Practise Radioactive decay and nuclear radiation. 14 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Why some nuclei are unstable, what alpha, beta, gamma and neutron radiation are, and how alpha, beta and gamma compare in penetration, range in air and ionising power. Expect recall, 'compare' questions, and 'explain which radiation is best for a use' questions.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name the types of nuclear radiationAlpha particles (α), beta particles (β), gamma rays (γ) and neutrons (n).
4
Define radioactive decay and activityUnstable nuclei give out radiation to become more stable; activity is decays per second, in becquerels (Bq).
4
State what absorbs each radiationAlpha: paper; beta: a few mm of aluminium; gamma: reduced by thick lead or concrete.
5
Describe what each radiation isAlpha: 2 protons and 2 neutrons; beta: a fast electron from the nucleus; gamma: electromagnetic radiation.
6
Explain how a beta particle is formedA neutron in the nucleus turns into a proton, and a high-speed electron is ejected.
6
Compare penetration, range and ionising powerAlpha is the most ionising with the shortest range; gamma is the least ionising with the longest range.
7
Choose the best radiation for a useJustify the choice using penetration and ionising power, and say why the others are unsuitable.
8
Link ionising power to rangeThe more strongly a radiation ionises, the faster it loses energy, so the shorter its range.
Notes
Radioactive decay
Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is radioactive decay.
Decay is random: you cannot predict which nucleus will decay next, or when.
Activity is the rate at which a source of unstable nuclei decays. It is measured in becquerels (Bq): 1 Bq = 1 decay per second.
Count rate is the number of decays recorded each second by a detector, such as a Geiger–Müller tube.
Types of nuclear radiation
Alpha particle (α): two protons and two neutrons, the same as a helium nucleus. Relative charge +2.
Beta particle (β): a high-speed electron ejected from the nucleus when a neutron turns into a proton. Relative charge −1.
Gamma ray (γ): electromagnetic radiation from the nucleus. No mass and no charge.
Neutron (n): some nuclei emit a neutron. It has no charge.
Comparing alpha, beta and gamma
Penetration: alpha is stopped by a sheet of paper or the outer layer of skin; beta by a few millimetres of aluminium; gamma is only reduced by several centimetres of lead or thick concrete.
Range in air: alpha travels a few centimetres; beta about a metre; gamma a very long way.
Ionising power: alpha is strongly ionising, beta is less ionising, gamma is weakly ionising.
The more ionising a radiation is, the more quickly it loses its energy as it passes through a material. That is why alpha has the shortest range and gamma the longest. grade 7+
Choosing a source for a use
Smoke alarms use alpha: it strongly ionises the air inside the alarm, so a small current flows. Smoke absorbs the alpha particles, so the current falls and the alarm sounds. Alpha's short range keeps it inside the alarm.
Thickness gauges for paper use beta: some passes through, and the amount changes when the thickness changes. Alpha would all be absorbed; gamma would nearly all pass through whatever the thickness.
Sterilising medical equipment uses gamma: it passes through the packaging and kills microorganisms.
Cheatsheet
Activity = decays per second, unit becquerel (Bq)
Count rate = decays recorded per second by a detector
Beta β = high-speed electron from the nucleus (neutron → proton), charge −1
Gamma γ = electromagnetic radiation from the nucleus, no mass, no charge
Absorbed by: α paper; β a few mm aluminium; γ reduced by thick lead or concrete
Range in air: α a few cm; β about 1 m; γ very far
Ionising power: α > β > γ
How to answer each type of question
Identify a radiation from absorption data
2 marks5
See which material stops the radiation (count rate falls to background).
Match it: paper → alpha; a few mm of aluminium → beta; only thick lead reduces it → gamma.
Quote the evidence from the question as your reason.
Example. A radioactive source emits one type of radiation. A sheet of paper placed in front of the source does not change the count rate. A 3 mm sheet of aluminium reduces the count rate to the background level. Which type of radiation does the source emit? Give a reason for your answer.
Show the model answer
Beta (1). It passes through paper but is absorbed by 3 mm of aluminium (1).
Describe what each radiation is
1 to 3 marks5
Alpha: say what particles it is made of.
Beta: say it is an electron AND that it comes from the nucleus.
Gamma: say it is electromagnetic radiation from the nucleus.
Example. (a) Describe the structure of an alpha particle. (1 mark) (b) Describe how a beta particle is produced. (2 marks)
Show the model answer
(a) Two protons and two neutrons (1) (b) A neutron in the nucleus changes into a proton (1) and a high-speed electron is ejected from the nucleus (1).
Explain which radiation is best for a use
3 to 4 marks7
State the property the use needs (e.g. partly absorbed, penetrates packaging).
Explain why the chosen radiation has that property.
Explain why each of the other radiations would not work.
Example. A paper mill uses a radioactive source and a detector on opposite sides of a sheet of paper to monitor its thickness. Explain why a beta source is used rather than an alpha source or a gamma source.
Show the model answer
Alpha would be completely absorbed by the paper, so the count rate would not change with thickness (1). Gamma would pass through almost completely, so small changes in thickness would hardly change the count rate (1). Beta is partly absorbed, and more is absorbed when the paper is thicker, so the count rate shows changes in thickness (1).
Explain: ionising power and range
2 marks8
Compare the ionising power of the radiations.
Link strong ionisation to losing energy quickly, and so a short range.
Example. Explain why alpha particles have a much shorter range in air than gamma rays.
Show the model answer
Alpha particles are much more strongly ionising than gamma rays (1), so they lose their energy over a much shorter distance (1).
Shortcuts and memory tricks
PAL: Paper stops alpha, Aluminium stops beta, Lead reduces gamma.
Ionising power and range go opposite ways: the most ionising (alpha) travels the least far.
Alpha is heavy with a +2 charge, so it ionises everything it passes and is stopped quickly.
Beta is an electron, but it is made in the NUCLEUS, not taken from the electron shells.
Where marks are lost
Saying beta particles are electrons from outside the nucleus. They are created in the nucleus.
Saying gamma is 'stopped' by lead. Thick lead greatly reduces it, but some still gets through.
Confusing activity (decays per second in the source) with count rate (what the detector records).
Saying gamma rays have mass or charge. They are electromagnetic waves.
In 'best source' questions, only explaining why the chosen one works, and not why the others do not.
Exam technique
Use comparatives in 'compare' questions, e.g. 'alpha is more ionising than beta'.
Quote data from the question, such as the thickness of material or count rates, in your answer.
Give activity in Bq and name the detector (Geiger–Müller tube) if asked how radiation is measured.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the sentence. Radioactive decay is a ................ process, so it is not possible to predict when a particular nucleus will decay.
random
What is meant by ionising radiation?
Radiation that knocks electrons out of atoms, turning them into ions.
State the material (or substance) needed to stop alpha particles.
Paper (or skin, or a few cm of air).
A different school, using the same type of source and the same method, gets similar count-rate readings to the teacher's class. Which term describes results that are similar when a different experimenter repeats the method: repeatable or reproducible?
Reproducible.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What is an alpha particle made of? Tick (✓) one box.[1]
Two protons and two neutrons
A high-speed electron
Electromagnetic radiation
A single neutron
(b) What is a gamma ray? Tick (✓) one box.[1]
A helium nucleus
A high-speed electron from the nucleus
Electromagnetic radiation from the nucleus
A neutron from the nucleus
(c) Complete the sentence. Radioactive decay is a ................ process, so it is not possible to predict when a particular nucleus will decay.[1]
(d) What is the unit of activity?[1]
Show the answer and mark scheme
(a)Answer: Two protons and two neutrons
(b)Answer: Electromagnetic radiation from the nucleus
(c)Answer: random
random
(d)Answer: Becquerel (Bq)
becquerel (Bq)
Question 2Medium6 marks
(a) What is meant by ionising radiation?[2]
(b) Alpha particles are the most strongly ionising type of nuclear radiation. Explain why alpha particles have the shortest range in air.[2]
(c) Which type of nuclear radiation is the least ionising?[1]
(d) As well as alpha particles, beta particles and gamma rays, an unstable nucleus can emit one other type of nuclear radiation. Name it.[1]
Show the answer and mark scheme
(a)Answer: Radiation that knocks electrons out of atoms, turning them into ions.
radiation that can remove electrons from atoms
turning the atoms into (positive) ions
(b)Answer: They ionise a lot of air molecules along their path, so they lose their energy within a few centimetres.
alpha particles collide with and ionise many air molecules / cause a lot of ionisation along their path
so they lose their energy quickly and are stopped after a few centimetres
(c)Answer: Gamma
gamma (rays)
(d)Answer: A neutron
neutron(s)
Question 3Hard7 marks
A teacher has a sealed radioactive source. It emits one or more types of nuclear radiation. The teacher also has:
a Geiger–Müller tube connected to a counter
a stopwatch
a sheet of paper
a sheet of aluminium 3 mm thick
a sheet of lead 5 cm thick
(a) Describe how the teacher could find out which types of radiation the source emits. Your answer should include how the teacher would use the results.[6]
(b) Explain why the distance between the source and the Geiger–Müller tube must be kept the same throughout the experiment.[1]
Show the answer and mark scheme
(a)Answer: Measure background; then count rate with no absorber, paper, aluminium and lead at a fixed distance and time; subtract background. A drop with paper shows alpha, a further drop with aluminium shows beta, a count above background through aluminium or lead shows gamma.
measure the background count with no source present over a set time (e.g. 5 minutes) and calculate the background count rate
place the source a fixed, short distance (e.g. 2 cm) from the Geiger–Müller tube and measure the count rate with no absorber
place the paper between the source and the tube and measure the count rate again, keeping the distance and counting time the same
repeat with the aluminium sheet and then with the lead sheet
subtract the background count rate from each reading
if the paper reduces the count rate significantly, alpha is emitted
if the aluminium reduces the count rate significantly more than the paper, beta is emitted
if the count rate with the aluminium or the lead is still above background, gamma is emitted
safety: handle the source with tongs, keep it away from the body and return it to its lead box as soon as possible
Marked with levels of response: the full level descriptors are in the app.
(b)Answer: So that any change in count rate is due only to the absorber, not to a change in distance.
the count rate also depends on the distance (air absorbs some radiation), so keeping it the same means any change in count rate is caused only by the absorber