Section 1 of 5Detectors
Each detector responds to ionising radiation in a different way.
Geiger–Müller tube
- Detects individual ionising particles by producing a pulse of current when radiation ionises the gas inside.
Cloud chamber
- Shows the tracks of radiation as condensation trails in supersaturated alcohol vapour.
Photographic film
- Darkened by ionising radiation, so the degree of blackening indicates how much radiation has reached the film.
Background response
- Every detector responds to natural background radiation, which must be taken into account in any measurement.
Section 2 of 5How a GM Tube Works
A GM tube turns each ionising particle into a short electrical pulse.
1
Radiation enters the tube through a thin window and ionises the low-pressure gas inside the tube.
2
The ions and electrons move towards electrodes held at a high voltage, producing a brief pulse of current.
3
The pulse is registered by a counter, so each ionising particle that enters the tube is recorded.
4
Because a separate pulse is produced for each particle, the counter gives a count rate in counts per second or per minute.
5
Alpha particles are detected only if the window is thin enough for them to pass through it.
Section 3 of 5Choosing a Detector
The right detector depends on whether you need a count, a track or a record of exposure.
Choosing a Detector
Cloud chamber
- Shows the path of each particle, where alpha tracks are short and thick, and beta tracks are long and thin.
Film
- Records a permanent exposure, which is useful for monitoring people, but it does not show individual particles.
GM tube
- Gives a quick electronic count, but it does not show the particle path or identify the type of radiation directly.
Background
- All detectors respond to natural background radiation, so it must be measured and subtracted from source readings.
Section 4 of 5Worked Example
Question Explain why a GM tube with a thick aluminium window cannot detect alpha particles.
- Alpha particles have a short range and are stopped by a few centimetres of air or by a thin sheet of material.
- A thick aluminium window absorbs the alpha particles before they reach the gas inside the tube.
- No ionisation occurs inside the tube, so no pulse is produced and no alpha particles are counted.
Answer The thick window absorbs the alpha particles before they can ionise the gas inside the tube, so no pulses are produced.
Section 5 of 5Exam Checklist & Tips
7.1.6 – Key Points for the Exam
- Geiger–Müller tube A radiation detector that produces an electrical pulse each time an ionising particle enters it.
- Cloud chamber A device that shows the tracks of ionising radiation as trails of condensed vapour.
- Ionisation The process of removing electrons from atoms or molecules to produce ions.
- Background radiation Natural radiation present in the environment, which is detected even when no source is nearby.
Common Mistakes
- Saying a cloud chamber counts individual particles electronically.
- Stating that a GM tube detects radiation through a thick wall with no window.
- Ignoring background radiation when interpreting readings from any detector.
- Describing alpha tracks in a cloud chamber as long and thin; alpha tracks are short and thick.