b
(b) \(\frac{N}{{{N_0}}} = {\left( {\frac{1}{2}} \right)^n}\)

\(\Rightarrow \frac{1}{{64}} = {\left( {\frac{1}{2}} \right)^6}\)

\(= {\left( {\frac{1}{2}} \right)^n} \Rightarrow n = 6.\)

After \(6\) half lives intensity emitted will be safe.  

Total time taken \( = 6 \times 2 = 12\,hrs.\)