ch43-p029 - 29. Let t be the present time and t = 0 be the...

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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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