Gamma rays are a kind of electromagnetic radiation, the family of waves that includes visible light and radio waves. They have the shortest wavelengths and the most energy of any part of that family. The wavelength shown here is 1 picometer, about the wavelength of the gamma rays given off by radioactive cobalt-60. A picometer is a trillionth of a meter, or a thousandth of a nanometer.

Wavelength

A wavelength is the distance from one crest of a wave to the next. NASA counts any electromagnetic wave shorter than about 10 picometers as a gamma ray. The explorer's figure, 1 picometer, is a tenth of that boundary. The shorter the wave, the more energy it carries, so astronomers who study gamma rays usually describe them by their energy instead.

Gamma rays overlap with X-rays, and the two are told apart in different ways. The U.S. Environmental Protection Agency separates them by where they come from: gamma rays start inside an atom's nucleus, while X-rays come from outside it. Astronomers separate them by energy alone.

Nearly every time a cobalt-60 atom decays, it releases two gamma rays, with wavelengths of about 1.06 and 0.93 picometers. The explorer's 1 picometer lies between them.

Size comparisons

A hydrogen atom, about 0.1 nanometers across, is about 100 of these wavelengths wide. An angstrom, a unit of length equal to 100 picometers, is exactly 100.
X-ray wavelengths run from about 10 picometers to 10 nanometers, 10 to 10,000 times as long as this gamma ray.
Visible light has wavelengths of 400–700 nanometers, from violet to red. That is 400,000–700,000 times as long.
About 80–100 million of these wavelengths, laid end to end, would span the width of a human hair.
If this wavelength were stretched to 1 millimeter (0.04 inches), a hydrogen atom would be about 11 centimeters (4 inches) across, and a human hair would be 80–100 kilometers wide.

Where gamma rays come from

In space, gamma rays come from the hottest and most energetic objects: neutron stars, exploding stars called supernovae, and the regions around black holes. The most powerful flashes, called gamma-ray bursts, can release more energy in 10 seconds than the Sun will give off in its whole lifetime of about 10 billion years.

On Earth, gamma rays come from lightning, nuclear explosions and radioactive decay. Cobalt-60, for example, forms when steel inside a nuclear reactor is struck by neutrons, and half of any sample decays every 5.27 years. Hospitals use its gamma rays for radiation therapy.

How gamma rays are stopped and detected

Gamma rays can pass completely through the human body, damaging tissue and DNA on the way. Stopping them can take several inches of a dense metal such as lead, or a few feet of concrete. Their wavelengths are so short that they can pass through the empty space inside the atoms of a detector.

Unlike light and X-rays, gamma rays cannot be caught and reflected by mirrors. Instead, gamma-ray detectors hold blocks of densely packed crystal. A gamma ray passing through knocks into electrons in the crystal, and those collisions make a signal the detector can record. Earth's atmosphere absorbs gamma rays from space, so gamma-ray telescopes such as NASA's Fermi work from orbit.

Sources

Wavelength, Frequency, and Energy. NASA Imagine the Universe, 2013.
Electromagnetic Spectrum: Introduction. NASA Imagine the Universe, 2013.
Gamma Rays. NASA Science, 2023.
Radiation Basics. U.S. Environmental Protection Agency, 2026.
Radionuclide Basics: Cobalt-60. U.S. Environmental Protection Agency, 2026.
Recommended Gamma-Ray Energies and Emission Probabilities. IAEA Nuclear Data Section.
Planck Constant in eV/Hz. NIST, CODATA 2022.
Bohr Radius. NIST, CODATA 2022.
Just How Small Is "Nano"?. National Nanotechnology Initiative.

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