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Half-Life
In radioactive materials, the term "half-life" refers to the length of time it takes for a substance to reduce by half. In radioactive dating, this means that a quantity of a radioactive substance will reduce by half after a certain amount of time. For example, carbon-14 has an average half-life of 5,730 years, so it will reduce by half over that period of time, and then by a quarter after another 5,730 years.
Measuring Rate of Decay
Knowing a material's half-life, scientists can begin to measure a sample's age based on the rate of decay and its original chemical makeup. For example, quickly cooled magma that contains both potassium and argon is ideal for radiometric dating because of potassium's long half-life and the fact that argon becomes trapped in the samples. By comparing the amount of argon to the amount of potassium, researchers can get an accurate measure of the sample's age.
Radiocarbon Dating
Radiocarbon dating is the most common form of radioactive dating because the carbon-14 isotope that is used in this process occurs in nature, specifically in the atmosphere. For carbon-based objects, including plants and animals (and objects created with organic materials such as wood), this means that quantities of the isotope are found in remains such as bones and can be compared with atmospheric amounts of carbon-14 in order to estimate the age of the specimen.
Other Radiometric Dating
Carbon-14 is not the only material used for radiometric dating. Any radioactive element with a known half-life can be used to determine the age of a specimen, as long as that element is present in the sample. For example, the process of uranium-lead dating is used on minerals because of the presence of uranium and its decay in minerals such as zircon. Because the half-life of uranium is billions of years, this technique can determine the ages of much older minerals.