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 Module 2 - Where Radiation Comes From

The second RADUCATION module will look at where radiation comes from, keeping in mind that our focus is on “ionizing” radiation:

  1. Radioactive decay and half-life
  2. Sources of radiation
  3. Radiation in the environment
  4. Radiation in the body 

1. Radioactive Decay and Half-life

Before we look at sources of ionizing radiation we need to touch on two concepts that are important to understanding how some radioactive materials are formed: radioactive decay and half-life.

In Module 1 we briefly mentioned that an atom with too much energy in the nucleus is said to be unstable.  All radioactive atoms - also known as radionuclides - are unstable and give up their extra energy through various methods, all of which we refer to as radioactive decay.  

How the energy is released and the amount of time it takes to reach a stable status depends on the radionuclide.  Throughout the process a radionuclide may decay to a stable atom or to another radionuclide, which may itself decay to another different radionuclide, and so on.  Only after all the extra energy has been released will the result be a stable atom, which may be a different element than the original.

If you have a large number of atoms of a certain radionuclide it is impossible to be able to say exactly when any one of the atoms will decay.  However, scientists have been able to calculate how long it will take for half of the atoms of the same radionuclide to decay, and this time period is called the half-life.  The half-life may be as brief as a tiny fraction of a second, or as long as hundreds of thousands of years, and it is different for each radionuclide. 
In Module 1 we used properties of the element Carbon to define the terms atomic number and isotope. Since we are already familiar with this element, we will use radioactive Carbon-14 to demonstrate radioactive decay and half-life.    
  Let’s imagine that we have gathered 100 atoms of the radioactive isotope Carbon-14 in a large bag. The half-life of Carbon-14 is 5730 years, which means that half of the 100 radioactive Carbon-14 atoms originally in our bag will have decayed after 5730 years, leaving us with 50 radioactive atoms remaining.

Half of those 50 atoms will have decayed after the second 5730 years, leaving only 25 Carbon-14 atoms in the bag. This process will continue, leaving half of the previous number of atoms after each half-life has passed, until all the atoms of Carbon-14 have become stable. 

2. Sources of Radiation

For purposes of this RADUCATION module, we will look at the production of ionizing radiation from two sources: through the use of radiation-generating equipment (i.e. X-ray machines) or emitted from radioactive material.

  With radiation-generating equipment, the radiation is produced as a result of the interaction of a focused beam of highly-energetic electrons with target typically made of copper or tungsten. The energy of the X-rays produced may be varied by changing the amount of energy applied to the electron stream. There is no decay associated with the ionizing radiation produced in this manner; once the source of power has been shut-off or disconnected the device will no longer generate X-rays.

Radioactive materials may be present as a result of certain man-made activities. However, there is a very small quantity of radioactive material that is a natural part of our environment. These naturally-occurring radionuclides are present in the air and in the water, soil, and rocks throughout much of the earth. Whether the source is natural or man-made, radionuclides will continue to emit radiation until all the atoms have fully decayed away.  

3. Radiation in the Environment

Many people would probably be surprised to learn that there is radiation all around us and we are exposed to radiation from many different sources every day.  Some radionuclides have been present in materials making up the earth’s crust since our planet was created.  Some of the most common of these naturally-occurring radioactive materials are isotopes of Potassium (K-40), Thorium (Th-232), Radium (Ra-226), and Radon (Rn-222).  Many building products, like cement or bricks, may contain measurable quantities of these materials.

  Radionuclides can also be found in the air. Some, such as our previous example of Carbon-14, are a result of cosmic radiation from space interacting with Nitrogen and Oxygen atoms in the Earth’s atmosphere. Other radionuclides, such as Hydrogen-3, Cesium-137 and Iodine-131, are present primarily as a result of nuclear weapons testing. These radioactive particles typically remain suspended by air currents, or may be “washed” from the atmosphere by rain or snow onto the earth’s surface. 
Radioactive materials can also be detected in water. Some may be present as a result of being deposited from the atmosphere as described above. Other radionuclides may be collected as water moves on or through soil and rocks. Some naturally-occurring radioactive materials (i.e., Radium and Radon) may be released from the ground through erosion or seepage. How a radionuclide moves in water is dependent on several factors. If the radionuclide will dissolve easily in water it will likely stay in the water as it travels on or below the surface. Other radioactive materials may stick (adhere) to the surface of the materials they pass through, limiting the distance they will travel with the water.   

4. Radiation in the Body

Radiation is sometimes administered intentionally, usually for medical diagnosis or therapy.  The method may be by injection, implants, or when taking X-rays.  Outside of these medical uses there are three main pathways for radionuclides to enter the body:

Absorption – radioactive material is absorbed through the skin, or may enter through a wound or other break in the skin surface.

Inhalation – radioactive particles suspended in the air may enter the body through the normal breathing process.

Ingestion – radioactive materials may be present in water and many types of food and are taken in when an individual is eating or drinking. 

Absorption and inhalation are generally thought of as occupational hazards, where radionuclides are used in the workplace in such areas as research, industry, or pharmacies.  These materials may be accidentally spilled or become airborne during their use and then become absorption or inhalation hazards in that work area.

However, there is one common example of inhalation – smoking – that contributes to much of the radiation received by some members of the public. As the tobacco plants grow, they absorb radioactive Lead and Polonium from the soil. These radionuclides remain in the leaves through the drying and curing processes and are still present when the tobacco is used to make cigars, cigarettes, and other products. Each time a smoker inhales, they are taking in small quantities of these radionuclides (in addition to tar, nicotine, and other chemicals) where they are then deposited directly in the lungs.   

Other than from medical procedures or occupational use, the primary method for radioactive materials to enter the body is ingestion through the food chain.  As we described above, radionuclides may be present in many soils and minerals, and in the water.  Plants growing in these areas may absorb radionuclides from the soil or water and these materials will remain in the plants. Animals eating these plants, or drinking the water, will also absorb some radioactive materials, and they will remain stored in their bodies. 



If people eat the plants or animals that have absorbed the radionuclides, they will in turn, absorb them into their own bodies. Where the plants or animals are grown will determine what radionuclides and how much is present. Some examples of “radioactive” foods are bananas, Brazil nuts, carrots, lima beans, and beer. While the radionuclides are present in these foods in very small quantities, they are measurable and are part of all the different types of radiation received by the public every day. 

Our next RADUCATION module will look at how ionizing radiation is detected and measured, and the units used to quantify various properties of radiation.  Module 3 will also discuss how much radiation a typical member of the public is exposed to in everyday life as a result of some of the examples shown above.

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If you would like more information on the topics covered in this module, these links may be helpful:

Health Physics Society, Public Information Web-site:

The Ohio State University Agriculture Extension Service:


US Food and Drug Administration, Radiation-Emitting Products:

US Nuclear Regulatory Commission, Radioactive Materials:

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If you have questions about any of the information in this module, or want to suggest a topic for a future module, please send your request to:  and put “RADUCATION” in the subject header.

Mailing Address:
Ohio Department of Health
Bureau of Environmental Health and Radiation Protection
Radiation Protection Programs 
246 N. High St.
Columbus, OH 43215

Telephone: (614) 644-2727
Fax: (614) 466-0381

Last Updated: 12/03/2015