A 280 days old radioactive substance shows an activity of 6000 dps, 140 days later it’s activity becomes 3000dps. What was its initial activity.
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Question

A 280 days old radioactive substance shows an activity of 6000 dps, 140 days later it’s activity becomes 3000dps. What was its initial activity.

G1 Physics · Accepted Answer

Step 1: Use N(t) = N₀ exp(−λt), A = λN and T₁/₂ = ln2/λ. Step 2: For two samples, write the decay law separately and equate the required activities/numbers. Final: B: 24000 dps.