AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Radioactive decay and nuclear radiation
Atomic structure · Lesson 4 of 10
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What is an unstable nucleus?
A nucleus that will change to become more stable
2. What is an ion?
A charged atom
3. What is a Geiger-Müller tube used for?
Detecting radiation
4. What is a helium nucleus made of?
Two protons and two neutrons
5. What is the unit of activity?
The becquerel (Bq)
Learning Objectives
1. Describe radioactive decay as a random process.
2. Define activity and count rate.
3. Name the types of nuclear radiation and describe them.
4. Compare alpha, beta and gamma in penetration, range in air and ionising power, and apply this to uses.
Radioactive Decay
Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable: this is radioactive decay, and it is a random process.
Activity is the rate at which a source decays, measured in becquerel (Bq). Count rate is the number of decays recorded each second by a detector.
Penetrating Power
|
The more ionising, the less penetrating. |
Alpha stopped by paper, beta stopped by aluminium and gamma reduced by thick lead, with their ionising power.
Types of Radiation
Learn the properties.
|
Radiation |
What it is |
Stopped by |
Range in air |
Ionising power |
|---|---|---|---|---|
|
Alpha (α) |
Two protons and two neutrons (a helium nucleus) |
A sheet of paper or a few cm of air |
A few cm |
Strongly ionising |
|
Beta (β) |
A high-speed electron from a neutron changing into a proton |
A few mm of aluminium |
About 1 m |
Moderately ionising |
|
Gamma (γ) |
Electromagnetic radiation from the nucleus |
Thick lead or several metres of concrete |
Very long |
Weakly ionising |
|
Neutron (n) |
A neutron from the nucleus |
Thick concrete or water |
Long |
Indirectly ionising |
Choosing a Source
|
Which radiation would you use for a smoke detector, and why? |
1. Detector
Uses alpha radiation ionising the air between two plates to make a small current
2. Why alpha
It is strongly ionising but stopped by paper, so is safe outside the detector
3. Smoke
Absorbs the alpha particles, the current falls and the alarm sounds
Answer: Alpha: it ionises air strongly (to give a current) but cannot escape the device.
Monitoring Thickness
|
Which radiation could be used to monitor the thickness of aluminium foil? Explain. |
1. Alpha
Absorbed completely by the foil: no use
2. Gamma
Passes through almost unchanged, so the thickness has little effect
3. Beta
Partly absorbed: the count on the other side changes with thickness
Answer: Beta radiation: it is partly absorbed by thin foil, so changes in the count rate show changes in thickness.
Key Ideas
Getting the exam points.
▸ Random. It is not possible to predict when a particular nucleus will decay.
▸ Activity or count rate. Activity in becquerel (Bq); count rate is the number of counts per second on a detector.
▸ Ionising. Radiation that removes electrons from atoms to make ions can damage living cells.
▸ Choose the radiation. Match penetration and ionising power to the job, and think about safety.
Key Terms
|
Radioactive decay The random process in which an unstable nucleus emits radiation. |
Activity The rate at which a source decays, in becquerel. |
|
Count rate Number of decays detected each second. |
Ionising Able to knock electrons off atoms. |
|
Alpha particle Two protons and two neutrons. |
Gamma ray Electromagnetic radiation from the nucleus. |
Your Task: Which Radiation?
12 minutes
|
For each use, choose alpha, beta or gamma and explain: (a) a smoke detector, (b) checking for a leak in an underground pipe using a tracer, (c) monitoring the thickness of paper in a factory. 1. Match penetration to the job. 2. Consider safety. |
A good answer shows: (a) Alpha: strongly ionising, stopped by paper. (b) Gamma: penetrates soil and can be detected above ground. (c) Beta: partly absorbed by paper so the count changes with thickness.
Note: Ask learners to say why alpha would not work for (b).
Can I...?
☐ Define radioactive decay.
☐ Say decay is random.
☐ Define activity and count rate.
☐ Describe alpha, beta and gamma.
☐ State what stops each.
☐ Compare ionising powers.
☐ Choose a source for a use.
☐ Explain a choice.
Summary
✓ Alpha: helium nucleus; beta: fast electron; gamma: EM wave.
✓ Penetration: alpha < beta < gamma.
✓ Ionising power: alpha > beta > gamma.
✓ Activity in Bq; count rate per second.
|
EXAM FOCUS Describe what an alpha particle is. (1 mark) Two protons and two neutrons. |
Exam Practice: Radioactive decay and nuclear radiation
Answer all questions. Use the mark allocation as a guide to how much to write. · 18 minutes
▸ Question 1 · 2 marks · State. Describe what an alpha particle is and what a gamma ray is.
▸ Question 2 · 3 marks · Complete. Complete the sentences. Alpha radiation is stopped by a ______. Beta radiation is stopped by a few millimetres of ______. Gamma radiation…
▸ Question 3 · 3 marks · Explain. A smoke detector contains a source of alpha radiation. Explain why alpha radiation is used and not gamma radiation.
▸ Question 4 · 3 marks · Explain. A factory monitors the thickness of aluminium foil by measuring the radiation that passes through it. Explain why a source of beta…
▸ Question 5 · 6 marks · Evaluate. Radioactive sources are used in industry and medicine. Compare the properties of alpha, beta and gamma radiation and evaluate which is best…
Question 1 · 2 marks · State
|
“Describe what an alpha particle is and what a gamma ray is.” |
HOW TO ANSWER IT Command word: State. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Alpha: two protons and two neutrons. 1 mark
▸ Gamma: electromagnetic radiation. 1 mark
▸ Model answer. An alpha particle is two protons and two neutrons (a helium nucleus). A gamma ray is electromagnetic radiation from the nucleus.
Question 2 · 3 marks · Complete
|
“Complete the sentences. Alpha radiation is stopped by a ______. Beta radiation is stopped by a few millimetres of ______. Gamma radiation is reduced by thick ______.” |
HOW TO ANSWER IT Command word: Complete. Worth 3 marks, so plan before writing.
Question 2 · mark scheme
3 marks available. Award a mark for each point made.
▸ Paper. 1 mark
▸ Aluminium. 1 mark
▸ Lead or concrete. 1 mark
▸ Model answer. sheet of paper; aluminium; lead (or concrete)
Question 3 · 3 marks · Explain
|
“A smoke detector contains a source of alpha radiation. Explain why alpha radiation is used and not gamma radiation.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ Alpha strongly ionising. 1 mark
▸ Stopped by paper or air so safe. 1 mark
▸ Gamma penetrates and weakly ionising. 1 mark
▸ Model answer. Alpha is strongly ionising so it ionises the air to make a current, but is stopped by paper or a few cm of air so it is not a hazard outside the detector. Gamma would pass out of the detector and is weakly ionising.
Question 4 · 3 marks · Explain
|
“A factory monitors the thickness of aluminium foil by measuring the radiation that passes through it. Explain why a source of beta radiation is used and not alpha or gamma radiation.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Alpha absorbed completely. 1 mark
▸ Gamma passes through unchanged. 1 mark
▸ Beta partly absorbed so count rate shows the thickness. 1 mark
▸ Model answer. Alpha is stopped by the foil so no radiation is detected. Gamma passes through almost unchanged so the thickness makes little difference. Beta is partly absorbed, so the count rate changes as the thickness changes.
Question 5 · 6 marks · Evaluate
|
“Radioactive sources are used in industry and medicine. Compare the properties of alpha, beta and gamma radiation and evaluate which is best to use as a tracer inside the body.” |
HOW TO ANSWER IT Command word: Evaluate. Worth 6 marks, so plan before writing.
Question 5 · mark scheme
6 marks available. Award a mark for each point made.
▸ Level 3 (5 to 6 marks): properties of all three (penetration, range, ionising power), the tracer needs to pass out of the body to be detected outside, and a justified conclusion that gamma is best. 5 to 6 marks
▸ Level 2 (3 to 4 marks): properties of at least two types with some link to use as a tracer. 3 to 4 marks
▸ Level 1 (1 to 2 marks): simple statements about the radiation. 1 to 2 marks
▸ Model answer. See levels-of-response scheme.