• # question_answer A freshly prepared radioactive source of half life 2 hr emits radiation of intensity, which is 64 times the permissible safe level. The minimum time after which it would be possible to work safely with this source is A)  6 hr                 B)  12 hrC)  42 hr                          D)  128 hr

Correct Answer: B

Solution :

b.$\frac{N}{{{N}_{0}}}={{\left( \frac{1}{2} \right)}^{t/T}}$or $\frac{1}{64}={{\left( \frac{1}{2} \right)}^{t/T}}$ or ${{\left( \frac{1}{2} \right)}^{6}}={{\left( \frac{1}{2} \right)}^{t/T}}$or $\frac{t}{T}=6$ $\therefore$$t=6T$or $t=6\times 2=12\,hr.$ Hence, the correction option is .

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