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13.Nuclei
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From a newly formed radioactive substance (Half life $2$ hours), the intensity of radiation is $64$ times the permissible safe level. The minimum time after which work can be done safely from this source is ..........$hours$
A
$6 $
B
$12$
C
$24$
D
$128$
(IIT-1983)
Solution
(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.$
Standard 12
Physics
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