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7.1.6Radioactivity and Particles

Detecting Radiation — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 17

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
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Part 1 of 7Why we need detectors

Humans cannot see, hear or feel ionising radiation, so detectors are needed. All detectors rely on ionisation: radiation passing through a material creates ions, and the effects of the ions are detected electrically, chemically or visually.

Part 2 of 7Geiger–Müller tube and counter

A GM tube is a sealed tube filled with a low-pressure gas, with a thin mica window at one end (thin enough for alpha particles to enter) and a central electrode at high voltage.

When radiation enters, it ionises the gas.
The ions are accelerated by the high voltage and cause an avalanche of further ionisation, producing a brief pulse of current.
Counter
A counter records each pulse as a count, or a ratemeter shows counts per second or per minute.
A loudspeaker may click with each count.
The GM tube detects alpha, beta and gamma, but does not tell you which kind; the type is found by using absorbers (7.1.4).
It also does not detect all gamma rays, as many pass through without interacting.

Part 3 of 7Photographic film

Radiation darkens photographic film in the same way as light does.

Film badges
Workers in hospitals and nuclear facilities wear film badges containing a film behind windows of different materials (plastic, aluminium, lead).
The pattern of blackening after a month shows how much radiation the wearer has been exposed to and what type it was.
The film badge gives a record of total dose, not an immediate reading.

Part 4 of 7Cloud chamber

A cloud chamber contains air saturated with alcohol vapour, cooled by dry ice.

Cloud chamber
Charged particles passing through ionise the vapour, and tiny droplets form along their track, leaving visible white trails.
Alpha tracks are short, thick and straight; beta tracks are thin and longer and may be wavy or curve; gamma produces very few, faint, sparse tracks,
  • as gamma rays ionise poorly.
In a magnetic field the tracks curve in a direction showing the sign of the charge.

Part 5 of 7Count rate and background correction

The count rate is the number of counts detected per second or minute.

The detector only records a fraction of the radiation emitted since the radiation spreads in all directions.
Background radiation
The measured count rate has to be corrected for background radiation (7.1.3) by subtracting the background count rate.
Since decay is random, the count varies from one measurement to the next; use long counting times and repeated readings for reliable results.

Part 6 of 7Choosing a detector

Different detectors suit different jobs.

1
A GM tube with a counter gives an immediate count and is used for surveys and experiments.
2
A film badge is worn all month by a radiographer to monitor the total dose.
3
A cloud chamber is used in demonstrations to make particle tracks visible, which helps to show the differences between alpha and beta.
4
Modern hospitals also use scintillation counters and semiconductor detectors, which convert radiation to light or an electrical signal.
5
Whatever the type, a detector records only a fraction of the emitted particles,
  • so a measured count rate is always less than the activity of the source.

Part 7 of 7Worked examples

  1. A GM tube records 450 counts in 5.0 minutes with a source and 60 counts in 5.0 minutes without. Calculate the corrected count rate in counts per minute.

  2. Describe what a cloud chamber would show for an alpha source.

Key terms

TermMeaning
GM tubeGas-filled detector that produces a pulse of current for each ionising particle.
RatemeterDevice displaying count rate.
Film badgePhotographic film worn to monitor radiation exposure.
Cloud chamberDevice in which ionising particles leave visible vapour trails.
Count rateCounts per unit time.

Avoid these mistakes

1
Thinking a GM tube identifies the type of radiation.
2
Forgetting to correct for background radiation.
3
Thinking the film badge gives a real-time reading.
4
Saying gamma leaves thick, dense tracks in a cloud chamber.

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