# Nuclear u-235 and u-238 isotope half life question

1. Apr 20, 2012

### debwaldy

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.

Thanks,
Debbie

2. Apr 20, 2012

### daveb

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?

3. Apr 20, 2012

### debwaldy

As in the equation N(t) equations? will the No be different in each case?

4. Apr 20, 2012

### debwaldy

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?