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A freshly prepared radioactive source of half life 2 hours emits radiation of intensity which is 64 times the permissible safe level.
The minimum times after which it would bepossible to work safely with the source is ;

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6 hours

12 hours

24 hours

128 hours

answer is B.

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Detailed Solution

$\frac{A}{A_{0}} = \frac{N}{N_{0}}$

Let safe-level activity be A. Initial activity =64 A.

Hence $\frac{N}{N_{0}} = \frac{A}{A_{0}} = \frac{A}{64 A} = \frac{1}{64}$

$or {(\frac{1}{2})}^{n} = \frac{1}{64} or n = 6$

$Hence \frac{t}{T} = n = 6$

But T = 2 hours,

$∴$ t = 12 hours

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