Half-life calculation (4 marks)

A sample of iodine-131 has an initial activity of 4800 Bq. The half-life of iodine-131 is 8 days.

(a) Define half-life. (1) (b) Calculate the activity of the sample after 32 days. Show your working. (3)

Mark scheme (4 marks): (a) 1 mark: time taken for half the (undecayed) nuclei in a sample to decay / for the activity (count rate) to halve.

(b)

• 1 mark: number of half-lives = 32 ÷ 8 = 4.
• 1 mark: activity = 4800 × (½)⁴ = 4800 ÷ 16.
• 1 mark: = 300 Bq.

Accept correct answer with valid alternative working (e.g. successive halving 4800 → 2400 → 1200 → 600 → 300).