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Head to savemyexams.com for more awesome resourcesCambridge O Level Physics Your notes
5.2 Radioactivity
Contents
Detection of Radioactivity
Background Radiation
T he T hree Types of Emission
Radioactive Decay
Fission & Fusion
Half-Life
Uses of Radiation
Dangers of Radiation
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Detection of Radioactivity
Your notes
Detection of Radioactivity
Ionising nuclear radiation is measured using a detector connected to a counter
Detection of Alpha Particles
α-particles are detected using a cloud chamber or spark counter
Cloud Chambers
There are several different types of cloud chambers
A cloud chamber contains:
A metal plate on the bottom, cooled using dry ice
A transparent viewing film at the top with a light source at one side, allowing the observer to
identify what is happening
A thin film of alcohol evaporated inside the chamber forming a vapour
A radioactive source placed in the chamber
A magnetic field at right angles to the chamber
A Simple Cloud Chamber Set-Up
A cloud chamber contains vapour obtained from the cooling of alcohol using dry ice
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When charged particles pass through the cloud chamber:
They collide with gas molecules
Creating ions as the alcohol condenses Your notes
Forming a path of condensation within the chamber
α-particles can be identified because they produce thick short tracks and curl slowly in one
direction
An Alpha Particle Path in a Cloud Chamber
Alpha particles can be identified by their short thick tracks that bend slightly in one direction
Spark Counters
A spark counter consists of a thin metal gauze mounted very close (about a mmm) to a thin wire
A large voltage (around 5000 V) is applied across the gauz e and the wire to cause sparking
between them
The voltage is then reduced until no more sparking occurs
When an alpha radiation source is brought close to the gauz e it will ionise the air around it
Creating sparks between the gauz e and the wire
A Spark Counter Set-Up
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Your notes
A spark counter contains a high voltage placed between a gauze and a wire to create sparking
Count Rate
The count rate is the number of decays per second recorded by a detector and recorded by
the counter
It is measured in counts/s or counts/min
The count rate decreases the further the detector is from the source
This is because the radiation becomes more spread out the further away it is from the source
Detection of Beta Particles and Gamma Radiation
The Geiger-Müller tube is the most common device used to measure and detect β-particles
and γ-radiation
Each time it absorbs radiation it transmits an electrical pulse to a counting machine called
a counter
Page 4 of 53
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