Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 8% of your exam marks.
stable
Low
Stable8%
Properties of alpha, beta and gamma radiation and nuclear equations tested consistently.
Why radiation is emitted
- An unstable nucleus can become more stable by emitting radiation: a high-energy particle or wave that carries away mass, charge or energy from the nucleus
- This process is called radioactive decay. It happens spontaneously and at a random moment for any given nucleus: you can predict the average behaviour of a large sample, but you can never say when one particular nucleus will decay
The three main types of radiation
- Alpha (α) particles are emitted from very large unstable nuclei
- Beta (β⁻) particles are emitted when a neutron in the nucleus converts into a proton and an electron
- Gamma (γ) rays are emitted as a high-energy electromagnetic wave when a nucleus needs to shed surplus energy without changing its composition
- A fourth, less common process is neutron emission: a single neutron is ejected without any change in proton number
What each one is made of
| Radiation | Symbol | Nature | Charge | Relative mass |
|---|---|---|---|---|
| Alpha | α (⁴₂He) | Two protons + two neutrons (a helium nucleus) | +2 | 4 |
| Beta | β⁻ (⁰₋₁e) | A high-speed electron ejected from the nucleus | −1 | 1/2000 (negligible) |
| Gamma | γ | A high-energy electromagnetic wave (photon) | 0 | 0 |
Penetrating power
- Alpha is the most massive and most highly charged, so it interacts strongly with matter. It ionises everything it meets very effectively but is stopped after only a few centimetres of air or by a single sheet of paper
- Beta is light and singly charged. It penetrates several tens of centimetres of air and is stopped by a few millimetres of aluminium foil. Its ionising effect is moderate
- Gamma has no charge and no mass, just energy. It penetrates the furthest (effectively infinite range in air; only reduced, never fully blocked, by even a few centimetres of lead). Its ionising effect on a given molecule is the weakest of the three because it interacts least
Ionising power (the flip side)
- The same property that makes alpha stop quickly is what makes it ionise heavily: a charged particle that loses its energy in a short path knocks lots of electrons off molecules on its way
- A useful summary:
- alpha: short range, strongly ionising
- beta: medium range, moderately ionising
- gamma: very long range, weakly ionising
- Inside the body, the dangers run in the opposite order: alpha is most dangerous inside the body (because it ionises heavily over a short range and damages nearby cells) but harmless outside (the skin stops it). Gamma is most dangerous outside the body because it can reach internal organs