The nucleus ejects the beta particle. However, the nucleus still has too much energy and ejects a gamma photon (gamma radiation) to become more stable.

How does gamma radiation change in the environment?

Gamma rays exist only as long as they have energy. Once their energy is spent, whether in air or in solid materials, they cease to exist. The same is true for x-rays.

How are people exposed to gamma radiation?

Most people's primary source of gamma exposure is naturally occurring radionuclides, particularly potassium-40, which is found in soil and water, as well as meats and high-potassium foods such as bananas. Radium is also a source of gamma exposure. However, the increasing use of nuclear medicine (e.g., bone, thyroid, and lung scans) contributes an increasing proportion of the total for many people. Also, some man-made radionuclides that have been released to the environment emit gamma rays.

Most exposure to gamma and x-rays is direct external exposure. Most gamma and x-rays can easily travel several meters through air and penetrate several centimeters in tissue. Some have enough energy to pass through the body, exposing all organs. X-ray exposure of the public is almost always in the controlled environment of dental and medical procedures.

Although they are generally classified as an external hazard, gamma emitting radionuclides do not have to enter the body to be a hazard. Gamma emitters can also be inhaled, or ingested with water or food, and cause exposures to organs inside the body. Depending on the radionuclide, they may be retained in tissue, or cleared via the urine or feces.

Does the way a person is exposed to gamma or x-rays matter?

Both direct (external) and internal exposure to gamma rays or X-rays are of concern. Gamma rays can travel much farther than alpha or beta particles and have enough energy to pass entirely through the body, potentially exposing all organs. A large protion gamma radiation largely passes through the body without interacting with tissue--the body is mostly empty space at the atomic level and gamma rays are vanishingly small in size. By contrast, alpha and beta particles inside the body lose all their energy by colliding with tissue and causing damage. X-rays behave in a similar way, but have slightly lower energy.

Gamma rays do not directly ionize atoms in tissue. Instead, they transfer energy to atomic particles such as electrons (which are essentially the same as beta particles). These energized particles then interact with tissue to form ions, in the same way radionuclide-emitted alpha and beta particles would. However, because gamma rays have more penetrating energy than alpha and beta particles, the indirect ionizations they cause generally occur farther into tissue (that is, farther from the source of radiation).