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Why do nuclei like Uranium and Thorium have such long half lives?

  1. Feb 23, 2013 #1
    Why is it that those atoms, with atomic numbers of 90 and 92 have much, much longer half lives than atoms like Radon, Radium, and Polonium? I do realize that atoms with even atomic numbers are more stable than ones with odd numbers, so it makes sense why atoms like astatine, actinium, and francium are really unstable, but why do elements 90 and 92 have half lives in the billions of years? No other unstable element, besides bismuth (but its really close to lead, so it makes sense why its almost stable) have such extraordinary lives. It seems like these two elements have some kind of a boost in stability - they don't follow the general trend towards instability at all.
     
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  3. Feb 23, 2013 #2

    mfb

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    There are many isotopes with lifetimes of billion of years or even more.
    If you compare isotopes with other isotopes nearby, nuclear shell effects become relevant.
     
  4. Feb 23, 2013 #3

    Bill_K

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    For more details, see the Wikipedia page on this.
     
  5. Feb 23, 2013 #4
    I'm mainly talking about the general trend of half lives shortening in elements heavier than lead. They all decrease in a curve, with the exception of uranium and thorium. They're outliers. Why?
     
  6. Feb 23, 2013 #5

    Bill_K

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    No, they're not outliers. But they are an extreme case. If you use the semi-empirical mass formula, you see a well-known trend, that in this part of the table, binding energies fall off with increasing A. As a result, Th-232 and U-238 have Q ≈ 4 MeV for alpha decay, and by the Geiger-Nuttall rule this implies a very long half-life. Beyond Uranium, nuclei are unstable for other decays such as fission, so U and Th are the last cases where the α-decay half-life dominates.
     
  7. Feb 23, 2013 #6

    mfb

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    Most (or at least many, did not count all of them) transuranium element isotopes have alpha and beta decays as dominant modes.

    That is a general trend beyond uranium (and might be wrong if there is an island of stability beyond Z=118), but not between uranium and lead, see the chart of nuclides, for example.
     
  8. Feb 24, 2013 #7

    Bill_K

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    True, but misleading. The issue is, which isotopes? Here's a list of approximate half-lives:

    Th-228 1 y
    Th-230 104 y
    Th-232 1010 y
    Th-234 rapid β decay

    U-234 105 y
    U-236 107 y
    U-238 109 y
    U-240 rapid β decay

    Pu-238 102 y
    Pu-240 104 y
    Pu-242 105 y
    Pu-244 rapid β decay

    My point is that Pu-244 and Pu-246 would have continued the long half-life trend, but the β decay instability took over.
     
  9. Feb 24, 2013 #8
    Manifestly false!
    Pu-244 108 y!!

    Pu-246 does have rapid beta decay (10 days)

    But elements 90 and 92 are not "outliers". They are the tip of a sizable maximum.

    Looking at even elements:
    82 - stable
    84 - 103 years
    86 - 3,8 days

    but that was the minimum, next

    88 - 1600 years
    90 - 14 milliard years
    92 - 4,5 milliard years

    But the maximum goes on:

    94 - 82 million years
    96 - 16 million years

    Then falling deeper:
    98 - 900 years (still longer than 84)
    100 - 100 days - but existence of other isotopes possible

    Odd elements have a similar pattern:

    81 - stable
    83 - 1019 years
    85 - 8 hours
    87 - 22 minutes

    And again a rise to a maximum:

    89 - 21 years
    91 - 32 000 years
    93 - 2,1 million years

    And then a (slower) fall:

    95 - 7400 years
    97 - 1400 years
    99 - 470 days
     
  10. Feb 24, 2013 #9

    Bill_K

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    Sorry Snorkack, I stand corrected. I hope I didn't upset you too much. :wink:
     
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