First slide

Radio activity

Question

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

Moderate

A

12 days
B

7 days
C

18 days
D

21 days

Solution

$A = A_{0} {(\frac{1}{2})}^{t / T_{1 / 2}} \Rightarrow 5 x 10^{- 6} = 64 x 10^{- 5} {(\frac{1}{2})}^{t / 3} \Rightarrow \frac{1}{128} = {(\frac{1}{2})}^{t / 3} \Rightarrow t = 21 days$

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