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

From: Kirk Bertsche <Bertsche@aol.com>
Date: Thu Oct 18 2007 - 00:59:48 EDT

On Oct 17, 2007, at 9:12 AM, Steven M Smith wrote:

>
> 2)  A large number of the reports of C-14 in Carbon "dead" samples
> that Baumgardner cites come from articles that describe the
> methodology & instrumental set-up of new AMS facilities.  Note some of
> the following titles: "Accelerator mass spectrometry of 14C at Nagoya
> University", "Accelerator radiocarbon dating at SFU", "AMS 14C
> measurements and preparative techniques at NIES-TERRA", "AMS at the
> TTT-3 tandem accelerator in Naples", "AMS sample handling in
> Groningen", "An assessment of laboratory contamination at the IsoTrace
> radiocarbon facility", etc.  (That is just the articles with titles
> that start with the letter A.)  As I read these articles, my weird
> sense of humor kicks in and I am reminded of a bunch of kids at school
> comparing their cars or stereos.  "My tandem AMS can get a date as old
> as 85 ka on a blank aluminum sample holder with the ion gate closed."
>  "Yeah, well my Tandetron AMS get >90 ka!"  

Most of the references are from the international AMS or Radiocarbon
conferences held every 3 years, where there is a bit of this flavor of
a friendly rivalry. I would describe it more like a bunch of computer
or ham radio hobbyists getting together and comparing notes. Everyone
wants to show off how well they've done, but they also want to help
other labs solve their problems (and have their methods adopted by the
others as a stamp of approval).

> These are articles about experiments that are testing the absolute
> limits of individual laboratory facilities, not experiments designed
> to measure "intrinsic" or "in-situ" C-14.  

Yes, and many are reports on new instruments that aren't yet working
quite right, or new techniques that aren't yet shaken out, etc. So
many of the numbers (especially in the NIM-B references) are not the
best that can be obtained.

> For some reason, Baumgardner did not list C-14 dates of 85,000 years
> for blank aluminum sample holders in his Table 1.

True, but empty aluminum sample holders are not a very accurate measure
of instrument background. 12C and 13C can produce "tails" which give
false 14C counts. Empty aluminum holders will underestimate the
background, because when they are used there is essentially no 12C or
13C entering the system. (But empty holders are easy to try, and are
useful for getting a lower limit on instrument background.)

Kirk

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Received on Thu Oct 18 01:03:21 2007

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