>The problem is that the half-life of C-14 is too short to be
>useful beyond historic times:
>
>"Most carbon atoms have six protons and six neutrons (shown
>as 12C, carbon twelve) and are not radioactive. However, 14C
>(carbon fourteen) has six protons and eight neutrons and is
>radioactive. With the release of radioactivity, 14C spontaneously
>breaks down over a period of time into 14N (nitrogen fourteen).
>A small amount of 14C is present in all living things. Scientists
>can determine the age of a fossil because the rate at which 14C
>breaks down is known. If the fossil still contains organic matter
>the relative amounts of 14C and 14N are measured and the age
>is calculated. However, this method is unreliable for measuring
>ages in excess of twenty thousand years. The radioactivity becomes
>so slight that it is difficult to make an accurate determination."
>(Mader S., "Biology", 3rd Ed., 1990, Wm. C. Brown, Indiana, p25)
The method described here (counting the decay rate of 14C) has not been used
for several years. The current method uses mass spectroscopy to count the
14C atoms rather than wait for them to decay. This new method is so
superior that it forced a reexamination of carbonaceous artifacts. It is
now possible to go back 35-40 thousand years or more using these new
techniques (some dates as old as 75,000 years have been reported but I have
trouble believing them).
There are other errors in this text. The old method of 14C dating never
used a measure of the relative amounts of 14C and 14N to determine the age
of a sample. Nitrogen 14 is so common (nearly 80% in our atmosphere) it
would be impossible to distinguish the 14N produced through 14C decay from
that found naturally. I'm afraid that Wm. Brown is wrong (but I am not
calling him a liar! ;-)
Steve