BCECE Engineering BCECE Engineering Solved Paper-2005

  • question_answer
    If half-life of radium is 77 days, its decay constant will be:

    A)  \[\text{3 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{-3}}}\text{/day}\]                   

    B)         \[\text{9 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{-3}}}\text{/day}\]

    C)         \[\text{1 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{-3}}}\text{/day}\]                   

    D)         \[\text{6 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{-3}}}\text{/day}\]

    Correct Answer: B

    Solution :

    The time required for the number of parent nuclei to fall to 50% is called half-life \[{{T}_{1/2}}\] and may be related to \[\lambda \]as follows. Since,    \[0.5\,{{N}_{0}}={{N}_{0}}{{e}^{-{{T}_{1//2}}}}\] we have,             \[\lambda {{T}_{1/2}}=\ln (2)=0.693\] or                            \[{{T}_{1/2}}=\frac{0.693}{\lambda }\] or                            \[\lambda =\frac{0.693}{{{T}_{1/2}}}\] Given,                   \[{{T}_{1/2}}=77\] days \[\therefore \]  \[\lambda =\frac{0.693}{77}=9\times {{10}^{-3}}/days\]


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