Radioactivity
The Atom and the Nucleus
An atom consists of a tiny, dense, positively charged nucleus (containing protons and neutrons) surrounded by electrons in shells. Almost all of the atom's mass is in the nucleus; almost all the volume is empty space.
Nuclide notation: ᴬ_Z X where A = mass number (protons + neutrons), Z = atomic number (protons).
Isotopes: atoms of the same element with the same number of protons but different numbers of neutrons. Some isotopes are unstable and undergo radioactive decay.
Types of Radiation
| Type | Nature | Symbol | Charge | Mass | Penetration | Stopped by |
|---|---|---|---|---|---|---|
| Alpha | 2 protons + 2 neutrons (helium nucleus) | α | +2 | 4 | Low | Paper / few cm air |
| Beta | Fast electron from nucleus | β | −1 | ~0 | Medium | 3 mm aluminium |
| Gamma | EM wave (photon) | γ | 0 | 0 | High | Several cm lead / thick concrete |
Alpha decay: ᴬ_Z X → ᴬ⁻⁴_(Z−2) Y + ⁴₂He Beta decay: ᴬ_Z X → ᴬ_(Z+1) Y + ⁰₋₁e
Background Radiation
Naturally occurring radiation from: cosmic rays, rocks/soil (radon gas), food/drink, medical procedures. Average in UK ≈ 2.5 mSv/year. Radon is the largest contributor in many areas.
Half-Life
Half-life (t₁/₂): the time taken for the number of radioactive nuclei (or the activity) to halve.
Half-life is constant and unaffected by physical/chemical conditions (temperature, pressure, chemical bonding).
After n half-lives: N = N₀ × (½)ⁿ
Example: if t₁/₂ = 30 years and initial activity = 800 Bq, after 90 years (3 half-lives): 800 → 400 → 200 → 100 Bq.
Irradiation vs Contamination
| Irradiation | Contamination | |
|---|---|---|
| Definition | Exposure to radiation from an external source | Radioactive material on/in the body |
| Risk | Stops when source removed | Continues even after leaving area |
| Example | Medical X-ray | Swallowing radioactive particles |
Protection from irradiation: increase distance, reduce exposure time, use shielding. Protection from contamination: protective clothing, gloves, masks; decontamination procedures.
Nuclear Fission and Fusion
Fission: a heavy nucleus (e.g. U-235) absorbs a neutron and splits into two smaller nuclei + 2–3 neutrons + energy. The neutrons can trigger further fissions → chain reaction. Used in nuclear reactors and weapons.
Fusion: two light nuclei (e.g. hydrogen isotopes — deuterium and tritium) combine to form a heavier nucleus, releasing large amounts of energy. Powers the Sun. Not yet achieved for net energy gain in reactors (extremely high temperature/pressure required).
Mass defect: the total mass of the products is less than the reactants → mass converted to energy: E = mc².
Uses of Radioactivity
- Medical: sterilisation of equipment (gamma), cancer treatment (gamma targeted at tumours), PET scans (beta emitters), thyroid treatment (iodine-131).
- Industrial: thickness gauges (beta), smoke detectors (americium-241, alpha emitter), carbon-14 dating (archaeological).
AI-generated · claude-opus-4-7 · v3-edexcel-physics