Detection of radioactivity

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:

1. A Detector: Usually a Geiger-Müller (GM) tube. When radiation enters the tube, it ionises the gas inside, creating an electrical pulse.
2. 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.

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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:

1. Remove the radioactive source from the room.
2. Measure the background radiation over a period of time (e.g., 10 minutes) and calculate the average background count rate.
3. Bring the source in and measure the total count rate.
4. 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

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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

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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.

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Exam Tips

1. 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.
2. 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.
3. 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.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0625 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A student is investigating the background radiation in their school laboratory. They use a Geiger-Muller (GM) tube connected to a counter.

(a) State two sources of background radiation. [2]

(b) Define the term count rate. [1]

(c) The student measures a count rate of 15 counts per minute with no radioactive source present. Explain why it is important to measure background radiation when performing experiments with radioactive sources. [2]

Worked Solution:

(a)

1. Radon gas found in the air, from rocks in the ground.
2. Cosmic rays from space.

How to earn full marks:

• Give two correct sources of background radiation.
• Do not give vague answers like "the environment".

(b)

1. Count rate is the number of counts (or events) detected per unit time. This is the definition.

How to earn full marks:

• Mention both "counts/events" and "per unit time" for the mark.

(c)

1. Background radiation will affect the results. This sets the context.
2. By measuring the background radiation, it can be subtracted from the readings to give a more accurate result for the radioactive source. This explains the need to subtract.

How to earn full marks:

• State that background radiation affects results.
• Explain that it needs to be subtracted for accurate results.

Common Pitfall: Students often forget that background radiation is always present. It's crucial to understand that any measurement of a radioactive source includes this background level, which must be accounted for to get an accurate reading of the source itself.

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Exam-Style Question 2 — Short Answer [6 marks]

Question:

A teacher sets up an experiment to measure the radioactivity of a sample. A detector is placed near the sample, and the count rate is recorded over a period of time.

(a) Identify the type of detector that is commonly used to detect ionising radiation. [1]

(b) State two sources of background radiation that could affect the experiment. [2]

(c) The detector records a count rate of 60 counts per minute when the sample is present. The background count rate is 20 counts per minute. Calculate the corrected count rate for the sample. [3]

Worked Solution:

(a)

1. Geiger-Muller (GM) tube. This is the standard detector.

How to earn full marks:

• State the full name or the abbreviation "GM tube".

(b)

1. Rocks and buildings. Many building materials contain radioactive isotopes.
2. Food and drink. Small amounts of radioactive isotopes are present in food and drink.

How to earn full marks:

• Give two correct sources of background radiation.
• Do not give vague answers like "the environment".

(c)

1. Corrected count rate = Total count rate - Background count rate. State the relationship.
2. Corrected count rate = 60 counts/minute - 20 counts/minute. Substitute the values.
3. Corrected count rate = $\boxed{40 \text{ counts/minute}}$. Calculate the final answer with units.

How to earn full marks:

• State the correct relationship between count rates.
• Substitute the correct values.
• Calculate the correct answer with the correct units.

Common Pitfall: Make sure you always subtract the background count rate from the total count rate to find the corrected count rate. Forgetting to do this will lead to an overestimation of the sample's radioactivity. Also, always include the correct units (counts/minute or counts/second) in your final answer.

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Exam-Style Question 3 — Extended Response [8 marks]

Question:

A group of students are investigating the background radiation levels at different locations around their school. They use a Geiger-Muller (GM) tube connected to a counter and record the count rate at each location for 5 minutes. The table below shows their results:

Location	Count Rate (counts/minute)
School Field	12
Science Laboratory	18
Basement	25

(a) Explain why the count rate is different at each location. [3]

(b) Suggest two reasons why the experiment should be repeated several times at each location. [2]

(c) The students discover that the science laboratory contains a small sample of uranium ore. The count rate from the uranium ore alone is 35 counts/minute. Calculate the total count rate that would be measured in the laboratory, assuming the background radiation remains constant. [3]

Worked Solution:

(a)

1. Different locations have different amounts of radioactive materials in the surrounding rocks and soil. This links location to radioactivity.
2. The basement may have higher levels of radon gas due to less ventilation. This explains a specific location.
3. The science laboratory may have radioactive sources used for experiments. This explains a specific location.

How to earn full marks:

• Relate differences in count rates to differences in radioactive materials in the environment.
• Explain why specific locations might have higher or lower levels.

(b)

1. To calculate an average count rate. Averaging reduces random errors.
2. To identify and discard any anomalous results. Anomalous results skew averages.

How to earn full marks:

• State both reasons relate to improving the reliability/accuracy of the data.

(c)

1. Total count rate = Count rate from uranium ore + Background count rate. State the relationship.
2. Total count rate = 35 counts/minute + 18 counts/minute. Substitute the values.
3. Total count rate = $\boxed{53 \text{ counts/minute}}$. Calculate the final answer with units.

How to earn full marks:

• State the correct relationship between count rates.
• Substitute the correct values.
• Calculate the correct answer with the correct units.

Common Pitfall: When explaining variations in background radiation, be specific about the sources. Saying "the environment" is too vague. Also, remember that repeating experiments improves reliability by allowing you to calculate an average and identify outliers.

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Exam-Style Question 4 — Extended Response [9 marks]

Question:

A scientist is investigating the radioactivity of a rock sample. They use a Geiger-Muller (GM) tube and counter to measure the count rate. The rock sample is placed at a fixed distance from the GM tube.

(a) State four sources of background radiation that could contribute to the measured count rate. [4]

(b) The scientist measures a total count rate of 85 counts per minute. The background count rate is 20 counts per minute. Calculate the corrected count rate from the rock sample. [2]

(c) The scientist then places a thin sheet of aluminium between the rock sample and the GM tube. The count rate drops to 5 counts per minute. Explain what this suggests about the type of radiation emitted by the rock sample. [3]

Worked Solution:

(a)

1. Radon gas in the air. From the decay of uranium in rocks.
2. Rocks and buildings. Containing radioactive isotopes.
3. Food and drink. Naturally occurring radioactive isotopes.
4. Cosmic rays. From space.

How to earn full marks:

• Give four correct sources of background radiation.
• Do not give vague answers like "the environment".

(b)

1. Corrected count rate = Total count rate - Background count rate. State the relationship.
2. Corrected count rate = 85 counts/minute - 20 counts/minute = $\boxed{65 \text{ counts/minute}}$. Substitute the values and calculate.

How to earn full marks:

• State the correct relationship.
• Calculate the correct answer with the correct units.

(c)

1. The aluminium sheet has absorbed a significant portion of the radiation. This sets the context.
2. Alpha particles are stopped by a thin sheet of aluminium. This links aluminium to alpha.
3. This suggests that the rock sample is emitting alpha particles. This draws the conclusion.

How to earn full marks:

• State that aluminium absorbs the radiation.
• Link aluminium absorption to alpha particles.
• Conclude that the rock emits alpha particles.

Common Pitfall: It's easy to forget the specific properties of alpha, beta, and gamma radiation. Remember that alpha particles are easily stopped by thin materials like paper or aluminium. If a count rate drops significantly after placing aluminium, it's a strong indication of alpha emission.