O-Level and SEC G3 Physics K323
P20: Radioactivity
Compare emissions, write decay changes, interpret half-life, and balance useful applications against exposure risk.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Radioactivity combines atomic and nuclide structure, random spontaneous decay, alpha, beta and gamma properties, nuclear equations, background correction, half-life, applications, hazards, fission and fusion.
Nuclides and emissions
An atom has a small positive nucleus containing protons and neutrons, with electrons outside. Proton number (Z) identifies the element, nucleon number (A) counts protons plus neutrons, and isotopes share (Z) but have different neutron numbers.
Alpha particles are helium nuclei, beta-minus particles are fast electrons emitted during nuclear change, and gamma is electromagnetic radiation. Alpha is strongly ionising and weakly penetrating; gamma is weakly ionising and strongly penetrating; beta lies between for these comparisons.
Decay equations, background and half-life
Radioactive decay is random for one nucleus and spontaneous, so ordinary temperature, pressure and chemical changes do not control it. In a nuclear equation, conserve total nucleon number and total proton number.
Background count must be measured and subtracted from measured count before interpreting source activity. Half-life is the time for undecayed nuclei or corrected activity to halve. Repeated halving gives an exponential trend, not a constant subtraction.
Uses, risks, fission and fusion
Choose a source for a use by half-life, penetration and ionisation. A tracer needs detectable radiation and a suitable half-life; radiotherapy uses ionising radiation to damage cancer cells while controlling dose to healthy tissue.
Fission splits a heavy nucleus and can support a chain reaction. Fusion joins light nuclei and powers stars. Both release nuclear energy. K323 does not require energy-mass calculations or detailed nuclear power-station technology.
Formulae and relationships
| Relationship | Use |
|---|---|
| Apply repeated half-lives when (n) half-lives have elapsed. | |
| Remove background radiation. |
Worked examples
Example 1: A source gives (820) counts per minute including background. Background is (20) counts per minute. After (12 h), the measured rate is (220) counts per minute. Find the half-life.
- Initial corrected rate: counts per minute.
- Final corrected rate: counts per minute.
- (800 to 400 to 200) is two half-lives in (12 h).
Answer: Half-life .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Distinguish alpha, beta, and gamma emissions by nature, charge, penetration, and ionisation.
- Balance nuclear equations and interpret half-life from counts, activity, or graphs after accounting for background.
- Compare uses, risks, fission, and fusion within the stated K323 boundary.
Official outcome coverage
K323 P20: 10 mapped outcomes, references P20(a), P20(b), P20(c), P20(d), P20(e), P20(f), P20(g), P20(h), P20(i), P20(j). Check the official K323 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Interpret count-rate data with background correction, repeated readings, and time, distance, and shielding controls rather than handling an unapproved source.
Exam traps and retrieval check
Avoid these traps
- Using uncorrected count rate in a half-life calculation.
- Treating half-life as the time for all nuclei to decay.
- Confusing gamma emission with a change in proton or nucleon number.
Check from memory
What identifies an element?
Its proton number.
Which emission is most ionising in the K323 comparison?
Alpha.
How do fission and fusion differ?
Fission splits a heavy nucleus; fusion joins light nuclei.
Pure versus Combined scope
Combined Physics revisits part of this core under CP16 Radioactivity, but with reduced outcome scope. Combined students should follow the component checklist rather than assume every K323 outcome is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K323. Open the related IP explanation.