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Alpha Particle

  1. May 16, 2003 #1
    I've searched the web madly, but I can't find a thorough explanation of alpha decay.

    When a nucleus is unstable, it emits a clump of 2 neutrons and 2 protons, and this is called an alpha particle. Is this right?

    As for the 2nd question. Why are alpha particles deadly if ingested? After all, the alpha particle is in the form of a helium nucleus, and if helium is inhaled it will just make your voice high pitched. Does anybody have an answer?

    Any help would be greatly appreciated.:smile:
  2. jcsd
  3. May 16, 2003 #2
    Some answers

    The Helium nucleus has a high binding energy and that is why it is thrown out during decay as opposed to other nuclei.

    It's deadly if you ingest an alpha emitter because the alpha particle can tear up vital molecules in a cell, disrupting cell funtion and even causing cancers.
  4. May 16, 2003 #3
    Hi McLaren,
    You are right about the composition of an alpha particle; It is easily stopped by the skin or a sheet of tissue paper where it becomes a neutral Helium atom having collected two electrons. Often with alpha emission [also with beta (an electron) emission] there is one or more gamma rays which are really penetrating. With U-238 there is 4.2 MeV alpha coincident with a 45 - 50 kilovolt gamma ray. With U-235 the alpha is about 4.4 MeV along with a 53 KeV gamma ray. Cheers, Jim
  5. May 17, 2003 #4
    There is indeed no satisfactory alpha-decay theory because there is no satisfactory theory of nuclear forces yet (QCD is still in development phase).

    So, explanation of sharp dependance of decay time versus energy of alpha particle goes as follows. Alpa particle is kinda "already there", in the nucleus, on some state (energy level) which is well above "ground" (~few MeV) if by "ground" here we mean its energy far away from nucleus. It is kinda moving back and forth inside nucleus in compliance with HUP, and is bouncing from tall walls of nuclear potential in desperation to leak out. The barrier it has to overcome sharply depends on energy of particle, because the outside wall of this barrier is simply a Coulomb potential of repulsion of alpha particle from the rest of the nucleus (U~1/r hyperbola) - it is thick and fat at low energies and quite thin at high.

    So, if energy state of alpha particle is 7-8 MeV above "ground", it leaks via barrier quite fast (in seconds and less), if 5-6 MeV - years, and if less than 3-4 MeV - billions and trillions of years.

    Of course, there is no alpha particle as an isolated entity in the nucleus before it desintegrates. (And strictly speaking so can be said about protons and neutrons themselves - in nucleus they constantly swap color exchanging by gluons). So it is not clear why this explanation kinda works (predicting relation between energy of alpha particle and decay time, which matches observations).

    And I don't know if there is better (or alternative) explanation for alpha decay to date. May be somebody else here knows?
    Last edited by a moderator: May 17, 2003
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