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Using the decay rate of rubidium isotope to determine age of fossils

  • Thread starter Helena_88
  • Start date
  • #1
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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 :-)
 

Answers and Replies

  • #2
mgb_phys
Science Advisor
Homework Helper
7,774
12
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 ?
 
  • #3
5
0
half life = ln(2)/decay constant = τln2
 
  • #4
5
0
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?
 
  • #5
Char. Limit
Gold Member
1,204
14
Hmm...

Do you mean [itex]\frac{log(2)}{\lambda}[/itex]?
 
  • #6
5
0
yes i did.
 

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