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Homework Help: Rearrangement of a complex radiation formula

  1. Apr 24, 2012 #1
    1. The problem statement, all variables and given/known data

    6. If the exposure rate from 137Cs at one metre is 2.50 mGy/h, how many bequerels are present?

    2. Relevant equations

    X=5.263 x 10-6 A∑(yiEi(μen/ρ)i)/r2

    3. The attempt at a solution

    what im looking to do isnt to solve the question, i need to rearrange the formula above so that A is the subject and i have no idea where to begin due to its complexity.

    i would also like to know what the symbols represent, my interpretation from what i have found is that:

    X = the exposure measured in Roetengens
    A = The activity of the source measured in Bequerels
    ∑ = this sigma sign is equal to a complex value. To gather this number the individual energy of each photon that this radiation source can produce must be known and multiplied by the rate that they are produced. This is due to the fact that a gamma source does not produce a single energy level but instead there is a range of energies produced.
    yi = The initial Yield from this source before any decay
    Ei = the initial energy of this source before any decay
    E = the energy of the photons
    i = The value of i on its own refers to the fact that the calculation must use the initial values of μen/ρ.
    r^2 = the distance away that the measurement is taken

    please could someone check that this is correct. we were not given this formula or taught how to use it so im struggeling.
  2. jcsd
  3. Apr 24, 2012 #2
    If you weren't given that formula, where did you get it from? It's far easier to use the gamma constant for Cs-137, especially since they didn't give you the value for μen/ρ, which changes depending on photon energy. Your formula should read

    X=5.263 x 10-6A∑(yiEien/ρ)i)/r2, where the summation is over all photon energies. However, when you use the gamma constant, this summation is already calculated for you.
  4. Apr 24, 2012 #3
    We weren't given anything for this calculation, as a group we found an online calculator that said this was the formula for it. Although it'd be fantastic if there is a simpler way, I'm not sure how to do what you suggested either
  5. Apr 25, 2012 #4
    There are numerous sources for the gamma constant (G) for Cs-137. The Rad Health Handbook lists the gamma constant as 3.3 R-cm2/hr-mCi, and the formula would be X = AG/r2, so all you need to do is perform some algebra and unit conversions to get the activity.
  6. Apr 25, 2012 #5
    this is FAR easier, thank you
  7. Apr 25, 2012 #6
    A = xr2 / G

    so then i need to convert the units for G to bequerels? but what is R in the R-cm2/hr-mCi

    does that mean its 3.3 x 1002 is equal to the value in milicuries?

    1002 due to my calculation being at 1m
  8. Apr 26, 2012 #7
    Yes, millicuries to Becquerels, and R (Roentgen) to mGy. 1 rad = 0.877 roentgen
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