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
BPolonium-215
0.0018 seconds
ismuth-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:
The half-life
of radioisotopes varies from seconds to billions of years.
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.
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.
