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KVPY Sample Paper KVPY Stream-SX Model Paper-27

  • question_answer
    From a newly formed radioactive substance (half-life 2 hours), the intensity of radiation is 64 times the permissible safe level. The minimum time after which work can be done safely from this source is

    A) 6 hours

    B) 12 hours

    C) 24 hours

    D) 128 hours

    Correct Answer: B

    Solution :

    \[N={{N}_{0}}{{\left( \frac{1}{2} \right)}^{n}}\]Or \[\frac{{{N}_{0}}}{64}={{N}_{0}}{{\left( \frac{1}{2} \right)}^{n}}\]
    \[\therefore \]\[N=6\]
    Now, \[t=n\,\,{{t}_{1/2}}\] 
    \[=\text{ }6\times 2\]
    = 12 h.


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