Types of Radiation (Cambridge (CIE) IGCSE Physics): Revision Note
Exam code: 0625 & 0972
Radioactive decay
The emission of radiation from a nucleus is spontaneous and random in direction
This random process of radioactive decay means:
There is an equal probability of any nucleus decaying
It cannot be known which particular nucleus will decay next
It cannot be known at what time a particular nucleus will decay
The rate of decay is unaffected by the surrounding conditions
It is only possible to estimate the probability of a nuclei decaying in a given time period
Unstable nuclei
Some atomic nuclei are unstable
This is because of an imbalance in the forces within the nucleus
Forces exist between the particles in the nucleus
Instability is commonly due to:
The nucleus having too many protons or neutrons
The nucleus being very large
An example of an unstable nucleus is carbon-14
This is an isotope of carbon
It has two extra neutrons compared to stable carbon-12
Isotopes of carbon
Unstable nuclei can emit radiation to become more stable
Radiation can be in the form of a high-energy particle or wave
Unstable nuclei emit radiation
As the radiation moves away from the nucleus, it takes some energy with it
This reduces the overall energy of the nucleus
This makes the nucleus more stable
Worked Example
Which of the following statements is not true?
A Isotopes can be unstable because they have too many or too few neutrons
B The process of emitting particles or waves of energy from an unstable nucleus is called radioactive decay
C Scientists can predict when a nucleus will decay
D Radiation refers to the particles or waves emitted from a decaying nucleus
[1]
Answer: C
Consider what you know about the statements above:
Isotopes |  |
|---|---|
Radioactive decay |  |
Nuclei decay prediction |  |
Radiation |  |
Answer A is true — the number of neutrons in a nucleus determines the stability
Answer B is true — this is a suitable description of radioactive decay
Answer D is true — radiation is about emissions. It is different to radioactive particles
Answer C is not true
Radioactive decay is a random process
It is not possible to predict precisely when a particular nucleus will decay
Therefore, the correct answer is C [1 mark]
Examiner Tips and Tricks
The terms unstable, random and decay have very particular meanings in this topic. Remember to use them correctly when answering questions!
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
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 following trends down the table:
The range increases
Penetrating power increases
Ionisation decreases
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 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
Examiner Tips and Tricks
It is important to note that beta particles are only stopped by aluminium if it is a few mm thick. They can pass though aluminium which is thinner than this. This concept often comes up in exam questions.
Worked Example
A student has an unknown radioactive source and is trying to determine which type of radiation it emits. Using a Geiger-Muller tube, they measure the count rate when the source is placed behind different materials.
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?
A Alpha particles
B Beta particles
C Gamma rays
D Neutrons
[1]
Answer:
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, this does not need to known for the exam)
Therefore, the source must be beta particles and the correct answer is B [1 mark]
Examiner Tips and Tricks
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 on these.
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