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ATOMIC STRUCTURE · CHAPTER 6 OF 12
How do we detect radiation?
A detector can register radiation that you cannot see. But some counts come from the surroundings—even when the source you are investigating has been removed. How do we separate those counts?
By the end, you’ll be able to…
- describe how a Geiger–Müller tube and photographic film detect ionising radiation
- identify natural and human-made sources of background radiation
- calculate count rate and correct readings for background radiation
- explain random fluctuations and how to improve measurements.
Detect the effect, not the invisible particle
Ionising radiation can remove electrons from atoms. Detectors turn the effects of these interactions into something we can measure: electrical pulses or a change in photographic film.
A Geiger–Müller tube, often called a GM tube, connects to a counter or rate meter. A registered event produces a pulse. The electronics may display a count or make a click.
From radiation to a count
Watch radiation enter the Geiger–Müller tube. Each detected event produces a pulse and increases the count.
How does the counter register radiation?
Radiation ionises the gas, producing electrons and positive ions. Electrons move towards the positive central wire, while positive ions move towards the negative casing, where they can gain electrons and become neutral again. The resulting electrical pulse is registered as one count.
* Simplified animation. The count repeats from 0 to 6; real radioactive events occur randomly.
Counts and count rate are different
A count is a registered detection. Count rate is the number of counts per unit time. A reading of 120 counts in 60 s gives 2 counts per second.
To calculate count rate:
Use counts/s when the time is in seconds, or counts/min when it is in minutes. Always show the unit.
Why does it count with no source nearby?
Background radiation is ionising radiation present in our surroundings. Removing the source being investigated does not remove radiation from rocks, the atmosphere, space or other sources.
Where does radiation come from?
Watch each object travel from left to right and pause beneath the scanner. Look at where the radiation particles originate.
Bananas
Bananas contain potassium, including a tiny amount of radioactive potassium-40.
Rocks
Some rocks contain naturally radioactive isotopes. Their unstable nuclei can decay.
Brickwork
Some bricks and other building materials contain naturally radioactive substances.
Flying
At high altitude, less atmosphere shields us from cosmic radiation, so exposure is greater.
Cosmic rays
Cosmic rays originate mainly from energetic processes beyond our Solar System and reach Earth's atmosphere.
Medical X-rays
X-ray machines produce ionising radiation for imaging. This is not natural background radiation.
Nuclear power
Nuclear reactors produce radiation during fission. Shielding and containment protect people outside.
* The scanner is imaginary. Pink particles are a simplified illustration of ionising radiation, not a literal representation of every type. Nuclear reactors are designed with shielding and containment.
Measure the background first
Measure background counts with the source removed or stored well away from the detector. Use a known counting time. Repeat the measurement, or collect counts for longer, to reduce the effect of random fluctuations on the estimated average rate.
Then measure with the source present, keeping the arrangement fixed. The total includes source detections and background detections.
To estimate the source’s contribution:
Convert both readings to the same time unit before subtracting. You are subtracting an estimate of the background rate, not identifying exactly which individual detections came from the source.
What is the detector actually measuring?
Even without a radioactive source nearby, a Geiger–Müller tube can detect background radiation from the surroundings.
Select a stage to see what the GM tube measures. Compare the readings, then calculate the count rate due to the source alone.
3 · Your turn: correct the reading
Total: 140 counts/min · Background: 20 counts/min
What is the estimated count rate due to the source alone?
Reveal the calculation
Corrected count rate = total − background
140 − 20 = 120 counts/min
120 ÷ 60 = 2 counts/s
* Illustrative measurements. Counts fluctuate randomly in real experiments. Background radiation cannot be switched off; the source is removed to measure it separately. The GM tube does not detect every emitted particle, so 2 counts/s is not necessarily an activity of 2 Bq.
Did the source suddenly change?
The same detector records background counts of 18, 22 and 20 in three equal one-minute intervals.
Why measure for longer?
A longer interval usually collects more counts. Random variations are then a smaller fraction of the total, so the average rate is more reliable.
It does not make individual decays predictable or guarantee an identical result each time. If intervals differ, divide each count by its own counting time before comparing rates.
Three worked examples
Calculate count rate:
A radiation detector records 360 counts in 120 s. Calculate the count rate in counts per second.
Count rate = counts ÷ time
Count rate = 360 ÷ 120
Count rate = 3 counts/s
Calculate corrected count rate:
A GM tube measures a total count rate of 48 counts/s. The background count rate is 3 counts/s. Calculate the corrected count rate due to the radioactive source.
Corrected rate = total rate − background rate
Corrected rate = 48 − 3
Corrected rate = 45 counts/s
Calculate corrected count rate:
A detector records 600 counts in 2 minutes with a radioactive source present. The background count rate is 30 counts/min. Calculate the corrected count rate due to the source in counts per minute.
Corrected rate = (counts ÷ time) − background rate
Corrected rate = (600 ÷ 2) − 30
Corrected rate = 270 counts/min
Photographic film keeps a record of exposure
Ionising radiation can affect photographic film. After development, greater exposure produces more darkening under otherwise comparable conditions.
A film badge keeps film in a light-tight holder, so ordinary light does not expose it. Sections behind different filters help trained staff interpret exposure. Film records accumulated exposure over time; it does not provide an immediate count or a click for each event.
What changes after exposure?
Compare the developed film for lower and higher exposure. Assume the same radiation type, film and development conditions.
Which detector fits the job?
When might a film badge be useful?
It can provide a record of a worker’s accumulated exposure over a monitoring period. It cannot warn the wearer instantly that the radiation level has just increased.
Make the comparison fair
- Measure background with the source removed or stored well away. Record the counting time.
- Position the source and detector at a fixed separation. Keep their orientation the same.
- Collect counts for a known interval. Repeat comparable readings or use a longer interval.
- Calculate rates using consistent units, then subtract the mean background rate.
- Change only the variable being investigated. For absorption, keep source distance and alignment fixed while changing the absorber.
* A real radioactive-source investigation is a supervised school activity with approved equipment and procedures. These models allow the measurements to be explored without a source.
Check your understanding
Choose one answer for each question. Feedback appears underneath.
A student subtracts 40 background counts, collected over 2 minutes, from 180 total counts, collected over 1 minute. They report 140 counts/min from the source. Explain and correct the mistake.
Compare the counting times before subtracting. Suggest one way to improve the measurement.
Reveal the answer
The counts were collected over different times, so they cannot be subtracted directly.
Background rate = 40 ÷ 2 = 20 counts/min. Total rate = 180 ÷ 1 = 180 counts/min.
Corrected count rate = 180 − 20 = 160 counts/min.
Collect counts over a longer interval, or repeat readings and calculate an average rate, to reduce the relative effect of random fluctuations. Keep the source–detector arrangement fixed.
CHAPTER 6
Chapter summary
- A GM tube detects radiation through ionisation of gas and produces electrical pulses.
- Photographic film darkens after exposure and development; it records accumulated exposure.
- Background radiation comes from natural and human-made sources.
- Count rate = counts ÷ counting time.
- Subtract background rate from total rate using matching units.
- Random fluctuations are expected. Longer or repeated measurements improve estimates of average rates.
- Detector count rate is not automatically the source’s activity.