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I Actual measurement of radioactive decay

  1. Mar 2, 2016 #1
    Hello,

    Are there any anthropologists, archaeologists, or geologists around on this board for help? I am trying to teach myself about radioactive decay via beta emission whereby a neutron spontaneously transmutes into a proton, releasing an electron. But, I do not understand the practical side. For instance, carbon-14 decays to become nitrogen on average about every 5,730 years. When an animal dies, its carbon-12 remains constant, carbon-14 decays to nitrogen. A scientist compares a sample of carbon-14 to nitrogen. I just can get the academic material -- if I had 14 grams of carbon-14 and 14 grams of nitrogen in an animal bone, I can guess that it started with 28 grams of carbon-14, 0 grams of nitrogen (a huge assumption). But, how does a scientist actually do this? How does a scientist get started? What exactly is done? I find an animal bone. What exactly do I do to find out how much material of carbon-14 there is and how much nitrogen there is, to compare? How does someone find out exactly how much of each there is? Creationists claim this first step is fraught with so many assumptions that the first step establishes a weak protocol.
     
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  3. Mar 2, 2016 #2

    mfb

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    You don't compare carbon to nitrogen. You compare C-14 to C-12 by putting samples in a mass spectrometer.
    The ratio of C-14 to C-12 is roughly constant in living material, but goes down after death with a known rate given by the lifetime.

    To get a very naive and rough approximation (but more than sufficient to see that some samples are tens of thousands of years old), you can measure the C-14 to C-12 ratio of old samples and compare it to the ratio in living things directly after their death today: If you see half of today's ratio, the sample is roughly 6000 years old, if you see 1/4, the sample is roughly 12000 years old, and so on.

    What is actually done is way more sophisticated, of course, to increase the accuracy. The C-14 to C-12 ratio in the biosphere varies a bit over time., so you need a track record of the ratio as function of time. Tree rings allow to measure that: take a tree that is 2000 years old (counted by rings) and still living. You can measure the C-14 to C-12 ratio in each ring and extrapolate it back to the time this ring was made. That improves the age determination for everything younger than 2000 years. Now consider a tree that lived e.g. from 1000 BC to 1000 AD: you know the C-14 to C-12 ratio for the second half of its life, so you can determine when the tree died with great accurary. The first 1000 rings then allow to determine the C-14 to C-12 ratio between 1000 BC and 0 AD. And so on. Some samples also have an age that can be determined independently via other methods.

    And then you include all the other effects that have to be considered (also including those). And use all the other dating methods as cross-check. It is impossible to describe hundreds of pubications in a single forum post.

    Ignore crackpots. It is just pointless to listen to them.
     
  4. Mar 2, 2016 #3
    Thank you, that helps a lot. I am printing your response, and looking again at the Wikipedia summaries.

    Now, why can't you compare carbon-14 to nitrogen? Is it because the sample can already have nitrogen in it, contaminating it, which invalidates the ability for it to be aged? Is it errant to claim that if I measure 10 grams of C-14 and 10 grams of N, then the sample started with 20 grams of C-14 and 0 grams of N and eventually will be 0 grams C-14 and 20 grams N? Is that mixed up logic?

    Yes, the crackpots approach me already with their half-formed truths, only cite the evidence that backs up their opinion, ignore evidence, and ignore what I say. It is a waste of time, but I really need to understand this so that evidence can sway the 'borderline' people who are privy to both sides.

    Thanks.
     
  5. Mar 2, 2016 #4

    mfb

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    Every cell has nitrogen in it, mainly N-14. And you have to consider the scale: a sample that contains 10 g of N-14 has something like picograms (10-12 g) of C-14. The additional nitrogen from C-14 decays is completely negligible. You would have to know the initial N-14 to C-14 ratio very well, and you would get large uncertainties from that. To make it worse, the carbon to nitrogen ratio is different in every sample, even within the same species. You also have nitrogen gas in the atmosphere, and the amount that is dissolved is tricky to estimate.

    C-14 to C-12 is much easier, as both carbon atoms have (nearly) identical chemical properties.
     
  6. Mar 2, 2016 #5

    Vanadium 50

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    One of the implications of this is that one does not measure the C-14 decay to measure the amount of C-14 (any more). It's the historic decay that matters.

    The decay rate is very low - looks to be about 1 decay per four seconds per gram of modern carbon.
     
  7. Mar 3, 2016 #6

    mfb

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    The decays can be measured, but that needs much larger samples than mass spectrometry.
     
  8. Mar 3, 2016 #7

    Vanadium 50

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    That's why I said "any more".
     
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