Q1(i). Identifying radioactive decay

Answer in brief

The curved, exponentially decreasing trace represents radioactive decay.

Key points

• For a single radionuclide, −dN/dt = λN; λ is the decay constant.
• Integrating gives ln(N/N₀) = −λt, hence N = N₀e^−λt.
• Activity A = λN therefore satisfies A = A₀e^−λt. Its slope is −λA and becomes progressively less negative.
• The curve is convex upwards (d²A/dt² = λ²A > 0), approaching zero asymptotically, not the straight lines.
• A straight-line decay plot requires ln A against t; its slope is −λ and t₁/₂ = ln 2/λ.