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Calculate the number of half lives elapsed , at the endof which , the activity of a radioactive sample decrease by 90% . |
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Answer» Solution : The change in activity with time follows the same LAW as that of number of radioactive atoms. `:.` Percentage of radioactive atoms left undecayed. 100 - 90% = 10% i.e., `N/N_(0) = 1/10 ` From `(N/N_0)=(1/2)^n` [where n = no . of half lives ELAPSED] `(1/2)^(n)=1/10" or ", 2^(n)=10` or n log 2 = log 10 or `n=1/(0.3010) = 3.3` Thus , at the end of 3.3 half lives, the activity (and hence the number of atoms ) WOULD have DECREASED by 90%. |
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