Home - Education Resources - Science of NDT - Radiography


After this reading this section you will be able to do the following:

  • Define radioactive half-life.
  • Explain how you measure the decay of radioactive isotopes.
  • Compare two radioactive sources and determine their specific activities in curies.

Not all of the atoms of a radioisotope decay at the same time, but they decay at a rate that is characteristic to the isotope. The rate of decay is a fixed rate called a half-life. The half-life of a radioisotope describes how long it takes for half of the atoms in a given mass to decay. Some isotopes decay very rapidly and, therefore, have a high specific activity. Others decay at a much slower rate.

How do you measure the decay of radioactive isotopes?

Now that we have an idea of how radioactive isotopes decay, let's look at how this is measured and apply the terms we just learned.

The basic unit of measure for describing the activity (radioactivity) of a quantity of radioactive material is the curie, named after Marie Curie. A quantity of radioactive material is considered to have an activity of 1 curie or 1 C, when 37 billion of its atoms decay (disintegrate) in one second. In scientific terms, this is expressed by the equation: 1C = 3.7 X 1010 disintegrations/sec. Remember that we said each isotope has its own decay pattern. If the rate of decay is greater than 37 billion atoms in one second, then the source would have an activity greater than one curie, and if that source had fewer than 37 billion atoms decaying in one second, its activity would be less than one curie.

Now that you know that the activity of a radioactive source is the measure of the number of atoms that decay each second and that the activity varies as a function of the size of the source, let's see why half-life is important.


  1. The term half-life describes how long it will take for half of the atoms to decay, and is constant for a given isotope.
  2. The curie the unit of measure used to describe the radioactivity of radioactive material. (1C = 3.7 X 1010 disintegrations/sec)
  3. The disintegration of the atoms from different isotopes can produce different amounts of radiation.