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Radiation Detectors

Key idea: Learn how GM tubes, film badges and cloud chambers detect radiation, handle background count-rate, and pick the right detector for common scenarios (O Level).

  • SEC G3 Physics 2027
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Learning objectives

  • Describe atomic composition
  • Use proton number, nucleon number and isotope
  • Use and interpret nuclide notation
  • Explain random and spontaneous nuclear decay
  • Describe alpha, beta-minus and gamma radiation
  • Compare ionising effect and penetrating power
  • Use nuclide equations for radioactive decay
  • Explain background radiation
  • Use half-life in tables and decay curves
  • State radioactivity applications and hazards
  • Evaluate uses and hazards using half-life and radiation properties
  • Relate fission and fusion to nuclear-energy release

1. Definition

Radiation detectors are instruments used to detect ionising radiation (alpha, beta, gamma).

For O Level, the most common detector you should know is the Geiger–Müller (GM) tube.

2. Key Ideas

  • A GM tube produces a count-rate in counts per second or counts per minute. With geometry fixed, a larger net count-rate usually indicates a larger activity, but count-rate is not identical to activity.
  • Always subtract background: net count-rate = measured - background
  • A film badge/dosimeter records cumulative exposure over time.
  • A cloud chamber can make tracks visible (useful to distinguish alpha vs beta by track thickness/length).

3. Detailed Explanations

A. Geiger–Müller (GM) tube

Geiger–Müller tube and counting circuitA cross-section of a Geiger–Müller tube showing a thin end window, low-pressure gas, central anode wire, cylindrical cathode and a connection to a counter.Ionising radiationThin windowLow-pressure gascentral anode wireMetal tube acts as cathodePulsecounterH.V.Each detected ionisation event produces one electrical pulse
Ionising radiation enters through the thin window and ionises the low-pressure gas. The resulting pulse is counted; the GM tube does not measure the energy of each emission.

When ionising radiation enters the tube, it ionises the gas. This causes a brief current pulse, which is counted electronically.

GM tubes are useful for:

  • comparing radiation intensity at different positions
  • plotting decay curves (for half-life)

The recorded count-rate also depends on source–detector distance, orientation, absorbers and detector efficiency. Keep these fixed when comparing sources or measurements.

B. Photographic film / film badge

Film darkens when exposed to ionising radiation. A film badge can be worn to monitor exposure over a period of time (e.g. a month).

C. Cloud chamber (visual tracks)

Comparing cloud-chamber tracksTwo labelled chamber panels compare several short thick alpha tracks with several longer thin zig-zag beta-minus tracks.Alpha tracksBeta-minus tracksshort and thickstrong ionisation; short range in airlonger and thinnerweaker ionisation; more easily deflectedTrack appearance supports identification; a GM tube alone mainly counts events.
Scroll diagram horizontally to read all labels.
Alpha radiation produces short, thick, nearly straight tracks because it ionises strongly and loses energy quickly. Beta-minus radiation produces longer, thinner, more irregular tracks.

The radiation ionises air along its path. Vapour condenses along the ions, making the track visible.

Optional video

4. Common Mistakes

  • Not subtracting background count-rate before interpreting results.
  • Thinking a GM tube tells you the type of radiation directly (you usually need absorbers/deflection tests).
  • Confusing count-rate (cpm) with radiation dose (different concept).
  • Treating one short count as exact. Random decay causes statistical fluctuations; count for longer or repeat and average when the method allows.

5. Exam Tips

  1. If the question uses a GM tube, write “subtract background” early.
  2. If asked “which detector?”, match the purpose:
    • quick checking: GM tube
    • long-term monitoring: film badge
    • visualising tracks: cloud chamber

6. Worked Examples

Modelled example 1

Net count-rate

Core

Problem

Background is 25 cpm. With a source in the fixed setup, the meter reads 145 cpm. Find the net count-rate.
Study the worked solution

  1. Separate measured and background rates

    Method

    Identify the source-plus-background reading and the background-only reading.

    Reason

    The detector continues to count environmental background when the source is present.

    Working

    Measured = 145 cpm; background = 25 cpm.

  2. Subtract background

    Method

    Remove the background contribution from the measured rate.

    Reason

    The remainder estimates the count-rate attributable to the source in this geometry.

    Working

    net = 145-25 = 120 cpm

Guided practice 2

Choosing a detector

About 5 min

Problem

A hospital wants to monitor each staff member’s cumulative radiation exposure over several weeks. Choose between a GM tube, film badge and cloud chamber, and justify the choice.

Match detector to purpose

Detector

Hints

Hint 1: focus on the timescale
The device must remain with one worker and record accumulated exposure rather than an immediate count.
View solution step by step

  1. Match the measurement role

    Method

    Select a film badge or equivalent personal dosimeter.

    Reason

    It records cumulative exposure over the period it is worn.

    Working

    A GM tube is suited to immediate count-rate checks; a cloud chamber visualises tracks.

  2. State the operational reason

    Method

    Assign and review the badge for each worker over the monitoring interval.

    Reason

    The record must correspond to that person’s accumulated exposure.

    Working

    The badge is processed or read after the stated monitoring period.

Common misconception 3

Find the background count-rate

Find and correct the mistake

Learner response

A setup gives a measured rate of 200 cpm and a net rate of 150 cpm. A learner adds them and reports a background rate of 350 cpm. Locate the first error and correct it.

Diagnose before viewing the correction

First error
Unit: cpm

View solution step by step

  1. Write the count-rate relationship

    Method

    Express measured rate as net source rate plus background.

    Reason

    The measured reading contains both contributions.

    Working

    measured = net + background

  2. Rearrange and calculate

    Method

    Subtract the net rate from the measured rate.

    Reason

    The background is the remaining contribution.

    Working

    background = 200-150 = 50 cpm

Examiner practice 4

Using absorbers to identify radiation

3 marks

Examination question

After background subtraction, a source produces a clear GM count. With a sheet of paper inserted, the net count-rate falls to zero. Identify the most likely radiation and explain the evidence. [3 marks]

Write the inference and evidence

View solution step by step

  1. Use the corrected observation

    1 mark

    Method

    State that the source signal disappears after paper is inserted.

    Reason

    Background subtraction distinguishes stopping from the detector’s ordinary environmental count.

    Working

    The net rate changes from a clear positive value to zero.

  2. Connect absorber and penetration

    1 mark

    Method

    Recall that paper stops alpha radiation.

    Reason

    Alpha has low penetrating ability compared with beta and gamma.

    Working

    Paper between source and detector prevents alpha particles reaching the GM tube.

  3. Make the bounded inference

    1 mark

    Method

    Identify alpha as the most likely radiation.

    Reason

    The absorber evidence supports the identification, whereas a GM count alone does not.

    Working

    Most likely radiation: alpha.

Challenge 5

Converting cpm to cps

Minimal support

Rate-unit transfer

A GM tube measures 420 counts per minute. Convert this to counts per second and explain the factor used.

Convert the time basis

Unit: cps

Hints

Hint 1: change from one minute to one second
Spread the 420 expected counts across 60 equal seconds.
View solution step by step

  1. Convert the denominator interval

    Method

    Divide the per-minute count by 60.

    Reason

    A per-second interval is one sixtieth of a minute, so the numerical rate is smaller.

    Working

    420 cpm÷60 = 7 cps

  2. Interpret the rate

    Method

    State that 7 cps is an average rate.

    Reason

    Radioactive counts fluctuate randomly from one second to the next.

    Working

    It does not predict exactly seven counts in every individual second.

7. Mind Stretchers

Mind stretcher 1: Fluctuating count-rateExtension

Why can the count-rate fluctuate even if the source and setup do not change?

Show Answer

Radioactive decay is random, so the number of decays detected in each time interval varies. If you measure for a longer time, the average count-rate becomes more stable (smaller percentage fluctuation).

Mind stretcher 2: Can a GM tube tell you the type?Extension

Why can a GM tube usually not identify whether the radiation is alpha, beta or gamma by itself?

Show Answer

The GM tube mainly counts ionisation events, so it measures count-rate, not the radiation type. To identify the type, you usually need extra tests such as absorbers (paper/aluminium/lead) or field deflection.

8. Practice and next step

Plan a fair absorber comparison with matched counting times and background subtraction, then continue to Nuclear Fission and Fusion.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027