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B Ionized trail in bubble chamber

  1. Aug 4, 2017 #1
    Question about bubble chambers.
    How can particles with small mass, such as electrons, create ionized path without having their trajectory disturbed by the ions they are creating? Seems like we should see them scattered around somewhat rather than following that nice circular pattern. Is this because their energy is very high and head on collisions are rare?
     
  2. jcsd
  3. Aug 4, 2017 #2

    mfb

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    They are scattered, but large scattering angles are rare. A "head on collision" is not well-defined in the microscopic world as the particles don't have a well-defined position.
     
  4. Aug 6, 2017 #3
    Thanks,
    Just seems strange looking at those perfectly smooth inward spirals that just stop abruptly imagining what is going on at the subatomic level. The high energy electron is apparently interacting with many atoms, enough to ionize them. And it does this without any noticeable change in its own direction, but with a definite change in its energy (thus the inward spiral). Then at some point it just disappears like it got absorbed by an atom. The electron never seems to scatter.
    Do you know of any good papers of books that cover this in any detail? My Quantum Physics (Eisberg/Resnick) text that I had in college only goes into some detail about the expected scattering of two neutrons as they collide.
     
  5. Aug 6, 2017 #4

    jtbell

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    A Google search for "multiple scattering of electrons" turns up some hits that look promising. Check them out and see which ones are accessible at your level.
     
  6. Aug 6, 2017 #5

    mfb

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  7. Aug 6, 2017 #6
    Each electron creates ions by collision at the expense of its kinetic energy and therefore every time there is an ionising collision the electron slows down a bit. Eventually the kinetic energy reaches a value that is too small to create further ions and because there will be no ions for the bubbles to form on the electron tracks will no longer be visible.
     
  8. Aug 6, 2017 #7
    Thanks for this helpful image,

    Looking at it more closely, it is occurring to me that when the electron has slowed down enough to exhibit the meandering trajectory of the low-energetic electron, its gyroradius in the magnetic field has become very small, so it only appears to be stopping at that point.
     
  9. Aug 7, 2017 #8
    The electron might appear to be "stopping" but as I tried to explain above each electron eventually reaches an energy which is too small to create further ions by collision. We see tracks because bubbles form on the ions produced. Or in the case of cloud chambers vapour condenses on the ions produced.

    Each electron will be losing energy at different collision points along its track and when its kinetic energy becomes smaller than the ionisation energy it can continue moving for a while but no longer be able to create ions and therefore tracks.
     
  10. Aug 7, 2017 #9

    mfb

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    A rough estimate: Let's take liquid hydrogen. It has a radiation length of 9 meters (PDG).
    To have a curvature radius of 9 cm in a 1 T magnetic field, we need an electron with 27 MeV. With the multiple scattering formula from GEANT, over 9 cm we get a typical scattering angle of about 0.04 rad. That is quite small compared to the curvature from the magnetic field (1 rad by the choice of the length).
     
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