O Level Physics: Nuclear physics (Cambridge 5054)

This guide is aligned to the Cambridge O Level Physics 5054 syllabus for examination in 2026, 2027 and 2028. The whole of topic 5 is here, Nuclear physics: 5.1 The nuclear model of the atom and 5.2 Radioactivity (detection, the three types of emission, decay, fission and fusion, half-life and safety). The syllabus does not split this topic into tiers, so all of it applies to every candidate.

Use it with the revision notes, the practice questions, the Cambridge O Level Physics hub and the printable checklist. When you finish, try a free 10-minute diagnostic to find what still needs work.

What this topic covers

Syllabus section	You must be able to
5.1.1 The atom	Describe the nuclear atom; explain what alpha-particle scattering shows
5.1.2 The nucleus	Protons and neutrons, ions, Z and A, nuclide notation, isotopes
5.2.1 Detection	Cloud chamber, spark counter, Geiger-Müller tube; count rate; background
5.2.2 Three emissions	Nature, ionising effect, penetration and deflection of alpha, beta, gamma
5.2.3 Radioactive decay	Spontaneous and random; decay equations
5.2.4 Fission and fusion	Both processes; chain reaction; coolant, moderator, control rods
5.2.5 Half-life	Definition, calculations, carbon-14 dating, choosing isotopes for uses
5.2.6 Safety	Effects on living things; exposure time, distance, shielding

5.1 The nuclear model of the atom

The atom (5.1.1)

In the nuclear model, an atom is a tiny central nucleus carrying positive charge, with negatively charged electrons orbiting it. Almost all of the atom’s volume is empty space.

Alpha-particle scattering. In this experiment a narrow beam of alpha particles is directed onto an extremely thin metal foil and detectors record where the particles come out. The results and what they show:

Observation	Conclusion
Most particles go straight through, almost undeviated	Most of the atom is empty space and the nucleus is tiny
A few particles are deflected through large angles, some almost straight back	A small nucleus holds most of the mass: only something heavy could turn an alpha particle round
Deflected particles are repelled	The nucleus is positively charged, like the alpha particles

Give all three conclusions when asked for evidence, and link “dense” to particles bouncing back.

The nucleus (5.1.2)

The nucleus contains protons (positive) and neutrons (no charge). Together they are called nucleons.

• Proton number Z (atomic number): the number of protons.
• Nucleon number A (mass number): protons and neutrons added together.
• Neutrons = A − Z.

A nuclide is a particular kind of nucleus with a given Z and A. It is written ᴬ_ZX, for example ⁶³₂₉Cu, where the top number is A and the bottom number is Z.

An atom is neutral because protons and electrons are equal in number. An atom becomes a positive ion by losing electrons and a negative ion by gaining electrons. The nucleus does not change. A magnesium ion Mg²⁺ made from ²⁴₁₂Mg has 12 protons, 12 neutrons and 10 electrons.

Isotopes are nuclei of the same element (same Z) with different numbers of neutrons (different A). An element can have more than one isotope. Chlorine-35 and chlorine-37 both have 17 protons, but 18 and 20 neutrons.

Worked example 1. Copper has two common isotopes, ⁶³₂₉Cu and ⁶⁵₂₉Cu. State how they are alike and different.

• Same Z = 29, so both have 29 protons and (when neutral) 29 electrons.
• Neutrons: 63 − 29 = 34 and 65 − 29 = 36.
• They differ only in neutron number, so they are isotopes of copper.

5.2.1 Detecting radioactivity

• Alpha particles: a cloud chamber shows alpha particles as short, thick, straight tracks of condensed vapour along the path of ionisation. A spark counter has a charged wire above a metal gauze; an alpha particle ionises the air and a spark jumps across.
• Beta particles and gamma radiation: a Geiger-Müller (GM) tube connected to a counter. Radiation entering the tube ionises the gas and each event gives an electrical pulse that is counted.

The count rate is the number of counts detected per unit time, in counts per second or counts per minute.

Background radiation

A GM tube with no source nearby still counts. This is background radiation. Significant sources:

• cosmic rays arriving from space
• radon, a gas found in the air
• rocks and buildings
• the food and drink we consume

To find the corrected count rate from a source, subtract the background count rate from the measured count rate. Make sure both are per the same time unit first.

Worked example 2. A GM tube records 156 counts in 2 minutes with a source in place. With the source removed it records 18 counts in 3 minutes. Find the corrected count rate.

• Measured rate: 156 ÷ 2 = 78 counts per minute.
• Background rate: 18 ÷ 3 = 6 counts per minute.
• Corrected rate: 78 − 6 = 72 counts per minute.

5.2.2 The three types of emission

Unstable nuclei give out radiation. The emission is spontaneous (it needs no outside trigger) and random in direction.

	Alpha (α)	Beta (β)	Gamma (γ)
What it is	2 protons and 2 neutrons (a helium nucleus)	Fast electron from the nucleus	Very high-frequency electromagnetic wave
Relative ionising effect	Strongest	Weaker	Weakest
Relative penetrating power	Least: stopped by paper or a few centimetres of air	Moderate: stopped by a few millimetres of aluminium	Greatest: only reduced by thick lead or concrete

Beta here means β⁻, the electron. Strong ionisation and weak penetration go together: an alpha particle loses its energy quickly because it ionises so many atoms along a short path.

Deflection. Alpha particles are positive, beta particles are negative and gamma rays are uncharged.

• In an electric field, alpha bends towards the negative plate, beta towards the positive plate, and gamma does not bend. Beta bends much more than alpha because its mass is far smaller.
• In a magnetic field, alpha and beta curve in opposite directions (they are opposite charges moving the same way) and gamma goes straight.

5.2.3 Radioactive decay

Decay happens when an unstable nucleus changes and gives out alpha particles, beta particles and/or gamma radiation. Decay is spontaneous and random: you cannot predict when a given nucleus will decay, only the chance that it will in a given time. This is why we describe large samples by half-life.

Decay equations

Write each particle in nuclide notation and make the top numbers balance and the bottom numbers balance.

• Alpha: ⁴₂He. A falls by 4, Z falls by 2.
• Beta: ⁰₋₁e. A is unchanged, Z rises by 1 (a neutron changes into a proton).
• Gamma: no change to A or Z. It often accompanies alpha or beta decay, carrying away surplus energy.

Worked example 3. Write equations for (a) alpha decay of americium-241, ²⁴¹₉₅Am, and (b) beta decay of carbon-14, ¹⁴₆C.

• (a) Daughter A = 241 − 4 = 237, Z = 95 − 2 = 93: ²⁴¹₉₅Am → ²³⁷₉₃Np + ⁴₂He.
• (b) A stays 14, Z = 6 + 1 = 7: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.
• Check (b): top 14 = 14 + 0; bottom 6 = 7 + (−1).

Worked example 4. Uranium-238, ²³⁸₉₂U, emits an alpha particle, and the product then emits a beta particle. Find the nuclide after both decays.

• After alpha: ²³⁴₉₀Th.
• After beta: Z = 90 + 1 = 91, A = 234, giving ²³⁴₉₁Pa.

5.2.4 Fission and fusion

Fusion is the joining of two smaller nuclei to form a larger nucleus, releasing energy. Fusion of hydrogen into helium is the energy source of stars.

Fission happens when a nucleus such as uranium-235 absorbs a neutron, becomes unstable and splits into daughter nuclei, releasing two or more neutrons and energy. One possible reaction is

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy

Balance: the mass numbers give 236 on the left and 141 + 92 + 3 = 236 on the right; the proton numbers give 92 on the left and 56 + 36 = 92 on the right.

Chain reaction and the reactor

Each fission releases neutrons, and these can be absorbed by other uranium-235 nuclei, each causing a further fission. This is a chain reaction. If it were uncontrolled, the rate of fission would grow rapidly. In a reactor it is controlled:

Part	Action
Moderator (for example graphite or water)	Slows fast neutrons so they are far more likely to be absorbed and cause fission
Control rods (for example boron or cadmium)	Absorb neutrons; moved in to slow the reaction and out to speed it up
Coolant	Flows through the core, takes thermal energy away and carries it to where steam is made to drive turbines

To keep the power steady, on average one neutron from each fission must go on to cause one more fission.

5.2.5 Half-life

The half-life of an isotope is how long it takes for half of the nuclei of that isotope present, whatever the sample size, to decay. The background-corrected count rate also halves in this time.

Worked example 5. A sample gives these readings. Background is 20 counts per minute throughout.

Time (min)	0	15	30	45
Measured (counts per minute)	660	340	180	100

• Correct each reading: 640, 320, 160, 80.
• Each 15 minutes the rate halves, so the half-life is 15 minutes.
• After 60 minutes (four half-lives) the corrected rate is 640 ÷ 16 = 40 counts per minute.
• A sample of mass 2.4 g of the isotope would have 2.4 ÷ 16 = 0.15 g left after the same time.

Reading a decay curve: pick a starting value, find half of it, read the time between the two. Repeat from a second starting point to check the half-life is the same.

Carbon-14 dating

Living things take in carbon, including a small constant fraction of radioactive carbon-14, so the proportion in a living organism stays steady. Once the organism dies, its intake stops and the carbon-14 left in it decays, with a half-life of 5730 years. By comparing the carbon-14 activity of an old wooden object with that of living wood, you can estimate its age.

Worked example 6. A wooden tool has one quarter of the carbon-14 activity of living wood. Estimate its age.

• One quarter means two halvings.
• Age = 2 × 5730 = 11 460 years.

Choosing an isotope for a use

Match the type of radiation (penetration and ionisation) and the half-life (long enough to last, or short enough to be safe) to the job.

Application	Radiation and half-life reasoning
Household fire (smoke) alarm	Alpha: ionises the air in a small chamber so a small current flows; smoke soaks up alpha particles, so the current drops and the alarm goes off. Weak penetration is safe indoors; long half-life means no frequent replacement
Irradiating food to kill bacteria	Gamma: penetrates packaging and food; food is not made radioactive and bacteria are killed
Sterilising equipment	Gamma: passes through sealed packs and kills microorganisms on surgical tools
Measuring and controlling thickness	Sheet material: the radiation must be partly absorbed. Thin aluminium foil suits beta; a thick steel plate needs gamma. Long half-life keeps readings steady
Diagnosing and treating cancer	Gamma: a gamma emitter swallowed or injected can be tracked outside the body, and a short half-life means it soon disappears; a focused beam of gamma from several directions delivers a high dose to a tumour and a lower dose to tissue around it

5.2.6 Safety precautions

Ionising radiation damages living cells. Effects include cell death, mutations (changes to DNA, which may be passed on) and cancer.

To use, move and store radioactive materials safely:

• Reduce exposure time: keep the source out only while needed.
• Increase distance: handle with long tongs, never by hand, and stay well back.
• Use shielding: keep sources in lead-lined containers; use lead or concrete walls to absorb radiation.

Common errors

• Writing that alpha is “the most penetrating” because it is the biggest. It is the most ionising and the least penetrating.
• Forgetting to subtract background before halving counts.
• Writing the proton number on top in nuclide notation.
• Saying fission “uses up atoms” without a neutron being absorbed and further neutrons released.
• Saying a short half-life means the sample decays “faster in one go”. It means the activity falls by half in less time.

Frequently asked questions

Which type of radiation is the most penetrating? Gamma is the most penetrating and alpha is the least penetrating, although alpha is the most ionising. Calling alpha the most penetrating because it is the biggest is a common error.

What do control rods and the moderator do in a nuclear reactor? The moderator slows neutrons down, control rods (for example boron or cadmium) absorb neutrons and are moved in to slow the reaction or out to speed it up, and the coolant carries the heat away.

What does a short half-life mean? A short half-life means the activity of a sample falls by half in less time. It does not mean the sample decays all at once.

How do you reduce exposure to radiation? Reduce the exposure time, increase the distance (for example by using tongs) and use shielding such as lead-lined containers or thick walls.

Official syllabus

Cambridge O Level Physics 5054 syllabus for examination in 2026, 2027 and 2028, Cambridge University Press & Assessment. Section 5, Nuclear physics, 5.1 to 5.2.6.