From: D. F. Siemens, Jr. (dfsiemensjr@juno.com)
Date: Fri Oct 03 2003 - 14:58:34 EDT
On Thu, 02 Oct 2003 17:33:20 -0700 allenroy <allenroy@peoplepc.com>
writes:
>
> From 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
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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