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Module 4 - Ionizing Radiation Detection and Measurement

This module of RADUCATION will discuss detection and measurement of ionizing radiation:

  1. Exposure versus Contamination
  2. Detecting Ionizing Radiation
  3. Units for Measuring Ionizing Radiation  

1.  Exposure versus Contamination   

There are two important differences that we need to discuss before we look at detection and measurement of ionizing radiation:  Exposure and Contamination.  We can get a general idea of what these two terms mean if we look at what happens if we aren't careful with a cup of hot coffee.

Exposure (sometimes called Irradiation) refers to the transfer of energy from the radioactive material.  The radiation may penetrate the body, as in the case of gamma radiation or an x-ray, but there is no transfer of material that takes place.  Looking at our cup of coffee, we could say that we are receiving exposure from the heat being givenSpilled cup of coffee off by the hot liquid (although we know from earlier RADUCATION modules that heat is not a type of ionizing radiation).

Contamination means that radioactive material is present, but it is someplace other than where we want it to be.  The material may be present on an object or person and must be removed.  Generally, such situations are complications to the normal work being performed, are not life-threatening, and are no longer an issue once the radioactive material has been removed.  Using our coffee analogy, a full cup waiting to be enjoyed would be the typical situation.  If coffee is spilled from the cup into the saucer (or onto a table, newspaper, or computer keyboard) the spilled liquid would be considered contamination – the coffee isn’t where we want or expect it to be for its intended use.  Cleaning up the spilled beverage (and maybe replacing the tablecloth, newspaper or keyboard) will correct the problem.

2. Detecting Ionizing Radiation
Person Looking at a Radiation Symbol
Ionizing radiation is not detectable simply by any of the human senses – it cannot be seen, tasted, smelled or touched.  However, a number of measuring devices that have been developed to detect and measure ionizing radiation.  These devices measure the interaction of ionizing radiation on some type of element or media, and convert those interactions into electrical data that may be read on a screen, graph or monitor.

Detection devices may be used in academic, research, and laboratory settings, or in field locations in the form of portable, hand-held units.  Below are some of the most common types of detection devices, with brief descriptions of how they operate:

Geiger Counter (also known as a Geiger-Mueller or GM tube) – This is a gas-filled device that uses a high voltage.  When radiation interacts with the tube wall or the gas in the tube, an electrical pulse is created.  The electrical pulses are read on a meter installed on the device. 

Liquid Scintillation Counters – These devices are primarily used in research or laboratory settings to measure for low-energy beta radiation.  Samples are placed in vials that contain a liquid detection media, or cocktail.  This media gives off light which is collected and measured in a similar manner to the sodium iodide detector mentioned above.   A liquid scintillation counter is typically able to detect very low amounts of radiation present in the samples through the use of shielding, cooling methods, and electronic enhancements.

3.  Units for Measuring Ionizing Radiation   
Calculator, Compass, Ruler
We use many different units of measure in our everyday life to define things like size, shape, speed, etc.   Height or width may be measured in inches, feet, or meters.  How fast something is traveling may be expressed in miles per hour or revolutions per minute; the distance traveled in kilometers, furlongs, or light years.  Area and volume are measured using units like square feet, cubic centimeters, or acres.

Just like these applications mentioned above, there are special units we use to define quantities of ionizing radiation. Those who work in the field of radiation safety, or health physics, use terms like: half-value layers; becquerel; action levels; quality factors; committed effective dose; and derived air concentration.  However, for this RADUCATION module, we will focus on two of the most common and relevant terms:

             Activity – the amount of a radioactive material present. 

   Dose – the amount of ionizing radiation exposure that has been received

 Marie Curie

Activity is measured in a unit called a Curie.  The curie is named after Marie Curie, and is based on the observations she made with her husband, Pierre, in their pioneering work with radium in the late 1800’s. The curie is named after Marie Curie, and is based on the observations she made with her husband, Pierre, in their pioneering work with radium in the late 1800’s.


A curie is defined as the amount of radioactive atoms present that will have 37 billion disintegrations in one second.

Pictorial of Boxes Emitting Gamma RadiationThis number of atoms will be different for each radionuclide.  A large amount of material can have a very small amount of radioactivity; a very small amount of material can have a lot of radioactivity.  To help us better understand this, let’s look at two different non-radioactive materials: marbles and feathers. 

Bag of Marbles

 If we were to put a pound of glass marbles in a box, we would probably have a relatively small box.  However, if we wanted to box up a pound of feathers, we would need a much larger box, because each feather weighs substantially less than each marble and it would require a lot more of them to make up a pound.

The same idea holds true when talking about a curie of radioactive material – the number of atoms necessary will be different for each radionuclide and will require a different size “box” to hold them.

Dose is measured in rads, a unit that describes how much ionization occurs in air.  Rad is actually an acronym for the term “radiation absorbed dose”. 

When referring to the dose received by a person, we use a unit called the remA rem is essentially the same as a rad, with a “quality factor” added to account for the type of ionizing radiation exposed to (i.e., alpha, beta, gamma) and the ease in which the different types of radiation can transfer their energy to the body.

The next RADUCATION module will discuss some typical doses received by the public through natural and man-made sources of ionizing radiation.

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

    The Ohio State University Agriculture Extension Service:

    Health Physics Society website:

    U.S. Department of Labor, OSHA website:

    Centers for Disease Control and Prevention website:

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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: 12/28/2015