1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    mgb_phys

    User Avatar
    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

    User Avatar
    Gold Member

    Hmm...

    Do you mean [itex]\frac{log(2)}{\lambda}[/itex]?
     
  7. Mar 12, 2010 #6
    yes i did.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook