5. Detecting Radiation

Detecting Radiation

Ionizing radiation cannot usually be detected directly by our senses. We cannot reliably:

• see it
• hear it
• smell it
• taste it
• feel low levels of it

Instead, radiation detectors use the interactions between radiation and matter to produce measurable signals.

Different detectors can measure:

• the presence of radiation
• count rate
• radiation energy
• accumulated dose
• sometimes the type of radiation

One of the most common detectors introduced in school physics is the Geiger-Müller counter.

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Why Radiation Can Be Detected

Alpha, beta and gamma radiation are all ionizing radiation.

This means they can cause atoms or molecules to become ions.

A detector can use this ionization to produce an electrical signal.

For example:

radiation enters detector → ionization occurs → electrical pulse produced → pulse counted

This allows invisible radiation to be converted into a measurable signal.

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The Geiger-Müller Counter

A Geiger-Müller counter, often called a Geiger counter, is an instrument used to detect ionizing radiation.

It usually consists of:

• a Geiger-Müller tube
• a high-voltage supply
• electronic counting circuitry
• a display or meter

Some instruments also produce an audible:

click

for each detected event.

The faster the clicks occur, the greater the measured count rate.

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Inside a Geiger-Müller Tube

A simplified Geiger-Müller tube contains:

• a metal cylindrical wall
• a thin central wire
• low-pressure gas
• a high potential difference
• sometimes a thin window

The central wire acts as an:

anode

The outer wall acts as a:

cathode

A large potential difference exists between them.

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How a Geiger-Müller Tube Works

The basic sequence is:

Radiation enters the tube

Ionizing radiation passes into the detector.

Gas atoms are ionized

Radiation transfers energy to gas atoms and can remove electrons.

This produces:

• positive ions
• free electrons

Charges accelerate

The electric field inside the tube accelerates the charged particles.

Electrons move toward the positive anode.

Positive ions move toward the cathode.

More ionization occurs

Accelerated electrons can collide with additional gas atoms and produce more ions.

This creates a rapid:

ionization avalanche

An electrical pulse is produced

The movement of charge produces a short electrical pulse.

The pulse is counted

The electronics record the event.

The detector may:

• display a count
• calculate count rate
• produce a click

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What Does One Click Mean?

A click represents a detected ionizing event in the tube.

It does not necessarily mean:

• exactly one radioactive atom decayed
• exactly one particle left the source
• all radiation from the source was detected

Only some emitted radiation reaches and interacts with the detector.

Therefore, the Geiger counter measures:

detected events

rather than directly counting every nuclear decay occurring in the source.

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Count and Count Rate

Two related measurements are commonly used.

Count

The total number of detected events during a measurement.

Example:

620 counts

Count rate

The number of detected events per unit time.

For example:

310 counts/min

or:

5.2 counts/s

The SI unit of count rate is:

s⁻¹

although counts per minute (counts/min or cpm) are also widely used.

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Calculating Count Rate

Use:

count rate = number of counts / measurement time

Suppose a detector records:

840 counts

during:

3 minutes

Then:

count rate = 840/3

count rate = 280 counts/min

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Example

A detector records:

450 counts

during:

30 s

Count rate:

= 450/30

= 15 counts/s

If expressed approximately in counts per minute:

15 × 60 = 900 counts/min

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Background Radiation

Even when no laboratory radioactive source is deliberately placed near a detector, the detector will usually record some radiation.

This is called:

background radiation

Background radiation is naturally and artificially present in our environment.

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Sources of Background Radiation

Background radiation can come from:

• cosmic rays
• rocks
• soil
• radon gas and its decay products
• naturally occurring radioactive isotopes
• building materials
• food
• medical sources
• residual human-made radioactive materials

The amount of background radiation varies with:

• location
• altitude
• geology
• building materials
• weather and ventilation
• recent medical procedures
• detector type and sensitivity

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Cosmic Radiation

High-energy particles arrive at Earth from space.

They interact with Earth's atmosphere and produce secondary radiation.

At higher altitudes there is generally less atmosphere above the observer to absorb this radiation.

Therefore, cosmic-radiation exposure generally increases with:

altitude

This is one reason radiation exposure during frequent high-altitude flying can differ from exposure at sea level.

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Radon

Radon is a naturally occurring radioactive gas produced in decay chains involving uranium in rocks and soil.

Radon can enter buildings and accumulate, particularly in poorly ventilated spaces under suitable geological conditions.

Some radon isotopes and their decay products emit alpha radiation.

Radon is an important contributor to natural background radiation in many regions.

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Measuring Background Radiation

Before measuring a radioactive source, background radiation should usually be measured first.

A simple procedure is:

1. Remove the test source from the measurement area.
2. Place the detector in the chosen position.
3. Measure the background count for a fixed time.
4. Calculate the background count rate.
5. Repeat if greater reliability is required.
6. Calculate an average background rate.

This value can then be used to correct measurements made with a source present.

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Corrected Count Rate

The detector measures:

source contribution + background

Therefore:

corrected count rate = measured count rate − background count rate

This is an important equation in radiation experiments.

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Example: Background Correction

Suppose the detector records:

360 counts/min

with a radioactive source present.

The background count rate is:

40 counts/min

Then:

corrected count rate = 360 − 40

corrected count rate = 320 counts/min

This gives a better estimate of the count rate associated with the source under those experimental conditions.

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Example: A Weak Source

Suppose:

measured rate = 75 counts/min

background rate = 58 counts/min

Then:

corrected rate = 75 − 58

corrected rate = 17 counts/min

The source signal is relatively small compared with the background.

This means repeated measurements and longer counting times may be particularly important.

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Why Background Must Be Considered

Suppose two students measure the same source.

Student A reports:

500 counts/min

Student B reports:

450 counts/min

At first, it appears that Student A measured a stronger source.

But suppose:

Student A background = 100 counts/min

Student B background = 50 counts/min

Corrected A:

500 − 100 = 400 counts/min

Corrected B:

450 − 50 = 400 counts/min

After background correction, the results agree.

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Radiation Counts Vary

Radioactive decay is:

random

It is impossible to predict exactly when a particular unstable nucleus will decay.

Therefore, repeated measurements do not normally give exactly the same number of counts.

For example, five measurements might produce:

118, 125, 121, 115, 126 counts

This variation does not automatically mean the detector is faulty.

It is an expected feature of radioactive decay and counting statistics.

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Improving Reliability

Reliability can be improved by:

• counting for longer periods
• repeating measurements
• calculating averages
• keeping the experimental setup unchanged
• measuring background radiation carefully

Longer counting times usually reduce the importance of random fluctuations relative to the total count.

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Example: Average Count Rate

Suppose five one-minute measurements are:

240, 252, 246, 238, 249

Average:

= (240 + 252 + 246 + 238 + 249)/5

= 1225/5

= 245 counts/min

If background is:

35 counts/min

then:

corrected average = 245 − 35

= 210 counts/min

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Distinguishing a Source from Background

Suppose a detector normally measures approximately:

30 counts/min

in the room.

A suspected radioactive sample is placed near the detector.

The detector repeatedly records around:

420 counts/min

The large increase strongly indicates that the sample is contributing additional radiation.

However, if the detector changes from:

30 counts/min

to:

33 counts/min

a single measurement would not provide strong evidence by itself because radioactive counts naturally fluctuate.

Repeated or longer measurements would be needed.

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Experimental Controls

When comparing radiation measurements, important variables should be kept constant.

These include:

• source-to-detector distance
• detector orientation
• counting time
• source position
• shielding
• detector type

Changing these variables can change the measured count rate.

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Distance and Count Rate

Moving a radiation detector farther from a source generally decreases the measured count rate.

For an approximately point-like source in suitable open conditions, radiation intensity may approximately follow:

I ∝ 1/r²

where r is distance from the source.

If distance doubles:

intensity ≈ 1/4

If distance triples:

intensity ≈ 1/9

Background radiation should still be considered when analyzing the measurements.

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Example: Distance Investigation

Suppose the corrected count rate at:

10 cm

is:

800 counts/min

If the source behaves approximately like a point source, doubling the distance to:

20 cm

might produce an idealized prediction of:

800/4 = 200 counts/min

At:

30 cm

the idealized prediction would be:

800/9 ≈ 89 counts/min

Real experimental results may differ because of:

• detector geometry
• scattering
• absorption
• background radiation
• source size
• measurement uncertainty

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Detecting Different Types of Radiation

A detector can also be used with absorbers to investigate the type of radiation present.

A simplified procedure might compare readings with:

• no absorber
• paper
• plastic or aluminium
• thicker dense shielding

Typical behaviour:

Alpha

Strongly reduced by paper or a short distance of air.

Beta

Passes through paper but can be strongly reduced by suitable plastic or thin aluminium.

Gamma

Can pass through both and requires much more substantial shielding for significant attenuation.

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The Detector Window

Some Geiger-Müller tubes contain a very thin window.

This is especially important when detecting alpha particles.

Alpha radiation has very low penetrating ability.

A thick detector wall could stop the alpha particles before they entered the sensitive gas.

A thin window therefore allows alpha particles to enter the tube more easily.

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Detector Efficiency

A detector does not normally record every radioactive decay.

Some emitted radiation:

• travels in another direction
• is absorbed before reaching the detector
• passes through without producing a detectable event
• falls outside the detector's sensitive region

The fraction of radiation successfully detected is related to:

detector efficiency

This means:

measured count rate ≠ total activity of the source

unless detector efficiency and geometry are properly accounted for.

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Count Rate vs Activity

These terms should not be confused.

Count rate

Number of events detected per second.

Unit:

s⁻¹

Activity

Number of nuclear decays occurring per second in the radioactive material.

Unit:

becquerel (Bq)

1 Bq means:

1 nuclear decay per second

A source with an activity of 1000 Bq undergoes:

1000 nuclear decays each second

but a nearby detector may record far fewer than 1000 counts/s.

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Why Count Rate May Be Lower Than Activity

Suppose a source has:

activity = 5000 Bq

but a detector records only:

250 counts/s

This does not necessarily indicate an error.

Many emitted particles or photons may never reach the detector.

Others may reach it but fail to produce a recorded event.

Therefore:

activity describes the source

while:

count rate describes what the detector records

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Other Radiation Detectors

The Geiger-Müller counter is only one type of radiation detector.

Other detectors include:

• scintillation detectors
• semiconductor detectors
• cloud chambers
• photographic film
• personal dosimeters

Different detectors are useful for different purposes.

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Scintillation Detectors

A scintillation detector uses a material that produces tiny flashes of light when ionizing radiation deposits energy.

The light is converted into an electrical signal.

Scintillation detectors can be very sensitive and may provide information about radiation energy.

They are widely used in:

• nuclear physics
• medical imaging
• environmental monitoring
• particle physics

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Semiconductor Detectors

Semiconductor detectors use materials such as silicon or germanium.

Radiation passing through the semiconductor produces mobile charge carriers.

These charges create electrical signals that can be measured.

Semiconductor detectors can provide precise information about radiation energy and are widely used in:

• nuclear spectroscopy
• particle physics
• medical technology
• scientific research

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Cloud Chambers

A cloud chamber allows the paths of some charged particles to become visible.

Ionizing particles create trails of ions through a supersaturated vapour.

Droplets form around these ions.

The resulting condensation trail reveals the particle's path.

Typical observations include:

• alpha → short, thick tracks
• beta → thinner, longer, more irregular tracks

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Personal Dosimeters

People who work around ionizing radiation may use personal monitoring devices called:

dosimeters

Dosimeters measure accumulated radiation exposure or dose over time.

They may be used by:

• medical workers
• nuclear-industry workers
• researchers
• radiographers
• other radiation workers

A dosimeter is different from a simple Geiger counter because it is primarily designed to monitor accumulated exposure.

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Safe Radiation Measurements

Radioactive materials used in education or laboratories must be handled according to the institution's approved radiation-safety procedures.

Three important general principles are:

time

distance

shielding

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Minimize Time

Reduce unnecessary time near a radioactive source.

A measurement should be:

• planned before bringing out the source
• carried out efficiently
• completed without unnecessary delays

Reducing exposure time generally reduces radiation dose.

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Maximize Appropriate Distance

Radiation exposure generally decreases as distance from the source increases.

Therefore:

• do not hold a source unnecessarily
• use appropriate handling equipment where required
• keep sources away from the body except when the procedure specifically requires a measured distance

The experimental distance should still be controlled accurately.

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Use Appropriate Shielding

Shielding depends on radiation type.

Alpha

Paper or thin barriers can stop alpha particles externally, but preventing contamination is especially important.

Beta

Plastic or acrylic is often appropriate; thin metal may also reduce beta radiation depending on the source.

Gamma

Substantial shielding such as lead or concrete may be required for strong attenuation.

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Do Not Touch Radioactive Sources Unnecessarily

Radioactive sources should not be directly handled unless specifically designed and approved for such handling under the relevant procedures.

In controlled educational settings, sealed sources may be manipulated using appropriate equipment.

The goal is to:

• increase distance
• prevent contamination
• reduce unnecessary exposure

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Never Pointlessly Bring a Source Close to the Body

Radioactive sources should not be placed near:

• eyes
• face
• skin
• clothing

unless part of an approved professional procedure.

They should never be treated as ordinary classroom objects.

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Return Sources to Secure Storage

After measurements are complete, radioactive sources should be returned promptly to their approved:

shielded storage location

They should not be left unattended on laboratory benches.

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Keep the Setup Consistent

Safe measurement is also good experimental practice.

When taking repeated readings:

• keep the detector fixed
• keep source distance fixed
• use the same counting time
• avoid unnecessary movement
• record results immediately

This reduces both:

• exposure time
• experimental uncertainty

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A Safe Measurement Procedure

A simplified school investigation might follow this sequence:

1. Set up the detector before bringing the source to the measurement position.
2. Measure background radiation.
3. Record the background count rate.
4. Position the source using the approved handling procedure.
5. Maintain a fixed source-to-detector distance.
6. Measure for a predetermined time.
7. Record the count.
8. Repeat measurements if required.
9. Remove the source promptly.
10. Return it to approved storage.
11. Calculate average and background-corrected count rates.

This combines radiation safety with good experimental design.

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Interpreting a Data Table

Suppose a student obtains:

Correct the readings.

At 10 cm:

460 − 28 = 432 counts/min

At 20 cm:

145 − 28 = 117 counts/min

At 30 cm:

82 − 28 = 54 counts/min

The corrected count rate decreases as distance increases.

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Interpreting Absorber Data

Suppose:

Paper has almost no effect.

Plastic produces a large reduction.

This suggests a substantial:

beta component

The small remaining signal above background after dense shielding could arise from more penetrating radiation or experimental/statistical effects, depending on the source and setup.

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Random Variation in Data

Suppose repeated one-minute measurements are:

153, 141, 158, 147, 151

The readings vary because radioactive decay is random.

It would be incorrect to expect:

153, 153, 153, 153, 153

every time.

Instead, scientists use:

• repeated measurements
• averages
• statistical uncertainty

to interpret radiation data.

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Why Longer Measurements Help

Suppose a detector records approximately:

2 counts/s

In 5 seconds, we might expect only about:

10 counts

Random variation could be a large fraction of this result.

In 100 seconds, we might expect about:

200 counts

The random fluctuations are still present, but they are usually a smaller fraction of the total.

Therefore:

longer counting times generally improve the relative statistical reliability of count-rate measurements.

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Detecting a Weak Radioactive Source

A weak source may produce a signal only slightly above background.

Suppose:

background = 52 counts/min

source + background = 61 counts/min

Corrected rate:

61 − 52 = 9 counts/min

Because the difference is small, a single short measurement is not convincing.

Better practice would include:

• longer measurement times
• repeated measurements
• average values
• statistical analysis where appropriate

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Common Mistakes

Mistake: Assuming a Geiger counter measures only artificial radioactive sources.
It also detects background radiation.

Mistake: Forgetting to subtract background radiation.
The detector reading includes both the source contribution and background.

Mistake: Assuming every click equals one decay in the source.
The detector records only radiation that reaches it and produces a detectable event.

Mistake: Confusing activity with count rate.
Activity describes nuclear decays in the source; count rate describes events detected.

Mistake: Expecting identical readings every time.
Radioactive decay is random.

Mistake: Changing the distance during repeated measurements.
Distance can strongly affect count rate.

Mistake: Measuring background for only a very short time when precision matters.
Longer measurements can reduce relative random uncertainty.

Mistake: Handling radioactive sources casually because they are small.
Radioactive materials require approved handling procedures regardless of physical size.

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Key Terms

Radiation detector — device used to detect ionizing radiation.

Geiger-Müller counter — instrument that detects ionizing radiation using electrical pulses produced in a Geiger-Müller tube.

Geiger-Müller tube — gas-filled detector in which radiation causes ionization and an electrical avalanche.

Ionization — process in which atoms or molecules gain or lose electrons.

Count — total number of detected radiation events.

Count rate — number of detected events per unit time.

Background radiation — ionizing radiation naturally and artificially present in the environment.

Corrected count rate — measured count rate after subtracting background count rate.

Activity — number of nuclear decays occurring each second.

Becquerel (Bq) — SI unit of activity; 1 Bq = 1 decay/s.

Detector efficiency — measure of how effectively a detector records radiation associated with a source.

Random decay — unpredictable decay of individual unstable nuclei.

Scintillation detector — detector that converts energy deposited by radiation into flashes of light and then electrical signals.

Semiconductor detector — detector that uses radiation-produced charge carriers in a semiconductor.

Cloud chamber — device that makes tracks of ionizing charged particles visible.

Dosimeter — device used to monitor accumulated radiation exposure or dose.

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Key Takeaways

• Ionizing radiation cannot normally be detected directly by human senses.
• Radiation detectors convert interactions between radiation and matter into measurable signals.
• A Geiger-Müller counter is commonly used to detect ionizing radiation.
• Radiation ionizes gas inside a Geiger-Müller tube.
• The resulting ionization avalanche produces an electrical pulse.
• Each detected event may produce a count or audible click.
• Count rate is the number of detected events per unit time.
• Background radiation is present even when no deliberate radioactive source is nearby.
• Background radiation comes from sources including cosmic radiation, rocks, soil and radon.
• Corrected count rate = measured count rate − background count rate.
• Radioactive decay is random, so repeated counts naturally vary.
• Longer counting times and repeated measurements improve reliability.
• Distance, detector orientation and counting time should be controlled during experiments.
• Count rate is not the same as source activity.
• Activity is measured in becquerels.
• A detector does not normally record every nuclear decay.
• Absorbers can help distinguish between alpha, beta and gamma radiation.
• Safe radiation measurements follow approved laboratory procedures.
• Time, distance and shielding are fundamental radiation-safety principles.
• Good experimental design improves both measurement quality and radiation safety.

Check Your Understanding

1. Why can't people normally detect ionizing radiation directly with their senses?
2. What is the purpose of a Geiger-Müller counter?
3. Describe the basic structure of a Geiger-Müller tube.
4. Explain how radiation entering a Geiger-Müller tube produces an electrical pulse.
5. What does a count represent?
6. Define count rate.
7. A detector records 900 counts in 3 minutes. Calculate the count rate.
8. What is background radiation?
9. Identify four sources of background radiation.
10. Why should background radiation be measured before investigating a radioactive source?
11. A detector measures 480 counts/min with a source and 35 counts/min without it. Calculate the corrected count rate.
12. Why do repeated radiation measurements normally give slightly different results?
13. Explain two ways to improve the reliability of radiation count data.
14. Explain why source-to-detector distance must be controlled during an experiment.
15. Distinguish between activity and count rate.
16. What does an activity of 500 Bq mean?
17. Explain why a detector might record far fewer counts per second than the activity of the source.
18. Describe how absorbers could be used with a detector to help distinguish alpha, beta and gamma radiation.
19. State the three general radiation-protection principles of time, distance and shielding.
20. Design a safe procedure for measuring the count rate from a sealed radioactive source, including background correction and steps to improve reliability.