Types of Radiation (Edexcel IGCSE Physics): Revision Note
Exam code: 4PH1
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Types of radiation
Some atomic nuclei are unstable and radioactive
This is because of an imbalance of protons or neutrons in the nucleus
Carbon-14 is an example of an isotope of carbon which is unstable
This is because it has two extra neutrons compared to a stable nucleus of carbon-12
Stable and unstable isotopes of carbon
Carbon-12 is stable, whereas carbon-14 is unstable because it has two extra neutrons
Unstable nuclei can emit radiation to become more stable
Radiation can be in the form of a high-energy particle or wave
This process is known as radioactive decay
As the radiation moves away from the nucleus, it takes some energy with it
This makes the nucleus more stable
Radioactive decay of a nucleus
Unstable nuclei decay by emitting high energy particles or waves
When an unstable nucleus decays, it emits radiation
The different types of radiation that can be emitted are:
Alpha (α) particles
Beta (β-) particles
Gamma (γ) radiation
These changes are spontaneous and random
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
ANSWER: C
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
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!
Properties of alpha, beta and gamma radiation
Alpha particles
The symbol for alpha is α
An alpha particle is the same as a helium nucleus
This is because it consists of two neutrons and two protons
Beta particles
The symbol for beta is β−
Beta particles are high-energy electrons
They are produced in nuclei when a neutron changes into a proton and an electron
Gamma rays
The symbol for gamma is γ
Gamma rays are electromagnetic waves
They have the highest energy of the different types of electromagnetic waves
Alpha, beta & gamma radiation
Alpha particles, beta particles and gamma waves can be emitted from unstable nuclei
Properties of alpha, beta & gamma
Alpha (α), beta (β) and gamma (γ) radiation can be identified by their:
Nature (what type of particle or radiation they are)
Ionising ability (how easily they ionise other atoms)
Penetrating power (how far can they travel before they are stopped completely)
Alpha, beta and gamma penetrate materials in different ways
This means they are stopped, or reduced, by different materials
Penetrating power of alpha, beta and gamma
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 rays can pass through aluminium but are only partially stopped by thick lead
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 |
Worked Example
A student has an unknown radioactive source. They are trying to work out which type of ionising radiation is being emitted.
They measure the count rate, using a Geiger-Muller tube, when the source is placed behind different materials. Their results are recorded in a table.
| no material between source and detector | thin sheet of paper between source and detector | 5 mm aluminium foil between source and detector | 5mm lead plate between source and detector |
|---|---|---|---|---|
Count-rate | 4320 | 4318 | 256 | 244 |
Which type(s) of ionising radiation is/are emitted by the source?
A Alpha particles
B Beta particles
C Gamma rays
D Alpha, beta and gamma radiation
ANSWER: B
The answer is not A or D because the radiation passed through the paper almost unchanged
This means it is not alpha as alpha is stopped by a thin sheet of paper
It is usual to find some variance between readings, you need to look for a significant change
The answer is not C because the lead plate did not decrease the count rate significantly
It was the aluminium that decreased the count rate significantly
Beta particles are blocked by a few mm of aluminium, therefore the source must be beta particles
Examiner Tips and Tricks
Students often get confused about whether beta particles can pass though aluminium foil. Beta particles can be stopped by aluminium foil if it is thick enough (a few mm thick); if the foil is thin enough, they can pass through. This is the basis of using beta radiation to measure the thickness of aluminium foil (a common exam question!). The thicker the foil, the fewer beta particles pass through and are measured by a detector on the other side of the foil.
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