24.2. Half-Lives http://www.ck12.org
24.2 Half-Lives
Lesson Objectives
- Define half-life, and be able to perform calculations relating the half-life of an isotope to the radioactivity of
that isotope over time.
Lesson Vocabulary
- half-life: The amount of time needed for one-half of a sample to spontaneously decay.
Check Your Understanding
- What tools can be used to determine the rate at which a radioactive substance is undergoing decay?
Half-Life
As radioactive substances gradually decay, there is less and less of the original material present, Thehalf-lifeof an
isotope is defined as the period of time needed for one-half of a sample to spontaneously decay. Some isotopes have
long half-lives, such as uranium-234, which has a half-life of 245,000 years. Other isotopes have shorter half-lives.
Iodine-131, which is used in thyroid scans, has a half-life of 8.02 days, and oxygen-15 has a half-life of only 2
minutes. Information on the half-life of an isotope can be used to calculate how much of that isotope will be present
after a certain period of time. TheTable24.1 includes the half-lives of a few other radioactive isotopes.
TABLE24.1:
Isotope Decay Mode Half-Life
Cobalt-60 beta 5.3 years
Neptunium-237 alpha 2.1 million years
Polonium-214 alpha 0.00016 seconds
Radium-224 alpha 3.7 days
Tritium (H-3) beta 12 yearsFigure24.6 illustrates a typical decay curve for a radioactive substance. The amount decreases by one-half after
each successive half-life.
One way to measure the amount of radioactive material present is by looking at the frequency of radioactive
emissions with a Geiger counter or other device. In this section, we will assume that we are dealing with a nuclear
decay process in which the parent nucleus is radioactive but the resulting nucleus is a stable isotope. When this is the