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Author Topic:   Several specific questions about RadioCarbon Dating using AMS
PhiloNibbler
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Posts: 8
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Message 1 of 30 (777252)
01-28-2016 2:23 AM


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.

Specifically, I was reading paul giem's page http://www.grisda.org/origins/51006.htm

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.

So far I have found two articles with some information about empty sample holders but need some clarification. First, Gillespie and Hedges mention in their report "LABORATORY CONTAMINATION IN RADIOCARBON ACCELERATOR MASS SPECTROMETRY":

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?

Also, I came across and article by D. L. KIRNER,1 R. E. TAYLORI'2 and JOHN R. SOUTHON3 "REDUCTION IN BACKGROUNDS OF MICROSAMPLES FOR AMS 14C DATING"

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, ≥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" 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.

Edited by Admin, : Correct portion between quotes in next to last paragraph.


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Message 2 of 30 (777254)
01-28-2016 8:04 AM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Several specific questions about RadioCarbon Dating using AMS thread in the Proposed New Topics forum.
    
Pressie
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Message 3 of 30 (777256)
01-28-2016 8:29 AM
Reply to: Message 1 by PhiloNibbler
01-28-2016 2:23 AM


I'm specifically looking for articles on machine background with empty sample holders.
Really? Empty sample holders? Articles on them? Why don't you start those yourself?

Edited by Pressie, : No reason given.


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RAZD
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Message 4 of 30 (777257)
01-28-2016 9:02 AM
Reply to: Message 1 by PhiloNibbler
01-28-2016 2:23 AM


calibration of equipment
Hello PhiloNibbler, and welcome to the fray

Again, I'm just trying to find out more information to better understand the process. Thanks

From what I see and know, this is about calibrating the machines to determine their potential error envelope

... 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. ...

So the potential error in measurement of an actual sample would be +/-0.05% of the i4C/i3C ratio for the modern reference standard. Seeing as this measurement of age is based on exponential decay this does not result in a major error in ages <50,000 yr BP, which is considered the current limit of 14C/13C testing.

Again, I'm just trying to find out more information to better understand the process. Thanks

There are a couple of members that are involved with 14C testing\measurements. PurpleYouko comes to mind, as does Coyote, and they may be able to help.

Enjoy


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Percy
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(1)
Message 5 of 30 (777261)
01-28-2016 9:19 AM
Reply to: Message 1 by PhiloNibbler
01-28-2016 2:23 AM


Hi PhiloNibbler,

I'll start with simple answers and more details can be filled in as necessary.

First, about the age of your references, they date from the 1980's and 1990's, more than 20 years ago when use of AMS (Accelerator Mass Spectrometry) for radiocarbon dating was still being researched and refined. More recent research about background measurement levels does seem to exist, but not too much as this issue has long been considered settled. Research would only resume if substantive questions were raised.

Your discussion and excerpts mention counting 14C atoms but don't mention that 12C and 13C atoms are also counted, so since I can't be sure you understand that radiocarbon dating actually measures a ratio, let me briefly describe it. Just counting 14C atoms won't provide a date. Counting 12C is also necessary, and the ratio of 14C atoms to 12C atoms yields the date. The ratio of 14C atoms to 13C can also be used (the levels of both 12C atoms and 13C in the atmosphere are relatively constant).

PhiloNibbler writes:

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?

This background measurement is very unlikely to come from "stray carbon molecules that are stuck in the AMS." They aren't specific about the cause, but it likely was just how their machine behaved. Their counts of 14C atoms are evidently in some way dependent upon the beam current, and what they were saying is that the beam current never goes to zero, even when no carbon sample is present. Keep in mind that they were working more than 30 years ago with the technology available in 1984.

The reason this background measurement is unlikely to stem from "stray carbon molecules" is because those carbon molecules would most likely be carbon dioxide from the air. The carbon in the air is constantly replenished with 14C and so has high levels of 14C compared to old carbon in ancient biological material. Hence carbon dioxide would produce a relatively high 14C count relative to the 12C count and date to zero years old, not 61,000 years old. The more 14C relative to 12C the younger the sample.

My only question here is what is meant by "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" in laymens terms.

The "ion source" is the part of the AMS machine that turns the carbon molecules in the sample into ions (charged molecules). Closing off the "ion source" must mean that the connection of the "ion source" to the rest of the machine is in some way blocked. It's slightly different from having the "ion source" unblocked but with no sample in place.

If we give the label "new carbon" to carbon samples with recent interactive contact with the air, then new carbon contamination would cause samples to date newer, not older.

If we give the label "old carbon" to ancient carbon sources with no recent interactive contact with the air (e.g., long-buried carbon such as in coal that has no exposure to radioactivity), then old carbon contamination could cause samples to date older, but it would have to be a great deal of contamination. For example, a 5,000 year-old sample would require contamination larger than the sample itself to make it date to 50,000 years old. There's also the question of where so much stray old carbon would come from and yet not be detected during cleaning and calibration.

--Percy


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PhiloNibbler
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Posts: 8
Joined: 01-29-2015


(1)
Message 6 of 30 (777320)
01-29-2016 2:44 AM
Reply to: Message 5 by Percy
01-28-2016 9:19 AM


First of all thanks to everyone for the quick responses. Really appreciated.

Pressie: My first response was to lol cause my area of study is IT not radiometric dating methods. Also, a year ago I would have been on here telling how much "ya'll were wrong" about this and "how much you didn't know what you talking about". Now, I am learning more and know better. Having said that, I do intend to help others in my circle of influence by presenting as factually as I can informational pieces about the processes and methods and why YEC is misleading/wrong. That's why I'm here asking questions to make sure I understand it well enough to present it correctly even though I'm not in the field.

Razd: that makes sense. I was reading a post on here (can't remember where now) by Coyote about this being similar to a scale and finding the limits.

Percy: regarding

This background measurement is very unlikely to come from "stray carbon molecules that are stuck in the AMS." They aren't specific about the cause, but it likely was just how their machine behaved....

That makes sense. I had in my mind the idea that the machine background was actual c14 but if I understand correctly now it's just the limits of the machines themselves (like a scale with set limits).

Now to make sure I understand correctly, the following article
"THE CARBONATE 14C BACKGROUND AND ITS COMPONENTS AT THE LEIBNIZ
AMS FACILITY" (1998) by MARKUS SCHLEICHER, PIE TER M. GROOTES, MARIE JOSEE NADEAU and AXEL SCHOON which states:

We tested the background of the AMS system using a pure graphite powder with a very low 14C concentration… which eliminates all but storage and pressing as possible blank contributions. It consistently gives the lowest 14C concentrations we observe in any target, and is measured weekly to check the level and stability of the machine background. These measurements yield a background of 0.03 ± 0.015 pMC, equivalent to 65 ka 14C, similar to machine backgrounds reported for other AMS systems (e.g., Vogel et a1. 1987 Beukens 1990; van der Borg et al. 1997; Kretschmer et al. 1997; Kirner et al. 1995; Grootes et al. 1986; Gillespie and Hedges 1984).
Beam stops inserted at the mid-section of the recombinator (Nadeau et al. 1997; Fig. 1) allow the breakdown of this background into counts from the scattering of ions that reach the accelerator via the mass-12 or mass-13 path, and ions that follow mass 14 (12CH2, 13CH), under otherwise normal operating conditions. No counts were observed in the detector over a 40-min period when the mass 13 and 14 paths were blocked, while a normal 12C- beam was injected into the accelerator. This puts an upper limit of 0.0013 pMC, equivalent to 90.5 ka 14C, if one count had been observed, to the combined background contributions of mass-12 charge exchange/scattering and electronic noise. Blocking only the mass 14 beam, four counts were detected in a 67-min period, resulting in a background of 0.0031 pMC and equivalent age of 83.4 ka. This is similar to the observations of Beukens (1990). The observed 0.03 pMC machine background is thus for ca. 90% made up by contributions from mass-14 charge exchange/scattering and true 14C. These contributions appear to be of similar magnitude but will vary with the degree of contamination of the ion source. As stated above, the 0.03 ± 0.015 pMC is an upper limit for the machine background, since the carbon powder may also contain some residual 14C, and additional 14C may have been introduced during storage and target pressing.

indicates that they were testing the machine for background readings using a known powder with low c14 concentration and received a total background of .03 ± .015 Pmc (percent modern Carbon) total due to electronic noise, stray particles, and possible introduction of (modern?) carbon14 during preparation of the sample. So the background here measured is mostly possible contamination and machine background from scattered particles?

"stray carbon molecules that are stuck in the AMS."
I think I was thinking of ion source memory here which I understand as absorbtion of C02 in the reduction system where the sample is reduced to graphite?. I originally thought they meant in the AMS sputtering source, but think I misunderstood at first.

I want to thank everyone again for the great responses and help. I look forward to reading your replies.

Edited by PhiloNibbler, : No reason given.


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ThinAirDesigns
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Message 7 of 30 (777322)
01-29-2016 3:47 AM


Connections
PhiloNibbler, I found it interesting that your first post involved the GRI. SDA background? If not cool answering -- that's understood. Many generations deep of such here.

JB

Edited by ThinAirDesigns, : No reason given.


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Percy
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(1)
Message 8 of 30 (777334)
01-29-2016 8:28 AM
Reply to: Message 6 by PhiloNibbler
01-29-2016 2:44 AM


PhiloNibbler writes:

indicates that they were testing the machine for background readings using a known powder with low c14 concentration and received a total background of .03 ± .015 Pmc (percent modern Carbon) total due to electronic noise, stray particles, and possible introduction of (modern?) carbon14 during preparation of the sample. So the background here measured is mostly possible contamination and machine background from scattered particles?

We need someone more knowledgeable to chime in, but here's what I think is going on. The negative ions from the sample are accelerated to a moderate kinetic energy1, and ions of different mass split into their own ion beams based upon atomic weight. This means the 12C, 13C and 14C ions are split into three separate beams. The recombinator uses a carefully designed magnetic field to recombine the ion beams into a single beam, which then enters the mass spectrometer portion of the machine.

Before the three beams are recombined the individual beams can be individually blocked, allowing researchers to determine the base level ion counts in the mass spectrometer for each carbon isotope. For example, by blocking the 12C ion beam they can determine how many 12C ions the mass spectrometer counts when the ion source is providing no 12C ions.

It bears repeating that by far the most abundant sources of environmental carbon are comparatively rich in 14C, and the more 14C reaching the mass spectrometer the younger a sample will date. This makes contamination an unlikely cause of very old carbon dates. They are most likely caused by very old samples.

--Percy

1: Mass recombinator for accelerator mass spectrometry


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PhiloNibbler
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Joined: 01-29-2015


Message 9 of 30 (777341)
01-29-2016 10:41 AM
Reply to: Message 7 by ThinAirDesigns
01-29-2016 3:47 AM


Re: Connections
Thin, by GRI do you mean Global Research Inititive? and SDA Seventh Day Adventists?
If so, not sure about the GRI connection to my first post but my background is more evangelical non-demon christian. In HS I went to a private Christian school and that's where most of my exposure to YEC (mostly bad, old arguments) came from. Also I was an avid AIG fan until Ken Ham's debate and I fact-checked him on the e-coli issue.
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Percy
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From: New Hampshire
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(1)
Message 10 of 30 (777353)
01-29-2016 1:32 PM
Reply to: Message 9 by PhiloNibbler
01-29-2016 10:41 AM


Re: Connections
PhiloNibbler writes:

Thin, by GRI do you mean Global Research Inititive? and SDA Seventh Day Adventists?

He's referring to the link to the GRI SDA (Geoscience Research Institute of the Seventh Day Adventists) that you provided in your Message 1:

PhiloNibbler in Message 1 writes:

Specifically, I was reading paul giem's page http://www.grisda.org/origins/51006.htm

--Percy


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PurpleYouko
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Posts: 713
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(6)
Message 11 of 30 (777354)
01-29-2016 1:39 PM
Reply to: Message 8 by Percy
01-29-2016 8:28 AM


Somebody looking for a mass spectrometer expert?

I've been designing them, teaching about them and fixing them for about 20 years. Does that count?

In all that time though I have never worked directly with a spectrometer specifically designed for Carbon ratio measurement.
However the method by which any isotope is measured is completely the same as for any other isotope. The only difference is in the ion source.

The most simplified version of a ratio measurement mass spectrometer would be something like this


https://en.wikipedia.org/.../Isotope-ratio_mass_spectrometry

The accelerated ions are passed through a magnetic field where the ions of different mass (momentum mV technically but mass is near enough) are separated and then each is sent into a different detector where the number of ions arriving over a period of time is counted.
This kind of spectrometer is known as a magnetic sector instrument. It will have other electrical filters which also focus the beam by kinetic energy (1/2 mV squared). An ion beam typically contains a spread of kinetic energy that follows a Gaussian bell curve so these are used to reject ions with energy outside of an arbitrarily set acceptable range.

The AMS works a little differently in that it uses acceleration energy in the region of MV rather than the usual 6 to 10 KV.
In an AMS the ions start out as KV level negatively charged ions which can often include charged molecules.
They are passed through a sector magnet to separate the ions out by mass before combining the beams with a MV particle beam created in a nuclear particle accelerator. This effectively smashes all the molecular species into atoms and also reverses their charge to positive.
The ions are now traveling extremely fast.
A series of carefully tuned 'filters', either magnetic, electrostatic or both, are used to remove unwanted species leaving only the isotopes that we are interested in.
The isotopes will then be measured in one of several different ways.
They may be slowed and captured in Faraday cups.
They may be fired into a TOF (Time Of Flight) lens
or their kinetic energy can be directly measured to determine the mass of each ion.
This wiki article describes AMS pretty well.
https://en.wikipedia.org/wiki/Accelerator_mass_spectrometry

As for the issue of background measurements and empty targets....
Background noise has always been the analytical chemist's worst nightmare.
Carbon is everywhere. Little tiny bits of it will work their way into every place they possibly can so we always have to clean, clean some more and then clean again a few more times just for luck before we analyze anything.
Atmospheric C02 will stick to samples just because it can.
Sodium is even worse but that's kind of beside the point in this topic.

So we always have to measure the instrumental backgrounds before, during and after every analysis of any kind in order to determine the BEC (Background Equivalent Concentration) of whatever isotopes we are measuring.

Does that help at all?
So my image didn't show up. Not too sure why

Edited by PurpleYouko, : No reason given.

Edited by Admin, : Fix image.


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jar
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Message 12 of 30 (777357)
01-29-2016 2:34 PM
Reply to: Message 11 by PurpleYouko
01-29-2016 1:39 PM


image is a svg. change it to a jpg and it will show up.

Anyone so limited that they can only spell a word one way is severely handicapped!

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Coyote
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(4)
Message 13 of 30 (777359)
01-29-2016 2:53 PM


Interesting subject but...
I've been following the discussion but haven't contributed as my specialties within C14 dating are sample selection and interpretation of the results. I leave the mechanics to the physicists and other bright folks.

But you'd be amazed at how complex, and how little understood, sample selection can be! As an example, in one area I work we have a lot of abalone and mussel shells in the archaeological sites. The oldest abalone shell dated so far is about 5900 BP (n=390), while mussel shells take you back beyond 9400 BP (n=403). What this means is if you select just abalone shells for your samples you are introducing a horrible bias and missing the oldest dates.


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PhiloNibbler
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Posts: 8
Joined: 01-29-2015


Message 14 of 30 (777367)
01-29-2016 6:54 PM
Reply to: Message 10 by Percy
01-29-2016 1:32 PM


Re: Connections
Percy: ah, lol. my only reason for using that source is AIG/ICR quote Giem as a study of sources and not the sources directly. I working my way through his material but lot of details and I plan to look at all 70 of his cited sources directly as well

Edited by PhiloNibbler, : No reason given.


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PhiloNibbler
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Posts: 8
Joined: 01-29-2015


Message 15 of 30 (777382)
01-29-2016 11:41 PM
Reply to: Message 13 by Coyote
01-29-2016 2:53 PM


Re: Interesting subject but...
Purple: Thanks very helpful. Glad to have your expertise. So from what I read I understand that when Snelling from AIG says:
Among their proposed explanations is that the AMS instruments do not properly reset themselves between sample analyses. But if this were true, why would the instrument find zero atoms when no sample is in it?
(he doesn's source this claim but I think it'd be from Giem based on other articles)

that's wrong because there is always some possible contamination [as well as background noise] and what you actually are measuring is a baseline measurement of these before and after to ensure good reliability of the the sample readings.

I know I'm repeating myself but I also know my YEC friends will challenge me when I challenge them


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