First slide

Radio activity

Question

After 280 days, the activity of a radioactive sample is 6000 dps. The activity reduces to 3000 dps after another 140 days. The initial activity of the sample in dps is

Difficult

A

6000
B

9000
C

3000
D

24000

Solution

Here the activity of the radioactive sample reduces to half in 140 days. Therefore, the half life of the sample is 140 days. 280 days is it’s two half lives. So before two half lives it’s activity was $({2^2} \times$ present activity).
$\therefore Initial\,\,activity\,\,{2^2} \times 6000 = 24000\,dps.$

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