The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated.
Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated.
Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14.
After another 5,730 years only one-quarter of the original carbon-14 will remain.
The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
Other radiometric dating techniques are available for earlier periods.
One of the most widely used is potassium–argon dating (K–Ar dating).
Potassium-40 is a radioactive isotope of potassium that decays into argon-40.
The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice.After yet another 5,730 years only one-eighth will be left.By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.This technique is based on the principle that all objects absorb radiation from the environment.This process frees electrons within minerals that remain caught within the item.Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.Because of these and other factors, Thermoluminescence is at the most about 15% accurate.Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics.Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.Heating an item to 500 degrees Celsius or higher releases the trapped electrons, producing light.This light can be measured to determine the last time the item was heated. Fluctuating levels can skew results – for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item.