Understanding Radiation

The information on this page is obtained by courtesy of U.S. Environmental Protection Agency.

We cannot eliminate radiation from our environment. We can, however, reduce our risks by controlling our exposure to it. Understanding radiation and radioactivity will help you make informed decisions about your exposure.

What is radiation?

Radiation is energy that travels in the form of waves or particles. When we hear the word ' radiation,' we generally think of nuclear power plants, nuclear weapons, or radiation treatments for cancer. We would also be correct to add 'microwaves, radar, electrical power lines, cellular phones, and sunshine' to the list. There are many different types of radiation that have a range of energy forming an electromagnetic spectrum. However, when you see the word 'radiation' on this Website, we are referring to the types of radiation used in nuclear power, nuclear weapons, and medicine. These types of radiation have enough energy to break chemical bonds, and are referred to as 'ionizing radiation.'

What is radioactivity?

The radioactivity is the property of some atoms to spontaneously give off energy as particles or rays. The atoms that make up the radioactive materials are the source of radiation. To be able to understand radiation and radioactivity, you need to understand the language of atomic structure.

What is an Atom?

Atoms are the extremely small particles of which we, and everything around us, are made. A single element, such as oxygen, is made up of similar atoms. Different elements, such as oxygen, carbon, and uranium contain different kinds of atoms. There are 92 naturally occurring elements and scientists have made another 17, bringing the total to 109. Atoms are the smallest unit of an element that chemically behaves the same way the element does. When two chemicals react with each other, the reaction takes place between individual atoms--at the atomic level. The processes that cause materials be radioactive--to emit particles and energy--also occur at the atomic level.

Atomic Structure

In the early 20th century, a New Zealand scientist, Ernest Rutherford, and a Danish scientist, Niels Bohr, developed a way of thinking about the structure of an atom that described an atom as looking very much like our solar system. At the center of every atom was a nucleus, which is comparable to the sun in our solar system. Electrons moved around the nucleus in "orbits" similar to the way planets move around the sun. (While scientists now know that atomic structure is more complex, the Rutherford-Bohr model is still a useful approximation to begin understanding about atomic structure.)

What holds the parts of an atom together?

Opposite electrical charges of the protons and electrons do the work of holding the nucleus and its electrons together. Electrons closer to the nucleus are bound more tightly than the outer electrons because of their distance from the protons in the nucleus. The electrons in the outer orbits, or shells, are more loosely bound and affect an atom's chemical properties. A delicate balance of forces among nuclear particles keeps the nucleus stable. Any change in the number, the arrangement, or energy of the nucleons can upset this balance and cause the nucleus to become unstable or radioactive. (Disruption of electrons in the inner orbits can also cause an atom to emit radiation.) The amount of energy required to break up the nucleus into its parts is called the binding energy; it is often referred to as "cosmic glue". This is the same amount of energy given off when the nucleus formed.

Nuclides & Isotopes

An atom that has an unbalanced ratio of neutrons to protons in the nucleus seeks to become more stable. The unbalanced or unstable atom tries to become more stable by changing the number of neutrons and/or protons in the nucleus. This can happen in several ways:

• converting neutrons to protons
• converting protons to neutrons
• ejecting an alpha particle (two neutrons and two protons) from the nucleus.

Whatever the mechanism, the atom is seeking a stable neutron to proton ratio. In changing the number of nucleons (protons and neutrons), the nucleus gives off energy in the form of ionizing radiation. The radiation can be in the form of alpha particles (2 protons and 2 neutrons), beta particles (either positive or negative), x-rays, or gamma rays.

Is the atom still the same element?

Only sometimes. If there is a change in the number of protons, the atom becomes a different element with different chemical properties. If there is a change in the number of neutrons, the atom is the same element, but becomes a different isotope of that element. All isotopes of one element have the same number of protons but different numbers of neutrons. All isotopes of a certain element also have the same chemical properties but have varying radiological properties such as half-life, or type of radiation emitted.

What if the protons and electrons of an atom are unbalanced?

Normally, the number of electrons and protons is the same, so the atom is balanced electrically. Sometimes electrons are added or removed, and the atom carries a negative or positive charge. These charged forms of an element are called 'ions' of the element. This change affects the way the atom reacts chemically, but does not affect the stability of the nucleus--the atom's radioactivity.

What are nuclides and radionuclides?

Nuclide is a term used to categorize different forms of atoms very specifically. Each nuclide has a unique set of characteristics:

• number of protons
• number of neutrons
• energy state.

If any of these change, the atom becomes a different nuclide. Approximately 3,700 nuclides have been identified. Most of them are radionuclides, meaning they are unstable and undergo radioactive decay.

Radiation Protection Basics

Three basic concepts apply to all types of ionizing radiation. When we develop regulations or standards that limit how much radiation a person can receive in a particular situation, we consider how these concepts might affect a person's exposure.

Basic Concepts of Radiation Protection

Time • Distance • Shielding

Time Distance Shielding Image


The amount of radiation exposure increases and decreases with the time people spend near the source of radiation. In general, we think of the exposure time as how long a person is near radioactive material. It's easy to understand how to minimize the time for external (direct) exposure. Gamma and x-rays are the primary concern for external exposure. However, if radioactive material gets inside your body, you can't move away from it. You have to wait until it decays or until your body can eliminate it. When this happens, the biological half-life of the radionuclide controls the time of exposure. Biological half-life is the amount of time it takes the body to eliminate one half of the radionuclide initially present. Alpha and beta particles are the main concern for internal exposure.


The farther away people are from a radiation source, the less their exposure. How close to a source of radiation can you be without getting a high exposure? It depends on the energy of the radiation and the size (or activity) of the source. Distance is a prime concern when dealing with gamma rays, because they can travel long distances. Alpha and beta particles don't have enough energy to travel very far. As a rule, if you double the distance, you reduce the exposure by a factor of four. Halving the distance, increases the exposure by a factor of four.


The greater the shielding around a radiation source, the smaller the exposure. Shielding simply means having something that will absorb radiation between you and the source of the radiation (but using another person to absorb the radiation doesn't count as shielding). The amount of shielding required to protect against different kinds of radiation depends on how much energy, mass, and charge of the particle.


A thin piece of light material, such as paper, or even the dead cells in the outer layer of human skin provides adequate shielding because alpha particles can't penetrate it. However, living tissue inside body, offers no protection against inhaled or ingested alpha emitters.


Additional covering, for example heavy clothing, is necessary to protect against beta-emitters. Some beta particles can penetrate and burn the skin.


Thick, dense shielding, such as lead, is necessary to protect against gamma rays. The higher the energy of the gamma ray, the thicker the lead must be. X-rays pose a similar challenge, so x-ray technicians often give patients receiving medical or dental X-rays a lead apron to cover other parts of their body.