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Types of Radioactive Decay (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))
Revision Note
Types of radioactive decay
- Radioactive decay is a change in an unstable nucleus that can result in the emission of one of the following types of radiation:
- Alpha (α) particles
- Beta (β-) particles
- Gamma (γ) radiation
- Remember that these changes are spontaneous and random
Alpha particles
- The radiation symbol for alpha is α
- An alpha particle is the same as a helium nucleus
- This is because they consist of two neutrons and two protons
- Alpha particles have a charge of +2
- This means they can be affected by an electric field
Beta particles
- The radiation symbol for beta is β-
- Beta particles are fast-moving electrons
- They are produced in nuclei when a neutron changes into a proton and an electron
- Beta particles have a charge of -1
- This means they can be affected by an electric field
Gamma rays
- The radiation symbol for gamma is γ
- Gamma rays are electromagnetic waves
- They have the highest energy of the different types of electromagnetic waves
- Gamma rays have no charge
Types of radioactive decay
Alpha particles, beta particles and gamma waves can be emitted from unstable nuclei
Alpha, beta & gamma emission
- α, β and γ radiation can be identified by the emission from a nucleus by recalling their:
- Nature (what type of particle or radiation they are)
- Their relative ionising effects (how easily they ionise other atoms)
- Their relative penetrating abilities (how far can they travel before they are stopped completely)
- The properties of alpha, beta and gamma are given in the table which shows the trend down the table as:
- The range increases
- Penetrating power increases
- Ionisation decreases
Summary of the properties of nuclear radiation
| Particle | Nature | Range in air | Penetrating power | Ionising ability |
| Alpha (α) |
helium nucleus (2 protons, 2 neutrons) |
a few cm | low; stopped by a thin sheet of paper | high |
| Beta (β) | high-energy electron | a few 10s of cm | moderate; stopped by a few mm of aluminium foil or Perspex | moderate |
| Gamma (γ) | electromagnetic wave | infinite | high; reduced by a few cm of lead | low |
Penetrating power
- Alpha, beta and gamma radiation have different properties
- So they penetrate materials in different ways
- This means they are each stopped by different materials
Penetrating power of alpha, beta and gamma radiation
Alpha, beta and gamma are different in how they penetrate materials. Alpha is the least penetrating, and gamma is the most penetrating
- Alpha is stopped by paper, whereas beta and gamma pass through it
- Beta is stopped by a few millimetres of aluminium
- Gamma can pass through aluminium
- Gamma rays are only partially stopped by thick lead
Worked example
A student has an unknown radioactive source. They are trying to work which type of radiation is being given off:
A Alpha particles
B Beta particles
C Gamma rays
D Neutrons
They measure the count-rate, using a Geiger-Muller tube, when the source is placed behind different material. Their results are shown in the table below:
| No material between source and detector | Paper between source and detector | 5 mm aluminium between source and detector | 5 mm lead between source and detector | |
| Count rate | 4320 | 4218 | 256 | 34 |
Which type of radiation is being given off by the source?
Answer: B
- Consider the diagram showing penetrating power from above
- The answer is not A because the radiation passed through the paper almost unchanged
- This means it is not alpha
- The answer is not C or D because the aluminium decreased the count-rate significantly
- This means it is not gamma (gamma penetrates aluminium)
- This also means it is not neutrons (neutrons penetrate aluminium, however you do not need to know this for your GCSE)
- Therefore, the source must be beta particles
Exam Tip
Remembering the type of particle, penetration and ionising power for alpha, beta and gamma radiation is very important for your exam! Often the exam question will give some clues and you will have to choose which type of radiation it could be based off these.
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