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Homework Help: Radioactivity homework question

  1. Oct 26, 2011 #1
    1. The problem statement, all variables and given/known data

    A nuclear power plant breeds 1mg of 239Pu per week. What activity, in Bq, does that create?

    2. Relevant equations

    [itex]R_0 = \lambda N_0[/itex] (initial activity of the sample)

    [itex]R = R_0 e^{-\lambda t}[/itex] (exponential behavior of the decay rate)

    [itex]T_{1/2}=\frac{ln 2}{\lambda}[/itex] (Half-life)

    3. The attempt at a solution

    (a) First I find the number of moles in 1mg of 239Pu:

    [itex]n= \frac{1\times10^{-3}}{239}=4.184 \times 10^{-6}[/itex]

    Now, I think to find N0 I have to times n by 6.02x1023 nuclei/mol. Which gives us N0=2.518x1018.

    But how can I find the decay constant λ, if we are not given the half life? :confused:

    I tried out the 1 week as the half life but I didn't get the correct answer:

    [itex]\lambda = \frac{0.693}{604800 \ s} = 1.1458 \times 10^{-6}[/itex]

    [itex]R_0 = \lambda N_0 = 2.88 \times 10^{12} \ Bq[/itex]

    [itex]R = R_0 e^{-\lambda t} = 1.44 \times 10^{12}[/itex]

    So, what should I do? The correct answer must be: [itex]2.3 \times 10^6 \ Bq[/itex].
  2. jcsd
  3. Oct 27, 2011 #2


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    Re: Radioactivity

    The half-life is usually something that you look up. Back in my time (:-p) we had a book of tables where you could find this, nowadays we have Wolfram Alpha.
  4. Oct 27, 2011 #3
    Re: Radioactivity

    Right, I see. So there's no equation or anything you can use in this situation if you don't have a table?
  5. Oct 27, 2011 #4


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