AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Radioactive decay and nuclear radiation
Atomic structure · Lesson 4 of 10
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.
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. |