Learning outcomes
- Describe alpha detection using a cloud chamber or spark counter.
- Describe beta and gamma detection using a GM tube and counter.
- Explain how ionisation produces a detectable signal.
- Select suitable apparatus and measurement times.
4.1 Detection depends on ionisation
Ionising radiation removes electrons from atoms or molecules, producing ions. Detectors turn this microscopic ionisation into a visible track, spark, electrical pulse or count. Radiation itself is not normally seen directly.
Different detectors suit different radiation types. Alpha particles ionise strongly but have a very short range and are easily stopped, so the source and detector window must be close and separated by very little material.
4.2 Cloud chamber
A cloud chamber contains supersaturated vapour. When an ionising particle passes through, ions provide centres on which droplets condense, leaving a visible track. Alpha tracks are relatively thick and short because alpha particles produce intense ionisation and lose energy rapidly.
The chamber demonstrates individual particle paths and random directions. It is primarily qualitative in this syllabus. Conditions must be stable and the chamber should be viewed with suitable lighting.
4.3 Spark counter
A spark counter has closely spaced wires or electrodes at high potential difference. A passing alpha particle ionises the air, and the ions allow a brief spark to form. Each spark indicates a detected particle.
Alpha particles are suitable because they create dense ionisation. The detector is not a general-purpose substitute for a GM tube, and safe operation requires supervised low-current high-voltage equipment.
4.4 Geiger–Müller tube
A GM tube contains low-pressure gas and a central electrode at high potential. Radiation entering the tube ionises gas atoms. The freed electrons accelerate and cause further ionisation, producing a short electrical pulse. A counter or rate meter records the pulses.
A thin end window allows beta particles to enter. Gamma rays may interact with the tube wall or gas and can also be detected, although not every gamma photon produces a count. Alpha particles can be detected by suitable thin-window tubes at very short range, but the syllabus specifically associates cloud chambers or spark counters with alpha detection and GM tubes with beta and gamma detection.
4.5 Reliable counting
Place the source and detector in fixed positions, use the same counting interval, and take repeated measurements. A longer interval gives more counts and usually lowers percentage random uncertainty. Measure background separately and subtract it.
A GM counter does not measure particle energy directly; it mainly counts events. A louder series of clicks means a higher count rate, not necessarily more energetic individual radiation.
Worked examples
Choosing a detector
To compare absorption of beta radiation by aluminium, use a thin-window GM tube and counter because it provides numerical count rates.
Counting interval
If a weak source gives about 12 counts in 10 s, measure for 100 s or longer so the total is larger and relative random variation is smaller.
Practical focus
Investigation
Use a simulation or approved sealed source under teacher supervision. Record counts for equal intervals, repeat, calculate means, and subtract background. Never touch or reposition a source by hand; follow local radiation rules and use tongs or a source holder.
Examination guidance
- Mention ionisation when explaining detection.
- A cloud chamber makes condensation tracks; it does not photograph the nucleus.
- A GM tube produces electrical pulses that are counted.
- Keep geometry and counting time constant during comparisons.
- Do not infer radiation energy simply from the number of clicks.
Check your understanding
- Why does an alpha particle leave a thick track in a cloud chamber?
- What produces the electrical pulse in a GM tube?
- Why should weak-source counts be measured for a longer time?
Answers
- It causes strong ionisation along a short path, creating many condensation droplets.
- Ionisation of the gas followed by an avalanche of charge creates a pulse.
- More counts are collected, reducing the percentage effect of random fluctuations.
