Radioactive Decay

Objectives

Lesson outcomes

Use conservation of nucleon number and charge in nuclear processes.
Describe the composition, mass, and charge of alpha, beta-minus, beta-plus, and gamma radiation.
Explain the role of positrons, neutrinos, and antineutrinos in beta decay.
Represent alpha and beta decay using balanced decay equations.

Syllabus

CIE 9702 syllabus points

7 linked

11.1.6 understand that nucleon number and charge are conserved in nuclear processes
11.1.7 describe the composition, mass and charge of α-, β- and γ-radiations (both β – (electrons) and β+ (positrons) are included)
11.1.8 understand that an antiparticle has the same mass but opposite charge to the corresponding particle, and that a positron is the antiparticle of an electron
11.1.9 state that (electron) antineutrinos are produced during β – decay and (electron) neutrinos are produced during β+ decay
11.1.10 understand that α-particles have discrete energies but that β-particles have a continuous range of energies because (anti)neutrinos are emitted in β-decay
11.1.11 represent α- and β-decay by a radioactive decay equation of the form 238 92 U " 234 90 Th + 24 α
11.1.12 use the unified atomic mass unit (u) as a unit of mass

Lesson Notes

Student guidance and lesson notes

Overview

This lesson treats radioactive decay as a nuclear process governed by conservation rules. You should be able to identify the radiation emitted, balance decay equations, and explain why beta decay involves particles beyond just the visible beta particle.

What You Need to Know

Nucleon number and charge are conserved in nuclear processes.
Alpha radiation is a helium nucleus: two protons and two neutrons. It has charge +2e and a mass of about 4 u.
Beta-minus radiation is an electron emitted from the nucleus. Beta-plus radiation is a positron, the antiparticle of the electron.
Gamma radiation is electromagnetic radiation emitted by the nucleus. It has no charge and no rest mass.
An antiparticle has the same mass as its corresponding particle but opposite charge.
Electron antineutrinos are produced in beta-minus decay. Electron neutrinos are produced in beta-plus decay.
Alpha particles have discrete energies. Beta particles have a continuous range of energies because an antineutrino or neutrino is also emitted.
In decay equations, balance the upper nucleon numbers and lower proton numbers on both sides.

How to Work Through It

Start by revisiting nuclide notation and the meaning of the upper and lower numbers.
Compare alpha, beta-minus, beta-plus, and gamma radiation using composition, charge, and mass.
Balance decay equations by conserving nucleon number and charge, not by guessing the daughter nucleus.
Use beta decay examples to explain why neutrinos or antineutrinos are needed.

Check Your Understanding

What happens to nucleon number and proton number in alpha decay?
Why does beta-minus decay increase the proton number by one?
What is the antiparticle of the electron?
Why do beta particles have a continuous range of energies?

Common Mistakes

Balancing only the nucleon number and forgetting to balance charge.
Treating gamma emission as a change in proton number or nucleon number.
Saying a positron is a proton because both have positive charge.
Forgetting the neutrino or antineutrino when explaining beta decay energy.

Next Steps

Practise writing alpha, beta-minus, and beta-plus equations from nuclide notation.
Carry the idea of antiparticles into the next lesson on fundamental particles.