RADIOACTIVE HALF-LIFE

Radioactive Half-Life

The radioactive half-life for a given radioisotope is the time for half the radioactive nuclei in any sample to undergo radioactive decay. After two half-lives, there will be one fourth the original sample, after three half-lives one eight the original sample, and so forth.

RADIOISOTOPE DECAY

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.
(1) Take this link to learn how to determine radioactive sources in curies:
Comparing Radioactive Activity
(2) Take this link to learn how to assess how much radiation is emitted from a source:
Assessing the Amount of Radiation Coming From a Source

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. 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
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.

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.