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. There are three types — alpha (α), beta (β) and gamma (γ).
They differ in what they are, how fast they go, 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.
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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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