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Homework Help: Help needed in calculating Flux of point source (NucEng)

  1. Apr 5, 2012 #1
    1. The problem statement, all variables and given/known data

    A 2 [itex]\mu[/itex]g sample of [itex]{}^{47}Ca[/itex] is encased in a lead sphere. What is the photon intensity 5 cm from the source? Use some of the tables in your book to find the required information.

    2. Relevant equations

    Flux/Intensity of photons from a point source:
    [itex]\phi(r)=\frac{S_p}{4\pi r^2} e^{-\mu r} [/itex]
    Where [itex]\mu[/itex] is the total microscopic cross section, r is the radius(distance) from point source and [itex]S_p[/itex] is the particles per unit time.

    3. The attempt at a solution

    I can and have done these problems when the energy of the sample is given. However I have never done one where the weight of the sample is given. I am lost on this problem. My main question here is: How can I go from weight to photons/second?

    I tried calculating [itex]S_p[/itex] by using 1047.1 keV/decay listed in the textbook but I still don't know where to go from there. Ci:=decays/second. How many photons per keV?? Or am I over thinking this?
  2. jcsd
  3. Apr 6, 2012 #2


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    Staff: Mentor

    Radioactive calcium 47 has a half-life of 4.7 days and emits a high-energy gamma ray (photon) of 1.3 Mev as well as a beta particle in its decay to scandium 47. - reference

    If you know the sample's mass you can calculate the number of atoms present, and based on its half-life you can determine the expected number of photons the sample is emitting per unit of time.
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