# Alpha, beta and gamma

Alpha is a helium nucleus (2 protons, 2 neutrons), stopped by paper. Beta is a fast electron, stopped by thin aluminium. Gamma is an electromagnetic wave, reduced by thick lead or concrete.

Physics (school) · Part 08: Atoms and radioactivity · Card 4 of 8 · Last updated 2026-10-19

## Analogy

Alpha is a bowling ball: heavy but stopped easily. Gamma is a ghost that walks through walls.

## Example

**Penetration**

Alpha: stopped by paper
Beta: stopped by aluminium
Gamma: only reduced by
thick lead

## In practice

Smoke alarms use alpha because it is stopped by a few centimetres of air. Gamma is used in medical sterilising.

## Practice

### Flashcards

- **Which radiation can be stopped by paper?** Alpha.

### Interview questions

**Compare alpha, beta and gamma radiation in terms of what they are and how far they penetrate.**

- Alpha is a helium nucleus, stopped by paper
- Beta is a fast electron, stopped by thin aluminium
- Gamma is an electromagnetic wave, reduced by thick lead
- Alpha is most ionising and gamma least

## Questions people ask

### Which type of radiation is most ionising?

Alpha is the most ionising, because it is large and strongly charged, so it knocks electrons off atoms easily. Gamma is the least ionising. This also means alpha is the most harmful inside the body, but the least harmful outside, where paper or skin stops it.

### How does the nucleus change in alpha and beta decay?

In alpha decay the nucleus loses 2 protons and 2 neutrons, so its mass number drops by 4 and its atomic number by 2. In beta decay a neutron turns into a proton, so the atomic number goes up by 1 and the mass number stays the same.

## Related concepts

- [Ionising radiation and safety](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-6)
- [Radioactive decay](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-3)
- [Half-life](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-5)

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