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Alpha Tunneling

  1. Nov 17, 2011 #1
    Hi,
    In case of alpha decay from Uranium atoms, i thought the barrier is the strong nuclear force, but i had seen many textbooks saying that it is electrical barrier,

    and i did rethink the subject and i imagined it as a barrier that have electrical force from one side and strong nuclear force from the other side.

    i don't know,
    please explain
     
  2. jcsd
  3. Nov 18, 2011 #2

    Bill_K

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    The nuclear force is attractive, like a potential well from which one must escape. The Coulomb potential is positive (repulsive) and surrounds the nuclear well. The simplest model of alpha decay (which works surprisingly well) is that the particle forms within the nucleus, repeatedly strikes the barrier, and eventually penetrates it and escapes.
     
  4. Nov 18, 2011 #3
    But the coulomb potential can't be considered as a barrier, because it help the alpha to escape away from the nucleus, so i feel it much clearer to say that the barrier is the strong nuclear force.
    i still need to know why they consider the coulomb fore as the barrier?
     
  5. Nov 19, 2011 #4
    i got this from the Wikipedia
    http://en.wikipedia.org/wiki/Alpha_particle" [Broken]
    In classical physics, alpha particles do not have enough energy to escape the potential well from the strong force inside the nucleus (this well involves escaping the strong force to go up one side of the well, which is followed by the electromagnetic force causing a repulsive push-off down the other side).
    However, the quantum tunnelling effect allows alphas to escape even though they do not have enough energy to overcome the nuclear force. This is allowed by the wave nature of matter, which allows the alpha particle to spend some of its time in a region so far from the nucleus that the potential from the repulsive electromagnetic force has fully compensated for the attraction of the nuclear force. From this point, alpha particles can escape, and in quantum mechanics, after a certain time, they do so.


    now it become more complicated for me, it seems to me that for an alpha particle inside the nucleus to escape from it, it has to overcome two barriers one due to the strong nuclear force, while the other is due to the electrical repulsion from the other particles around it
    ???
     
    Last edited by a moderator: May 5, 2017
  6. Nov 21, 2011 #5

    Bill_K

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    If you Want to Learn, pay attention to what I said:
    The nuclear force is a well (negative), the Coulomb force is a barrier (positive)
    To escape first the nuclear well, the alpha particle needs to have an energy E > 0. The Coulomb barrier V(x) is positive with a maximum Vmax > E. Then as you get farther away from the nucleus it tapers off. So the alpha particle must penetrate this barrier out to a distance where V(x) ≤ E. The Coulomb force only helps the alpha particle to escape after it has penetrated the barrier.
     
  7. Nov 21, 2011 #6
    These are much clear statements, i got it
    many thanks
     
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