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In-flight neutron capture by Cobalt-59

  1. Jun 15, 2013 #1
    Can anyone help me with an approximate answer to this or point me in the direction of avenues to get an answer to this? This has practical applications and is potentially worrying.

    Roughly how much (or how many atoms) of cobalt-59 exposed to the average neutron radiation at in-flight altitudes (about 11,000 meters) -- neutron radiation makes up about half of the in-flight radiation dose due to cosmic radiation -- will be transmuted into Cobalt-60 via neutron capture over a given amount of time? Alternatively, and maybe this would be easier to answer, roughly what duration or range of time would it take for such in-flight neutron radiation to transmute one atom of cobalt-59 into one atom of cobalt-60?

    I have a plain chrome-cobalt dental crown (composed mainly of cobalt-59) that is not covered with any porcelain or ceramic, and so I am concerned about this.

    Thanks in advance for any approximate answers, guesses, thoughts, pointing to other sources, etc.
     
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  3. Jun 15, 2013 #2

    mfb

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    I would not worry about this.
    In airplanes, only some fraction of the radiation comes from neutrons. Only a tiny fraction of the neutrons will reach your dents, and from that only some fraction gets absorbed by Co-59. Compared to the overall neutron radiation, the radiation from Co-60 should be negligible.

    On the technical side:
    According to this handbook, the thermal neutron absorption cross-section is ~57 barn.
    If the neutron hits a massive 1cm3-block of Co-59, absorption is a very likely result (as 5>1).

    Do you have numbers for the neutron flux in airplanes?
     
  4. Jun 15, 2013 #3
    Hi mfb,

    Thanks so much for your reply, but if I may, I would like to ask you to help me understand things a little more. To start out, I should point out that the fraction of the in-flight radiation that comes from neutrons is actually on average more than 50% of all the in-flight radiation (according to the World Health Organization). Second, I am somewhat of a novice when it comes to nuclear physics terms, so may I clarify, when you said "absorption", did you mean neutron capture in the cobalt-59 nuclei whereby transmutation to cobalt-60 would occur? I assume you did, but just checking to be sure. Related to this, on what basis did you say that only a tiny fraction of the neutrons will reach one's dents? This is very interesting, and I would like to know more. Then, out of the neutron radiation that reaches the dents, on what basis do you say that only some fraction of that gets absorbed by Co-59? Did you say this simply on the basis of the proportion of cobalt to chrome in the dental crown? I am not necessarily doubting your assertions, but I am looking to know the reasons for them, so that I can better understand the situation.

    Further, you said that thermal neutron absorption by Co-59 is a very likely result. Are neutrons at in-flight altitudes considered to be thermal neutrons? I'm not sure of their energy level. (Does it make a big difference to the likelihood of their absorption by Co-59 if they are not thermal neutrons?)

    I too was interested to get neutron flux numbers in airplanes, but I'm not sure where to get them. Might you be able to suggest a possible avenue to find them? More importantly, again because I am somewhat of a novice, could you enlighten me as to how we could then use the neutron flux numbers to help us understand this problem better? How could we use them in a calculation or formula?

    Finally, can you share with me your reasons for saying that compared to the overall neutron radiation, the radiation from Co-60 would be negligible? I'm thinking that the neutron exposure is basically a "one-off", albeit significant, exposure only during the time of a flight but that any transmutation of Co-59 to Co-60 by neutron capture would expose the mouth to long-term high-energy gamma radiation (as Co-60's half-life is more than 5 years, and because it is fixed in the surface of the dental crown, there would be no biological half-life to help out.)

    Sorry for the long post and thanks in advance for helping me explore this issue further.
     
  5. Jun 15, 2013 #4

    Vanadium 50

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    A neutron either directly interacts with an atom in your body and immediately deposits its energy there, or it interacts with a cobalt atom in your tooth which decays, possibly years later, and then deposits its energy.

    Why do you think the second is more dangerous than the first?
     
  6. Jun 16, 2013 #5
    Hi Vanadium 50,

    I should clarify that I don't know whether one is more dangerous than the other. They are different kinds of danger, and I'm trying to get more information about the actual danger of possible cobalt-59 to cobalt-60 transmutation. I can't do anything about the neutron radiation, but I can do something about my cobalt dental crown. Theoretically at least, if at least say a dozen or two atoms of cobalt-59 become cobalt-60, then there is a high probability that some of those atoms would start to decay and bombard my mouth with high energy gamma much earlier than a few years later. But I'm still trying to understand roughly how much or even whether any of the cobalt-59 would be transmuted into cobalt-60.
     
  7. Jun 16, 2013 #6

    mfb

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    Right
    Simple: the dents are a small part of your body. And not all your dents are pure Co-59.

    Absorption cross-sections tend to go down with the neutron energy. For high-energetic neutrons, I would not expect any special effects of Co-59 unless they get slowed down first (and then we have thermal neutrons again). Anyway, neutron damage usually comes from thermal neutrons.

    I am sure there are some studies about that, but I don't know where.

    See Vanadium 50: For the total dose, it does not matter when the decays occur.
    For high radiation doses (some orders of magniture more than we are discussing here), a short-term exposure is even worse than a long-term exposure with the same total dose.
     
  8. Jun 16, 2013 #7

    Vanadium 50

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    A dozen extra decays works out to a total exposure of 0.00000005% of an average daily exposure. The additional risk is the same as driving 1/4000 of an inch. It's the same risk as being killed by lightning on a sunny day if you are outdoors for 1/20 of a second.

    In short, the risk is so small, you are taking a risk thousands or even millions of times larger just in getting to the dentist's to have the risk reduced.
     
  9. Jun 16, 2013 #8

    mfb

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    Oh, I didn't see that "dozen".
    It is important to see that radioactive decays are common everywhere in nature - in a human body, about 100-1000 decays occur per second. In addition, a few high-energetic muons per second are crossing the human body.

    But if you drive those 1/4000 inch in a closed car, you are protected from lightning strikes!
     
  10. Jun 26, 2013 #9
    Vanadium and mfb,

    I just threw out "at least dozen or two" as a totally hypothetical random number. For all I know, it could be 12 thousand or 12 trillion. That's the problem. I don't know enough to make even a rough guess of what order of magnitude of cobalt-59 atoms would be transmuted into cobalt-60 atoms. To help me approach a better range of a guess, may I trouble you to help me with some more basic novice questions?

    From information online I reckon that neutrons at in-flight altitudes are probably categorized as high-energy neutrons, but would the interactions of those neutrons with air molecules (before reaching the airplane) and with the metal, plastic and glass of the airplane cabin (before reaching a passenger) cause the neutrons to slow down enough to fall into the thermal neutron range?

    Does a neutron usually pass easily through soft biological matter (like the cheek of one's face) without being absorbed (not being captured) by such biological matter? Does passage through such matter slow down the neutron? (reduce its energy level?)

    However, I also read in wikipedia: "neutrons produced from cosmic radiation in the Earth's atmosphere ... can be significantly higher energy than those encountered in reactors. Most of them activate a nucleus before reaching the ground; a few react with nuclei in the air. The reactions with nitrogen-14 lead to the formation of carbon-14...."

    As an aside, I should add that I just saw now that each atom of cobalt-60 yields two decays, both a beta and a gamma (and the former, like the latter, is also an issue of concern inside one's mouth.)

    Thanks again for all your assistance in better understanding this.
     
  11. Jun 26, 2013 #10

    Vanadium 50

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    Is there anything we can write to convince you that your worries are unfounded?
     
  12. Jun 27, 2013 #11

    mfb

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    High-energetic neutrons don't care about the isotope so much, they can interact with all atoms (but those interactions are rare), so you don't have to worry about some specific isotope in your mouth.
     
  13. Jun 27, 2013 #12
    mfb,

    Thanks for your informative post again. When you say they can "interact" with all atoms, do you mean both absorption (capture) and other kinds of non-absorption interaction? If mainly the latter, then wouldn't it be more likely that they would just slice through lighter-nuclei atoms of biological material and, as you previously said, the large cross-section of Co-59 and its high probability of capturing neutrons would make the Co-59 the most likely capture point? If we assume the neutrons inside an airplane remain at high-energy levels, roughly how much more unlikely is it that high-energy neutrons will be captured than slow neutrons? Is it an order of magnitude difference or a small percentage difference?

    Vanadium 50, how can I know whether or not there is anything to be worried about before I have all the facts? We have not even seen the neutron flux numbers on airplanes for starters. And besides, this involves interesting research questions independent of worries.
     
  14. Jun 27, 2013 #13

    Vanadium 50

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    Yes, but you're not the one doing the research. We are.

    You asked a very specific question, I answered it, and you immediately changed the question. You're treating us like your personal Google.

    I notice you didn't answer my question. I didn't ask you if yoiu were convinced. I asked if there was anything we could write that would convince you. Is there?
     
  15. Jun 27, 2013 #14

    mfb

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    I do.

    No, the large cross-section applies for thermal neutrons only.

    Several orders of magnitude.

    I found a neutron activation calculator.
     
  16. Jun 28, 2013 #15
    Vanadium 50,

    If you look above, you will see that my original questions were:

    Roughly how much (or how many atoms) of cobalt-59 exposed to the average neutron radiation at in-flight altitudes (about 11,000 meters) -- neutron radiation makes up about half of the in-flight radiation dose due to cosmic radiation -- will be transmuted into Cobalt-60 via neutron capture over a given amount of time? Alternatively, and maybe this would be easier to answer, roughly what duration or range of time would it take for such in-flight neutron radiation to transmute one atom of cobalt-59 into one atom of cobalt-60?

    These have basically remained unanswered. I was not trying to change the topic, just trying to get as much information as I can related to those original questions. Believe me, I have tried getting -- and have gotten a good deal -- of information from the internet before I came here and asked this forum. I thank you again for your assistance and sharing your knowledge.
     
  17. Jun 28, 2013 #16
    mfb,

    Thanks again very much for your highly informative reply. And many thanks as well for providing the neutron calculator; I'm not sure how to use it in relation to my questions, but I'll try to figure it out.

    All the best,
    Ray1
     
  18. Jun 28, 2013 #17

    Vanadium 50

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    Ray, please either answer the question in #4 or in #10. Thanks.
     
  19. Jun 28, 2013 #18

    Vanadium 50

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    Let me sum up:

    The total exposure is small.
    The exposure from neutrons is smaller still.
    A neutron the hits your dental work is less damaging than one that hits your body.

    What more do you want? Don't worry about this.
     
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