Re: [asa] Kirk on C14 contamination

On Nov 30, 2007, at 1:47 PM, Jon Tandy wrote:

> So after reading Baumgardner's response in
> http://www.answersingenesis.org/articles/2007/11/30/feedback-rate-
> contaminat
> ion, I have listed several paragraphs below which are snips from the
> original that I see as the main ones having any serious merit toward
> affecting Kirk's conclusions. Anyone care to respond to them?

Sure, I'd be glad to provide clarification (I wish AIG would let me do
so on their website!)

> "He invokes the fact that over the years many AMS labs have
> established that
> their procedures routinely introduce tiny amounts of modern carbon
> (with
> today’s level of 14C), typically on the order of 1 µg, into the
> samples they
> process. This level of contamination becomes serious for tiny sample
> sizes,
> say, 1 mg or less, especially if the sample is old. On the other hand,
> 1 µg
> of contamination has negligible consequences when the sample size is
> on the
> order of 100 mg, as was the case for the samples we tested and
> reported.
> Bertsche fails to point out the very basic reality that AMS labs
> normally
> require a large enough sample such that this issue does not affect the
> precision of their measurement. For most of the 14C values reported in
> the
> peer-reviewed literature which I list in my chapter in the RATE book
> [Vardiman et al., 2005] the investigators had plenty of material
> available,
> and so, small sample size was just not an issue."

Baumgardner is speaking from the perspective of a sample-submitter, not
a sample-preparer. He is simply mistaken regarding typical sample
sizes.

I have no doubts that he submitted samples of about 100mg for his coal
(and maybe diamond) samples. Laboratories like to receive significantly
larger samples than they actually need. This allows them to select a
good piece of the sample and to re-do the measurement if something
happens to the sample. But the amount actually converted to CO2 and
graphitized is typically 0.5-5mg of carbon. Samples larger than about
10mg tend to explode CO2 combustion tubes in the furnace.
Graphitization lines generally can't handle quite this much (and larger
sample take longer to reduce to graphite, so cause bottlenecks in the
processing). A 1mg sample is generally considered “large”, not “tiny.”

But the context is not the samples which he submitted for analysis; it
is the measurements reported in his references at various AMS
laboratories. Most AMS labs use the “Vogel method” of graphitization,
using similar equipment, so they deal with similar sample sizes. The
typical sizes in his references (and used for his table and plots) are
about 1mg.

Support for my contention that graphitized samples are almost always
less than 5 or 10mg is best seen in tests looking for dependence of
background on sample size. Vogel et al had one sample at 20mg and all
others below 5 or 10mg (Radiocarbon 29 (1987) p323ff); Kirner et al had
all samples less than or equal to 1mg (Radiocarbon 37 (1995) p697ff).

> "Moreover, Bertsche is simply incorrect when he then claims “the
> graphitization process typically add[s] from 0.1 to 0.7 pMC (highly
> dependent on sample size and procedure).” This is absurd. He cannot
> support
> such a claim from any peer-reviewed source. "

This one is simple--just look at Baumgardner's references. I've posted
excerpts here:
http://www.theologyweb.com/campus/showpost.php?p=2147898&postcount=142

> "In regard to 14C production due to the presence of uranium in crustal
> environments, I treat that topic in detail in section 7 of my chapter
> and
> show the maximum plausible 14C production rate, given measured neutron
> fluxes in deep mines and measured reaction cross sections, is more
> than four
> orders of magnitude too small to account even for the small measured
> 14C
> levels in diamonds. This same analysis also applies to coal. "

Maybe Steve Smith can answer this better, but I believe that his
estimates came from granites, but that coals can have orders of
magnitude higher levels of uranium and thorium. Steve--any comments?

> "I emphasized the careful procedures applied in the collection and
> preservation of these samples. They were sealed under argon to preclude
> contamination from just moments after they were collected. For most of
> their
> lives these samples were sealed in argon in multi-laminate foil bags
> and
> refrigerated at 3 °C."

I agree that this should help, but coal is notorious for contamination.
  I mentioned a number of things which are known to cause contamination
in coal. In addition, Glenn Morton and others have pointed out that
coal tends to adsorb atmospheric CO2. Steve or others--any more
comments?

> "Instrument background involves a “14C signal registering in the
> detector
> circuitry when 14C-ion [is] not present.” This item is routinely and
> reliably tested by running the system with no sample in the aluminum
> sample
> holder. This test is the basis for the value of the ultimate AMS
> detection
> limit, about 0.0005 pMC, corresponding to about 100,000 14C years. "

This captures only one component of background. A major component is
the 12C and 13C which scatters into the detector and registers false
14C counts. This is not present unless 12C and 13C are injected into
the system. As Taylor and Southon say:
"instrument or machine background ... involves the registration of what
is interpreted by the detector circuitry and/or software as a 14C-ion
produced pulse when, in fact, a non-14C-ion mimics 14C or the detector
counts 14C which was not originally present in the sample matrix when
it
was introduced into the source." (Nucl Instr and Meth B259 (2007)
282ff.) One of the best characterizations of instrument background is
the diamond values from this reference.

Kirk

To unsubscribe, send a message to majordomo@calvin.edu with
"unsubscribe asa" (no quotes) as the body of the message.
Received on Fri Nov 30 21:47:14 2007