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Double-slit experiment: What is the speed of the neutrons?

  1. May 8, 2016 #1
    1. The problem statement, all variables and given/known data
    An experiment was performed in which neutrons were shot through two slits spaced 0.10 mm apart and detected 3.5 m behind the slits. The following figure shows the detector output. Notice the 100 μm scale on the figure. To two significant figures, what was the speed of the neutrons?
    38_49.jpg
    2. Relevant equations
    Δy = λL/d (spacing between maxima)
    λ = h/mv (de Broglie λ)
    m= 1.67*10^-27 Kg

    3. The attempt at a solution
    The spacing between the maxima appears to be 100*10^-6 m in the figure, so:
    (Δy*d)/L = λ = ((100*10^-6 m)*(0.1*10^-3 m))/3.5 m = 2.85714*10^-9 m = de Broglie λ

    v = h/(mλ) = (6.626*10^-34 J.s)/((1.67*10^-27 Kg)*(2.85714*10^-9 m) = 138.9 m/s

    This answer is incorrect. Can someone please give me a hint as to what I'm doing wrong?
     
  2. jcsd
  3. May 8, 2016 #2

    BvU

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    Could you check that once more for me ?

    (oh, and 138.9 is not what I would call 'to two significant figures' ! )
     
  4. May 8, 2016 #3
    Hum, on second look it looks more like (2/3)(100*10^-6m).

    Which gives Vneutron = 2.1*10^2 m/s; which is the correct answer! Thanks a lot!
     
  5. May 8, 2016 #4

    BvU

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    well done!
     
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