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Homework Help: Radioactivity question

  1. Sep 9, 2014 #1
    The question is: The activity of a source of Tc-99m (half-life = 6.01 hours) is counted in an ionization
    chamber in a radiopharmacy.

    (b) 44989345 counts are counted in 5.0 seconds. What is the activity of this
    sample and what is the error on this measurement?

    (c) A nearby hospital carries out a procedure which requires 6.0 MBq of Tc-99m
    radiolabelled to a phosphate analogue in order to perform a bone scan. What
    is the maximum amount of time that can be taken to transport the sample so
    that enough activity reaches the hospital?

    I did b but I'm not sure how to do this question but this is how I answered it, i got the 9 from part b:

    N= No x e^(-λt)
    6 = 9 x e ^ ( - 0.0013 x t)
    ln (2/3) = -0.0013 x t
    t = 311.9 s
  2. jcsd
  3. Sep 9, 2014 #2


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    Staff: Mentor

    Hi melissaaa, Welcome to Physics Forums.

    Please use the posting template provided to format your questions when you post the the Homework sections of PF.

    You haven't explained the "-0.0013" value that you've used for the decay constant. Where did that come from? (Hint: it's related to the half-life, so check your calculation...)
  4. Sep 9, 2014 #3
    I did it again and I think I got the decay constant wrong, is it meant to be:

    T(1/2)= ln(2)/λ

    λ= ln(2) /6.01 x 60 x 60
    = 0.0000320367...
  5. Sep 10, 2014 #4


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    Staff: Mentor

    Okay, that looks better. What units are associated with λ? Always provide units when you show results.

    This new value for λ should lead you to a more reasonable result for part (c)...
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