Science And Environment: How Carbon-14 Dating Works

Carbon-14 dating is a laboratory technique used to determine the age of organic materials from the past. Radioactive decay is measured in several ways.

Carbon-14 Dating is a method scientists use to determine the age of a variety of items, including ancient cultural artifacts and remains of extinct animals. Willard Libby invented it in the 1950s, and in 1960, won the Nobel Prize in Chemistry for his achievements.

In the upper atmosphere, cosmic radiation knocks neutrons out of atomic nuclei. These neutrons react with a small percentage of nitrogen in the atmosphere to become radioactive carbon - 14 (or radiocarbon). This happens through a process called "neutron capture." The vast majority of carbon molecules contain 6 protons and 6 neutrons, for an atomic weight of 12. But when the carbon has captured 2 extra neutrons from nitrogen, it becomes the radioactive isotope carbon -14.

(An "isotope" is a variation of an element that has a different number of neutrons and thus a different atomic weight.)

Plants survive by breathing in carbon dioxide from the atmosphere. Animals all eat either plants or other animals, which eat plants. For this reason, all living things ultimately take in carbon. That carbon should be in a proportion of C-14 to C-12 equivalent to that in the air. When a plant or animal dies, the "carbon clock" begins. As the dead organism decays, the carbon-14 in it decays through radiation to again become nitrogen. Nitrogen is a gas, and escapes into the air.

Therefore, the approximate length of time it has been since the organism died can be determined by measuring the proportion of C-14 to C-12 still existing in it.

Radioactive decay is measured in half-lives. The half-life of an isotope is the amount of time it takes for half of the substance to decay.

The half-life of carbon-14 is 5730 years. This means that half the carbon 14 in a sample will be gone in 5730 years, and in another 5730 years another half will be gone. In other words, ¾ of the Carbon 14 will be gone in about 11,460 years. Similarly, 7/8 will be gone after about 17,000 years. After 50,000 years, practically all the carbon 14 would have decayed. Thus, any ages greater than 50,000 years cannot be tested with Carbon-14 dating.

Any material that was once living is able to be carbon dated. For instance, paper, parchments, textiles, shells, charcoal, feathers, and any other remains of plants or animals can be carbon dated.

To test the procedure, Libby used a sample of wood from the tomb of one of the Egyptian pharaohs, the age of which was known. His results matched the expected age. The original method of C-14 dating involved burning some of the sample in order to convert it to carbon dioxide gas. A radiation detector (Geiger counter) then counted the electrons given off by the decaying C-14.

Three methods of carbon-14 dating are in use today. They are "gas proportional counting" (or GPC), "liquid scintillation counting" (or LSC), and "accelerator mass spectrometry" (or AMS). These methods are an improvement over Libby's original method, which involved counting individual radioactive decay events, because the new methods enable dating to be done with much smaller samples. With GPC or LSC, a sample as small as 100 milligrams is all that is required, and with AMS a single milligram of the substance can be tested. This is a sample 1000 times smaller than was required with Libby's conventional method.

In GPC, a sample is converted to carbon dioxide or methane gas. A proportional counter is filled with this gas, and the electrical discharge from the decay of the carbon-14 is counted electronically. In LSC, the sample is converted to benzene. This is mixed with certain organic chemicals and placed in a clear container. Whenever a C-14 atom decays, the liquid emits a pulse of light. Sensitive photo multiplier tubes are placed near the container. These tubes can count the pulses of light.

Since the decay of a radioactive atom is a random event, it takes a number of days to get a count accurate enough to find the age of the sample.

Both of these methods measure the radioactivity of the sample. In AMS dating, individual carbon-14 atoms are counted through the use of an accelerator mass spectrometer.

Samples to be dated are pretreated in different ways depending on the circumstances. The lab will first make a detailed analysis of the sample. They will consider all possibilities of contamination, such as the presence of rootlet intrusion, or glue or ink that found it's way into the sample through packaging. The lab will want to know detailed information about the original location of the sample, including a diagram of any layers of earth in which it was buried. They will also want to know of any pretreatment the collector has done, such as washing the sample in water.

At the lab, the sample is often crushed to reduce the volume and increase the surface area. It may then be cleansed by using chemicals.

While C-14 dating is widely accepted as accurate, some scientists are not convinced that the theory behind the method is without flaws. They raise questions about the assumptions that are made in carrying out the method. For instance, can we be sure that the rate of decay for C-14 hasn't changed in thousands of years? Can we be sure that the amount of C-14 in the atmosphere hasn't changed?

It is known that the C-14 ratio in the air is lower since the burning of fossil fuels has released extra carbon dioxide in the air. This means that creatures dying thousands of years ago may have had more C-14 in their tissues to begin with. This would yield a date older than it really is. Then in the 1950's, the testing of atomic bombs resulted in extra C-14. So samples dating from the "˜50's would yield too young an age. Is it not conceivable that other occurrences in the past could similarly have altered the C-14 ratio?

A recent study of fossils from different layers of strata presumed to represent widely differing eras of geologic time found the level of C-14 (determined by AMS technology, theoretically able to measure to 90,000 years ago) to be unrelated to the fossil's location in the strata. They all measured about the same. These scientists hypothesize that the earth and the life on it are not as old as is commonly believed, and that the theory behind the geologic column is faulty.

Be that as it may, C-14 dating can be used to date things that were once living and can generally measure ages only up to 50,000 years (and some scientists think that is a stretch.) C-14 dating finds its greatest usefulness when used to date historical artifacts.

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