6.1.2Radiation
Relative Ionising Effect — SQA National 5 Physics Revision Notes & Topical Questions
5 min read 5 parts Lesson 2 of 14
Part 1 of 5Ionisation
Ionising radiation is any radiation with enough energy to remove electrons from atoms.
Definition
- Ionisation is the removal of electrons from atoms, leaving positively charged ions and free electrons.
How it happens
- A radiation particle passing through matter transfers energy to atoms, which can knock an electron out of an atom.
Energy lost
- Each ionisation uses some of the radiation's energy, so the radiation slows and is eventually absorbed.
Detection
- Ionisation lets a Geiger–Müller tube detect radiation, because the freed charges make a small electrical pulse.
Cell damage
- Ionisation inside living cells can break chemical bonds in DNA, which is the reason ionising radiation is dangerous.
Part 2 of 5Comparing the Three Radiations
Comparing the Three Radiations
Alpha
- Heavy, slow and doubly charged, it collides with many atoms over a short path, so it is the most strongly ionising.
Beta
- Lighter and faster with a single negative charge, it ionises moderately and travels further before stopping.
Gamma
- Has no charge or mass and so interacts weakly, producing the least ionisation per unit distance.
Trade-off
- The more strongly a radiation ionises, the quicker it loses energy and the less far it penetrates.
Summary
- Ionising effect
- alpha greatest, then beta, then gamma;
- penetrating power is in the reverse order.
- alpha greatest, then beta, then gamma;
| Radiation | Ionising effect | Penetrating power |
|---|---|---|
| Alpha | Strong | Low |
| Beta | Medium | Medium |
| Gamma | Weak | High |
Part 3 of 5Why This Matters for Safety
1
An alpha source outside the body is relatively safe, since the radiation cannot get through skin or clothing.
2
If an alpha emitter is inhaled or swallowed it is in contact with living cells, where its strong ionisation causes concentrated damage.
3
Beta sources are a hazard both outside the body, where they can cause burns to skin, and inside it.
4
Gamma radiation passes through the body easily, so it is hard to shield against, but it deposits energy sparsely in any one cell.
5
Safe working therefore uses tongs, distance, short handling times and suitable shielding chosen for the type of radiation.
Part 4 of 5Worked Example
Question Explain why an alpha source is more dangerous than a gamma source when it is swallowed, but less dangerous when held a metre away from the body.
- Alpha particles are strongly ionising, so they deposit all their energy in a very small region of tissue.
- Inside the body there is no skin or air to absorb alpha, so this energy is delivered to living cells and can damage DNA.
- At a distance of a metre, alpha is absorbed by a few centimetres of air and cannot reach the body at all.
- Gamma rays pass through air and the body easily, so they remain a hazard at a distance but ionise weakly.
Answer
- Alpha is highly ionising but of low penetration, so it is dangerous only inside the body;
- gamma is weakly ionising but highly penetrating, so it is a hazard at a distance.
Part 5 of 5Exam Checklist & Tips
6.1.2 – Key Points for the Exam
- Ionisation The removal of electrons from atoms to form ions.
- Ion An atom that has lost or gained electrons and so carries a charge.
- Ionising radiation Radiation energetic enough to cause ionisation.
- Penetrating power How far a radiation can travel through a material before it is absorbed.
- Relative ionising effect A comparison of how strongly different radiations ionise.
Common Mistakes
- Saying that gamma is the most ionising because it is the most penetrating.
- Stating that alpha radiation is harmless because paper stops it, ignoring danger inside the body.
- Describing ionisation as a nucleus becoming radioactive; it is the removal of electrons from atoms.
- Mixing up the order of ionising effect with the order of penetration.
Last updated:
\n