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Homework Help: Using the decay rate of rubidium isotope to determine age of fossils

  1. Mar 11, 2010 #1
    Hello! i was just wondering if anyone could help me with this,

    The rubidium isotope 87-Rb is a beta emitter with a half life of 4.9 x 10^10 yr that decays into 87-Sr. It is used to determine the age of rocks and fossils. Certain rocks contain a ratio of 87-Sr to 87Rb of 0.0100. Assuming there was no 87-Sr present when the rocks were formed, calculate the age of these fossils.

    So far all i can think of doing is taking 1% of the half life as the answer but i'm very uncertain!

    Thanks for any help with this :-)
  2. jcsd
  3. Mar 11, 2010 #2


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    Science Advisor
    Homework Helper

    So the question asks how long does it take for 1% of the Rb to decay.

    First of all you need an estimated answer, otherwise with such large numbers you will make a mistake.
    The half life is the time it takes for 50% to decay, so we are looking for an answer much much less than the half-life.

    Do you know the equation linking half life and decay rate ?
  4. Mar 11, 2010 #3
    half life = ln(2)/decay constant = τln2
  5. Mar 11, 2010 #4
    can i say that
    because the ratio is Sr/Rb = 0.01
    N/No = e^-λt so
    0.01 = e^-λt

    then t1/2 =ln/λ then find t?
  6. Mar 12, 2010 #5

    Char. Limit

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    Gold Member


    Do you mean [itex]\frac{log(2)}{\lambda}[/itex]?
  7. Mar 12, 2010 #6
    yes i did.
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