Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Mean Current of Photomultiplier at anode

  1. Dec 14, 2011 #1
    1. The problem statement, all variables and given/known data
    A weak light source (wavelength 600 nm, mean power 66.2 pW) falls on a multiplier tube with a cascade of 8 dynodes. How large is the mean current at the anode with an amplification of 108 and a 50% quantum efficiency for the photoelectric effect.

    2. Relevant equations
    [itex]E_{kin} = h\nu - W[/itex]

    3. The attempt at a solution

    No. of incident photons = No. of electrons emitted ??
    [itex]N_{\gamma} = N_{e}[/itex]
    [itex] \frac{P * t * λ}{h * c} = \frac{66.2pW * 1s * 600nm}{h*c} = 1.66E29[/itex] (in one second)

    average current produced at first dynode = I0

    [itex]I_{0} = \frac{dq}{dt} = \frac{N_{e} * e}{1s} = 3.201E-11A[/itex]

    Then this is amplified by a factor 108 to give final average current at anode

    I = I_{0} * 1E8 = 3.201E-3A

    well, that was my idea but I don't see how the 50% efficiency fits into this. spot my mistake anyone?
    Last edited: Dec 14, 2011
  2. jcsd
  3. Jun 6, 2012 #2
    The mistake is right in the first line i.e the number of incident photons =no of electrons emitted. You should use the quantum efficiency to calcualte that, i.e no. of photoelectrons=no of incident photons *Quantum efficiency . It is usually a function of the incident wavelength.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook