CIE iGCSE Co-ordinated Sciences-P5.2.1 Detection of radioactivity- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-P5.2.1 Detection of radioactivity – Study Notes
CIE iGCSE Co-ordinated Sciences-P5.2.1 Detection of radioactivity – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
1. Know what is meant by the terms ionising nuclear radiation and background radiation
2. Know the sources that make a significant contribution to background radiation including:
(a) radon gas (in the air)
(b) rocks and buildings
(c) food and drink
(d) cosmic rays
3. Know that ionising nuclear radiation can be measured using a detector connected to a counter
4. Use count rate measured in counts/s or counts/minute
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Ionising Nuclear Radiation
Ionising radiation is radiation that has enough energy to remove electrons from atoms or molecules, creating ions.
Types:
- α-particles: Strongly ionising, cause dense ionisation along short paths.
- β-particles: Moderately ionising, penetrate further than α but less ionising.
- γ-rays: Weakly ionising, but penetrate deeply into materials.
Importance: Ionisation can damage living cells and DNA, leading to radiation sickness, mutations, or cancer. However, it can also be useful (e.g., killing cancer cells, sterilising equipment).
Background Radiation
Background radiation is the natural low-level radiation present everywhere in our environment.
Main Sources:
- Natural sources:
- Radioactive isotopes in rocks and soil (e.g., Uranium, Radon gas).
- Cosmic rays from outer space.
- Radioactive isotopes inside our bodies (e.g., Carbon-14, Potassium-40).
- Artificial sources:
- Medical uses (X-rays, radiotherapy).
- Nuclear power plants and past nuclear testing.
Typical Level: On average, a person is exposed to about 2–3 mSv per year, though this varies with location.
Significance: Background radiation must always be considered in radiation experiments (detectors are corrected for this value).
Example
Explain why a Geiger counter never shows zero counts, even when no obvious radioactive source is nearby.
▶️Answer/Explanation
The detector still registers counts because of background radiation from natural sources (radon gas, cosmic rays, rocks, etc.).
Final Answer: The counts are due to unavoidable background radiation present everywhere.
Sources of Background Radiation
(a) Radon Gas (in the air):
- The largest natural source of background radiation in many regions.
- Radon is a radioactive gas produced by the decay of Uranium in rocks and soil.
- It seeps into houses, especially basements, and can be inhaled.
(b) Rocks and Buildings:
- Many rocks (e.g., granite) contain small amounts of Uranium, Thorium, and Potassium-40.
- These materials emit α, β, and γ radiation.
- Radiation from building materials adds to daily exposure.
(c) Food and Drink:
- Certain isotopes like Carbon-14 and Potassium-40 are naturally present in food and inside the human body.
- Bananas, for example, are slightly radioactive due to potassium.
(d) Cosmic Rays:
- High-energy radiation arriving from outer space.
- Produces secondary particles when interacting with the atmosphere.
- Exposure increases with altitude (e.g., airplane flights, mountain regions).
| Source | Origin | Contribution to Background Radiation |
|---|---|---|
| Radon Gas | Decay of Uranium in rocks and soil, seeping into buildings | Largest natural contributor in many regions |
| Rocks & Buildings | Radioactive isotopes in rocks (granite, etc.) and construction materials | Significant local contributor, especially in granite areas |
| Food & Drink | Naturally occurring isotopes (C-14, K-40) in food and inside the body | Small but continuous exposure |
| Cosmic Rays | High-energy particles from outer space hitting Earth’s atmosphere | Varies with altitude — higher at mountains and in aircraft |
Example
Why would a person living in a granite-rich area and working as a pilot receive more background radiation than average?
▶️Answer/Explanation
Step (1): Granite-rich regions contain radioactive isotopes, increasing exposure from rocks and buildings.
Step (2): Pilots spend long hours at high altitudes, where cosmic ray exposure is much higher.
Final Answer: The combined effect of granite radiation and increased cosmic rays at high altitude leads to greater background radiation exposure.
Measuring Ionising Nuclear Radiation
Ionising radiation can be detected and measured using a radiation detector connected to a counter.
Typical detectors:
Geiger–Müller (GM) Tube: The most common detector.
- Contains low-pressure gas that becomes ionised when radiation enters.
- The ionisation creates a pulse of current.
- Pulses are sent to a counter or a ratemeter, which records the count rate (counts per second or per minute).
Scintillation Detector:
- Radiation strikes a phosphor, producing tiny flashes of light (scintillations).
- A photomultiplier converts the light flashes into electrical signals.
Counter / Ratemeter:
- Displays the number of radiation events detected.
- Can be used to calculate activity or to monitor background radiation.
Key Concept: Since radioactive decay is random, the count rate fluctuates. An average over time gives the most reliable value.
Count Rate in Radiation Measurements
The count rate is the number of ionising radiation events detected per unit time by a radiation detector (e.g., Geiger–Müller tube).
Units:
- Counts per second (cps) → \( \text{counts/s} \).
- Counts per minute (cpm) → \( \text{counts/min} \).
Corrections:
- Measured count rate always includes background radiation.
- To find the true activity of a source:
\( \text{Corrected count rate} = \text{Measured count rate} – \text{Background count rate} \).
Key Point: The count rate is proportional to the activity of the source, but not always equal (since some radiation may not be detected).
Example
A Geiger counter detects 360 counts in 3 minutes near a radioactive source. Background radiation is measured as 20 counts per minute. Calculate:
- The measured count rate in counts/s.
- The corrected count rate of the source.
▶️Answer/Explanation
(a) Measured count rate:
Total counts = 360 in 3 minutes = \( \dfrac{360}{3} = 120 \,\text{cpm} \).
In counts/s: \( \dfrac{120}{60} = 2.0 \,\text{cps} \).
(b) Background correction:
Background = 20 cpm = \( \dfrac{20}{60} = 0.33 \,\text{cps} \).
Corrected = \( 2.0 – 0.33 = 1.67 \,\text{cps} \).
Example
A GM tube connected to a counter records 120 counts in 60 seconds when placed near a radioactive source. Background radiation was measured separately as 20 counts in 60 seconds. Calculate the corrected count rate of the source.
▶️Answer/Explanation
Step (1): Total recorded counts = 120 counts in 60 s → \( \dfrac{120}{60} = 2.0 \,\text{counts/s} \).
Step (2): Background = 20 counts in 60 s → \( \dfrac{20}{60} = 0.33 \,\text{counts/s} \).
Step (3): Corrected count rate = \( 2.0 – 0.33 = 1.67 \,\text{counts/s} \).
Final Answer: The source has a corrected count rate of about 1.67 counts/s.