Radiocarbon dating also referred to as carbon dating or carbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon 14 Ca radioactive isotope of carbon. The method was developed by Willard Libby in the late s and soon became a standard tool for archaeologists. Libby received the Nobel Prize in Chemistry for his work in The radiocarbon dating method is based on the fact that radiocarbon is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen.
The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxidewhich is incorporated into plants by photosynthesis ; animals then acquire 14 C by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of 14 C it contains begins to decrease as the 14 C undergoes radioactive decay.
Measuring the amount of 14 C in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died. The older a sample is, the less 14 C there is to be detected, and because the half-life of 14 C the period of time after which half of a given sample will have decayed is about 5, years, the oldest dates that can be reliably measured by this process date to around 50, years ago, although special preparation methods occasionally permit accurate analysis of older samples.
The idea behind radiocarbon dating is straightforward, but years of work were required to develop the technique to the point where accurate dates could be obtained. Research has been ongoing since the s to determine what the proportion of 14 C in the atmosphere has been over the past fifty thousand years. The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age.
Other corrections must be made to account for the proportion of 14 C in different types of organisms fractionationand the varying levels of 14 C throughout the biosphere reservoir effects. Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the s and s.
Because the time it takes to convert biological materials to fossil fuels is substantially longer than the time it takes for its 14 C to decay below detectable levels, fossil fuels uses of radioisotopes carbon dating almost no 14 Cand as a result there was a noticeable drop in the proportion of 14 C in the atmosphere beginning in the late 19th century. Conversely, nuclear testing increased the amount of 14 C in the atmosphere, which attained a maximum in of almost twice what it had been before the testing began.
Measurement of radiocarbon was originally done by beta-counting devices, which counted uses of radioisotopes carbon dating amount of beta radiation emitted by decaying 14 C atoms in a sample. More recently, accelerator mass spectrometry has become the method of choice; it counts all the 14 C atoms in the sample and not just the few that happen to decay during the measurements; it can therefore be used with much smaller samples as small as individual plant seedsand gives results much more quickly.
The development uses of radioisotopes carbon dating radiocarbon dating has had a profound impact on archaeology. In addition to permitting more accurate dating within archaeological sites than previous dating sites for legally separated, it allows comparison of dates of uses of radioisotopes carbon dating across great distances.
Histories of archaeology often refer to its impact as the "radiocarbon revolution". Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice ageand the beginning of the Neolithic and Bronze Age in different regions. InMartin Kamen and Samuel Ruben of the Radiation Laboratory at Berkeley began uses of radioisotopes carbon dating to determine if any of the elements common in organic matter had isotopes with half-lives long enough to be of value in biomedical research.
They synthesized 14 C using the laboratory's cyclotron accelerator and soon discovered that the atom's half-life was far longer uses of radioisotopes carbon dating had been previously thought. Korffthen employed at the Franklin Institute in Philadelphiathat the interaction of slow neutrons with 14 N in the upper uses of radioisotopes carbon dating would create 14 C.
InLibby moved to the University of Chicago uses of radioisotopes carbon dating he began his work on radiocarbon dating. He published a paper in in which he proposed that the carbon in living matter might include 14 C as well as non-radioactive carbon. By contrast, methane created from petroleum showed no radiocarbon activity because of its age.
The results were summarized in a paper in Science inin which the authors commented that their results implied it would be possible to date materials containing carbon of organic origin. Libby and James Arnold proceeded to test the radiocarbon dating theory by analyzing samples with known ages. For example, two samples taken from the tombs of two Egyptian kings, Zoser and Sneferuindependently dated to BC plus or minus 75 years, were dated by radiocarbon measurement to an average of BC plus or minus years.
These results were published in Science in InLibby was awarded the Nobel Prize in Chemistry for this work. In nature, carbon exists as two stable, nonradioactive isotopes: The half-life of 14 C the time it takes for half of a given amount of 14 C to decay is about 5, years, so its concentration in the atmosphere might be expected to reduce over thousands of years, but 14 C is constantly being produced in the lower stratosphere and upper troposphere by cosmic rayswhich generate neutrons that in turn create 14 C when they strike nitrogen 14 N atoms.
Once produced, the 14 C quickly combines with the oxygen in the atmosphere to form carbon dioxide CO 2. Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is taken up by plants via photosynthesis. Animals eat the plants, and ultimately the radiocarbon is distributed throughout the biosphere.