When radiation passes through matter, it may knock electrons out of atoms, turning them into charged particles called ions. Radiation that can do this is called ionising radiation; radiation that cannot is called non-ionising radiation. The difference is the amount of energy the radiation carries.
Ionising radiation
The main types of ionising radiation from radioactive sources are alpha (α), beta (β) and gamma (γ). X-rays are also ionising. They differ in charge and in how far they can penetrate.
| Type | Charge | Stopped by |
| Alpha (α) | +2 | Paper |
| Beta (β) | −1 | Aluminium sheet |
| Gamma (γ) | 0 | Thick lead / concrete |
Key idea
Alpha is the most ionising but least penetrating. Gamma is the least ionising but most penetrating.
Because ionising radiation can damage the living cells in our bodies, it must be handled with care. It can be detected using instruments such as a Geiger-Muller counter, which clicks when radiation passes through it. The more ionising the radiation, the more damage it can do to cells inside a short distance, which is why alpha sources are especially dangerous if they get inside the body.
Non-ionising radiation
Non-ionising radiation carries less energy and cannot form ions. Examples include radio waves, microwaves, infrared, visible light and low-energy ultraviolet. It is generally less harmful, but strong doses can still heat or damage body tissue. We use non-ionising radiation every day, for example radio waves for broadcasting, microwaves for cooking, and infrared in television remote controls.
Remember
- Ionising: alpha, beta, gamma, X-rays.
- Non-ionising: radio, microwave, infrared, visible light.
- Penetrating power: gamma > beta > alpha.