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29. Let
t
be the present time and
t
= 0 be the time when the ratio of
235
U to
238
U was 3.0%.
Let
N
235
be the number of
235
U nuclei present in a sample now and
N
235, 0
be the number
present at
t
= 0. Let
N
238
be the number of
238
U nuclei present in the sample now and
N
238,
0
be the number present at
t
= 0. The law of radioactive decay holds for each specie, so
NN
e
t
235
235 0
235
=
−
,
λ
and
e
t
238
238 0
=
−
,
.
λ
238
Dividing the first equation by the second, we obtain
rr
e
t
=
−−
0
()
λλ
235
238
where
r = N
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 Spring '08
 Any
 Physics

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