Theme E — Nuclear and Quantum Physics

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

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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

Property	Alpha	Beta-minus	Gamma
Nature	Helium-4 nucleus	Electron	Photon
Charge	+2e	−e	0
Ionising power	Highest	Medium	Lowest
Range in air	Few cm	Up to about a metre or more	Very long
Stopped by	Paper	A few mm of aluminium	Thick lead or concrete
Energy spectrum	Discrete	Continuous	Discrete

Part 8 of 10Investigating the radiations

Measure the count rate of a source with a Geiger counter.

Place different materials between the source and the detector.

Measure the count rate again.

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

Radiation absorption testing

Radiation absorption testing placing paper, aluminium, or lead absorbers in particle paths provides a diagnostic means to identify radiation types.

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.

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.

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.

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.

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.

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.

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.

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.

Half-life

Time for half the nuclei of an isotope to decay.

Isotope

Atom of the same element with a different number of neutrons.

Isotopes

Nuclei with the same proton number but different neutron numbers.

Binding energy

The minimum energy required to separate the nucleons of a nucleus to infinity.

Background radiation

Radiation that is always present in the environment.

Nuclear notation

Symbol showing mass number and atomic number.

Decay equation

An equation in nuclide notation with balanced nucleon and proton numbers.

Key terms

Term	Meaning
Alpha particle	Helium-4 nucleus emitted in decay, charge +2e.
Beta particle	Electron (β⁻) or positron (β⁺) emitted from the nucleus.
Gamma ray	High-energy photon from a nucleus.
Ionisation	Removal of electrons from atoms, producing ions.
Penetrating power	How far radiation passes through a material before absorption.
Continuous spectrum	Beta energies spread over a range from zero to a maximum.

Avoid these mistakes

Saying gamma is deflected by a magnetic field — it has no charge.

Believing alpha is the most dangerous in every case — it is only the most dangerous if taken inside the body.

Stating that beta particles are emitted with a single fixed energy — they show a continuous spectrum.

Thinking more penetrating means more ionising — the reverse is true.

Confusing beta with orbital electrons — beta is created in the nucleus.

Last updated: 15 October 2026

Part 1 of 10Why nuclei decay

Part 2 of 10Alpha particles

Part 3 of 10Beta decay

Part 4 of 10Gamma radiation

Part 5 of 10Ionising ability and range

Part 6 of 10Deflection in fields

Part 7 of 10Comparing the radiations

Part 8 of 10Investigating the radiations

Part 9 of 10Hazards and uses

Part 10 of 10More from the specification

Key terms

Avoid these mistakes

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