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Why the helium nucleus?

  1. Jul 28, 2013 #1
    Why does alpha decay emit a helium nucleus.Why not a proton or gamma rays. Please help me understand the science behind why it emits this?
  2. jcsd
  3. Jul 28, 2013 #2
    Its a thing that we wont be able to solve ever i think? But maybe bosons can
  4. Jul 28, 2013 #3


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    It has to do with the balance of the EM force and the strong force felt on particles inside the nucleus. The repulsive force from all those protons is balanced by the attractive strong force. For the most part the strong force holds the entire nucleus together. But, due to quantum tunneling, at some point the nucleons, in the form of an alpha particle, spend enough time outside of the range of the strong force to be ejected from the nucleus, resulting in alpha decay.

    Why not a lone proton? I think that has to do with how tightly bound an alpha particle is.

    Why not a gamma ray? Well, many times the nucleus DOES emit a gamma ray after undergoing alpha or beta decay. After the initial decay, the nucleus is often left in an excited state. It can then fall to a lower energy state and emit a gamma ray in the process to carry away the excess energy.

    It doesn't release a gamma ray before the alpha or beta decay because it is not in an excited state, and thus has no energy to give up to form a gamma ray.
  5. Jul 28, 2013 #4


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    Proton emission can happen, it is one of the less common modes of radioactive decay.

    Alpha particles have a high binding energy, in many cases their emission is possible even if the emission of a single nucleus (proton or neutron) is not possible.
  6. Jul 29, 2013 #5
    First recall the radioactive decay is a process by which an unstable nucleus emits particles such that the sum of the new nucleus and emitted particle are more stable than the original particle.

    The helium nucleus is doubly magic, and it is very stable. Thus for large nuclei that are unstable because they are too big, its a ideal particle to emit.

    In chemistry you learned that when an atom has a certain number of electrons (2,10, 18, ...) it is extremely stable. This is why the noble gases don't react chemically.

    A similar thing happens in the nucleus of an atom. The interaction is more complex because you have to forces at play (instead of one), and you have both protons and neutrons (instead of just electrons). It turns out that when an atomic nuclei has a certain number (2,8,28,50 ... ) of protons or neutrons will tend to be more stable. Nuclei where both the protons and neutrons are on of these "magic" numbers are extremely stable. Helium for is one of these isotopes. Some other examples are oxygens-16 and lead-208.
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