Nuclear Emissions
Section: Nuclear Physics | Syllabus: Cambridge IGCSE Physics (0625)
The Three Types of Nuclear Emission
Unstable nuclei can emit three main types of ionising radiation:
- alpha (α) particles
- beta (β) particles
- gamma (γ) rays
- The emission of radiation from a nucleus is spontaneous, meaning:
- it happens by itself, without any external trigger
- random in direction (it is impossible to predict which direction any particular emission will travel)
Radioactive emission is spontaneous and random in direction
Alpha (α) Particles
An alpha particle consists of 2 protons and 2 neutrons - the same as a helium nucleus, 42He.
- Charge: +2 (from the 2 protons)
- Mass: 4 (relatively heavy)
- Speed: relatively slow, typically a few percent of the speed of light
- Ionising effect: strong
- alpha particles are the most strongly ionising of the three types
- due to their large charge and relatively low speed
- which mean they interact frequently with atoms along their path
- Penetrating power: weak - because they lose their energy quickly in frequent collisions
- alpha particles are the least penetrating of the three types
- Stopped by a sheet of paper, human skin, or a few centimetres of air
Beta (β) Particles
A beta (β⁻) particle is a fast-moving electron, emitted from the nucleus (not from the electron shells) when a neutron changes into a proton.
- Charge: −1
- Mass: negligible
- Speed: fast, up to about 90% of the speed of light
- Ionising effect: moderate - beta particles are moderately ionising, less than alpha but more than gamma
- Penetrating power: moderate - beta particles are moderately penetrating
- Stopped by a few millimetres of aluminium
Gamma (γ) Rays
A gamma ray is high-energy electromagnetic radiation emitted from an unstable nucleus - unlike alpha and beta radiation, gamma radiation is a wave, not a particle.
- Charge: 0
- Mass: 0
- Speed: the speed of light, 3 × 10⁸ m/s
- Ionising effect: weak - gamma rays are the least ionising of the three types, because they have no charge and rarely interact with atoms
- Penetrating power: strong - gamma rays are the most penetrating of the three types, and are never completely stopped, only reduced in intensity
- Reduced by several centimetres of lead, or metres of concrete
- There is an inverse pattern between ionising effect and penetrating power: the more strongly ionising a radiation type is, the less penetrating it is, because it loses its energy more quickly through frequent interactions with matter
Comparison of Alpha, Beta and Gamma Radiation
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Particle (2 protons + 2 neutrons) | Particle (fast electron) | Electromagnetic wave |
| Charge | +2 | −1 | 0 |
| Mass | 4 (heavy) | Negligible | 0 |
| Speed | Slow (few % of c) | Fast (up to ~90% of c) | Speed of light, c |
| Ionising effect | Strong | Moderate | Weak |
| Penetrating power | Weak | Moderate | Strong |
| Stopped/reduced by | Paper, skin, a few cm of air | A few mm of aluminium | Several cm of lead, or metres of concrete |
Deflection in Electric and Magnetic Fields
Extended Only
Because α-particles and β-particles are charged, they are deflected as they pass through electric and magnetic fields; γ-radiation, having no charge, is not deflected by either.
Deflection in an Electric Field
- Alpha particles are deflected towards the negative plate (since they are positively charged), but only slightly, because they are relatively heavy
- Beta particles are deflected towards the positive plate (since they are negatively charged) - in the opposite direction to alpha particles - and by a much larger amount, because they are very light
- Gamma rays are not deflected, since they carry no charge
Deflection of alpha, beta and gamma radiation in an electric field
Deflection in a Magnetic Field
- Alpha particles are deflected slightly, in one direction
- Beta particles are deflected strongly, in the opposite direction to alpha particles, because they carry the opposite charge and have far less mass
- Gamma rays are not deflected, since they carry no charge
Deflection of alpha, beta and gamma radiation in a magnetic field
Explaining Relative Ionising Effects
Extended Only
The relative ionising effects of alpha, beta and gamma radiation can be explained with reference to their electric charge and kinetic energy.
| Radiation | Electric Charge | Explanation of Ionising Effect |
|---|---|---|
| Alpha (α) | +2 (large) | The large charge attracts electrons from atoms strongly, so alpha particles are highly ionising |
| Beta (β) | −1 (smaller) | A smaller charge means a weaker interaction with atoms, so beta particles are moderately ionising |
| Gamma (γ) | 0 (none) | With no charge, there is little electrical attraction to pull electrons from atoms, so gamma rays are weakly ionising |
- An alpha particle's large charge and relatively low speed mean it transfers kinetic energy to the atoms it passes, ionising many of them in a short distance before it loses all its energy - this is why alpha particles are highly ionising but travel only a short range
- A gamma ray carries a great deal of energy but, having no charge, only occasionally transfers this energy to an atom - so it is weakly ionising but travels a long range
Identifying Radiation Types Using Absorbers
The type(s) of radiation emitted by a source can be identified by observing which absorbing materials reduce the count rate to background level.
- Measure the count rate with no absorber present
- Place paper between the source and detector - if the count rate drops to background level, the source emits alpha radiation
- Place a few mm of aluminium between the source and detector - if the count rate drops to background level, the source emits beta radiation
- Place a thick sheet of lead between the source and detector - if the count rate is reduced but not eliminated, the source emits gamma radiation
Worked Example: Identifying Unknown Radiation
Museli tests an unknown source. Background count rate = 30 counts/min. With no absorber: 500 counts/min. With paper: 500 counts/min (no change). With aluminium: 35 counts/min (drops to background).
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