Radioactivity (P4.3)
Radioactivity is a topic where Gateway A examiners love both recall questions (properties of emissions) and numerical half-life calculations. The irradiation vs contamination distinction is also a common 6-marker.
Radioactive decay
The nucleus of an unstable atom loses energy by emitting radiation. This is a random and spontaneous process — it cannot be controlled or predicted for any single nucleus. It is unaffected by temperature, pressure or chemical bonding.
Radioactive decay produces three main types of emission.
Types of radiation
| Emission | Symbol | What is it? | Charge | Mass | Range in air | Stopped by | Ionising power |
|---|---|---|---|---|---|---|---|
| Alpha (α) | ⁴₂He or α | 2 protons + 2 neutrons (helium nucleus) | +2 | 4 u | ~5 cm | Paper / skin | High |
| Beta (β) | ⁰₋₁e or β | Fast-moving electron from nucleus (neutron → proton + electron) | −1 | ~0 | ~1 m | 3 mm aluminium | Medium |
| Gamma (γ) | γ | Electromagnetic radiation (photon) | 0 | 0 | Many km | Several cm lead / thick concrete | Low |
⚠ Gamma has no mass and no charge — it is NOT a particle.
Nuclear equations
In a nuclear equation, the top numbers (mass numbers) and bottom numbers (atomic numbers) must each balance.
Alpha decay:
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
(Mass: 238 = 234 + 4 ✓; Atomic number: 92 = 90 + 2 ✓)
Alpha emission reduces atomic number by 2, mass number by 4.
Beta decay:
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
(Mass: 14 = 14 + 0 ✓; Atomic number: 6 = 7 + (−1) ✓)
Beta emission increases atomic number by 1 (neutron → proton), mass number unchanged.
Gamma emission:
Gamma photons are emitted alongside alpha or beta — the nucleus releases excess energy. No change to mass or atomic number.
Half-life
Definition: the time taken for half the nuclei in a sample to decay (or for the activity/count rate to halve).
Half-life is a statistical property — with billions of atoms, the average is very predictable even though individual decays are random.
Half-life calculations
If initial activity = A₀ and half-life = t½:
- After 1 half-life: A = A₀ / 2
- After 2 half-lives: A = A₀ / 4
- After 3 half-lives: A = A₀ / 8
- After n half-lives: A = A₀ / 2ⁿ
Example: A source has an initial activity of 800 Bq and a half-life of 5 days. What is the activity after 20 days?
- 20 / 5 = 4 half-lives.
- Activity = 800 / 2⁴ = 800 / 16 = 50 Bq.
Finding half-life from a graph:
Choose any activity value, find the corresponding time, then find when the activity has dropped to half that value. The difference in time = the half-life.
Irradiation vs contamination
| Feature | Irradiation | Contamination |
|---|---|---|
| Definition | Exposure to radiation from an EXTERNAL source | Radioactive material DEPOSITED ON or IN the body |
| Source in contact? | No — source is outside the body | Yes — source is inside or on the body |
| Stops when? | When you move away from the source | Continues until material is removed/decays |
| Best radiation type for concern | Gamma (penetrates to body from outside) | Alpha (highly ionising if inside body) |
Background radiation
Natural sources: radon gas (from soil/rock, especially granite), cosmic rays, food and drink, medical sources.
Background radiation must be subtracted from readings in experiments to get the true count rate from the source.
Uses of radioactivity
| Use | Type | Why |
|---|---|---|
| Medical tracers (PET) | Gamma (short half-life) | Passes through body; detected externally; short half-life reduces dose |
| Radiotherapy | Gamma (focused) | Kills cancer cells |
| Thickness control (paper) | Beta | Partially absorbed by paper; detects changes in thickness |
| Smoke detectors | Alpha | Ionises air to complete a circuit; smoke absorbs alpha and breaks the circuit |
| Carbon dating | Beta (C-14) | Long half-life (5,700 years) matches timescale of organic remains |
| Sterilisation | Gamma | Kills microbes in food/medical equipment; gamma penetrates packaging |
Common Gateway-paper mistakes
- Saying gamma is a particle — it is electromagnetic radiation.
- Mixing up what stops each emission: paper = alpha; aluminium = beta; lead = gamma.
- Getting beta decay wrong — forgetting that beta comes from a neutron changing to a proton (atomic number +1, mass number unchanged).
- Not subtracting background radiation from experimental count-rate data.
- Confusing irradiation (external source, stops when you leave) with contamination (internal or on-body source, continues).
AI-generated · claude-opus-4-7 · v3-ocr-combined-science