The isotope iodine-131 (¹³¹₅₃I) is radioactive with a decay constant of 8.6 × 10⁻² day⁻¹. β⁻ particles are emitted with a maximum energy of 0.61 MeV.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

1. [1] (a)(i) nucleus emits particles/EM radiation/ionising radiation
2. [1] (a)(i) emission/release from unstable nucleus or emission from nucleus is random and/or spontaneous
3. [1] (a)(ii) probability of decay (of a nucleus) or fraction of (number of undecayed) nuclei that will decay
4. [1] (a)(ii) per unit time
5. [1] (b) energy is shared with another particle
6. [1] (b) mention of antineutrino
7. [1] (c)(i) number = [(1.2 × 10⁻⁹) / 131] × 6.02 × 10²³ or number = (1.2 × 10⁻³ × 10⁻⁹)/(131 × 1.66 × 10⁻²⁷)
8. [1] (c)(i) (= 5.51 × 10¹²)
9. [1] (c)(i) A = λN
10. [1] (c)(i) = [0.086 / (24 × 3600)] × 5.51 × 10¹²
11. [1] (c)(i) = 5.5 × 10⁶ Bq
12. [1] (c)(ii) 1/50 = exp(-0.086t) or 1/50 = 0.5ⁿ
13. [1] (c)(ii) t = 45 days