Radioactive decay and nuclear radiation
Some atomic nuclei are unstable. They lose energy by emitting nuclear radiation — a process called radioactive decay. The decay is random: you can't predict when a particular nucleus will decay, only the probability per second.
Types of nuclear radiation
- Alpha (α) — a helium nucleus ($^4_2 He$): 2 protons + 2 neutrons.
- Beta-minus (β⁻) — a fast electron emitted when a neutron in the nucleus turns into a proton.
- Gamma (γ) — a high-energy electromagnetic wave (no mass, no charge).
- Neutron emission (n) — sometimes a free neutron is ejected, especially after fission.
Properties
| Type | Charge | Mass (u) | Penetration | Range in air | Stopped by |
|---|---|---|---|---|---|
| α | +2 | 4 | very low | a few cm | sheet of paper |
| β⁻ | −1 | ≈0 | medium | 1 m or so | thin aluminium |
| γ | 0 | 0 | very high | many metres | thick lead/concrete |
| neutron | 0 | 1 | high | varies | water/concrete (moderation) |
Ionising power
The ionising effect is the ability to knock electrons off atoms in matter:
- α — most ionising (heavy and slow → lots of interaction).
- β⁻ — moderately ionising.
- γ — least ionising (passes through with little interaction).
Highly ionising radiation tends to be the least penetrating (it's stopped by interactions before going far). Conversely, weakly ionising radiation is highly penetrating.
Background radiation
We're constantly exposed to small doses from:
- Cosmic rays.
- Rocks and soil (especially granite — radon gas).
- Food and drink.
- Medical procedures.
- Nuclear weapons fallout (small now).
This is natural background radiation and varies with location (Cornwall has more granite → more radon).
Random nature
Radioactive decay is fundamentally random:
- Each nucleus has a fixed probability of decaying per unit time.
- You can't predict which nucleus or when.
- Over many nuclei, the average rate is highly predictable — that's how half-life works.
⚠Common mistakes
- Saying alpha is "stopped by glass" — paper is enough.
- Thinking gamma is harmless — it's the most penetrating, but its lower ionising power per unit length makes it less dangerous per particle than alpha if both are inside the body.
- Confusing radiation type with ionisation — alpha is least penetrating but most ionising.
- Implying decay is predictable — it's stochastic.
AI-generated · claude-opus-4-7 · v3-deep-physics