What Are X-rays and Gamma Rays?

There are many different types of radiation – from the light that comes from the sun to the heat that is constantly coming off our bodies. But when talking about radiation and cancer risk, it is often x-rays and gamma rays that people think about.

X-rays and gamma rays can come from natural sources, such as radon gas, radioactive elements in the earth, and cosmic rays that hit the earth from outer space. But this type of radiation can also be man-made. X-rays and gamma rays are created in power plants for nuclear energy, and are also used in smaller amounts for medical imaging tests, cancer treatment, food irradiation, and airport security scanners.

X-rays and gamma rays are both types of high energy (high frequency) electromagnetic radiation. They are packets of energy that have no charge or mass (weight). These packets of energy are known as photons. Because X-rays and gamma rays have the same properties and health effects, they are grouped together in this document.

Both x-rays and gamma rays are forms of high-frequency ionizing radiation, which means they have enough energy to remove an electron from (ionize) an atom or molecule. Ionized molecules are unstable and quickly undergo chemical changes.

If ionizing radiation passes through a cell in the body, it can lead to mutations (changes) in the cell’s DNA, the part of the cell that contains its genes (blueprints). Sometimes this causes the cell to die, but sometimes it can lead to cancer later on. The amount of damage caused in the cell is related to the dose of radiation it receives. The damage takes place in only a fraction of a second, but other changes such as the beginning of cancer may take years to develop.

Gamma rays and x-rays aren’t the only kinds of ionizing radiation. Some types of ultraviolet (UV) radiation are also ionizing. Ionizing radiation can also exist in particle form, such as protons, neutrons, and alpha and beta particles.

Doses of radiation

Radiation exposure can be expressed in certain units.

The absorbed dose is the amount of energy deposited per unit of mass. Most often this is measured in grays (Gy). A milligray (mGy), which is 1/1000th of a Gy, may also be used.

The equivalent dose is the absorbed dose multiplied by a converting factor based on the medical effects of the type of radiation. It is often expressed in sieverts (Sv) or millisieverts (mSv), which is 1/1000th of a Sv.

For x-rays and gamma rays (and beta particles), the equivalent dose in Sv is the same as the absorbed dose in Gy.

Less common radiation dose units include rads, rems, and roentgens.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

National Toxicology Program. Ionizing radiation: x-radiation and gamma radiation. Rep Carcinog. 2011;12:237-240.


National Toxicology Program. Ionizing radiation: x-radiation and gamma radiation. Rep Carcinog. 2011;12:237-240.

Last Revised: February 24, 2015

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