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[Phys-L] C-14 dating.



Many years ago, when I visited a New Jersey lab performing commercial
C-14 dating the method used was different from what is now possible.
The C-14, as most of you know, is created in the atmosphere, due to
cosmic rays. It then becomes part of CO2 molecules used by living
plants. All living objects (trees, animals, etc.) contain roughly one
14C atom out of about 10^10 common 12C atoms.

14C is beta radioactive and its half-life is 5730 years. One gram of
carbon, extracted from a recently cut wood, for example, emits 15 beta
particles per minute. Carbon from wood cut 5730 years ago is expected
to emit 7.5 such particles per minute, on the average. The lower the
counting rate the longer is the time interval between the object's
death and present time, according to the well known law of exponential
decay. Determination of ages consisted of accurately measuring the
counting rates.

This approach has a limitation; it requires large samples, typically
several grams or more. New methods, based on mass spectroscopy, are
much better in that respect. Here only several milligrams of material
might be sufficient to perform an accurate age determination. The idea
is to accurately measure the 14C over the 12C atomic ratio. That ratio
also decreases exponentially and can thus be used to measure the age of
an old object of biological origin, for example, charcoal found in a
cave, a moomy, a preserved wooden tool, or a fragment of cloth.

The sample is introduced into an ion source and the ratio is determined
by measuring areas under the 14C (very small) and 12C (much larger)
peaks. This approach is simple in principle but complicated in
practice. All kind of potential errors are possible. The 14N ions, for
example, can be mixed with 14C ions. Likewise, the ionic 14C beam might
be contaminated with ionizes 12C1H2 molecules. Experts, however, know
how to avoid such errors. Some of them perform mass spectrometry by
using particle accelerators.

I learned about this from an old paper of A.E. Litherland (in Annual
Review of Nuclear and Particle Science; volume 30, 1980; pages
437-473).

Happy New Year,
Ludwik
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