Compare alpha, beta and gamma radiation
Compare radiation composition, relative mass and charge, and distinguish particle-antiparticle pairs.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 11. These original particle models and equations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use atomic number, nucleon number, electric charge and conservation with signed values.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the charge of an alpha particle?
What you will learn
- Describe alpha, beta-minus, beta-plus and gamma composition, mass and charge
- Explain the relationship between a particle and its antiparticle
- Identify the positron as the electron's antiparticle
Radiations differ in composition, mass and charge
An alpha particle contains two protons and two neutrons, has charge +2e and mass about 4 u. A beta-minus particle is an electron with charge −e and very small mass compared with a nucleon; beta-plus is a positron with charge +e and the same mass as an electron.
Gamma radiation is electromagnetic radiation: photons have zero charge and zero rest mass. Do not describe gamma as a charged particle or give it nucleons.
Antiparticles reverse charge but keep mass
An antiparticle has the same mass as its corresponding particle and opposite charge. The positron is the antiparticle of the electron; an electron has charge −e and a positron +e.
The neutrino and antineutrino are also particle-antiparticle partners. The syllabus requires recognising these identities, not a broad catalogue of antiparticle properties.
Pause and explain
How does a positron compare with an electron?
Worked example
Compare the charge and relative mass of an alpha particle and a positron.
Show the worked solution
- An alpha particle has charge +2e and mass about 4 u.
- A positron has charge +e and the same tiny mass as an electron.
- The positron is an antiparticle, not a positively charged proton.
Answer Alpha: +2e and ≈4 u. Positron: +e and electron-scale mass.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State composition, charge and approximate mass for an alpha particle.
Give me a hint
Count its nucleons and positive protons.
Compare my reasoning
- It contains two protons and two neutrons.
- Charge = +2e.
- Mass is approximately 4 u.
Two protons and two neutrons; charge +2e; mass about 4 u.
Look for these in your work
- Composition correct
- Charge correct
- Mass unit stated
A radiation has zero electric charge and zero rest mass and is electromagnetic. Identify it and contrast beta-plus charge.
Give me a hint
Separate a photon from the charged positron particle.
Compare my reasoning
- The electromagnetic radiation is gamma radiation.
- A gamma photon has zero charge and zero rest mass.
- A beta-plus positron has charge +e and electron mass.
Gamma radiation is uncharged and has zero rest mass; a positron has charge +e and electron mass.
Look for these in your work
- Gamma identified
- Photon properties stated
- Positron distinguished
A student says an antiparticle must have negative mass because its charge is opposite. Correct the statement using the electron-positron pair.
Give me a hint
Compare mass and charge as separate properties.
Compare my reasoning
- An antiparticle has the same mass as its particle partner.
- Its charge is opposite.
- A positron therefore has electron mass but charge +e instead of −e.
Opposite charge does not mean negative mass: a positron has the same mass as an electron and the opposite charge.
Look for these in your work
- Mass distinction corrected
- Charge distinction stated
- Pair applied
Common mistakes
- Calling beta-minus radiation a helium nucleus
- Giving a positron the proton's mass
- Assigning gamma photons non-zero electric charge
What can you explain now?
What is the charge of an alpha particle?
Compare with the explanation
+2e
It contains two protons and two neutrons.
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Make it stick
Which properties reverse, and which stay the same, for a particle and its antiparticle?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.