🚀 go-pugleaf

In article <9204080901.aa00587@Paris.ics.uci.edu>, kibler@turing.ICS.UCI.EDU (Dennis Kibler) writes...
>How is radioactive dating done?
> 
> I understand that by knowing the current % (say Z) of X in Y,
>and the half-life of X, that we can project backwards
>and figure out the % (say Z') of X in Y at any previous time.
>So to figure out the age of Y  we need to know the original
>percentage of X in Y.  But how is that known?
> 
There are a variety of methods for doing this.  It depends upon the chemistry
involved.  If you want to date something with Uranium, then you can do chemical
separation of the elements produced in the decay chain by straightforward
methods, and measure the total amount of Uranium vs. decay products.  Assuming
that all of the decay products are from original Uranium, you have an answer.

Sometimes, isotopic separation is required.  For example, for Carbon dating, 
you are interesting in the ratio of C13 to C12.  They are (almost entirely)
indistinquishable by their electronic structure, which means that their
chemistry is identical.  You can do mass spectroscopy, however to measure the
fraction of the carbon which is 13/12 times as massive.  

There are many other variations, but I'm no expert so I'll leave it at this.  


> Let me put it in a more concrete form. Suppose we use carbon
>dating to measure the age of a fossilized plant. I don't
>understand why we aren't measuring the age of the carbon,
>created I presume at the some time before the age of the earth,
>rather than the age of the plant.
> 

Good question.  The half-live of C13 is (roughly - please don't pick nits) 8000
years.  Clearly, if there were no new source of C13, we'd have none left on a 
4.5 Billion year old planet.  But that's not so.  The C13 is constantly being
replenished in the atmosphere by known reactions.  So the atmospheric C13/C12
is (roughly - please don't pick nits) constant over lime. 

But plants only "inhale" CO2 as long as they live.  As soon as a plant dies, 
the C13/C12 ratio begins to decrease as the C13 decays but is not replenished
by atmospheric C13.  This allows you to date the death of the plant, provided
that it did not happen too many C13 half-lives ago.  

-Scott