Topic 5.2.1: Detection of Radioactivity Revision Notes
1. Overview
Radioactivity is not just found in laboratories; it is a natural phenomenon occurring all around us. Understanding how to detect and measure this "background" radiation is essential for ensuring safety and for performing accurate scientific experiments involving radioactive sources.
Key Definitions
- Background Radiation: The low-level ionising radiation that is produced all the time by natural and artificial sources in the environment.
- Count Rate: The number of decays (pulses of radiation) detected per unit of time.
- Geiger-Müller (GM) Tube: A specialized device used to detect ionising radiation.
- Counter: An electronic device connected to a detector that tallies the number of ionising events.
- Ionising Radiation: Radiation that has enough energy to remove electrons from atoms, creating ions.
Core Content
Background Radiation
Background radiation is the radiation that is present everywhere at all times. It is unavoidable and comes from several key sources:
- Radon Gas: A radioactive gas that produced by the decay of uranium in rocks and soil. It seeps out of the ground and can build up in buildings with poor ventilation.
- Rocks and Buildings: Many natural rocks (like granite) contain radioactive isotopes. Since buildings are made from stone and concrete, they also emit low levels of radiation.
- Food and Drink: Naturally occurring radioactive isotopes, such as Potassium-40, are found in many foods (e.g., bananas, Brazil nuts).
- Cosmic Rays: High-energy particles (and some electromagnetic radiation) that strike the Earth’s atmosphere from outer space.
Detecting and Measuring Radiation
To measure radiation, you need two distinct components:
- A Detector: Usually a Geiger-Müller (GM) tube. When radiation enters the tube, it ionises the gas inside, creating an electrical pulse.
- A Counter: This device counts the electrical pulses sent by the detector.
Units of Measurement
The intensity of radiation is measured by the Count Rate.
- Common units: counts per second (counts/s) or counts per minute (counts/min).
- Because radioactive decay is a random process, the count rate on a counter will flicker and change slightly even when the source is steady.
Extended Content (Extended Only)
The Corrected Count Rate
When you use a GM tube to measure a specific radioactive source, the counter shows the Total Count Rate. This includes the radiation from the source PLUS the background radiation already in the room.
To find the radiation emitted by the source alone, you must calculate the Corrected Count Rate.
Step-by-Step Process:
- Remove the radioactive source from the room.
- Measure the background radiation over a period of time (e.g., 10 minutes) and calculate the average background count rate.
- Bring the source in and measure the total count rate.
- Subtract the background rate from the total rate.
Worked Example: A student measures the background radiation to be 30 counts/minute. When a radioactive rock is placed in front of the detector, the counter reads 150 counts/minute. Calculate the corrected count rate of the rock.
- Total Count Rate = 150 cpm
- Background Count Rate = 30 cpm
- Corrected Count Rate = 150 - 30 = 120 counts/minute
Key Equations
| Equation | Symbols | Units |
|---|---|---|
| Corrected Count Rate = Total Count Rate – Background Count Rate | $C_{corrected} = C_{total} - C_{back}$ | counts/s or counts/min |
Common Mistakes to Avoid
- ❌ Wrong: Stating that a "Geiger Counter" is the source of radiation.
- ✅ Right: A Geiger-Müller tube is a detector used to measure radiation emitted by a source.
- ❌ Wrong: Claiming the nuclear power industry is the main cause of background radiation.
- ✅ Right: Natural sources (radon, rocks, cosmic rays) make up over 99% of background radiation; the nuclear industry contributes less than 1%.
- ❌ Wrong: Saying that rocks "absorb" radiation from the ground.
- ✅ Right: Radioactive isotopes are inside the rocks, so the rocks emit radiation.
- ❌ Wrong: Trying to divide the total count by the background count.
- ✅ Right: Always subtract the background count from the total count to get the corrected rate.
Exam Tips
- The "Before" Measurement: If an exam question asks how to make an experiment accurate, always mention measuring the background radiation before bringing the radioactive source into the room.
- Units Matter: Always check if the question provides data in counts per second or counts per minute. Do not mix them in your calculations; convert them so they are the same.
- Randomness: If a table of results shows count rates like 32, 28, 31, and 29 for background radiation, explain that this variation is because radioactive decay is a random process.