This graph is a good illustration of the concept of half-life. If you have a sample of 100 atoms of a radioactive isotope with a half-life of 10 minutes this means that within that first 10 minutes an estimated 50% of the atoms will have gone through radioactive decay and transmuted into another element. After the first half-life, an estimated 50 atoms are still the original element. During the second half-life (in this example: 10 minutes) another estimated 50% will undergo radioactive decay leaving about 25 of the original element.
This process continues until there are no more atoms left of the original element. Sometimes the new element created is also radioactive. All radioactive elements undergo this half-life process. A single atom may transmute into several elements before finally undergoing a final radioactive decay that transmutes it into a stable non-radioactive element.
Each radioactive element's half-life is a different period of time. Below is a chart of uranium-238’s nuclear half-life cycle. About half of a sample of Uranium-238 atoms will spontaneously undergo alpha decay within 4.5 billion years. An estimated half of the sample will still be U-238, but the other half will now be Thorium-234. About half of the sample of this element undergoes beta decay every 24.1 days transmuting the atoms into Practinium-234. As you can see by this chart, the process continues until a stable isotope is reached. In Uranium-238's decay chain that stable isotope is Lead-206. Since Lead is stable and non-radioactive it does not go through the process of radioactive decay such as radioactive isotopes do.
A sample of Uranium atoms has about a 50% chance of undergoing radioactive decay at any time. Within the Uranium-238 decay cycle, half-life can range from billions of years to mere seconds. Radioactive waste disposal is another example of the use of radioactive half-life knowledge in creating estimates that ensure proper handling and disposal of this waste. As illustrated in the chart above, Uranium-238 can eventually decay into Lead-206 which is non-radioactive. It takes an estimated several hundred thousand years for U-238 to decay into a stable non-radioactive isotope. However, keep in mind we used U-238 as an example of a decay chain in this segment. Other radioactive isotopes of Uranium and Plutonium are used in nuclear energy and weapons production that have different half-life cycles and decay chains. What does not change is that this process of decaying into a stable element can take a long time and pose many challenges for radioactive waste disposal.
Radioactive half-life is also important to understand when dealing with the effects of a nuclear disaster in terms of how long an area may remain radioactive. Nuclear waste and clean-up will be the subject of a future segment of Nuclear 101.
Below is a chart the Uranium-235 decay chain. U-235 can be used to make nuclear weapons.
Lawrence Berkeley National Laboratory
World Nuclear Association
**I have no formal training in nuclear physics and will gladly accept any and all feedback and will update this series accordingly with accepted corrections.