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Is half-life inversely related to the radiation dosage of an element?

  1. Feb 26, 2015 #1
    Is half-life inversely related to the radiation dosage of an element? That is, if an element has a longer half-life is it safer? If so, why is plutonium so dangerous, even though it has a very long half life
     
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  3. Feb 26, 2015 #2

    e.bar.goum

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    Sure, the shorter the half-life, the greater the number of disintegrations per second, the larger the dose rate - e.g. μSv/h - but for a given quantity, the time integrated dose would end up being the same. In addition, if you let the material sit around for a while, the dose rate will drop below something with a longer half-life.

    For example, take two sources 57Co and 60Co that had 1 μCi activity in 2008 - in 2008, the activity of the 57Co would have been much higher, but now, the 57Co source will have a very low activity, and the 60Co source will still have a reasonably large activity.

    Plutonium is dangerous for a number of reasons: It accumulates in bone marrow and the liver, with a very long biological half life (on the order of 200 years), and is an alpha emitter - the kind of radiation matters, when it comes to radiation exposure - see the concept of "equivalent dose". Of course, it is also fissile, so large quantities can result in criticality incidents.
     
  4. Feb 27, 2015 #3
    There is something of a sweet spot in terms of half-life and difficulty handling. With a very short half-life material its very easy to simply wait for it to become safe to handle (for most applications). Give it 10 half-lives and the activity has dropped by 1024.

    Some materials have very long half-lives like uranium-238 which is around 4 billion years. Clearly you can't wait out 10 or more half-lives of U-238. For these materials the activity is so low that it is effectively stable (for some applications). Pure uranium is actually pretty easy to handle requiring only gloves and possibly air filters depending on the physical form.

    Things with medium half-lives are the most difficult. Too long to wait for it to decay but short enough to have a high activity. Pu-239 is ~24 000 years. Pu-238 is around 87 years. Pure alpha emitters can actually be easy to handle (in some physical forms). Since alpha particles are easily stopped by air, skin or most containers. HOWEVER, alpha emitters can be very dangerous if they get inside you. Some chemical elements are easily rejected from the body. Others, especially heavy metals can accumulate. Happens to have a moderate half-life (meaning significant activity) and chemical properties that mean it stays in your body near sensitive tissue.

    Not to downplay the seriousness of plutonium but, I would say there are far more dangerous materials on earth (chemical toxins particularly gases and infectious bio-hazzards like small-pox) which are 'harder' to handle safely.
     
  5. Mar 2, 2015 #4
    This is a good question.

    Just as a follow up to Holo's final paragraph, try googling "Bernard Cohen plutonium caffeine" for an interesting story.
     
  6. Mar 27, 2015 #5
    Uranium is also chemically poisonous.
    Is plutonium chemically poisonous? Pu-244 has 80 million year half-life, so the activity ought to be pretty low!
     
  7. Mar 27, 2015 #6

    Doug Huffman

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    See specific activity to calculate Becquerel gram^-1
     
  8. Mar 27, 2015 #7
    To give you an idea thorium has a half-life of 14 Billion years. Uranium-238 has a half-life of 4 billion years. U-235 is 700 million years. The most common types of plutonium in spent fuel is Pu-239, it has a halflife of 24 thousand years, Pu-240 is only 6.5 thousand years and Pu-241 is 14 years.

    I believe that Pu-244 would only be present in minute quantities (I'd be surprised if it is even measurable since it would require 6 consecutive neutron captures).
     
  9. Mar 27, 2015 #8

    Doug Huffman

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    Thanks for the mention of Bernard Cohen, an opponent of the Linear No Threshold hypothesis. Some of his web site is archived
    http://www.phyast.pitt.edu/~blc/
     
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