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Homework Help: Activity of I_131 after 2 days

  1. Apr 15, 2009 #1
    1. The problem statement, all variables and given/known data
    The initial activity of I_131 is 0.74MBq.
    The half time of I_131 is 8 days.

    How large is the activity after two days?

    2. Relevant equations

    [tex] A = A_0 e^{-\lambda t} [/tex]

    3. The attempt at a solution

    We know
    t = 2 days
    A_0 = 0.74 MBq
    T_0.5 = 8 days

    1. Solve the activity constant
    [tex] \lambda = ln2 / T_0.5 [/tex]

    2. Plug it to the equation
    [tex] A = A_0 e^{(-ln2 / T_0.5) * t} [/tex]

    I standardise the units to SI and then omit/cancel them
    [tex] A = 0.74E6 * e^{-ln2 / 4} [/tex]
    [tex] = 6.22E5 Bq [/tex]


    The right answer is 0.4 times what I get
    [tex] A = 0.4 * 6.22E5 Bq [/tex]
    [tex] = 250 kBq[/tex]

    I am not sure where the 0.4 is got.
  2. jcsd
  3. Apr 15, 2009 #2
    The problem is now solved.

    There is the following sentence two pages before the exercise
    "Only 40% of the activity in a human is of the form I_131"
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