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radio-carbon dating

Index

[LINK] Radio-carbon dating is a method of obtaining age estimates on
organic materials. It has been used to date samples as old as 50,000
years. The method was developed immediately following World War II by
Willard F. Libby and coworkers, and has provided age determinations in
archaeology, geology, geophysics and other branches of science.
Radiocarbon determinations can be obtained on wood; charcoal; marine
and fresh-water shell; bone and antler; peat and organic-bearing
sediments, carbonate deposits such as tufa, caliche, and marl; and
dissolved carbon dioxide and carbonates in ocean, lake and
ground-water sources.

Each sample type has specific problems associated with its use for
dating purposes, including contamination and special environmental
effects. While the impact of radiocarbon dating has been most profound
in archaeological research and particularly in prehistoric studies,
extremely significant contributions have also been made in hydrology
and oceanography. In addition, in the 1950s the testing of
thermonuclear weapons injected large amounts of artificial radiocarbon
("Radiocarbon Bomb") into the atmosphere, permitting it to be used as
a geochemical tracer.

Radioactive carbon, produced when nitrogen 14 is bombarded by cosmic
rays in the atmosphere, drifts down to earth and is absorbed from the
air by plants. Animals eat the plants and take C14 into their bodies.
Humans in turn take carbon 14 into their bodies by eating both plants
and animals. When a living organism dies, it stops absorbing C14 and
the C14 that is already in the object begins to disintegrate.
Scientists can use this fact to measure how much C14 has disintegrated
and how much is left in the object. Carbon 14 decays at a slow but
steady rate and reverts to nitrogen 14. The rate at which Carbon
decays (Half-life) is known: C14 has a half-life of 5730 years.
Basically this means that half of the original amount of C14 in
organic matter will have disintegrated 5730 years after the organisms
death; half of the remaining C14 will have disintegrated after another
5730 years and so forth. After about 50,000 years, the amount of C14
remaining will be so small that the fossil can't be dated reliably.

To discover how long an organism has been dead (to determine how much
C14 is left in the organism and therefore how old it is), we count the
number of beta radiations given off per minute per gram of material.
Modern C14 emits about 15 beta radiations per minute per gram of
material, but C14 that is 5730 years old will only emit half that
amount, (the half-life of C14) per minute. So if a sample taken from
an organism emits 7.5 radiations per minute in a gram of material,
then the organism must be 5730 years old. The accuracy of radiocoarbon
dating was tested on objects with dates that were already known
through historical records such as parts of the dead sea scrolls and
some wood from an Egyptian tomb. Based on the results of the Carbon 14
test the analysis showed that C14 agreed very closely with the
historical information.

The natural radiocarbon activity in the geologically recent
contemporary "pre-bomb" biosphere was approximately 13.5
disintegration's per minute per gram of carbon. A measurement of the
radiocarbon content of an organic sample will provide an accurate
determination of the sample's age if it is assumed that (1) the
production of radiocarbon by cosmic rays has remained essentially
constant long enough to establish a steady state in the 14C/12C ratio
in the atmosphere, (2) there has been a complete and rapid mixing of
radiocarbon throughout the various carbon reservoirs, (3) the carbon
isotope ratio in the sample has not been altered except by radiocarbon
decay, and (4) the total amount of carbon in any reservoir has not
been altered. In addition, the half-life of radiocarbon must be known
with sufficient accuracy, and it must be possible to measure natural
levels of radiocarbon to appropriate levels of accuracy and precision.

It has long been recognized that if the radiocarbon atoms could be
detected directly, rather than by waiting for their decay, smaller
samples could be used for dating and older dates could be measured. A
simple hypothetical example to illustrate this point is a sample
containing only one atom of radiocarbon. To measure the age (that is,
the abundance of radiocarbon), the sample can be placed into a mass
spectrometer and that atom counted, or the sample can be placed into a
Geiger counter and counted, requiring a wait on the average of 8000
years (the mean life of radiocarbon) for the decay. In practice,
neither the atoms nor the decays can be counted with 100% efficiency,
but the huge advantage for atom counting remains. Samples are also
processed in a lab such as the Beta Analytic Inc.

Reference

McGraw-Hill Encyclopedia of Science & Technology-volume 15 pages
136-144.