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Radioactive nucleus - why is alpha emitted and not proton

  1. May 3, 2013 #1
    So I recently heard of a simple way of thinking about how the emission of an alpha particle is "beneficial" to a nucleus being stable.

    The argument goes as follows:

    "Let's say we have a nucleus that is unstable because the electric repulsion of protons can't balance out the strong force that tries to keep the nucleus in one piece.

    Emitting an alpha particle reduces the ratio of protons to neutron in a nucleus so this will make a more stable nucleus. "

    So, I realize that this is a very simplified version of the story but I know nearly nothing about nuclear physics but this argument seems to be a useful way to rationalize why helium decay makes the nucleus more stable.

    My question is that, a proton emission is also beneficial to reduce the ratio of protons to neutrons, why is it that nature seems to go with alpha decay instead?

    I did some googling and had a look at the theory of alpha decay by Gamow; how he deduced it from QM. That's explicatory, but I still think the question is valid, and I couldn't really find anything else on it, possibly I don't know what to search for. As I say I am null with nuclear physics & this sort of advanced physics in general, but I'm curious and would appreciate it if you would point me in the right direction.
  2. jcsd
  3. May 3, 2013 #2
    I usually associated alpha decay with a desire to form smaller nuclei.

    If you recall, the strong nuclear force only acts over small distances, while electrostatic repulsion acts over long distances. A consequence of this is that large nuclei tend to be unstable. This is why all elements heavier than uranium are unstable.

    Alpha decay is a mode of decay that efficiently decreases the size of a nucleus. It is also more energetically favourable than other forms of decay because helium-4 is a doubly magic nucleolus and thus very very stable.

    Nuclei that are proton rich decay by positron emission (beta +) or electron capture. Both of these processes "convert" a proton into a neutron which is more efficient in approaching a favourable proton to neutron ratio than proton emission.
  4. May 3, 2013 #3

    Vanadium 50

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    There are rare single proton and double proton decays. However, these are usually energetically unfavorable.
  5. May 5, 2013 #4
    There is chart of nucleides that shows the radioactive isotopes of the elements. In the chart you can literally draw a line through the stable isotopes. It also tells you how the isotopes usually decay and their half lives. If I remember correctly the manner of decay is pretty constant for where the isotopes lie in reference to that line. I know it doesn't answer your question but it might help you.
  6. May 6, 2013 #5


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    Proton emission would be endothermic reaction for heavy nuclei. You would have to add considerable energy to the nucleus for it to be able to occur and thus would not happen spontaneously. You can calculate the mass defect yourself: http://www.nndc.bnl.gov/qcalc/
  7. May 6, 2013 #6
    look at the wiki on magic number
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