Radioactive decay practice questions.

Radium-226 (Z = 88) decays by emitting an alpha particle. What is the daughter nuclide?

• Francium-222 (Z = 87)
• Radon-222 (Z = 86)
• Radon-224 (Z = 86)
• Polonium-222 (Z = 84)

Answer: Radon-222 (Z = 86)

Alpha decay removes 2 protons and 2 neutrons: A decreases by 4 (226 → 222) and Z decreases by 2 (88 → 86), giving radon-222.

A nucleus undergoes β⁻ decay. Which row correctly gives the change in mass number A and atomic number Z?

• A decreases by 1; Z unchanged
• A unchanged; Z decreases by 1
• A unchanged; Z increases by 1
• A increases by 1; Z decreases by 1

Answer: A unchanged; Z increases by 1

In β⁻ decay a neutron converts into a proton, an electron and an antineutrino, so the nucleon number stays the same while the proton number rises by 1.

Which of the following lists alpha, beta and gamma radiation in order of increasing penetrating ability?

• γ, β, α
• β, α, γ
• γ, α, β
• α, β, γ

Answer: α, β, γ

Alpha is stopped by paper or a few cm of air, beta by a few mm of aluminium, and gamma requires thick lead or concrete — penetration increases as ionizing power decreases.

The activity of a radioactive source falls from 800 Bq to 100 Bq in 24 days. What is the half-life of the source?

• 8 days
• 3 days
• 6 days
• 12 days

Answer: 8 days

800 → 400 → 200 → 100 is three half-lives, so T½ = 24/3 = 8 days. Dividing 24 by the factor 8 instead of by the number of half-lives gives the wrong answer of 3 days.

The existence of the antineutrino was first proposed in order to explain which experimental observation?

• The discrete energies of alpha particles
• The emission of gamma-ray photons immediately after each alpha decay
• The continuous range of kinetic energies of beta particles
• The recoil of the nucleus in gamma emission

Answer: The continuous range of kinetic energies of beta particles

If beta decay produced only two particles, the electron would always carry a fixed energy. The observed continuous beta spectrum required a third, undetected particle sharing the released energy — Pauli's (anti)neutrino.

Which statement about radioactive decay is correct?

• Heating a sample increases its decay rate
• It is impossible to predict when an individual nucleus will decay
• All nuclei in a sample have decayed after two half-lives
• The half-life of a nuclide decreases as the sample gets smaller

Answer: It is impossible to predict when an individual nucleus will decay

Decay is random and spontaneous: each nucleus has a fixed probability of decaying per unit time, unaffected by temperature, pressure or the size of the sample.