1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy and Compton scattering

  1. May 19, 2017 #1
    1. The problem statement, all variables and given/known data

    Compton scattering can be used both to measure the direction and energy of photons in nuclear physics experiments. For a particular preparation a spectrum of Compton scattered electrons was measured which clearly corresponded to a generally monochromatic gamma radiation. The maximum electron energy was measured to 150 keV. Calculate the wavelength of the incoming monochromatic radiation.
    2. Relevant equations
    What do I know :

    Compton formula : λ'-λ = (h/mc) * (1-cos(θ))
    Energy conservation: hc/λ + m * c^2 = hc/λ' +γ*m *c^2
    E_befor = hc/λ , E_after = hc/λ'

    3. The attempt at a solution
    The correct solution is :
    E_max = 150 keV = 2.4 * 10-14

    λ'-λ = (h/mc) * (1-cos(θ)) and maximum occurs when cosθ = -1 then :
    λ'-λ = 2*(h/mc)
    Further we have :
    E_befor + m * c2 = E_after + γ*m *c2
    E_befor - E_after = (γ-1)mc2 = E_kinetic
    E_kinetic = hc((1/λ)-(1/λ')) †
    We take E_kinetic = 150 keV = 2.4 * 10-14
    and solve λ'= 2*(h/mc) + λ
    and just put everthing in † and solve λ which is = 0.044nm

    My question is why can't we solve this by thinking :

    hc/λ + m * c^2 = hc/λ' +γ*m *c^2 = Constant = 2.4 * 10-14 J

    and just take hc/λ + m * c^2 = 2.4 * 10-14 J ? I dont get the same λ as the solution and I know it's wrong but why is this wrong?!
     
  2. jcsd
  3. May 19, 2017 #2

    vela

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Why would the maximum electron energy be equal to the total energy of the photon-electron system? That's what you're claiming in your method.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted