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Question

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 D 20
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.

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