# physics

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The rubidium isotope 87Rb is a β emitter that has a half-life of 4.9 x 1010 y that decays into 87Sr. It is used to determine the age of rocks and fossils. Rocks containing the fossils of early animal have a ratio of 87Sr to 87Rb of 0.004. Assuming that there was no 87Sr present when the rocks were formed, calculate the age of the fossils.

• physics -

N=N₀•exp(-ln2•t/T)
N/N₀=1-0.004=0.996
ln0.996= - ln2•t/4.9•10¹º
t= ln0.996•4.9•10¹º/ln2= 2.83•10⁸ years

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