Cambridge IGCSE Physics 0625: Nuclear physics – Revision Notes

These notes condense Topic 5, Nuclear physics (sections 5.1 and 5.2), of the Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028. Core content is for every candidate; Supplement content, marked Extended only, is examined only on Papers 2 and 4. For full explanations and worked examples, use the study guide for this topic.

Other pages: 0625 course hub, printable 0625 checklist, Core and Extended practice questions on nuclear physics, Extended practice set on nuclear physics.

Key definitions

• Proton number (atomic number), Z: number of protons in the nucleus.
• Nucleon number (mass number), A: number of protons + neutrons.
• Isotopes: atoms of the same element with the same Z but different numbers of neutrons (different A).
• Ion: an atom that has lost electrons (positive ion) or gained electrons (negative ion).
• Background radiation: ionising radiation that is always present, from radon gas, rocks and buildings, food and drink, and cosmic rays.
• Count rate: counts per second or per minute recorded by a detector connected to a counter.
• Radioactive decay: a change in an unstable nucleus that can emit α or β particles and/or γ radiation; spontaneous and random.
• Half-life: the time taken for half the nuclei of a particular isotope in any sample to decay.
• Fission (Extended only): splitting of a nucleus. Fusion (Extended only): joining of nuclei.

Numbers to handle

Quantity	How to find it
protons	Z
neutrons	A − Z
electrons in a neutral atom	Z
electrons in an ion of charge +n	Z − n
electrons in an ion of charge −n	Z + n
relative charge of nucleus (Extended only)	+Z
relative mass of nucleus (Extended only)	A
corrected count rate (Extended only)	measured − background
fraction left after n half-lives	(1/2)ⁿ

Particle charges: proton +1, neutron 0, electron −1.

The nuclear model

• Small, positive nucleus (protons and neutrons); electrons orbit it.
• Alpha scattering (Extended only):
  • most pass straight through → mostly empty space, tiny nucleus
  • some deflected → nucleus is positive
  • a few bounce back → nucleus holds most of the mass

Alpha, beta and gamma

	α	β	γ
What it is	helium nucleus (2p + 2n)	electron from the nucleus	electromagnetic radiation
Symbol in equations	⁴₂He	⁰₋₁e	γ
Charge	+2	−1	0
Ionising	most	medium	least
Stopped by	paper, few cm air	few mm aluminium	thick lead/concrete reduces it
Electric field (Extended only)	towards − plate, small bend	towards + plate, large bend	no deflection
Magnetic field (Extended only)	bends one way, slightly	bends the opposite way, more	no deflection

• β always means β⁻ (an electron) in this syllabus.
• Why α ionises most (Extended only): largest charge and large kinetic energy, so it interacts strongly with atoms it passes and loses energy fast. γ has no charge, so it ionises least and penetrates most.

Decay equations (Extended only)

Method in steps

1. Write the parent nucleus on the left in nuclide notation.
2. Write the emitted particle on the right: ⁴₂He for α, ⁰₋₁e for β, γ for gamma.
3. Find the daughter: A and Z so that tops balance and bottoms balance.
4. α: A − 4, Z − 2. β: A same, Z + 1. γ: no change.
5. Name the daughter from its Z (use the symbol you are given).

Worked reminders

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
¹³¹₅₃I → ¹³¹₅₄Xe + ⁰₋₁e

• In β decay: neutron → proton + electron; the electron leaves as the β particle. That is why Z rises by 1 and the number of excess neutrons falls.
• Isotopes are unstable because of too many neutrons and/or a nucleus that is too heavy. Decay makes the nucleus more stable.
• α or β decay: the nucleus becomes a different element. γ alone: same element, less energy.

Fission and fusion (Extended only)

	Fission	Fusion
What happens	large nucleus splits into two smaller ones (often after absorbing a neutron)	two light nuclei join into a heavier one
Other products	extra neutrons	sometimes a neutron
Mass after vs before	slightly less	slightly less
Energy	released	released

Balance both rows in every nuclide equation. No numerical mass or energy values are required.

Half-life

Method in steps (from a table or curve)

1. (Extended only) Subtract the background from every reading first.
2. Pick a starting count rate and find when it has halved.
3. Or count how many halvings fit between two readings, then divide the time by that number.
4. From a curve, repeat from a second starting point and average.

Worked reminder. 800 → 400 → 200 → 100 counts/minute in 6 hours is 3 half-lives, so the half-life is 2 hours.

Reading a decay curve

• The curve falls steeply at first, then more slowly: each half-life always halves the count rate.
• A curve that levels off above zero is showing the background count rate (Extended only: subtract it before finding the half-life).
• Read values off the curve to half a small square; draw construction lines to show where you read.
• Core calculations will not include background; Extended ones may.

Detection

• Ionising radiation is measured with a detector connected to a counter.
• Record counts over a fixed time and divide to get counts/s or counts/minute.
• Emission is random, so repeated counts over the same time vary; a longer count gives a more reliable rate.
• Extended only: measure the background with the source removed, then subtract it from every reading.

Uses (Extended only)

• Smoke alarm: α (ionises air, cannot escape the casing), long half-life (steady activity).
• Irradiating food and sterilising equipment: γ (penetrates packaging), kills bacteria.
• Thickness control: β for paper and thin sheet; the radiation must be partly absorbed by the material.
• Cancer diagnosis: γ tracer with a short half-life (detectable outside the body, little lasting dose).
• Cancer treatment: γ beams aimed at the tumour.

Rule of thumb: justify each choice with penetration and half-life, not one of them alone.

Safety

• Effects of ionising radiation: cell death, mutations, cancer.
• Safe handling: tongs, lead-lined boxes, pointed away from people, short use, locked labelled storage.
• Extended only: explain each precaution as less time, more distance or shielding.

Must-know distinctions

• Isotope vs ion: isotopes differ in neutrons; ions differ in electrons.
• Ionising vs penetrating: α is the most ionising but the least penetrating; γ is the reverse.
• Nucleon number vs proton number: A counts protons + neutrons; Z counts protons only.
• Fission vs fusion: splitting vs joining; both release energy.
• Count rate vs corrected count rate: the corrected rate has the background removed.

Quick self-test

1. How many neutrons are in a nucleus of ²³⁸₉₂U?
2. (Extended) Write the equation for the alpha decay of ²³⁸₉₂U to thorium (Th).
3. (Extended) Write the equation for the beta decay of ¹³¹₅₃I to xenon (Xe).
4. A count rate falls from 960 counts/s to 60 counts/s in 32 hours. Find the half-life.
5. (Extended) A detector reads 75 counts/minute near a source; the background is 15 counts/minute. Find the corrected count rate.
6. What fraction of the original nuclei remains after 3 half-lives?
7. (Extended) Which radiation should be used to monitor the thickness of paper? Why not the other two?
8. (Extended) Give two reasons why α particles are the most ionising.
9. Define an isotope.
10. An atom of ²³₁₁Na loses one electron. State the charge on the ion and its number of electrons.
11. (Extended) Readings of 340 counts/minute and then, 12 minutes later, 100 counts/minute are taken. The background is 20 counts/minute. Find the half-life.
12. (Extended) State the relative charge and relative mass of the nucleus of ⁵⁶₂₆Fe.

Answers

1. 238 − 92 = 146.
2. ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.
3. ¹³¹₅₃I → ¹³¹₅₄Xe + ⁰₋₁e.
4. 960 → 480 → 240 → 120 → 60 is 4 half-lives; 32/4 = 8.0 hours.
5. 75 − 15 = 60 counts/minute.
6. (1/2)³ = 1/8.
7. β: paper partly absorbs it, so the count changes with thickness. α would be stopped completely; γ would pass through almost unchanged.
8. It has the largest charge (+2) and a large kinetic energy, so it interacts strongly with the atoms it passes.
9. Atoms of the same element (same proton number) with different numbers of neutrons.
10. Charge +1; 10 electrons.
11. Corrected: 320 and 80; 320 → 160 → 80 is 2 half-lives; 12/2 = 6.0 minutes.
12. Relative charge +26; relative mass 56.

Where marks are usually lost

• Writing “same number of protons and electrons” as the definition of an isotope; isotopes are about neutrons.
• In β decay, lowering Z or changing A; A stays the same and Z goes up by 1.
• Unbalanced nuclide equations; always check the top row and the bottom row.
• Using raw count rates for half-life when a background value is given (Extended).
• Describing half-life as “half the time for the source to decay”; it is the time for half the nuclei of that isotope to decay.
• Saying γ rays are deflected, or that α bends more than β in a field; γ is uncharged and β bends more.
• Justifying a use with only one factor; name both the radiation type and the half-life (Extended).
• Listing safety rules without linking them to time, distance or shielding when asked to explain (Extended).
• Forgetting to name the four main sources of background radiation.

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). Check your readiness with the 0625 Core diagnostic or 0625 Extended diagnostic.