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Homework Help: Nuclear u-235 and u-238 isotope half life question

  1. Apr 20, 2012 #1
    1. The problem statement, all variables and given/known data

    The present day value of the uranium isotopic ratio U-235/U-238 is 0.00723. The half life of U-238 is 4.51 x 10^9 and the half life of U-235 is 7.13 x 10^8 years. What was the U-235/U-238 isotopic ratio 2 billion years ago when the Oklo natural reactor was active?

    2. Relevant equations
    T1/2 = ln (2) / λ
    τ = 1/λ
    N(t) = No e^(-kt)

    3. The attempt at a solution

    Calculated the decay constant lambda for each isotope:

    U-235: λ = 9.72 x 10^-10 per year

    U-238: λ = 1.54 x 10^-10 years,

    I'm not sure how to proceed or what I need to do next

    New to this radioactive decay stuff so any tips or suggestions would be much appreciated.

  2. jcsd
  3. Apr 20, 2012 #2
    You have 2 equations, one for the activity of U-235 and one for the activity of U-238, each as a function of time. What happens when you divide one by the other?
  4. Apr 20, 2012 #3
    As in the equation N(t) equations? will the No be different in each case?
  5. Apr 20, 2012 #4
    If i do this and substitute in t = 2 billion years i get a ratio of 0.1947, although I don't think that this is correct?
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