Alpha, beta and gamma radiation
Key ideas
An unstable nucleus releases energy and moves toward a more stable arrangement by emitting radiation. There are three types: alpha, beta and gamma.
- Alpha () — a particle made of 2 protons and 2 neutrons (identical to a helium nucleus, ).
- Beta (, beta-minus) — a fast-moving electron, created when a neutron in the nucleus converts into a proton and an electron.
- Gamma () — electromagnetic radiation (a high-energy photon) with no mass and no charge.
Comparing the three types
| Property | Alpha () | Beta () | Gamma () |
|---|---|---|---|
| What it is | 2 protons + 2 neutrons | fast electron | EM radiation (photon) |
| Relative mass | 4 | (negligible) | 0 |
| Relative charge | 0 | ||
| Ionising ability | strong (highly ionising) | moderate | weak (weakly ionising) |
| Penetrating power | stopped by paper / a few cm of air | stopped by a few mm of aluminium | needs several cm of lead to greatly reduce |
| Range in air | a few cm | around a metre | very long (travels far) |
| Deflection in E/B field | deflects, small deflection (heavy, ) | deflects strongly, opposite direction to alpha (light, ) | not deflected (no charge) |
- Ionising ability and penetrating power trade off against each other: a particle that ionises strongly loses its energy quickly by colliding with atoms, so it does not travel far or penetrate deeply.
- Alpha is the most ionising but the least penetrating.
- Gamma is the least ionising but the most penetrating.
Deflection in electric and magnetic fields
- Alpha particles are positive, so they deflect toward the negative plate (in an electric field) — a small deflection because they are relatively heavy.
- Beta particles are negative, so they deflect toward the positive plate — a larger deflection because they are much lighter.
- Gamma rays carry no charge, so they are not deflected by electric or magnetic fields.
- Alpha and beta deflect in opposite directions to each other, since their charges are opposite.
Practice question
A radioactive source is placed near a detector. A sheet of paper blocks almost all the radiation, but a small amount still gets through and is stopped by a few millimetres of aluminium. What two types of radiation is the source most likely emitting?
Worked solution: the paper blocks the most ionising, least penetrating radiation — alpha. The remainder, stopped by a few mm of aluminium, is beta.
Test yourself
Practice by grade
One question each at Achieved, Merit and Excellence. Have a go, then compare with the model answer.
State what an alpha particle is made of, and state its relative charge.
A student places a Geiger counter next to a radioactive source. The count rate drops sharply when a single sheet of paper is placed in the way, but a further increase in shielding to several mm of aluminium makes almost no additional difference.
Explain what this tells you about the type(s) of radiation being emitted.
A radioactive source emitting all three types of radiation is placed between two charged parallel plates, with the beam initially travelling straight down the middle. Explain fully what happens to each type of radiation, and why they behave differently, referring to charge, mass and ionising ability.