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Estimate the number of mean lives elapsed, when the number of atoms in a radioactive sample decrease to 5% of the original value. |
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Answer» Solution :Let the REQUIRED number of mean lives ELAPSED be .m., then `t=m lt tau gt=m/lambda` N=5% of `N_0=1/20N_0` But, `N=N_0e^(-lambdat)` `therefore N_0/20 = N_0 e^(-m)`or , `e^m`=20 or , m=LN 20 = ln 2+ ln 10 = 0.693 + 2.303 `APPROX` 3 `therefore` At the end of about three average lives, the number of atoms (undecayed) would fall to 5%. |
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