Ohio State University Factsheet

What Is Ionizing Radiation?


Audeen W. Fentiman
Timothy A. Leet
Ronald J. Veley

Many kinds of radiation are found in our environment. They include visible radiation (light), radio waves, ultraviolet radiation, and cosmic rays. These are examples of electromagnetic radiation found in the electromagnetic spectrum shown in Figure 1. Also shown in the spectrum is gamma radiation, one kind of ionizing radiation.

Figure 1

Figure 1. The Electromagnetic Spectrum

Some types of radiation are detectable by our senses. We see visible light, feel ultraviolet radiation after it burns our bodies, and feel the warmth of infrared radiation. Ionizing radiation is not detected by any of our senses, but can be easily detected by electronic equipment.

Ionizing radiation is emitted by radioactive atoms. Three types of ionizing radiation are alpha particles, beta particles, and gamma rays. Even though these high-energy forms of radiation cannot be detected by our senses, they are important because they can damage our bodies. This fact sheet describes ionizing radiation and how it behaves.

Atoms and Ions

To understand ionizing radiation, it is helpful to understand the structure of an atom. An atom is composed of a positively charged nucleus orbited by negatively charged electrons (see Figure 2). Normally, the number of positive charges (protons) in the nucleus is equal to the number of negative electrons orbiting the nucleus, and the atom is electrically neutral.

Figure 2
Figure 2. Structure of an Atom

If an electron is "knocked" out of its orbit in an atom, the result is a free negative electron and a positively charged ion. When ionizing radiation passes through material, it can "knock" electrons out of their orbits, forming ions; hence its name (see Figure 3). Some energy from the ionizing radiation is used up each time an electron is "knocked" from its orbit.

Figure 3
Figure 3. Ionization of an Atom

The formation of ions is important for two reasons. First, if ions are formed in living tissue, such as the human body, they can cause damage. Second, because ions have an electrical charge, they are easy to detect. This makes it possible to measure the amount of radiation present even at extremely low levels.

Three important types of ionizing radiation emitted by radioactive atoms are alpha particles, beta particles, and gamma rays.

Alpha Radiation

An alpha particle is composed of two neutrons with no charge and two positively charged protons (see Figure 4). When alpha particles travel through solid material, they interact with many atoms within a very short distance. They create ions and use up all their energy in that short distance. Most alpha particles will use up their energy while traveling through a single sheet of ordinary notebook paper. The primary health concern associated with alpha particles is that when alpha-emitting materials are ingested or inhaled, energy from the alpha particles is deposited in internal tissues such as the lungs.

Figure 4
Figure 4. Types of Ionizing Radiation

Beta Radiation

A beta particle is an electron that is not attached to an atom. It is small, over 7000 times lighter than an alpha particle. The beta particle travels farther through solid material than an alpha particle. For example, a very high-energy beta particle will travel about half an inch through plastic before it uses up all its energy. Like alpha particles, beta particles lose energy with every interaction and no longer produce ions once all their energy is spent. Health concerns associated with beta particles arise primarily when beta-emitting materials are ingested or inhaled.

Gamma Radiation

Gamma radiation (gamma rays) can pass completely through the human body. Thus gamma rays emitted outside of the body may cause ionization, and possible health effects, in any organ in the body. But once a gamma ray loses all its energy, it can no longer cause damage. Most high energy gamma rays will lose all their energy in a few feet of soil, three feet of concrete, or six inches of lead.

For More Information

If you would like to read more about ionizing radiation, some of the references and other fact sheets listed below may be helpful.

American Chemical Society, Chemistry in the Community, 2nd edition, Kendall/Hunt Publishing Company, 1988. (Fifth section of the book, Nuclear Chemistry in Our World, discusses radiation)

Raymond L. Murray, Understanding Radioactive Waste, Battelle Press, 1989.

Other Fact Sheets:

RER-20 What is radioactive material and how does it decay?
RER-22 What are the sources of ionizing radiation?
RER-24 What are the health effects of ionizing radiation?
RER-26 How are people protected from ionizing radiation?

Author Notes:

Dr. Audeen W. Fentiman is an Assistant Professor in Nuclear Engineering at The Ohio State University. Timothy A. Leet is a Graduate Research Associate in Nuclear Engineering. Ronald J. Veley is a Graduate Research Associate, Ohio State University Extension.

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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

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