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Homework Help: Alpha Particle Spectroscopy - Is My Method Correct?

  1. Dec 29, 2014 #1
    1. The problem statement, all variables and given/known data
    Hey, first time poster here, I'm current writing up a lab report I just wanted to check my method is correct. I'm doing alpha particle spectroscopy using a cloud chamber setup. Most published reports seem to vary pressure (so the mass thickness varies). Our chamber let us vary pressure and distance, I decided to write my report explicitly around varying distance.

    I've used the classical Beth-Bloch equation as the model. By plotting x vs alpha energy E I used function fitting to get a third order polynomial out, by taking the derivative of this I found dE/dx as a second order polynomial. Substituting the energy at each point I evaulated -dE/dx at each point and thus, -dX/dE. By plotting E vs -dX/dE I can integrate this to determine the range.

    It gets complicated as theres a singularity at 160 KeV (From the log part of the BB equation). So far I overcame this by integrating to the limits of initial energy to the singularity point giving me an under estimate for the area (and hence range). Im not sure if this is the best approach, or how to quantify uncertainty. I used 2 integration methods, the trapezium rule and fitting the function as a high order polynomial and integrating that analytically, is that acceptable?

    2. Relevant equations

    3. The attempt at a solution

    Thanks, sorry if this is unclear, feel free to point that out.
  2. jcsd
  3. Dec 30, 2014 #2


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    I don't understand what you did.

    x is distance? How do you vary distance in a cloud chamber? Every track will have some distance, that is a measurement value.
    How can you plot distance vs alpha energy? Do you have some emitters with known energy and plot the distribution of distances?

    That is a problematic model for typical energies of radioactive decays. You get ##\beta \leq 0.05##, especially below 1 MeV the model won't work.

    But that was a measurement before?
  4. Dec 31, 2014 #3
    Basically we used an MCA/MCB to measure the peak energy at various distances. http://www.cityu.edu.hk/ap/nru/pub_j96.pdf very similar to this report here.

    By plotting the SRIM simulations and my model I saw that by approximating the equation as a third order polynomial doesnt work as below the low energy limit the BB model breaks down (the nuclear stopping power becomes the most prominent and overall stopping power increases) got some nice graphs that show this and outline why my method produces an underestimate.

    A quick estimate for the range was taken by measuring the distance at which the peak energy is not distinguishable with background noise levels.
  5. Dec 31, 2014 #4


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    Ah, I see.

    Why do you need Bethe-Bloch? As comparison, sure (without expecting a match for most of the energy range), but you have your own experimental values for dE/dx.

    That depends on your data and the fit. Do you get a reasonable agreement between the methods, and with the measured range value?
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