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Alpha vs. beta radiation?

  1. Apr 23, 2012 #1
    Hello all,
    I just built an expansion cloud chamber and noted that while my alpha source (americium 241) produced a plethora of tracks, the beta source (strontium 90) produced no tracks at all-just mist. Does anyone know if beta radiation can be seen in an expansion type cloud chamber? Why or why not? Details would be great!
    Thanks
     
  2. jcsd
  3. Apr 23, 2012 #2

    HallsofIvy

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    I'm no expert but alpha particles consist two protons and two neutrons while a beta particle is simply a single electron (or positron). That is, an alpha particle is much heavier than a beta particle and so carries more energy at low speeds.
     
  4. Apr 23, 2012 #3
    so you're saying that a beta particle wouldn't have enough energy to ionize the air molecules in a cloud chamber traveling at such a slow speed, whereas an alpha particle would?
     
  5. Apr 23, 2012 #4
    The alpha particles are very heavily ionizing particles, all of which have the same initial energy. So all of the tracks are about the same length. The ionization density of the track in the cloud chamber is high because the alpha particles are slow, and the charge is high (2 protons). (Track ionization density is proportional to Z2)

    The beta decay process involves the emission of both an electron and a (anti)neutrino. which share the energy of the decay of strontium 90 (and yttrium 90). So the energy of the betas ranges from 0 energy up to the maximum energy (about 2.28 MeV). Furthermore, because the betas are relativistic, the track ionization density is low. The energy of many of the betas is so high that they do not stop in the cloud chamber.
     
  6. Apr 23, 2012 #5

    Vanadium 50

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    Let's step back a bit. What is the activity of each source?
     
  7. Apr 24, 2012 #6
    Strontium 90 is around 140 Ci/g; americium 241 is 3.5 Ci/g.
     
  8. Apr 24, 2012 #7
    I would think that 10 visible tracks per sec might be about right, so maybe 1000 dps into 4 π. 1000 dps = 0.03 μCi. If you are interested in researching this more, look at http://pdg.lbl.gov/2011/reviews/rpp2011-rev-passage-particles-matter.pdf, especially the plot of stopping power on page 4. The beta particles are near the minimum (about 2 MeV per gram per cm2), while the alpha particles are near the peak on the left (x4 because Z=2).
     
  9. Apr 24, 2012 #8

    Vanadium 50

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    Yes, and how many grams of each do you have?
     
  10. Apr 24, 2012 #9
    I'm not quite so sure... I took the americium 241 out of a smoke detector, so I'd assume there would be 2.0 times 10^-4 mg, the standard amount in scientific notation. As for the strontium, I have no idea. The website, however, says the activity level of the strontium 90 source is .1uCi.
     
  11. Apr 24, 2012 #10
    Wow, looks like a great source, Bob S. I haven't really started looking through it yet, but I took a quick glance and it seems that restricted energy loss rates actually result in less visible ionization. Any quick explanations for why this is?
     
  12. Apr 24, 2012 #11

    Vanadium 50

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    Typical smoke detectors are 1 or 2 uCi. That's 10-20x the activity of your beta source.
     
  13. Apr 24, 2012 #12
    Ah, I see. Without that variable controlled its hard to make generalizations.
     
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