Detection of RadioactivityCambridge IGCSE Physics: Revision notes
Section 1
What is background radiation?
Background radiation is the constant, low-level ionising radiation that is present all around us in our environment at all times. It exists naturally and cannot be avoided. This radiation is always being detected, even when no radioactive sources are deliberately being used in an experiment.
Background radiation occurs because Earth and the atmosphere are exposed to natural sources of radiation continuously. It is important to understand that background radiation exists as a baseline that must be accounted for when measuring radioactivity from specific sources.
Examiners expect you to recognise that background radiation is always present and unavoidable. When answering questions, state clearly that it must be subtracted from experimental measurements to find the true count rate from a source.
Section 2
What are the sources of background radiation?
Background radiation comes from four main natural sources:
| Source | Description |
|---|---|
| Radon gas | A radioactive gas produced by decay of uranium in rocks and soil; seeps into buildings from the ground and is the largest contributor to background radiation |
| Rocks and buildings | Natural radioactive elements (uranium, thorium, potassium-40) in construction materials and geological formations emit radiation |
| Food and drink | Radioactive isotopes (potassium-40, carbon-14) are naturally present in consumed food and water, and ingested by living organisms |
| Cosmic rays | High-energy radiation from outer space that penetrates the atmosphere; more intense at higher altitudes |
These sources vary slightly depending on location (radon gas is higher in some geological regions) and altitude (cosmic rays increase with height above sea level).
In an experiment measuring radioactivity in a school laboratory, the count rate includes cosmic rays from space, radon gas seeping through the walls, and radiation from potassium-40 in the tiles. All of these must be included in the background radiation measurement.
Background radiation is like ambient noise in a busy street—it's always there in the background, so you must subtract it to hear the volume of a specific speaker clearly.
Section 3
How is ionising radiation detected and measured?
Ionising nuclear radiation is detected using a detector connected to a counter. The detector responds to ionising radiation by producing electrical signals, which the counter records and displays.
Common detectors include:
- Geiger-Müller (GM) tube: Most common; detects all types of ionising radiation (alpha, beta, gamma)
- Scintillation counter: Uses materials that emit light when struck by radiation
- Semiconductor detectors: Modern detectors offering high precision
The counter displays the result as a count rate, which is the number of radiation events detected per unit time. Count rate is expressed in two common units:
- Counts per second (counts/s) – used for high-intensity sources
- Counts per minute (counts/minute or cpm) – used for low-intensity sources or background radiation
The choice of unit depends on the magnitude of the count rate being measured.
When answering questions about detection, specify the unit of count rate (counts/s or counts/minute) in your answer. Examiners check that you understand both units are acceptable depending on context.
Students often forget to mention that a detector must be connected to a counter—they are not the same thing. The detector senses radiation; the counter records it.
Section 4
How do you measure background radiation?
To measure background radiation, place a detector and counter in a location with no deliberate radioactive source present. Record the count rate over a suitable time period (typically 5–10 minutes). This gives a measurement that includes all natural background sources: radon, cosmic rays, natural radioisotopes in materials, etc.
Why measure background radiation:
- Background counts are always detected, even without a radioactive source
- To find the true count rate from a specific source, background must be subtracted
- Background varies slightly between locations and times
Typical background count rate: In a typical school laboratory, background radiation produces a count rate of approximately 10–20 counts per minute (or 0.17–0.33 counts per second). This varies depending on local geology, altitude, and building materials.
A student measures background radiation and records 240 counts in 10 minutes = 24 counts/minute. When measuring a radioactive sample, they record 180 counts in 10 minutes = 18 counts/minute. This measurement includes background, so the sample measurement appears lower than the true source activity.
Section 5
How do you calculate corrected count rate?
The corrected count rate is the count rate from a specific radioactive source alone, with background radiation removed. It is calculated using:
Corrected count rate = Measured count rate − Background count rate
This calculation is essential because any measurement of a radioactive source includes background radiation. Subtracting background reveals the true activity of the source being investigated.
Step-by-step process:
- Measure the count rate with only background radiation (no source present) – record background count rate
- Measure the count rate with the radioactive source present – record total count rate
- Subtract: Corrected count rate = Total count rate − Background count rate
- Express the answer with appropriate units (counts/s or counts/minute)
Important notes:
- Both background and source measurements should be taken for the same time period for accurate comparison
- The corrected count rate is always less than the measured count rate (because background is subtracted)
- If measured count rate is very close to background, the corrected count rate is small and the source activity is weak
Background count rate = 12 counts/minute. Measured count rate with uranium sample = 50 counts/minute. Corrected count rate = 50 − 12 = 38 counts/minute. The uranium sample alone produces 38 counts/minute.
Always show your subtraction clearly: write both the background and measured values, then subtract. Examiners award marks for correct method even if you make an arithmetic error—showing your working is critical.
Must Know
- Background radiation is natural, constant ionising radiation present everywhere in the environment that cannot be avoided
- Four main sources of background radiation: radon gas (largest contributor), rocks and buildings, food and drink, and cosmic rays
- Detection and measurement: Ionising radiation is detected by a detector connected to a counter, producing a count rate in counts/s or counts/minute
- Corrected count rate formula: Corrected count rate = Measured count rate − Background count rate; this removes background to show only the source's activity
- Background measurement: Must be measured with no radioactive source present and subtracted from all source measurements to obtain true results
- Why correction matters: Background is always detected, so measured values include background; correcting ensures accurate determination of a source's radioactivity
That's the notes covered.
Carry on to the next subtopic.