NOVA Home Find out what's coming up on air Listing of previous NOVA Web sites NOVA's history Subscribe to the NOVA bulletin Lesson plans and more for teachers NOVA RSS feeds Tell us what you think Program transcripts Buy NOVA videos or DVDs Watch NOVA programs online Answers to frequently asked questions
Dirty Bomb

Sources of Radiation

Dirty Bomb homepage

sources of radiation main diagram - landscape with labeled items landfill sun house nuclear reactor sky bedrock lighthouse water

We all encounter many forms of radiation every day. Some of these are naturally occurring, and some are manmade. Some are harmful, while others are not.

Select the links below to find out about sources of radiation in the environment.

sun | lighthouse | nuclear reactor | bedrock | house | sky | landfill | water | atoms and particles


Many forms of radiation are not harmful to life. Visible light is a good example. Infrared radiation, which we feel as heat, and radio waves, which we can't sense at all, are two others. These are all forms of electromagnetic radiation. Forms of electromagnetic radiation that can be harmful to life include ultraviolet, x-rays, and gamma radiation.

Electromagnetic Radiation
Diagram of atom, showing how an electron moving from one energy state down to another causes the emission of an electron Electromagnetic radiation is carried by massless particles called photons. An atom emits a photon when one of its electrons or its nucleus "jumps" from a higher energy state to a lower energy state. How much energy the photon carries—that is, whether it's a radio wave, light, or gamma radiation—depends on the amount of energy released when the electron or nucleus makes its jump.

Microwaves, though less energetic than light waves, can heat tissue by causing water molecules within the tissue to vibrate rapidly (this is how microwaves are used to heat food), but microwaves do not have enough energy to affect chemical bonds unless they heat the material enough to cause thermal damage. Ultraviolet light, x-rays, and gamma rays can also damage tissue, but they do so by breaking the chemical bonds within cells, including those that hold DNA together.

Back to top

The Sun

The sun, powered by nuclear fusion, emits radiation that is both necessary and harmful to life on Earth. Light and infrared radiation (heat) are needed by almost all of Earth's life forms. The atmosphere filters out much of the harmful radiation emitted by the sun, though some radiation, such as ultraviolet light, does pass through. UV radiation can damage living tissues and cause cancer.

Nuclear Fusion
1) two protons collide (no electrons); 2) one of the protons becomes a neutron (i.e., changes color); 3) a positron is emitted from the neutron and combines with an electron; 4) positron and electron disappear (with a flash) and gamma radiation moves outward from where the collision took place At the sun's core is a nuclear furnace—a furnace that runs on the energy released when the fast-moving nuclei of atoms collide and "fuse" together. When a proton fuses to another proton, it turns into a neutron. In other words, it changes from a particle with a positive charge to one with no charge. As it makes the change, it emits a positive electron (a positron), which then quickly combines with a negatively charged electron. The positron and electron annihilate each other, releasing all of their energy in the form of gamma radiation.

Gamma radiation in high doses is potentially lethal to life on Earth, but the sun releases relatively little gamma radiation. The gamma radiation created deep within the sun is absorbed and re-emitted by other atoms as it works its way toward the surface. By the time it leaves the sun's surface, most of it is no longer high-energy gamma rays but other forms of electromagnetic radiation.

Back to top

Nuclear Reactor

Nuclear fission is a useful energy source because of its ability to create heat and sustain a chain reaction. Unfortunately, fission produces waste products that are radioactive, including cesium-137 and strontium-90. Strontium-90 emits powerful beta radiation, which is a spray of negatively charged electrons.

1) uranium atom, with electron cloud; 2) neutron comes in from outside of atom, strikes the uranium nucleus and splits the nucleus in two; 3) the splitting releases several neutrons from the nucleus To initiate a chain reaction, a "slow" neutron strikes the nucleus of an atom of uranium-235, which causes the atom to split, or fission. As the atom splits, its nucleus releases two fast-moving neutrons. These neutrons are slowed when they collide with hydrogen in surrounding water. They then strike other uranium atoms, which split and release more neutrons. A chain reaction ensues. The two newly formed atoms initially carry a great deal of energy, which is converted to heat as the atoms move through the fuel. In a power plant, the heat is used to create steam, and the steam runs turbines.

Most of the radioactivity in spent nuclear fuel is in the form of strontium-90 and cesium-137. These atoms are unstable and decay to other atoms through the emission of high-energy gamma rays. Strontium-90 and cesium-137 are chemically separated from the spent fuel and sold to various companies.

Back to top


Although most of the radioactive substance tritium (an isotope of hydrogen) is created naturally by cosmic ray interactions, slightly higher concentrations of the element have been found in the waters surrounding nuclear reactors that use uranium-238 as a fuel and "heavy water" as a coolant. Tritium emits beta radiation, which sprays negatively charged electrons.

Beta Decay
1) tritium atom with two neutrons and one proton; 2) one of the neutrons turns into a proton (changes color); 3) electron is emitted from nucleus Tritium is an atom of hydrogen that consists of a proton and two neutrons. (Deuterium, a non-radioactive form of hydrogen, is not radioactive.) Water molecules made with tritium or deuterium forms of hydrogen are called heavy water.

When tritium decays, one of its neutrons turns into a proton. The change from neutral state to positive causes the neutron to emit an electron. With two protons rather than one, the atom becomes a helium atom. This type of decay is known as beta-minus decay because it emits a negative electron. Beta-plus decay occurs when a proton turns into a neutron, which causes the emission of a positively charged electron (a positron).

Back to top


The Earth contains various forms of naturally occurring radioactive sources, including uranium, potassium, and carbon. One of the products of uranium decay is the gas radon, which sometimes collects inside enclosed spaces, such as homes. Radon decays quickly by alpha decay and can easily find its way into the body through inhalation. Prolonged exposure to high levels of radon may cause lung cancer.

Alpha Decay
uranium atom decays, releasing an alpha particle (helium nucleus When an atom decays by emitting an alpha particle, it changes into another element. For example, the release of alpha particles causes uranium to become thorium, thorium to become radium, and radium to become radon.

When the atom decays, a particle consisting of two protons and two neutrons—a helium nucleus—shoots out of the nucleus of the radioactive atom. Once it leaves the atom, it often picks up two free electrons and becomes a helium atom. An alpha particle doesn't travel very far: a single sheet of paper or the dead outer layer of your skin cells will stop it.

In addition to alpha radiation, naturally radioactive elements in the Earth also emit gamma and beta radiation.

Back to top

Radioactivity within the House

There are several potential sources of radiation inside the house. One is radon-222, which can seep from soil and concrete and into the house. (For more about radon, see Radioactivity in the Earth.) Others include smoke detectors (americium), watches (tritium), and even you (carbon-14 and potassium-40).

Radioactivity within the House
With the exception of radon, which has been linked to lung cancer, the radioactive substances just listed pose little threat to human health. The radioactive material within smoke detectors emits essentially no radiation, as the detector itself absorbs the alpha particles emitted by the radioactive americium. Some old watches make use of tritium to make the numerals on the watch's dial glow, but although tritium emits beta particles when it decays, the particles do not have enough energy to pass through the watch's glass cover.

There's radioactive material within your body—carbon-14 and potassium-40—but no one knows for sure if these elements pose a great threat to health. If natural radiation does hurt health, the effects are so small that they have been impossible to measure. Moreover, there's little understanding of how the human body repairs radiation damage or what stimulates or degrades the repair process.

Back to top

Sky and Space

Earth's atmosphere protects us from cosmic radiation—fast-moving particles such as protons—that come from the sun and from outside the solar system. Like other forms of harmful radiation, cosmic radiation can damage cells by breaking the chemical bonds within cells.

Cosmic Radiation
fast-moving hydrogen nuclei collides with a nitrogen atom to create a shower of smaller particles Cosmic radiation consists of protons, alpha particles, electrons, and the nuclei of atoms. Within our solar system they originate in the sun. They also come from sources outside the solar system, such as pulsars and supernovae.

Some of these particles, which can travel at speeds close to that of light, make their way to Earth's surface, but most collide with the atoms and molecules in the atmosphere long before they reach the surface. These collisions produce a shower of fragments that then rain down toward the surface.

Back to top


Although it's not commonly done and is, in fact, against the law, the disposal of radioactive materials at landfills not licensed for radioactive waste does occur. Sometimes material is thrown out inadvertently. For example, a lost or stolen medical or industrial instrument containing cesium-137 may be found by someone unaware of its radioactivity and brought to a landfill.

Gamma Radiation
1) cesium atom; 2) one of the neutrons within the nucleus turns into a proton (changes color); 3) electron and photon of gamma radiation emitted Cesium-137 emits beta and gamma radiation. When it decays, it expels a beta-minus particle (an electron) and sometimes a gamma photon. It emits the beta particle when one of its neutrons changes into a proton. This changes the cesium atoms into an atom of barium. If, from the conversion, there's extra energy within the nucleus, the nucleus sends out a high-energy photon—gamma radiation.

Unlike beta and alpha particles, gamma rays have great penetrating ability.

Back to top

Atoms and Particles

a helium atom with 2 protons, 2 neutrons, and two electrons, and a photon leaving the atom; also, a positron electron: a particle with a negative charge, often found near an atom's nucleus; carrier of beta-minus radiation

positron: an electron with a positive charge; carrier of beta-plus radiation

photon: a massless particle that travels at the speed of light; carrier of electromagnetic radiation

proton: a particle with a positive charge, often found inside the nucleus of an atom

neutron: a particle with no electrical charge, often found within the nucleus of an atom

nucleus: the central region of an atom, made up of protons and neutrons (nuclei are more than one nucleus)

Radiation is the emission of particles or electromagnetic waves.

Back to top

Dirty Bomb Web Site Content
Preparing for Terrorism

Preparing for Terrorism
Graham Allison discusses both radiological and nuclear terrorism.

Ask the Expert

Ask the Expert
Charles Ferguson of the Center for Nonproliferation Studies answers your questions.

Chronology of Events

Chronology of Events
Review incidents involving unprotected radioactive materials.

Sources of Radiation

Sources of Radiation
Explore radioactive sources both harmful and beneficial, natural and manmade.

Send feedback Image credits
NOVA Home Find out what's coming up on air Listing of previous NOVA Web sites NOVA's history Subscribe to the NOVA bulletin Lesson plans and more for teachers NOVA RSS feeds Tell us what you think Program transcripts Buy NOVA videos or DVDs Watch NOVA programs online Answers to frequently asked questions