Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic P5.2.3 — Radioactive decay
• Topic P5.2.4 — Half-life
• Topic P5.2.5 — Applications and safety precautions
• Topic P6.2.2 — Life cycle of stars
(a) ¹⁴C → ⁰₋₁β + ¹⁴₇N
Beta decay involves a neutron converting into a proton and emitting a beta particle (electron). The mass number remains unchanged, but the atomic number increases by one.
(b) beta ; (beta) penetrates through thin aluminium foil (to be detected) / (beta) absorbed by thick aluminium foil (so not detected)
Beta radiation has intermediate penetrating power. It is suitable for thickness monitoring because the amount of radiation reaching the detector changes with foil thickness. Alpha would be completely absorbed, while gamma would pass through unaffected.
(c) 11,400 years
Activity has fallen from 28 to 7 counts/min
28 → 14 → 7 (2 half-lives)
Age = 2 × 5700 = 11,400 years
Each half-life reduces the activity by half. After two half-lives, the activity is a quarter of the original.
(d)(i) fusion
Stars produce energy through nuclear fusion, where lighter nuclei combine to form heavier nuclei, releasing enormous amounts of energy.
(d)(ii) helium ; and any named product heavier than hydrogen but lighter than iron ; energy
In stable stars, hydrogen nuclei fuse to form helium. In larger stars, further fusion produces heavier elements up to iron. Energy is released during these fusion reactions.
