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Ionizing & Non-Ionizing Radiation
Understanding Radiation:  

Gamma Rays

Ionizing & Non-Ionizing Radiation

A gamma ray is a packet of electromagnetic energy 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.

On this page:

The Basics

Exposure to Gamma Radiation

Health Effects

Protecting People from Gamma Radiation

The Basics

Who discovered gamma radiation?

Physicists credit French physicist Henri Becquerel with discovering gamma radiation. In 1896, he discovered that uranium minerals could expose a photographic plate through a heavy opaque paper. Roentgen had recently discovered x-rays, and Becquerel reasoned that uranium emitted some invisible light similar to x-rays. He called it "metallic phosphorescence.

In reality, Becquerel had found gamma radiation being emitted by radium-226. Radium-226 is part of the uranium decay chain and commonly occurs with uranium.

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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.

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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 kind of hazard, they differ in their origin. Gamma rays originate in the nucleus. X-rays originate in the electron fields surrounding the nucleus or are machine-produced.

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. When 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.

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How do we use gamma emitters?

Gamma emitting radionuclides are the most widely used radiation sources. The penetrating power of gamma photons has many applications. However, while gamma rays penetrate many materials, they do not make them radioactive. The three radionuclides by far most useful are cobalt-60, cesium-137, and technetium-99m.

Uses of Cesium-137:
Uses of Cobalt-60:
Uses of Technetium-99m:

TC-99m is the most widely used radioactive isotope for diagnostic studies. (Technetium-99m is a shorter half-life precursor of technetium-99.) Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies.

In manufacturing, gamma radiation from cobalt-60 or cesium-137 can improve the physical characteristics of materials. For example, exposure to gamma radiation improves the durability of some wood and plastic composites. Treated materials can be used for flooring in high-traffic areas of department stores, airports, and hotels, because they resist abrasion and ensure low maintenance.

Another process, industrial radiography, uses gamma radiation to inspect metal parts and welds for defects. A sealed radiation source, usually iridium-192 or cobalt-60, beams gamma radiation at the part. Any gamma radiation passing through a crack or incomplete weld exposes special photographic, or radiographic, film. (The process is similar to taking an x-ray of a broken arm.) For example, manufacturers use radiography to inspect jet engine turbine blades.

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How does gamma radiation change in the environment?

Gamma rays travel at the speed of light and 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.

Exposure to Gamma Radiation

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. Gamma and x-rays can easily travel great distances through air and penetrate several centimeters in tissue. Most 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 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.

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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 portion of 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. X-rays behave in a similar way, but have slightly lower energy. By contrast, alpha and beta particles inside the body lose all their energy by colliding with tissue and causing damage.

Gamma rays can ionize atoms in tissue directly or cause what are known as "secondary ionizations." Ionizations are caused when energy is transferred from gamma rays to atomic particles such as electrons (which are essentially the same as beta particles). These energized particles then interact with tissue to form ions through secondary ionizations. Because gamma rays are photons and thus interact less frequently with matter than alpha and beta particles, they are more penetrating and the damage they cause can occur much farther into tissue (that is, farther from the source of radiation).

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Health Effects of Gamma Radiation

How can gamma radiation affect people's health?

Because of the gamma ray's penetrating power and ability to travel great distances, it is considered the primary hazard to the general population during most radiological emergencies. In fact, when the term "radiation sickness" is used to describe the effects of large exposures in short time periods, the most severe damage almost certainly results from gamma radiation.

Protecting People from Gamma Radiation

How do I know I'm near gamma emitters and gamma radiation?

You need specialized equipment to detect gamma radiation. You cannot see, or feel radiation hitting your body. However, you should be familiar with radiation warning symbols. You can protect yourself by avoiding devices with this symbol, and not entering areas where the symbol is posted.

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How do I protect myself from x-ray and gamma radiation?

Your exposure to x-rays is almost entirely from dental and medical x-rays, including mammograms. The best way to protect yourself from excessive radiation from x-rays is to make sure the technician performing the procedure has the proper qualifications, and to simply ask questions. You might inquire about the necessity of having an x-ray, or receive assurance the x-ray machine has been inspected recently and that it is properly calibrated. You should be aware of steps taken to prevent exposures to other parts of your body (for example, through the use of a lead apron).

It is possible that you or a member of your family may encounter an industrial instrument or device containing a gamma radiation source. Every year, hundreds of devices containing radiation sources are lost, stolen, or otherwise enter the general public by mistake. For example:

These devices should be avoided. You may recognize them by the radiation symbol, which means the device is radioactive. You should also look for identifying information such as "Nuclear Regulatory Commission" or the name of a radionuclide. Sometimes the radioactive markings may be covered over and not visible.

If you find a device you think may be radioactive, stay away from it, and promptly call your state radiation control program Exit EPA Disclaimeror 1-800-999-7879.

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