JEE Main & Advanced Physics Nuclear Physics And Radioactivity Question Bank Critical Thinking

  • question_answer
    The half life period of a radioactive element X is same as the mean life time of another radioactive element Y. Initially both of them have the same number of atoms. Then                    [IIT 1999; UPSEAT 2003]

    A)            X and Y have the same decay rate initially

    B)            X and Y decay at the same rate always

    C)            Y will decay at a faster rate than X

    D)            X will decay at a faster rate than Y

    Correct Answer: C

    Solution :

               \[{{({{T}_{1/2}})}_{x}}={{({{t}_{mean}})}_{y}}\]                    \[\Rightarrow \frac{0.693}{{{\lambda }_{x}}}=\frac{1}{{{\lambda }_{y}}}\Rightarrow {{\lambda }_{x}}=0.693\,{{\lambda }_{y}}\] or lx < ly                    Also rate of decay = lN                    Initially number of atoms (N) of both are equal but since \[{{\lambda }_{y}}>{{\lambda }_{x}},\] therefore, y will decay at a faster rate than x.


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