Activity of a sample is 64 × 10^{-5} Ci . Its half-life 3 days. activity will become 5 × {10

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13.Nuclei

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The activity of a sample is $ 64 \times 10^{-5}\ Ci$ . Its half-life $3$ days. The activity will become$5 \times {10^{ - 6}}\ Ci$ after ........... $days$

A

$12$

B

$ 7$

C

$18 $

D

$21$

Solution

$A=A_{0}\left(\frac{1}{2}\right)^{t / T_{1 / 2}} \Rightarrow 5 \times 10^{-6}=64 \times 10^{-5}\left(\frac{1}{2}\right)^{t / 3}$

$\Rightarrow \frac{1}{128}=\left(\frac{1}{2}\right)^{t / 3} \Rightarrow t=21 \mathrm{\,days}$

Standard 12

Physics

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