From: Mccarrick Alan D CRPH (MccarrickAD@nswccd.navy.mil)
Date: Fri Oct 03 2003 - 15:17:50 EDT
The preferred method of C14 dating is to use a mass spectrum to actually separate the carbon isotopes. This does not involve counting radioactive decays which would be subject to background decays. Atomic Mass Spec (AMS) in principle would be much more sensitive. The claim of the ICR publication is that those samples which are expected to have zero C14 left appear to have residual C14.
I still wonder whether the problem still lies in the noise when looking at the lowest amounts of C14. They do report attempts to exclude contamination in several ways. They hint that isotopically pure "zone refined, reactor grade graphite samples" were tested by the anonymous laboratory - although the results were not given (p. 10).
Al McCarrick
-----Original Message-----
From: D. F. Siemens, Jr. [mailto:dfsiemensjr@juno.com]
Sent: Friday, October 03, 2003 2:59 PM
To: allenroy@peoplepc.com
Cc: asa@calvin.edu
Subject: Re: RATE
On Thu, 02 Oct 2003 17:33:20 -0700 allenroy <allenroy@peoplepc.com> writes:
>
> From http://www.icr.org/pubs/imp/imp-364.htm <http://www.icr.org/pubs/imp/imp-364.htm> (ICRs Impact #364)
> "The AMS (Accelerator Mass Spectrometer) method improved the
> sensitivity of the
> raw measurement of the 14C/12C ratio from approximately 1% of the
> modern value
> to about 0.001%, extending the theoretical range of sensitivity from
> about
> 40,000 years to about 90,000 years. The expectation was that this
> improvement in
> precision would make it possible to use this technique to date
> dramatically
> older fossil material.1"
>
From http://www.c14dating.com/agecalc.html <http://www.c14dating.com/agecalc.html>
It is vital for a radiocarbon laboratory to know the contribution to routine sample activity of non-sample radioactivity. Obviously, this activity is additional and must be removed from calculations. In order to make allowances for background counts and to evaluate the limits of detection, materials which radiocarbon specialists can be fairly sure contain no activity are measured under identical counting conditions as normal samples. Background samples usually consist of geological samples of infinite age such as coal, lignite, limestone, ancient carbonate, athracite, marble or swamp wood. By measuring the activity of a background sample, the normal radioactivity present while a sample of unknown age is being measured can be accounted for and deducted.
In an earlier section we mentioned that the limit of the technique is about 55-60 000 years. Obviously, the limit of the method differs between laboratories dependent upon the extent to which background levels of radioactivity can be reduced. Amongst accelerator laboratories there has been mooted the theoretical possibility of extended range dating to 75 000 yr +, at present this seems difficult to attain because of the problems in accurately differentiating between ions that mimic the mass and charge characteristics of the C14 atom. Beukens (1994) for instance has stated that this means the limit of the range for his Isotrace laboratory is 60 000 yr which is very similar to the conventional range.
End quote.
Looks to me as though the ICR report exaggerates the limit and involves mistaking background for age.
Dave
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