The big idea: A smoke detector on the ceiling has a speck of radioactive material quietly ticking away — unstable nuclei calming down by throwing out radiation.
Three types, four differences
- There are three types — alpha (α), beta (β) and gamma (γ).
- They differ in what they are and how fast they go.
- They differ in how far they get through stuff (penetration) and how strongly they ionise (knock electrons off atoms).
- Know these differences and you can answer almost any question on the types of radiation.
What 'ionising' means: To ionise an atom is to knock an electron off it, leaving a charged ion.
The more ionising a radiation is, the more damage it does as it passes through matter — but the quicker it runs out of energy, so the less far it gets.
| Property | Alpha (α) | Beta-minus (β⁻) | Gamma (γ) |
|---|---|---|---|
| What it is | A helium nucleus (2 protons + 2 neutrons), ⁴₂He | A fast electron, emitted from the nucleus | A high-energy photon (electromagnetic wave) |
| Charge | +2 | −1 | 0 (neutral) |
| Mass | Heavy (4 u) | Tiny (≈ 1/1800 u) | None (massless) |
| Speed | Slow (≈ 5% of c) | Fast (up to ≈ 99% of c) | Speed of light, c |
| Penetration | Lowest — stopped by paper / a few cm of air / skin | Medium — stopped by a few mm of aluminium | Highest — needs thick lead or concrete |
| Ionising ability | Strongest — knocks out the most electrons | Medium | Weakest |
| Deflected by a field? | Yes (small deflection, +) | Yes (large deflection, opposite way, −) | No (neutral, not deflected) |
The pattern to lock in: Going α → β → γ:
- penetration goes UP (paper → aluminium → lead) - ionising power goes DOWN (α strongest → γ weakest)
The best ioniser travels the shortest distance — α dumps its energy fastest, so it's stopped first.
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Penetration = how far the radiation gets before it is stopped. It is the property exam questions test most, because it decides what shields you and what is dangerous.
The penetration ladder: - Alpha (α) — stopped by a sheet of paper, a few cm of air, or your skin. - Beta (β⁻) — passes paper, but is stopped by a few mm of aluminium. - Gamma (γ) — the hardest to stop; needs thick lead or concrete, and even then is only reduced, not fully blocked.
Why ionising power is the opposite order: Alpha is +2 and slow and heavy, so it interacts strongly with the atoms it meets — it ionises the most, but loses its energy in a short distance.
Gamma is a neutral photon, so it interacts weakly — it ionises the least, which is exactly why it travels the furthest.
Inside vs outside the body: Outside the body, alpha is the least dangerous — your skin stops it.
Inside the body (breathed in or swallowed), alpha is the most dangerous — its strong ionising power damages tissue with no skin to shield it. Examiners often catch students out with this inside-vs-outside sign-flip.
Stopping and bending the three radiations
Choosing a source for a job: Thickness of paper or foil: beta, partly absorbed, so the count rate changes with thickness. Leaks in underground pipes: a gamma emitter (it gets through soil) with a short half-life. Medical tracers: gamma, which leaves the body to reach a camera, short half-life. Smoke detectors: alpha, strongly ionising and stopped by the casing and a few cm of air.
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How this is tested — 5.3.1 is a recall-and-compare topic:
Paper 1A
- Given three statements comparing two radiations (speed, penetration, ionising ability), pick which are correct.
- Or: which radiation a description matches.
Paper 2
- Outline or state a property — e.g. why an alpha source in a smoke detector is safe.
- Or what shield stops each type.
The classic trap: Thinking the most penetrating radiation is the most dangerous — it depends on whether the source is inside or outside the body.
How to answer a 'safe?' question: Name the radiation, say how far it penetrates, then say what stops it. For a sealed alpha source held away from you: α is stopped by a few cm of air and by the skin / detector casing, so it never reaches living tissue.
IB-style questionOutline[3 marks]
A household smoke detector contains a small sealed source that emits alpha radiation. Outline why this source is safe for people in the house.
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