Energy and particles · GCSE Physics
Types of radiation
Teacher-written GCSE Physics revision on alpha, beta and gamma radiation: ionising power, penetration, deflection in fields, nuclear equations, and a use matched to each type.
Alpha is a helium nucleus — highly ionising, stopped by paper. Beta is an electron from the nucleus — stopped by aluminium. Gamma is an EM wave — the most penetrating, the least ionising.
The important bits
What you need to know
- 1
Unstable nuclei decay, emitting radiation and becoming more stable. Decay of a single nucleus is random; a large sample is statistically predictable.
- 2
Alpha (α): two protons and two neutrons (helium nucleus, ⁴₂He), charge +2. Highly ionising, short range in air (a few centimetres), stopped by paper or skin.
- 3
Beta (β⁻): a high-speed electron emitted when a neutron turns into a proton. Charge −1. Moderately ionising, stopped by a few millimetres of aluminium.
- 4
Gamma (γ): electromagnetic radiation from the nucleus. No charge, no mass. Weakly ionising, very penetrating, reduced by thick lead or concrete.
- 5
In an electric field, alpha is deflected towards the negative plate, beta towards the positive plate (more easily, because it is lighter), gamma is not deflected.
- 6
Nuclear equations must balance mass number A and atomic number Z. Alpha: A falls by 4, Z by 2. Beta: A unchanged, Z rises by 1. Gamma: A and Z unchanged.
- 7
Ionisation means knocking electrons off atoms, which can damage DNA. Alpha is most dangerous inside the body; gamma is the shielding problem outside.
- 8
Uses: smoke alarms (alpha); thickness control of paper or foil (beta); sterilising equipment and treating tumours (gamma). Match range and ionisation to the job.
Quotations worth analysing
Short evidence. Real method.
“Alpha is the most ionising and the least penetrating; gamma is the least ionising and the most penetrating.”
Do not say gamma is “stronger”. Stronger at what? Penetration and ionisation trade off. Learn a shield and a use for each.
“Beta radiation is a high-speed electron emitted from the nucleus.”
It is not an orbiting electron. A neutron changes to a proton, so the element moves one place up the periodic table.
Go deeper
Ionising versus penetrating is a trade-off
Alpha dumps a lot of energy in a short track, so it is dangerous if swallowed or breathed in, but a dead layer of skin stops it from outside. Gamma goes through the body, so it is useful as a tracer or for treating a tumour from outside, but it is harder to shield. Beta sits in the middle, which is why it is used to monitor paper thickness: some is absorbed, some gets through to a detector, and the count rate changes if the sheet is too thick. Learn a source, a range, a shield and a use for each type. “Radiation is dangerous” without the type and the route (contamination or irradiation) will not score. In tables, keep ionisation and penetration as opposite columns so you cannot mix them up.
Go deeper
Nuclear equations are bookkeeping, not magic
Write the parent nucleus, the particle, and the daughter. Alpha takes 4 from A and 2 from Z: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. Beta keeps A the same and adds 1 to Z: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e. Gamma is ⁰₀γ, so the nuclide is the same isotope in a lower energy state. If A and Z do not balance, the equation is wrong — that check is worth doing before you move on. Students invent “beta is a proton” because Z increased; the emitted particle is the electron, and the extra proton stayed in the nucleus. Practise two alpha and two beta equations until the arithmetic is automatic.
See the idea in action
Americium-241 in a smoke alarm is an alpha emitter: ²⁴¹₉₅Am → ²³⁷₉₃Np + ⁴₂He. A and Z both balance (241 = 237 + 4, 95 = 93 + 2). Alpha ionises air in the detector; smoke disrupts the current and the alarm sounds. Paper would stop the alphas from leaving the casing, which is why the source is safe from outside but must not be smashed and inhaled. A beta thickness gauge would be the wrong choice here: beta would pass through the casing and would not ionise the small air gap as strongly.
Exam technique
Turn knowledge into marks
For comparisons, mention ionisation, penetration and a shield. Write nuclear equations with A and Z balanced. Name a use that matches the range: alpha inside a detector, beta through thin sheets, gamma through the body or packaging.
Common mistakes
Do not give these marks away
- 01
Saying gamma is the most ionising because it is the most penetrating — it is the least ionising.
- 02
Treating beta as an orbiting electron, or forgetting to increase Z by 1 in a beta decay equation.
- 03
Using gamma for a smoke alarm, or alpha as an external medical tracer.
Which statement about alpha radiation is correct?
AIt is the most penetrating and the least ionising
BIt is a helium nucleus, highly ionising, and is stopped by paper
CIt is an electron from the nucleus
DIt is not deflected by an electric field
Show the answer
It is a helium nucleus, highly ionising, and is stopped by paper. Alpha is ⁴₂He, charge +2. It ionises strongly, has a short range, and is stopped by paper or skin. Gamma is the penetrating, weakly ionising one.
Quick questions
If this is the bit you searched
What is the range of each type of radiation in air?
Alpha: a few centimetres. Beta: around a metre (order of magnitude). Gamma: effectively unlimited in air; intensity falls with distance and shielding.
How are alpha, beta and gamma deflected in an electric field?
Alpha towards the negative plate, beta towards the positive plate (greater deflection), gamma not at all because it has no charge.
Why is alpha used in smoke alarms?
It ionises air strongly over a short range, producing a small current. Smoke particles disrupt that current. The alphas cannot escape the plastic case.
What must balance in a nuclear equation?
Mass number A and atomic number Z on both sides. That is how you identify the daughter nucleus if the particle is given.