Re: [asa] Denver RATE Conference (Thousands...Not Billions)_Part 2

> It sounds like what you are saying is that the measurement process
> (instrument?) itself yields a "background" of C14 roughly equal to an
age of
> 40,000 years. Thus a measurement of 40,000 years old for a diamond
equals
> the measurement of C14 from the AMS background, plus zero C14 from the
> diamond itself. Is this about right?

Yes, this is about right.
Contamination/background can come from three main sources:
1) contamination of the original sample itself
2) contamination from the sample preparation process
3) background from the instrument

The first two sources interact. If a sample is suspected of
contamination, it can be cleaned more aggressively with chemical
procedures. But this runs the risk of introducing contamination from
the chemicals. For this reason, reputable labs want to know as much as
possible about a sample so they can tailor the sample prep
appropriately to it. The YEC crowd does not understand this,
suspecting bias. So they generally hide the provenance of their
samples, sometimes deceptively. This leads to dating errors due to
improper sample prep.

For modern AMS measurements, the process is somewhat involved:
1) chemical/mechanical cleaning (often HCl and NaOH)
2) sealed combustion of sample to CO2, then reduction to graphite
3) mechanical pressing into sample holder, insertion into negative ion
source
4) generation of C- ion beam (this avoids contamination from N14, which
does not form stable negative ions)
5) pre-elimination and measurement of C12 (and perhaps C13) in magnetic
spectrometer (this reduces backgrounds)
6) injection of remaining C ions into tandem van de Graaf particle
accelerator
7) acceleration to a few MeV, stripping to +3 (or more) at center,
acceleration out of accelerator to a few more MeV
8) injection through a Wien filter (or perhaps a bend) to eliminate
particles of wrong velocity
9) detection in a dE/dx type gas detector to eliminate particles of
wrong Z

Thus, there are lots of possibilities for instrument background.

> If so, I suppose the process of calibrating the instrumentation is
much more
> involved and has much more history and "check and balance" than what
> Baumgardner has represented. But still (and I'm not disputing the
age of
> the earth), the last paragraph below seems on the surface a little
like the
> scientific "circular reasoning" that YEC claims to expose. If the
diamond
> is used to calibrate the instrument because it is "known" to have no
C14
> (due to its great age), isn't this assuming the conclusion as part of
the
> proof? What if the diamond really isn't that old, and/or really does
have
> some C14 in it? How would we know, if that assumption is taken at the
> first?

It's not so circular as it sounds, and is much more careful than
Baumgartner suggests. For example, at LLNL we used very old (nearly
petrified) wood as our "C14-free" blank. This had been dated
independently to about 55k years. We prepared samples in batches,
always processing at least one blank and one or two OX1 NIST-traceable
modern C14 standards in parallel. Thus, our blanks and standards saw
the same chemical sample prep (and chemical contamination) as did our
unknowns. Our blanks generally measured 40k-50k years old in the
instrument. Thus, the backgrounds were dominated by the instrument and
sample prep process, not the actual C14 in the blank. If we had tried
to date something that was >50k years old, we would have had to take
the finite C14 content of the blank into account. Otherwise, the blank
can be assumed to have no C14 because this correction is in the noise.

Here's a more thorough explanation of background correction:
http://www.llnl.gov/tid/lof/documents/pdf/229581.pdf

Kirk
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Received on Tue Sep 25 15:39:14 2007