Hello, I am a former YEC and am trying to find specific information about carbon dating methods and procedures. After searching the internet for the last couple of days and reading several freely available scholarly articles I have failed to find specific information explaining some things I'm curious about.
If one defines machine background as carbon-14 equivalent counts without a sample in place, the predictions of zero background turn out to be largely correct. Schmidt et al. (1987) were able to run their machine with an empty aluminum target holder without finding any atoms of carbon-14 in a 30-minute run, which would be equivalent to >90,000 radiocarbon years (<0.0014 pmc) if they had had a standard current of ordinary carbon. Van der Plicht et al. (1995) found an equivalent age of >100,000 radiocarbon years, and Kirner et al. (1995) obtained an equivalent age of >104,000 years.
For the record, I've seen the posts about Giem's lack of relevant background (http://talkrational.org/showthread.php?t=25497) and I'm not interested in that line of response. I'm specifically looking for articles on machine background with empty sample holders. I've been unable to find much. I can't even find a way to pay for the quoted article by Schmidt (1987) but from reading the abstract it looks more like they were testing their limits of the AMS.
It is convenient to begin with the background of the measurement system itself; this includes the sample holder, ion source, accelerator and detector as one unit. When no sample carbon is present (either an empty holder or a bare tantalum wire) a beam current of l-5 nA is generated, with a 14C count rate less than 10% of that from the best “dead” sample. The lowest 14C/13C ratio we have measured is for geological graphite, which yields lo-15 PA of beam current at less than 0.05% of the i4C/i3C ratio for the modern reference standard. This “machine background” is equivalent to an age of about 61000 years.
Now I read this as saying that they ran the test without a sample and measured an age of 61000 yrs age. And as I understand the process the tantalum wire is normally used to hold the graphite in the sample holder but in this case it's bare or the sample chamber is empty. Right? And the machine background measured is just stray carbon molecules that are stuck in the AMS?
AMS 14C measurements were done at the Center for Accelerator Mass Spectrometry (CAMS), University of California Lawrence Livermore National Laboratory (LLNL) (Davis et a1.1990; Southon et a1.1990; 1992). Experiments carried out with the CAMS FN tandem accelerator using a GIC Model 846 ion source determined that the machine background as defined by Schmidt, Balsley and Leach (1987), i.e., with the ion source closed off from the remainder of the beam line, ≥104 ka (0 counts in 20.5 min of counting). With the ion source containing an empty aluminum target holder open to the beam transport system, a 14C count rate equivalent to .009 pMC (74 ka) was measured. The average (N=2) 14C value obtained at LLNL on natural graphite used by the UCR radiocarbon laboratory to monitor contamination from loading graphite into their target holders is 64,460 ± 3200 BP (CAMS-12338; 64,500 ± 1800 BP and CAMS-12339; 58,800 ± 1000 BP). The average (N=7) 14C value obtained on graphite powder used by the LLNL laboratory is 57,900 ± 1500 BP (.074 ± .014 pMC)
My only question here is what is meant by "with the ion source closed off from the remainder of the beam line ka (0 counts in 20.5 min of counting). With the ion, is 2104 source containing an empty aluminum target holder open to the beam transport system, a 14C count rate equivalent to .009 PMC (74 ka) was measured" in laymens terms.
Again, I'm just trying to find out more information to better understand the process. Thanks
Edited by Admin, : Convert last two textual article titles into links. Also, fix quote from last article, it looks like edits to improve the presentation accidentally garbled portions, or it was typed in manually.