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Homework Help: STD of Poisson distributed particle intensity

  1. Nov 14, 2015 #1
    1. The problem statement, all variables and given/known data
    Say I have a detector that detects alpha-particles of some decay process. Let N be the amount of particles detected and let N be Poisson distributed. If I now define the intensity to be the amount of detected particles per second I = N/t, what would the standard deviation of I be? Also derive the standard deviation of ln I using propagation of error (error in t can be disregarded)..

    2. Relevant equations
    Poisson distribution

    3. The attempt at a solution
    If I let the amount of particles detected in a time interval t be n = μ/t, the probability of detecting x particles is given by the Poisson probability function with mean (parameter) μ = nt. The expectation value is therefore just μ and the std is sqrt(μ) = sqrt(nt). This means that the std for the intensity is sqrt(nt)/t = sqrt(n/t). That's the first part done.

    As for the std of ln I, I don't really know what is meant by deriving it using propagation of error or maybe I'm just too tired. Any help is appreciated.
  2. jcsd
  3. Nov 18, 2015 #2


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    Science Advisor
    2017 Award

    Shouldn't this be n = µt?


    Instead of defining a new variable µ in part 1, you should try applying the formula above to I = N/t in order to calculate sI in terms of N and t.
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