A) 6 hours
B) 12 hours
C) 24 hours
D) 128 hours
Correct Answer: B
Solution :
\[\frac{N}{{{N}_{0}}}={{\left( \frac{1}{2} \right)}^{n}}\Rightarrow \frac{1}{64}={{\left( \frac{1}{2} \right)}^{6}}={{\left( \frac{1}{2} \right)}^{n}}\Rightarrow n=6.\] After 6 half lives intensity emitted will be safe. \ Total time taken \[=6\times 2=12\,hrs.\]
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