Radioactive HalfLife (Continued)
After this reading this section you will be able to do the following:
 Describe carbon dating and how halflife information is used.
 Explain how a radiographer uses halflife 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 halflife. The term halflife is defined as the time it takes for onehalf of the atoms of a radioactive material to disintegrate. Halflives 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 halflives.
Radioisotope

Halflife

Polonium215

0.0018 seconds

Bismuth212

60.5 seconds

Sodium24

15 hours

Iodine131

8.07 days

Cobalt60

5.26 years

Radium226

1600 years

Uranium238

4.5 billion years

How does the halflife affect an isotope?
Let's look closely at how the halflife affects an isotope. Suppose you have 10 grams of Barium139. It has a halflife of 86 minutes. After 86 minutes, half of the atoms in the sample would have decayed into another element, Lanthanum139. Therefore, after one halflife, you would have 5 grams of Barium139, and 5 grams of Lanthanum139. After another 86 minutes, half of the 5 grams of Barium139 would decay into Lanthanum139; you would now have 2.5 grams of Barium139 and 7.5 grams of Lanthanum139.
How is halflife information used in carbon dating?
The halflives 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 halflife of Carbon14 to determine the approximate age of organic objects less than 40,000 years old. By determining how much of the carbon14 has transmutated, scientist can calculate and estimate the age of a substance. This technique is known as Carbon dating. Isotopes with longer halflives such as Uranium238 can be used to date even older objects.
You will learn more about carbon dating in the next subunit.
Uses of the halflife in NDT
In the field of nondestructive testing radiographers (people who produce radiographs to inspect objects) also use halflife information. A radiographer who works with radioisotopes needs to know the specific halflife to properly determine how much radiation the source in the gamma camera is producing so that the film can be exposed properly. After one halflife 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 radiographer will need to expose the source for twice as long as when the gamma camera was new. It is important to recognize that while the amount of radiation is decreasing due to age, the penetrating energy of the radiation remains constant.
Review:
 The halflife of radioisotopes varies from seconds to billions of years.
 Carbondating uses the halflife of Carbon14 to find the approximate age of an object that is 40,000 years old or younger.
 Radiographers use halflife information to make adjustments in the film exposure time due to the changes in radiation intensity that occurs as radioisotopes degrade.