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
- When charged particles pass through the cloud chamber:
- They collide with gas molecules
- Creating ions as the alcohol condenses
- 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 gauze 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 gauze it will ionise the air around it
- Creating sparks between the gauze and the wire
A Spark Counter Set-Up
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
- This makes a clicking sound or displays the count rate on a screen
- The greater the frequency of clicks heard, or the higher the count rate seen, the more radiation the Geiger-Müller tube is absorbing
- Therefore, it matters how close the tube is to the radiation source
- The further away from the source, the lower the count rate detected
A Geiger-Müller Counter
A Geiger-Müller tube (or Geiger counter) is a common type of radiation detector
Worked example
A Geiger-Müller tube is used to detect radiation in a particular location. If it counts 16,000 decays in 1 hour, what is the count rate?
Answer:
Step 1: Identify the different variables
- The number of decays is 16 000
- The time is 1 hour
Step 2: Determine the time period in seconds
- 1 hour is equal to 60 minutes, and 1 minute is equal to 60 seconds
Time period = 1 × 60 × 60 = 3600 seconds
Step 3: Divide the total counts by the time period in seconds
Counts ÷ Time period = 16 000 ÷ 3600 = 4.5
- Therefore, there are 4.5 decays per second
Examiner Tip
If asked to name a device for detecting radiation, the Geiger-Müller tube is a good example to give. You can also refer to it as a GM tube, a GM detector, GM counter, Geiger counter etc. (The examiners will allow some level of misspelling, providing it is readable). Don’t, however, refer to it as a ‘radiation detector’ as this is too vague and may simply restate what was asked for in the question.
