AFMC AFMC Solved Paper-2008

  • question_answer
    Activity of a radioactive sample decreases to (1/3) rd of its original value in 3 days. Then, in  9 days its activity will become

    A) (1/27) of the original value       

    B) (1/9) of the original value         

    C) (1/18) of the original value

    D) (1/3) of the original value

    Correct Answer: A

    Solution :

    \[R={{R}_{0}}{{e}^{-\lambda t}}\] \[\Rightarrow \]     \[\left( \frac{1}{3} \right)={{e}^{-\lambda \times 3}}={{e}^{-3\lambda }}\] Again,  \[\frac{R'}{{{R}_{0}}=}{{e}^{-\lambda \times 9}}={{e}^{-9\lambda }}={{({{e}^{-3\lambda }})}^{3}}\]        \[={{\left( \frac{1}{3} \right)}^{3}}\]                                [From Eq. (i)]         \[=\frac{1}{27}\] \[\Rightarrow \]      \[R'=\frac{{{R}_{0}}}{27}\] Hence , in 9 days activity will become \[\left( \frac{1}{27} \right)\]of the original value.


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