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Theme E — Nuclear and Quantum Physics

E.3 Radioactive Decay — IB Physics SL Revision Notes & Topical Questions

5 min read 11 parts Lesson 7 of 30 1 quiz

📚 IB Physics SL Hub / Topical Revision
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Curriculum Equation Key Radioactive Decay & Half-Life FormulasFast Exam Solution
N(t) = N₀ × (1/2)t / t½ | λ = ln(2) / t½ ≈ 0.693 / t½ | E = Δm × c²
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Lesson summary

E.3 Radioactive Decay — IB PHYSICS SL notes: Describe nuclear stability, binding energy and mass defect, the properties of alpha, beta and gamma radiation, and the random nature of decay including half-life.Radioactive decay — the random, spontaneous emission of particles or radi…

  • Conservation
  • Isotopes and stability
  • Strong nuclear force
  • Conversion
  • Binding energy per nucleon
  • Beta-minus

Part 1 of 8Nature of radioactive decay

Radioactive decay
  • Radioactive decay the random, spontaneous emission of particles or radiation from an unstable nucleus
Conservation
  • Conservation nucleon number and charge are conserved in every decay
Radiations
  • Alpha is a helium nucleus.
  • Beta-minus is an electron plus an antineutrino.
  • Beta-plus is a positron plus a neutrino.
  • Gamma is a high-energy photon.

Part 2 of 8Nuclear stability

Isotopes and stability
  • Isotopes and stability light stable nuclei have roughly equal numbers of neutrons and protons
  • Heavier stable nuclei need extra neutrons to offset the growing repulsion between protons.
  • Nuclei away from the line of stability decay.
Strong nuclear force
  • Strong nuclear force an attractive force between nucleons
  • It has a very short range, a few femtometres.
  • It is much stronger than the electric force at that range.
  • It is the same for protons and neutrons.

Part 3 of 8Mass defect and binding energy

1Mass defect
  • Mass defect a nucleus has less mass than its separate nucleons
2Binding energy
  • Δmc2, the energy needed to separate the nucleus completely into its nucleons
3Conversion
  • 1 u corresponds to 931.5 MeV c−2
4Binding energy per nucleon
  • Binding energy per nucleon it peaks near A ≈ 56–62, the most stable nuclei

Part 4 of 8Alpha decay

Equation
  • AZX → A−4Z−2Y + 42α
Energies
  • Energies alpha particles have discrete energies
Properties
  • Properties strongly ionising and stopped by paper or a few centimetres of air

Part 5 of 8Beta decay

Beta-minus
  • Beta-minus a neutron becomes a proton, emitting an electron and an antineutrino, so Z rises by 1
Beta-plus
  • Beta-plus a proton becomes a neutron, emitting a positron and a neutrino, so Z falls by 1
Energies
  • Energies beta particles have a continuous range of energies, which was evidence for the neutrino
Properties
  • Properties stopped by a few millimetres of aluminium

Part 6 of 8Gamma emission

1Process
  • Process an excited nucleus emits a photon, and A and Z are unchanged
2Properties
  • Properties weakly ionising and very penetrating, with intensity reduced by thick lead or concrete

Part 7 of 8Radiations compared

PropertyAlphaBeta-minusGamma
NatureHelium nucleusElectronPhoton
Charge+2e−e0
Ionising abilityHighMediumLow
Stopped or reduced byPaperA few mm of aluminiumThick lead
Energy spectrumDiscreteContinuousDiscrete

Part 8 of 8Measuring a half-life

1
Record the background count rate with no source present.
2
Measure the count rate at regular intervals.
3
Subtract the background from each reading.
4
Plot the corrected count rate against time.
5
Read off the time for the rate to halve at several points and average.

Key terms

TermMeaning
Radioactive decayThe random, spontaneous emission of particles or radiation from an unstable nucleus.
Half-lifeThe time taken for half the radioactive nuclei in a sample to decay.
ActivityThe number of nuclei decaying per unit time.
Mass defectThe difference between the mass of a nucleus and the total mass of its separate nucleons.
Binding energyThe energy required to separate a nucleus completely into its nucleons.
Strong nuclear forceThe short-range attractive force between nucleons.
Background radiationRadiation from natural and artificial sources that is always present.

Avoid these mistakes

Avoid these mistakes
Saying alpha is the most penetrating radiation — it is the least penetrating.
Forgetting to subtract background radiation from count rates.
Thinking half-life is affected by temperature or chemical state.

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