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The decay constant of a radio isotope is λ. If A₁ and A₂ are its activities at times t₁ and t₂ respectively, the number of nuclei which have decayed during the time (t₁ − t₂) is

1. λ(A₁ − A₂)
2. A₁/λ − A₂/λ
3. A₁ − A₂
4. (A₁ − A₂)/λ

Correct answer: A₁/λ − A₂/λ

Solution

The activity of a radioactive substance is proportional to the number of undecayed nuclei. The number of nuclei that have decayed during the time interval is given by the difference in the number of nuclei at t₁ and t₂, which can be expressed as (A₁/λ - A₂/λ).

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