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Explanation of particle physics phenomenon - multiple Coulomb scattering

  1. Apr 4, 2008 #1
    1. The problem statement, all variables and given/known data

    1) I read the following article: http://pdg.lbl.gov/2005/reviews/passagerpp.pdf

    On page 12 of the .pdf there is a figure, figure 27.8, which shows how a charged particle deviates from it's path when scattered by the Coulomb force that comes from the nuclei of the matter in the bubble chamber.

    I don't understand the figure. I know I am supposed to use formula 27.15 (my teacher told me), but I don't get the geometry in figure 27.8 - the bold line is the particle's trajectory and the whole grey area is the matter it is moving it (the bubble chamber). S_plane - which I must find - is the distance between the trajectory and the "dotted" line - but what does the dotted line represent?

    2) When talking about Coulomb-forces; if an electron in a bubble-chamber gets near a nuclei in the champers matter (lets say hydrogen, H_2). Then does the hydrogen-atom attract the eletron? And vice versa - if a proton gets near the H_2-nuclei, does it get repelled?

    Do the particles have to pass next to the nuclei or can it be outside the atom?

    These were my two questions. I hope you can help me,

    Last edited: Apr 4, 2008
  2. jcsd
  3. Apr 4, 2008 #2


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    1) The dotted line is the effective path travelled.

    2) Like always repels like, and opposites attract.

    An atom may have zero net charge but a charged particle traversing matter will "see" non-zero charge most likely that of the atomic electrons. The bare nuclear charge is screened by the outer electrons and only a tiny fraction of interactions are with nuclei.

    So neglecting nuclear interactions: e is repelled by [tex]H_{2}[/tex](or indeed any atom) and p is attracted.

    This varies of course with the media and the traversing particle type, Nuclear affects are more significant at very low [tex]\beta \gamma[/tex] as you can see in fig 27.1
  4. Apr 4, 2008 #3
    First, thanks for replying and taking the time to read the article.

    1) "The effective path travelled": By this you mean the path it would have travelled if not being scattered by the Coulomb force?

    2) So the protons and electrons all pass the [tex]H_2[/tex] "outside" the nuclei? So the protons will not get repelled by the proton in [tex]H_2[/tex]?
    Last edited: Apr 4, 2008
  5. Apr 6, 2008 #4
    Is it possible that I can get you guys to take a last look at my questions?

    The thing is, I have to make a "presentation" for a professor about this topic - the only questions I have that do not get answered in that article are those two, so it's kinda crucial. I don't wish to say something that is false.

    I hope you understand.
    Last edited: Apr 6, 2008
  6. Apr 6, 2008 #5


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    1) The solid line is a typical actual path of a charged particle. The dashed line is a linear fit to that trajectory. The slope of the dahed line is tan\theta.

    2) Each Coulomb scattering is the same Rutherford scattering whether the force is attractive or repulsive. The Rutherford formula is the same for each. The individual scattering is not important, just the slope of the dashed line.

    3) The scattering is mostly off the nucleus, but it is only the slope of the dashed line that matters.
  7. Dec 6, 2008 #6
    Sorry to bump this again. I have a question about coulomb scattering. I may have done an experiment similar to OP. I measured scattering angle for MCS of muons through He filled spark chambers scattered by layers of Pb.

    I'm alittle confused about the coulomb interaction. When mu minus particles come near the Pb atoms, they would be repelled by the atomic electrons? And then when mu plus particles come near the Pb atoms, they would be first attracted by the atomic electrons, then repelled by the high Z nucleus? Eqn 27.12 in the PDG booklet doesn't really shed light on the different interaction for mu plus and mu minus. Could someone help me understand how coulomb scattering is different for mu plus and mu minus on a Pb atom? and how these different scattering angles will contribute differently to a summed total scattering angle (eqn 27.12)
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