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RADIOACTIVE HALF-LIFE (CONTINUED)

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

  • Describe carbon dating and how half-life information is used.
  • Explain how a radiographer uses half-life information.

As we have mentioned before each radioactive isotope has its own decay pattern. Not only does it decay by giving off energy and matter, but it also decays at a rate that is characteristic to itself. The rate at which a radioactive isotope decays is measured in half-life. The term half-life is defined as the time it takes for one-half of the atoms of a radioactive material to disintegrate. Half-lives for various radioisotopes can range from a few microseconds to billions of years. See the table below for a list of radioisotopes and each of unique their half-lives.

Radioisotope
Half-life
Polonium-215
0.0018 seconds
Bismuth-212
60.5 seconds
Sodium-24
15 hours
Iodine-131
8.07 days
Cobalt-60
5.26 years
Radium-226
1600 years
Uranium-238
4.5 billion years

How does the half-life affect an isotope?

Let's look closely at how the half-life affects an isotope. Suppose you have 10 grams of Barium-139. It has a half-life of 86 minutes. After 86 minutes, half of the atoms in the sample would have decayed into another element, Lanthanum-139. Therefore, after one half-life, you would have 5 grams of Barium-139, and 5 grams of Lanthanum-139. After another 86 minutes, half of the 5 grams of Barium-139 would decay into Lanthanum-139; you would now have 2.5 grams of Barium-139 and 7.5 grams of Lanthanum-139.

How is half-life information used in carbon dating?

The half-lives of certain types of radioisotopes are very useful to know. They allow us to determine the ages of very old artifacts. Scientists can use the half-life of Carbon-14 to determine the approximate age of organic objects less than 40,000 years old. By determining how much of the carbon-14 has transmutated, scientist can calculate and estimate the age of a substance. This technique is known as Carbon dating. Isotopes with longer half-lives such as Uranium-238 can be used to date even older objects.

You will learn more about carbon dating in the next sub-unit.

Uses of the half-life in NDT

In the field of nondestructive testing radiographers (people who produce radiographs to inspect objects) also use half-life information. A radiographer who works with radioisotopes needs to know the specific half-life to properly determine how much radiation the source in the camera is producing so that the film can be exposed properly. After one half-life of a given radioisotope, only one half as much of the original number of atoms remains active. Another way to look at this is that if the radiation intensity is cut in half; the source will have only half as many curies as it originally had. It is important to recognize that the intensity or amount of radiation is decreasing due to age but not the penetrating energy of the radiation. The energy of the radiation for a given isotope is considered to be constant for the life of the isotope.

Review:

  1. The half-life of radioisotopes varies from seconds to billions of years.
  2. Carbon-dating uses the half-life of Carbon-14 to find the approximate age of an object that is 40,000 years old or younger.
  3. Radiographers use half-life information to make adjustments in the film exposure time due to the changes in radiation intensity that occurs as radioisotopes degrade.