5.2.2 The three types of nuclear emission
- Syllabus
- 0625–2026–2027
- Topic
- 5.2.2
- Level
- —
Learning objectives
5.2.2.1Emission of radiation from a nucleus• Describe the emission of radiation from a nucleus as spontaneous and random in direction5.2.2.2Alpha (α), beta (β) and gamma (γ)• Identify alpha (α), beta (β) and gamma (γ) emissions from the nucleus by recalling: (a) their nature (b) their relative ionising effects (c) their relative penetrating abilities (β+ are not included, β-particles will be taken to refer to β–)5.2.2.3Deflection of α-particles, β-particles• Describe the deflection of α-particles, β-particles and γ-radiation in electric fields and magnetic fields5.2.2.4Their relative ionising effects with• Explain their relative ionising effects with reference to: (a) kinetic energy (b) electric charge
Understand spontaneous and random nuclear emission
Radiation emission from an unstable nucleus is spontaneous: it happens without being started by heating, pressure, a chemical reaction or another external trigger.
The emission is random. The exact nucleus that will emit next and the exact time of its emission cannot be predicted, so repeated detector readings naturally fluctuate.
The direction is also random. Emissions from many nuclei leave in different directions rather than following one preferred direction.
Although one event is unpredictable, a large sample can show a stable statistical pattern when counts are collected over suitable time intervals.
Random variation does not by itself mean the detector is faulty, and spontaneous does not mean that every nucleus emits at once or at a constant individual schedule.
Compare alpha, beta and gamma emissions
| Emission | Nature and charge | Relative ionising effect | Relative penetration and typical absorber |
|---|---|---|---|
| alpha (α) | helium nucleus: 2 protons + 2 neutrons; charge +2 | greatest | least; stopped by paper or a few centimetres of air |
| beta (β−) | fast electron emitted from the nucleus; charge −1 | intermediate | intermediate; stopped by a few millimetres of aluminium |
| gamma (γ) | electromagnetic radiation; no charge and no rest mass | least | greatest; reduced by thick lead or concrete |
In this syllabus, beta means β− only. β+ emission is explicitly outside scope.
The radiation that ionises most strongly loses energy most rapidly in matter, so it travels the shortest distance and has the lowest penetrating ability. Gamma interacts less frequently and is therefore the most penetrating.
Penetrating ability is not the same as speed. Gamma is most penetrating because it interacts less readily with matter, not because 'more penetrating' simply means 'faster'.
Predict deflection in electric and magnetic fields
| Emission | Charge | Behaviour in an electric field |
|---|---|---|
| α | +2 | bends towards the negative plate |
| β− | −1 | bends towards the positive plate, usually much more strongly than α |
| γ | 0 | remains undeflected |
An electric field exerts forces in opposite directions on positive and negative charges. Beta bends more because its mass is far smaller than the mass of an alpha particle.
| Emission | Behaviour in a magnetic field |
|---|---|
| α and β− | bend in opposite directions because their charges have opposite signs |
| γ | remains undeflected because it is uncharged |
For a magnetic-field diagram, use the beam direction, field direction and the force rule for a positive charge; reverse that force direction for β−. The beta path normally has the tighter curvature because beta has much smaller mass.
Identify whether the diagram shows an electric or magnetic field before predicting a path. A charged emission is deflected in both; gamma is undeflected in both.
Explain relative ionising effects
Ionisation occurs when an interaction removes an electron from an atom or molecule, leaving an ion.
A larger electric charge produces a stronger electrical interaction with electrons in nearby atoms. An alpha particle has charge +2, so it interacts more strongly than a beta particle with charge −1; gamma has no charge and interacts less frequently.
Kinetic energy is transferred during interactions that cause ionisation. For the same kinetic energy, the much more massive alpha particle travels more slowly than a beta particle, remains near atoms for longer and produces dense ionisation along a short path.
| Emission | Charge/interaction pattern | Relative ionising effect |
|---|---|---|
| α | +2; strong, frequent interactions and dense energy transfer | greatest |
| β− | −1; weaker charged-particle interactions | intermediate |
| γ | uncharged; less frequent interactions | least |
Do not explain the ranking using penetration alone. Greater ionisation causes faster energy loss and therefore lower penetration; it is the charge and transfer of kinetic energy in interactions that provide the explanation.