Radioactivity
The Nucleus and Isotopes
The nucleus contains protons (positive charge) and neutrons (neutral). The atomic number (Z) is the number of protons; the mass number A is protons + neutrons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (e.g. ¹²C and ¹⁴C).
Radioactive decay occurs when an unstable nucleus emits radiation to become more stable. It is a random process — we cannot predict which nucleus will decay or when, but we can predict statistical rates.
Types of Radiation
| Type | Symbol | Composition | Charge | Penetrating power | Stopped by |
|---|---|---|---|---|---|
| Alpha | α | 2 protons + 2 neutrons (He nucleus) | +2 | Low | Paper / a few cm of air |
| Beta | β | Fast-moving electron (from nucleus) | −1 | Medium | 3-5 mm aluminium |
| Gamma | γ | High-energy electromagnetic radiation | 0 | High | Several cm of lead / concrete |
Alpha decay: ᴬ/ᵤX → ᴬ⁻⁴/ᵤ₋₂Y + ⁴/₂α (mass number decreases by 4; atomic number decreases by 2) Beta decay: ᴬ/ᵤX → ᴬ/ᵤ₊₁Y + ⁰/₋₁β (mass number unchanged; atomic number increases by 1; a neutron converts to a proton) Gamma: no change in mass or atomic number — just releases energy.
Ionising Radiation
All three types can ionise atoms (knock out electrons), but alpha is the most ionising (large charge, slow), while gamma is least ionising (no charge).
Background radiation is always present from: cosmic rays, natural radon gas (from rocks, especially granite), food and drink, medical procedures.
Half-Life (WJEC Required Knowledge)
Half-life (t½) is the time for half the radioactive nuclei in a sample to decay (or for the count rate / activity to halve). It is constant for a given isotope and not affected by temperature, pressure, or chemical state.
After n half-lives, the fraction remaining = (½)ⁿ.
Activity A = change in number of nuclei / time. Unit: becquerel (Bq) = 1 decay per second.
Uses and Hazards
Uses: medical imaging (technetium-99m, gamma emitter); cancer radiotherapy (gamma/beta); carbon dating (C-14, beta); smoke detectors (americium-241, alpha); irradiation of food (gamma).
Hazards: ionising radiation damages DNA and cells, causing mutations, cancer, or death at high doses. Minimise exposure: use tongs; maintain distance; use shielding; limit exposure time.
Nuclear Fission and Fusion
Fission: a large nucleus (e.g. U-235) absorbs a neutron and splits into two smaller nuclei + 2-3 neutrons + energy. The released neutrons can trigger further fissions — a chain reaction. In a nuclear reactor, the chain reaction is controlled (control rods absorb neutrons to prevent runaway reactions).
Mass-energy equivalence: E = mc² — a small mass difference between reactants and products accounts for the large energy released.
Fusion: two small nuclei (e.g. hydrogen isotopes deuterium and tritium) combine to form a larger nucleus + energy. Releases more energy per kg than fission; produces no long-lived radioactive waste. Requires enormous temperatures and pressures (>100 million °C) to overcome electrostatic repulsion — this is the challenge for controlled fusion (e.g. JET/ITER reactors).
⚠Common mistakes
- Alpha is most ionising, not most penetrating: alpha is most ionising but stopped by paper; gamma is least ionising but most penetrating.
- Half-life is time to halve, not time to fully decay: after one half-life, 50% remains; after two half-lives, 25% remains — never exactly zero.
- Beta decay increases atomic number by 1: because a neutron converts to a proton (+1). Students often think atomic number stays the same.
- Random process: individual nuclei cannot be predicted. Only statistical statements about large numbers are meaningful.
AI-generated · claude-opus-4-7 · v3-wjec-physics