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Why is Cobalt-60 radioactive?

  1. Oct 4, 2013 #1
    I have heard that if n/p ratio of a substance becomes more than 1.56, it becomes radioactive. But for Cobalt-60, n/p= 1.22. Yet it is radioactive. Why is that so?

    Thanx in advance...
  2. jcsd
  3. Oct 5, 2013 #2

    Simon Bridge

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    That is only a rule of thumb.
    Co60 is radioactive because it is energetically favorable for it to be so.
  4. Oct 5, 2013 #3
    There is a simple rule named Mattauch rule with just 2 exemptions:

    Out of two isotopes with same mass and whose proton number differs by one, at least one must be radioactive.

    The 2 exemptions are:
    Antimony-123 and tellurium-123 are both stable
    Tantalum-180 and hafnium-180 are both stable (Also tungsten-180 decays by alpha decay, not electron capture as it should by Mattauch rule).

    A consequence of Mattauch rule is:
    By Mattauch rule, the only stable isotopes with odd number of protons and neutrons are the 4 light isotopes D, Li-6, B-10 and N-14. All others should be radioactive.
    Tantalum 180 is violating Mattauch rule and so is the 5th stable odd-odd isotope. There are still just these 5. Cobalt 60 is odd-odd, and is not one of these 5 stable isotopes, so it is radioactive.
  5. Oct 5, 2013 #4
    Thanx a lot...!
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