Ionisation

Ionisation is the process of removing one or more electrons from an atom, turning it into a positively charged ion. In IGCSE Physics, this term is used mainly in the context of nuclear radiation.

How ionising radiation causes ionisation

Alpha particles, beta particles, and gamma rays all have enough energy to knock electrons out of atoms they interact with. When a radiation particle (or photon) passes close to an atom, it can transfer energy to an orbital electron. If the energy is sufficient, the electron is ejected from the atom, creating an ion pair (a positive ion and a free electron).

Ionising ability of each type

Radiation	Ionising ability	Reason
Alpha	Strongest	Large, slow, +2 charge; interacts with many atoms in a short distance
Beta	Medium	Smaller, faster, -1 charge; interacts with fewer atoms
Gamma	Weakest	No charge, no mass; passes through most atoms without interaction

Alpha particles cause the most ionisation per centimetre of path. This is also why they have the shortest range: they lose energy quickly through many ionisation events.

Why ionisation is dangerous

When ionising radiation passes through living tissue, it ionises molecules inside cells. If the ionised molecule is DNA, the damage can cause:

• Cell death: if enough DNA is damaged, the cell cannot function and dies.
• Mutation: the DNA may be altered but the cell survives. Mutations can lead to uncontrolled cell division (cancer).

This is why exposure to ionising radiation must be minimised.

Applications of ionisation

Despite its dangers, ionisation is useful:

• Smoke detectors: alpha particles ionise air between two electrodes, creating a small current. Smoke absorbs the alpha particles, reducing the current, which triggers the alarm.
• Sterilisation: gamma rays ionise and destroy bacteria on medical equipment.
• Radiotherapy: controlled ionisation destroys cancer cells.