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

Options

A. 6000
B. 9000
C. 3000
D. 24000

Answer

D

Explanation

(d) 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 its two half-lives. So before two halflives its activity was `(2^2 xx "present activity")`.
`:.` Initial activity `=2^2 xx 6000 = 24000 dps`.