A freshly prepared radioactive source (half life =4 hrs) emits radiations of intensity which is 32 times the permissible level.The minimum time (in hr) after which it would be safe to work with this source is
A
4
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B
8
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C
10
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D
20
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Solution
The correct option is D20 Activity of substance remaining R=Ro2n where n=tT1/2=t4 Permissible level R=Ro32 ∴132=12n Or 2n=32 ⟹n=5 Or t4=5 ⟹t=5×4=20 hrs Correct answer is option D.