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Theme E — Nuclear and Quantum Physics

E.3.3 Alpha, Beta and Gamma Particles — IB Physics HL Revision Notes & Topical Questions

5 min read 12 parts Lesson 20 of 45 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

E.3.3 Alpha, Beta and Gamma Particles — IB PHYSICS SL notes: Compare the nature, decay changes, ionising power, penetration and deflection of alpha, beta and gamma radiation.Instability — an unstable nucleus has an imbalance of protons and neutrons or too much energyRadioactive d…

  • Radioactive decay
  • Parent and daughter
  • Three main radiations
  • Alpha (α) particle
  • Change in the nucleus
  • Beta-minus (β−) particle

Part 1 of 10Why nuclei decay

Instability
  • Instability an unstable nucleus has an imbalance of protons and neutrons or too much energy
Radioactive decay
  • Radioactive decay to become more stable the nucleus emits particles or radiation
Parent and daughter
  • Parent and daughter when a nucleus decays its composition changes, and a change in proton number gives a different element
Three main radiations
  • Three main radiations alpha, beta and gamma

Part 2 of 10Alpha particles

Alpha particles
Alpha (α) particle
  • Alpha (α) particle a helium-4 nucleus of 2 protons and 2 neutrons, charge +2e and mass 4 u
Emitters
  • Emitters large unstable nuclei with too many nucleons
Change in the nucleus
  • Change in the nucleus proton number falls by 2 and nucleon number falls by 4
Energies
  • Energies a given nuclide emits one or a few discrete energies, typically 4–9 MeV
  • This is why ranges in air are well defined.

Part 3 of 10Beta decay

Beta decay
Beta-minus (β−) particle
  • Beta-minus (β−) particle a fast electron created in the nucleus, with charge −e and mass 0.0005 u
  • A neutron turns into a proton, emitting an electron and an antineutrino.
  • The proton number rises by 1 and the nucleon number is unchanged.
  • It occurs in nuclei with too many neutrons.
Beta-plus (β+) particle
  • Beta-plus (β+) particle a positron, the antiparticle of the electron, with charge +e
  • A proton turns into a neutron, emitting a positron and a neutrino.
  • The proton number falls by 1 and the nucleon number is unchanged.
  • It occurs in nuclei with too many protons.
Memory aid
  • Memory aid beta plus is the proton that turns into the neutron
Energies
  • Energies beta particles show a continuous spectrum from zero to a maximum
  • The missing energy is carried by the neutrino or antineutrino.

Part 4 of 10Gamma radiation

Gamma (γ) ray
  • Gamma (γ) ray a high-energy photon of electromagnetic radiation with no charge and no rest mass
Origin
  • Origin emitted by nuclei that need to lose some energy
Change in the nucleus
  • Change in the nucleus proton number and nucleon number are both unchanged
Energies
  • Energies discrete, characteristic of the energy levels of the daughter nucleus

Part 5 of 10Ionising ability and range

Ionising ability
  • Ionising ability the amount of ionisation caused as radiation passes through matter, measured by ion pairs per cm of air
  • Highly ionising radiation may produce about 104 ion pairs per cm and weak radiation about 1.
Penetrating power
  • Penetrating power the distance radiation travels before losing all its energy
Trade-off
  • Trade-off the more ionising the radiation, the less penetrating, since it loses energy faster
Alpha
  • Alpha most ionising and least penetrating
  • It travels about 3–7 cm in air and is stopped by a sheet of paper or the dead outer layer of skin.
Beta
  • Beta moderately ionising and moderately penetrating
  • Its range in air is about 20 cm to 3 m, depending on energy, and a few millimetres of aluminium stops it.
Gamma
  • Gamma least ionising and most penetrating
  • Its intensity is reduced by several centimetres of lead or several metres of concrete, and it follows an inverse-square law in air.
  • The intensity falls exponentially with thickness, I = I0e−μx.
  • It can still damage cells, which is why it is used in cancer radiotherapy.

Part 6 of 10Deflection in fields

Electric field
  • Electric field alpha moves towards the negative plate, beta-minus towards the positive plate, and gamma is undeflected
Magnetic field
  • Magnetic field charged particles follow circles of radius r = mv/(Bq)
  • Alpha and beta bend in opposite senses because their charges have opposite signs.
  • Beta bends much more because it is far lighter and faster (about 7000 times smaller mass).
  • Alpha has the highest charge but also the greatest mass, so it deflects less.
Gamma
  • Gamma it is not deflected because it is neutral, though it can transfer energy to atomic electrons that are deflected

Part 7 of 10Comparing the radiations

PropertyAlphaBeta-minusGamma
NatureHelium-4 nucleusElectronPhoton
Charge+2e−e0
Ionising powerHighestMediumLowest
Range in airFew cmUp to about a metre or moreVery long
Stopped byPaperA few mm of aluminiumThick lead or concrete
Energy spectrumDiscreteContinuousDiscrete

Part 8 of 10Investigating the radiations

1
Measure the count rate of a source with a Geiger counter.
2
Place different materials between the source and the detector.
3
Measure the count rate again.
4
Judge the penetration from how much the material reduces the count rate.

Part 9 of 10Hazards and uses

1Outside the body
  • Outside the body alpha is least dangerous because it cannot penetrate skin, while gamma and fast beta are external hazards
2Inside the body
  • Inside the body alpha is most dangerous, for example inhaled radon or ingested plutonium, because all its energy is deposited in a tiny volume
3Handling
  • Handling keep sources in lead-lined containers, use tongs and minimise time while maximising distance
4Foil thickness gauges
  • Foil thickness gauges beta suits it, because it is partly absorbed
5Signals through thick steel pipes
  • Signals through thick steel pipes gamma is needed
6Smoke detectors
  • Smoke detectors alpha is used because a thin layer absorbs it, so it is safe inside the casing

Part 10 of 10More from the specification

1
Radiation absorption testing
  • Radiation absorption testing placing paper, aluminium, or lead absorbers in particle paths provides a diagnostic means to identify radiation types.
2
Strong Nuclear Force and Binding Energy
  • The strong nuclear force is a short-range attractive force that binds nucleons together within the nucleus against electrostatic repulsion, determining the binding energy and stability of an isotope.
3
Radioactive Half-Life Determination
  • The half-life T1/2 of a radioactive isotope is the time taken for half the unstable atoms in a radioactive sample to decay, related to the decay constant by T1/2 = ln(2) / λ.
4
Correcting for Background Radiation
  • When measuring the count rate of a radioactive sample, background radiation originating from cosmic sources and natural rocks must be measured independently and subtracted.
5
Radiometric Dating Across Geological Years
  • Measuring the ratio of parent radioisotopes to stable daughter atoms in ancient samples allows age determination spanning thousands to billions of years based on known half-life decay rates.
6
Strong Nuclear Force and Binding Energy
  • The strong nuclear force is a short-range attractive force that binds nucleons together within the nucleus against electrostatic repulsion, determining the binding energy and stability of an isotope.
7
Radioactive Half-Life Determination
  • The half-life T1/2 of a radioactive isotope is the time taken for half the unstable atoms in a radioactive sample to decay, related to the decay constant by T1/2 = ln(2) / λ.
8
Correcting for Background Radiation
  • When measuring the count rate of a radioactive sample, background radiation originating from cosmic sources and natural rocks must be measured independently and subtracted.
9
Radiometric Dating Across Geological Years
  • Measuring the ratio of parent radioisotopes to stable daughter atoms in ancient samples allows age determination spanning thousands to billions of years based on known half-life decay rates.
10
Half-life
  • Time for half the nuclei of an isotope to decay.
11
Isotope
  • Atom of the same element with a different number of neutrons.
12
Isotopes
  • Nuclei with the same proton number but different neutron numbers.
13
Binding energy
  • The minimum energy required to separate the nucleons of a nucleus to infinity.
14
Background radiation
  • Radiation that is always present in the environment.
15
Nuclear notation
  • Symbol showing mass number and atomic number.
16
Decay equation
  • An equation in nuclide notation with balanced nucleon and proton numbers.

Key terms

TermMeaning
Alpha particleHelium-4 nucleus emitted in decay, charge +2e.
Beta particleElectron (β⁻) or positron (β⁺) emitted from the nucleus.
Gamma rayHigh-energy photon from a nucleus.
IonisationRemoval of electrons from atoms, producing ions.
Penetrating powerHow far radiation passes through a material before absorption.
Continuous spectrumBeta energies spread over a range from zero to a maximum.

Avoid these mistakes

1
Saying gamma is deflected by a magnetic field — it has no charge.
2
Believing alpha is the most dangerous in every case — it is only the most dangerous if taken inside the body.
3
Stating that beta particles are emitted with a single fixed energy — they show a continuous spectrum.
4
Thinking more penetrating means more ionising — the reverse is true.
5
Confusing beta with orbital electrons — beta is created in the nucleus.

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