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    Double Magic Numbers: Not Always Guaranteed Stability
    By Enrico Uva | April 8th 2011 10:07 AM | 3 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    In the shell model of the nucleus, there are certain magic numbers of nucleons (protons or neutrons): 2, 8, 20, 28, 50, 82, 126 that are more tightly bound than the next higher integer. A magic number of nucleons is important because it increases the likelihood that isotopes will be stable. If the number of neutrons and protons are both  magic numbers (not necessarily the same one) we say the nucleus is doubly magic, and it is even more likely to be stable. Isotopes with doubly magic numbers include :
    4He
    16O
    40Ca, 48Ca
    56Ni, 48Ni
    132Sn
    208Pb

    None of the above isotopes are radioactive except for nickel's pair and 132Sn, which, in spite of its double magic number of 50 protons and 82 neutrons, has a half life of less than a day. So a while ago, I contacted a scientist to ask him why 132Sn is radioactive.
    Here is the answer from Alex Brown of the National Superconducting Cyclotron Laboratory http://www.nscl.msu.edu
    "Sn-132 is doubly-magic, but being doubly-magic does not guarantee stability. There are other features that can make a nucleus unstable. In this case Sn-132 is too far from the "valley of stability" - it has too many neutrons. The neutrons in Sn-132 beta decay and turn into protons eventally leading to the most stable mass 132 nucleus Xe-132.
    The "magic number" of 50 protons for Sn does show up by the fact that Sn has more stable isotopes than any other element - they are: Sn-112, Sn-114, Sn-115, Sn-116, Sn-117, Sn-118, Sn-119, Sn-120, Sn-122 and Sn-124. There are other nuclei that we predict to be doubly-magic that are unstable, such as Sn-100 and Ni-78.
    At our National Superconducting Cyclotron Laboratory are are trying to produce these nuclei and study their properties.
    "

    Comments

    rholley
    Interesting.  This got me thinking of supernovae.  Much of the energy of their collapse, I understand, gets carried off in the form of neutrinos, but their massive light output corresponds to the decay of nickel-56 formed in the shock wave which follows core collapse and rebound.  Digging around in Wikipedia, I find under Isotopes of Nickel:

    Nickel-48, discovered in 1999, is the most neutron-poor nickel isotope known. With 28 protons and 20 neutrons 48 Ni is "doubly magic" (like 208 Pb) and therefore unusually stable.

    Nickel-56 is produced in large quantities in type Ia supernovae and the shape of the light curve of these supernovae corresponds to the decay of nickel-56 to cobalt-56 and then to iron-56.

    Nickel-78 is the element's heaviest isotope and is believed to have an important involvement in supernova nucleosynthesis of elements heavier than iron. [1]

    Three unstable “wizards” in one element!
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    UvaE
    Thx, Robert. I made the adjustments . Nickel-56 is definitely radioactive with a half life of 6.10 days and experiences electron capture, thus lowering the atomic number by one to cobalt-56. Brown mentioned Ni-78 and Ni-48 probably is too neutron-poor to be stable. But you wrote,
    Most of their mass, I understand, gets carried off in the form of neutrinos
    My impression was that only about 10% of a supernova's mass gets carried off as neutrinos, even though they carry off an awful lot of energy.
    rholley
    My impression was that only about 10% of a supernova's mass gets carried off as neutrinos, even though they carry off an awful lot of energy.
    My web search bears out what you said.  I most probably have misread or mis-remembered something from years ago.  Correction made.
    Robert H. Olley / Quondam Physics Department / University of Reading / England