Cambridge IGCSE Physics 0625: Nuclear physics – Study Guide

This guide teaches Topic 5, Nuclear physics, of the Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028. It covers section 5.1 (the nuclear model of the atom) and section 5.2 (radioactivity). Core content is for every candidate. Supplement content is examined only on the Extended papers (Paper 2 and Paper 4) and is labelled Extended only.

Use it with the 0625 course hub and the printable 0625 checklist.

What this topic covers

Section	Core	Extended only
5.1.1 The atom	Nucleus and orbiting electrons; forming positive and negative ions	Alpha-particle scattering as evidence for the nucleus
5.1.2 The nucleus	Protons and neutrons; relative charges; Z, A, number of neutrons; nuclide notation; isotopes	Fission and fusion equations; Z and nuclear charge; A and nuclear mass
5.2.1 Detection	Background radiation and its sources; detector and counter; count rate	Corrected count rate
5.2.2 Types of emission	Spontaneous, random emission; nature, ionising effect and penetration of α, β, γ	Deflection in electric and magnetic fields; explaining ionising effect
5.2.3 Radioactive decay	Decay changes the nucleus; α or β decay gives a new element	Why isotopes are unstable; effect on the nucleus; decay equations
5.2.4 Half-life	Definition; simple calculations from tables or curves	Half-life with background included; choosing isotopes for uses
5.2.5 Safety	Effects on living things; safe moving, use and storage	Time, distance and shielding

5.1.1 The atom

An atom has a small, positively charged nucleus with negatively charged electrons in orbit around it. A neutral atom has equal numbers of protons and electrons.

• An atom that loses electrons becomes a positive ion.
• An atom that gains electrons becomes a negative ion.

For example, an atom that loses two electrons has a charge of +2.

Alpha-particle scattering (Extended only)

Alpha particles were fired at a very thin sheet of metal (gold). Each observation supports part of the nuclear model:

Observation	What it shows
Nearly all alpha particles pass straight through	The atom is mostly empty space; the nucleus is very small
A few are deflected	The nucleus is positively charged, so it repels the positive alpha particles
A very few bounce back	The nucleus contains most of the mass of the atom

Link each observation to one feature. “The nucleus is small, dense and positive” in one line will not earn separate marks for separate observations.

5.1.2 The nucleus

The nucleus contains protons and neutrons (together called nucleons).

Particle	Relative charge	Where
Proton	+1	nucleus
Neutron	0	nucleus
Electron	−1	orbits the nucleus

• Proton number (atomic number), Z: the number of protons.
• Nucleon number (mass number), A: the number of protons plus neutrons.
• Number of neutrons = A − Z.

Nuclide notation writes A at the top left and Z at the bottom left of the symbol: ᴬ_Z X. On this page it is printed with superscripts and subscripts, for example ¹³⁷₅₅Cs.

Worked example. State the numbers of protons, neutrons and electrons in a neutral atom of ¹³⁷₅₅Cs.

protons   = Z = 55
neutrons  = A − Z = 137 − 55 = 82
electrons = 55 (neutral atom: same as protons)

Isotopes are atoms of the same element (same proton number) with different numbers of neutrons (different nucleon numbers). An element can have more than one isotope: ¹²₆C and ¹⁴₆C are both carbon, with 6 and 8 neutrons.

Extended only. The relative charge on a nucleus equals its proton number Z; the relative mass of a nucleus is given by its nucleon number A. So the nucleus of ⁵⁶₂₆Fe has relative charge +26 and relative mass 56.

Fission and fusion (Extended only)

Nuclear fission is the splitting of a large nucleus into two smaller nuclei, usually after it absorbs a neutron. More neutrons are released and energy is released. For example:

²³⁵₉₂U + ¹₀n → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + 3 ¹₀n
top:    235 + 1 = 236      144 + 89 + 3 = 236
bottom:  92 + 0 = 92        56 + 36 + 0 = 92

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, with energy released. For example:

²₁H + ²₁H → ³₂He + ¹₀n
top:    2 + 2 = 4      3 + 1 = 4
bottom: 1 + 1 = 2      2 + 0 = 2

In both processes the total mass of the products is slightly less than the total mass before; the lost mass is released as energy (mainly kinetic energy of the products). The syllabus asks for this qualitatively, without values.

Always check that the top numbers balance and the bottom numbers balance. That check alone catches most errors.

5.2.1 Detection of radioactivity

Background radiation is the ionising radiation present all the time, from natural and artificial sources. Sources that make a significant contribution:

• radon gas in the air
• rocks and buildings
• food and drink
• cosmic rays

Ionising radiation is measured with a detector connected to a counter. The count rate is the number of counts per second (counts/s) or per minute (counts/minute).

Extended only. A reading near a source includes the background. The corrected count rate = measured count rate − background count rate. If a detector reads 146 counts/minute next to a source and 18 counts/minute with the source removed, the corrected count rate is 146 − 18 = 128 counts/minute.

5.2.2 The three types of emission

Emission from a nucleus is spontaneous (nothing triggers it) and random in direction.

	Alpha (α)	Beta (β)	Gamma (γ)
Nature	helium nucleus: 2 protons + 2 neutrons	fast-moving electron from the nucleus	electromagnetic radiation
Charge	+2	−1	0
Ionising effect	strongest	medium	weakest
Penetration	least: stopped by paper or a few cm of air	medium: stopped by a few mm of aluminium	most: reduced by thick lead or concrete

The syllabus does not include β⁺; “beta” means β⁻, an electron.

Extended only: explaining the ionising effect. Alpha particles have a large charge and a large kinetic energy and move relatively slowly, so they interact strongly with the atoms they pass and ionise many of them. They lose their energy quickly, which is why they are the least penetrating. Beta particles have a smaller charge, so they ionise less and travel further. Gamma rays have no charge, so they ionise least and penetrate most.

Extended only: deflection in fields. In an electric field, α particles bend towards the negative plate and β particles towards the positive plate; β particles bend much more because they are far lighter. In a magnetic field, α and β bend in opposite directions, again with β bending more. Gamma is not deflected by either field because it has no charge. For the direction of the force in a magnetic field, treat α as a current in its direction of motion and β as a current opposite to its motion, then apply Fleming’s left-hand rule.

5.2.3 Radioactive decay

Radioactive decay is a change in an unstable nucleus that can result in the emission of α particles or β particles and/or γ radiation. It is spontaneous and random. During α or β decay, the nucleus changes into that of a different element. Gamma emission alone does not change the element.

Extended only.

• An isotope may be radioactive because it has too many neutrons and/or because the nucleus is too heavy.
• Decay makes the nucleus more stable. Beta decay reduces the number of excess neutrons, because inside the nucleus: neutron → proton + electron, and the electron is emitted.
• Gamma emission leaves the nucleon number and the proton number unchanged; the nucleus loses energy.

Emission	Change to A	Change to Z	Emitted particle
α	−4	−2	⁴₂He (or ⁴₂α)
β	0	+1	⁰₋₁e (or ⁰₋₁β)
γ	0	0	γ (energy only)

Worked example (Extended). Write equations for the alpha decay of radium-226 and the beta decay of carbon-14.

²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He        top: 226 = 222 + 4   bottom: 88 = 86 + 2
¹⁴₆C   → ¹⁴₇N  + ⁰₋₁e           top: 14 = 14 + 0     bottom: 6 = 7 + (−1)

The proton number identifies the new element (86 is radon, 7 is nitrogen).

5.2.4 Half-life

The half-life of a particular isotope is the time taken for half the nuclei of that isotope in any sample to decay. After each half-life the count rate (and the number of undecayed nuclei) halves. After n half-lives, the fraction left is (1/2)ⁿ: 1/2, 1/4, 1/8, 1/16 …

Worked example (Core). An isotope has a half-life of 8.0 days. A sample gives a count rate of 1200 counts/s. Find the count rate 24 days later.

number of half-lives = 24 / 8.0 = 3
1200 → 600 → 300 → 150 counts/s

From a decay curve, read the time for the count rate to fall from any value to half that value. Do it twice, from different starting points, and take the mean.

Worked example (Extended). A detector reads 420 counts/minute near a source. 40 minutes later it reads 120 counts/minute. The background count rate is 20 counts/minute. Find the half-life.

corrected: 420 − 20 = 400;   120 − 20 = 100
400 → 200 → 100 is 2 half-lives
half-life = 40 / 2 = 20 minutes

If you do not subtract the background first, the halvings do not come out as a whole number and the half-life is wrong. If the data come as a curve that levels off above zero, the level it approaches is the background.

Choosing an isotope for a use (Extended only)

The choice depends on penetration and half-life.

Use	Radiation	Half-life	Reason
Smoke alarm	α	long	ionises air strongly but cannot escape the casing; activity stays steady for years
Irradiating food	γ	long enough to avoid frequent replacement	penetrates packaging and food to kill bacteria
Sterilising equipment	γ	long enough to avoid frequent replacement	penetrates sealed packs; kills microorganisms
Thickness of paper or thin sheet	β	long	partly absorbed, so the count changes when thickness changes (α would be stopped completely; γ would pass almost unchanged)
Cancer diagnosis (tracer)	γ	short	detected outside the body; decays soon so the patient is not exposed for long
Cancer treatment	γ	long enough for the source to last	beams from several directions focus on the tumour

For thickness control, match the radiation to the material: it must be partly absorbed by the thickness being measured.

5.2.5 Safety precautions

Ionising radiation can cause cell death, mutations and cancer.

Radioactive sources are moved, used and stored safely: kept in lead-lined containers, handled with tongs (never with hands), pointed away from people, used for the shortest time possible, and stored in a locked, labelled cabinet.

Extended only. Every precaution works by one of three ideas:

• reducing exposure time
• increasing the distance between the source and living tissue
• using shielding to absorb the radiation (lead, concrete)

When a question asks you to “explain” a precaution, name the precaution and link it to one of these three ideas.

Common errors

• Saying isotopes have different numbers of protons; they differ only in neutrons.
• Lowering Z in beta decay; Z goes up by 1.
• Forgetting that alpha decay removes 4 from A and 2 from Z.
• Saying gamma emission changes the element.
• Saying decay can be triggered or predicted for one nucleus; it is spontaneous and random.
• Leaving background in a half-life calculation (Extended).
• Saying alpha is “most dangerous” without context: it is most ionising, but outside the body it is stopped by skin or air.

Next steps

Test yourself with the revision notes, then try the Core and Extended practice questions on nuclear physics and the Extended practice set. Find weak spots across the course with the 0625 Core diagnostic or 0625 Extended diagnostic. The previous topic is Electricity and magnetism.

Official syllabus

Cambridge International, Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028, Topic 5 Nuclear physics (sections 5.1–5.2).