Cambridge IGCSE Physics 0625: Nuclear physics – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Tier note: questions 1 to 8 are on Core content of the 0625 syllabus (5.1 and 5.2 Core outcomes) and every candidate should do them. Questions 9 to 16, marked (Extended), are on Supplement content, examined only on Papers 2 and 4: alpha-particle scattering (5.1.1 Supplement 3), the charge and mass of a nucleus (5.1.2 Supplement 7–8), fission (5.1.2 Supplement 6), why isotopes are unstable, decay equations and the effect of decay on the nucleus (5.2.3 Supplement 3–5), ionising effect and deflection in a magnetic field (5.2.2 Supplement 3–4), corrected count rate and half-life with background (5.2.1 Supplement 5, 5.2.4 Supplement 2), choosing an isotope for a use (5.2.4 Supplement 3) and safety precautions (5.2.5 Supplement 3). Core candidates do not need questions 9 to 16. The marks shown are indicative only.

Related: nuclear physics study guide, revision notes, Extended practice set on nuclear physics, course hub and printable checklist.

After each answer there is a common mistake to avoid.

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Questions

1. This question is about the structure of atoms.

(a) Describe the structure of an atom. [2] (b) A neutral atom of magnesium, ²⁴₁₂Mg, loses two electrons. (i) State whether it forms a positive ion or a negative ion. [1] (ii) State the number of electrons in the ion. [1] (c) State how an atom forms a negative ion. [1]

2. A nucleus of cobalt-60 is written in nuclide notation as ⁶⁰₂₇Co.

(a) Name the two types of particle found in the nucleus. [1] (b) State the relative charges of a proton, a neutron and an electron. [2] (c) Define the terms proton number and nucleon number. [2] (d) Calculate the number of neutrons in a nucleus of cobalt-60. [1] (e) A nucleus of oxygen (symbol O) contains 8 protons and 9 neutrons. Write this nucleus in nuclide notation. [1]

3. Chlorine has two naturally occurring isotopes, ³⁵₁₇Cl and ³⁷₁₇Cl.

(a) Explain what is meant by isotopes. [2] (b) State how the nuclei of these two isotopes differ. Give the number of neutrons in each nucleus. [2]

4. This question is about detecting radiation.

(a) State what is meant by background radiation. [1] (b) Name two sources that make a significant contribution to background radiation. [2] (c) State what is used to measure ionising nuclear radiation. [1] (d) The counter records 540 counts in 3.0 minutes. Calculate the count rate in counts/minute and in counts/s. [2]

5. This question is about the three types of nuclear emission.

(a) The emission of radiation from a nucleus is described as spontaneous and random in direction. State what each of these two words means here. [2] (b) State the nature of (i) an alpha particle, (ii) a beta particle and (iii) gamma radiation. [3] (c) Write alpha, beta and gamma in order of their ionising effect, starting with the most ionising. [1] (d) A student places different absorbers between a radioactive source and a detector. Ignore background radiation in this part.

Absorber	Count rate / counts per minute
none	600
thin paper	600
5 mm of aluminium	150
2 cm of lead	40

Using the results, state which types of radiation the source emits. Give a reason for each type you include or rule out. [3]

6. This question is about radioactive decay.

(a) State what is meant by radioactive decay. [2] (b) A nucleus of radium emits an alpha particle. State what happens to the element. [1] (c) A nucleus emits a beta particle. State whether it becomes a different element. [1]

7. This question is about half-life. Background radiation can be ignored.

(a) Define the half-life of a particular isotope. [2] (b) The table shows the count rate from a sample of one isotope.

Time / minutes	0	10	20	30	40	50	60
Count rate / counts per minute	640	453	320	226	160	113	80

Use the table to find the half-life of the isotope. [2] (c) Predict the count rate at 80 minutes. [1] (d) A different isotope has a half-life of 5.0 hours. A sample contains 4.0 × 10¹² undecayed nuclei of this isotope. Calculate the number of these nuclei left undecayed after 15 hours. [2]

8. This question is about safety.

(a) State two effects of ionising nuclear radiation on living things. [2] (b) Describe three ways in which a radioactive source in a school laboratory is moved, used or stored safely. [3]

9. (Extended) In an alpha-particle scattering experiment, a beam of alpha particles is directed at a very thin sheet of metal in a vacuum. For each feature of the nuclear model below, describe the observation that provides evidence for it.

(a) The nucleus is very small and is surrounded by mostly empty space. [1] (b) The nucleus is positively charged. [1] (c) The nucleus contains most of the mass of the atom. [1]

10. (Extended) Polonium-210, ²¹⁰₈₄Po, decays by emitting an alpha particle and becomes an isotope of lead (Pb).

(a) State the relative charge and the relative mass of a polonium-210 nucleus. [2] (b) Suggest why this nucleus is unstable. [1] (c) Write the decay equation using nuclide notation. [2] (d) State how the stability of the nucleus changes as a result of the decay. [1]

11. (Extended) Cobalt-60, ⁶⁰₂₇Co, decays by emitting a beta particle and becomes an isotope of nickel (Ni). The new nickel nucleus then emits gamma radiation.

(a) Write the equation for the beta decay using nuclide notation. [2] (b) Describe the change that happens inside the nucleus during beta emission, and explain why this makes the nucleus more stable. [2] (c) State the effect of the gamma emission on the nucleon number and the proton number of the nickel nucleus, and state what the nucleus loses. [1]

12. (Extended) In a nuclear reactor, a nucleus of uranium-235, ²³⁵₉₂U, absorbs a neutron, ¹₀n. It splits into a nucleus of barium-141, ¹⁴¹₅₆Ba, a nucleus of krypton-92, ⁹²₃₆Kr, and some neutrons.

(a) Name this process. [1] (b) Work out the number of neutrons released, and write the complete nuclide equation. [2] (c) State how the total mass of the products compares with the total mass of the uranium nucleus and the neutron, and state what this tells you about energy. [2] (d) State one difference between nuclear fission and nuclear fusion. [1]

13. (Extended) This question is about the properties of alpha, beta and gamma radiation.

(a) Explain, with reference to kinetic energy and electric charge, why alpha particles are more strongly ionising than beta particles. [2] (b) A narrow beam containing alpha particles, beta particles and gamma rays travels, in a vacuum, into a uniform magnetic field at right angles to the field. Describe how the paths of the three types of radiation differ in the field. [3]

14. (Extended) A detector is placed near a radioactive source. The readings below include background radiation. With the source removed, the detector records a background count rate of 10 counts per minute.

Time / hours	0	4.0	8.0	12.0
Count rate / counts per minute	250	130	70	40

(a) Calculate the corrected count rate at time 0. [1] (b) Determine the half-life of the source. [2] (c) Many days later, the readings stop falling. State the count rate they approach and explain why. [2]

15. (Extended) The table gives information about five radioactive isotopes.

Isotope	Radiation emitted	Half-life
A	alpha	430 years
B	beta	29 years
C	gamma	6 hours
D	gamma	5.3 years
E	beta	15 hours

(a) State which isotope is best for monitoring and controlling the thickness of paper made in a factory. Explain your choice. [3] (b) State which isotope is best as a tracer injected into a patient to help diagnose cancer. Explain your choice. [3] (c) State which isotope is best for sterilising sealed packs of surgical equipment and for irradiating food to kill bacteria. Give one reason. [2] (d) State which isotope is suitable for a household smoke alarm. [1]

16. (Extended) A hospital technician prepares a gamma-emitting source for a patient. Explain how each precaution reduces the technician’s exposure to radiation.

(a) The source is handled with long tongs. [1] (b) When it is not in use, the source is kept in a thick lead container. [1] (c) The technician works quickly and returns the source to its container as soon as possible. [1]

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Answers

1. (a) A small, positively charged nucleus at the centre [1], with negatively charged electrons in orbit around it [1]. (b) (i) Positive ion [1]. (ii) Magnesium has proton number 12, so the neutral atom has 12 electrons; 12 − 2 = 10 electrons [1]. (c) It gains one or more electrons [1].

Common mistake: saying an atom becomes positive by gaining protons. Ions form when electrons are lost or gained; the nucleus does not change.

2. (a) Protons and neutrons [1]. (b) Proton +1, neutron 0, electron −1 [2] (all three correct for 2 marks; two correct for 1 mark). (c) Proton number (atomic number) Z is the number of protons in the nucleus [1]. Nucleon number (mass number) A is the number of protons plus neutrons in the nucleus [1]. (d) Neutrons = A − Z = 60 − 27 = 33 [1]. (e) Nucleon number A = 8 + 9 = 17, so the nucleus is ¹⁷₈O [1].

Common mistake: putting the numbers the wrong way round in nuclide notation. The nucleon number (the larger number) goes at the top; the proton number goes at the bottom.

3. (a) Isotopes are atoms of the same element, with the same number of protons (same proton number) [1], but different numbers of neutrons (different nucleon numbers) [1]. (b) Both nuclei have 17 protons; they differ in the number of neutrons [1]. Chlorine-35 has 35 − 17 = 18 neutrons and chlorine-37 has 37 − 17 = 20 neutrons [1].

Common mistake: saying isotopes have different numbers of protons or electrons. A different number of protons would make a different element.

4. (a) The ionising radiation that is always present around us, from natural and artificial sources, even when no source is being used [1]. (b) Any two of: radon gas in the air; rocks and buildings; food and drink; cosmic rays [1 each]. (c) A detector (for example a Geiger–Müller tube) connected to a counter [1]. (d) Count rate = 540 ÷ 3.0 = 180 counts/minute [1]; 180 ÷ 60 = 3.0 counts/s [1].

Common mistake: multiplying instead of dividing. A count rate is the number of counts divided by the time taken to record them.

5. (a) Spontaneous: the emission happens by itself; nothing outside the nucleus triggers it or changes when it happens [1]. Random in direction: the direction in which the radiation leaves the nucleus cannot be predicted [1]. (b) (i) Alpha: a helium nucleus (two protons and two neutrons) [1]. (ii) Beta: a fast-moving electron emitted from the nucleus [1]. (iii) Gamma: electromagnetic radiation [1]. (c) Alpha, beta, gamma (alpha most ionising, gamma least) [1]. (d) No alpha: the thin paper made no difference to the count rate, and paper would stop alpha [1]. Beta is emitted: 5 mm of aluminium reduced the count rate greatly (600 to 150), and a few mm of aluminium stops beta [1]. Gamma is emitted: radiation still passed through the aluminium, and the lead reduced it but did not stop it [1].

Common mistake: saying gamma is “stopped” by lead. Lead or thick concrete reduces gamma radiation a lot, but some still gets through.

6. (a) A change in an unstable nucleus [1] that can result in the emission of alpha or beta particles and/or gamma radiation; the change is spontaneous and random [1]. (b) The nucleus changes to that of a different element [1] (radium becomes radon). (c) Yes; in beta decay the nucleus changes to that of a different element [1] (its proton number changes, going up by 1).

Common mistake: thinking decay can be speeded up by heating or stirring the sample. Decay is spontaneous and random; outside conditions do not change it.

7. (a) The time taken for half the nuclei of that isotope in any sample [1] to decay [1]. (b) The count rate falls from 640 to 320 counts per minute in 20 minutes [1]. Check: it falls from 320 to 160 between 20 and 40 minutes, also 20 minutes. Half-life = 20 minutes [1]. (c) 80 minutes is one half-life after 60 minutes: 80 ÷ 2 = 40 counts per minute [1]. (d) Number of half-lives = 15 ÷ 5.0 = 3 [1]. 4.0 × 10¹² → 2.0 × 10¹² → 1.0 × 10¹² → 5.0 × 10¹¹ nuclei [1].

Common mistake: halving the time instead of the count rate. After each half-life the count rate (and the number of undecayed nuclei) halves; the half-life itself stays the same.

8. (a) Any two of: cell death; mutations; cancer [1 each]. (b) Any three of: moved and used with tongs, never held in the hand; kept in a lead-lined container when not in use; stored in a locked, labelled cabinet; pointed away from people when in use; used for the shortest possible time [1 each].

Common mistake: writing “wear gloves” as the main precaution. Gloves do not stop gamma or most beta radiation; tongs, shielding and short use are what reduce the dose.

9. (Extended) (a) Most alpha particles pass straight through the sheet without being deflected [1]. (b) Some alpha particles are deflected (repelled) as they pass close to a nucleus; the positive alpha particles are repelled, so the nucleus must be positively charged [1]. (c) A very few alpha particles bounce back (are deflected through more than 90°), which needs a nucleus with most of the atom’s mass [1].

Common mistake: giving the same observation for (b) and (c). Deflection shows repulsion by a positive charge; bouncing straight back shows the nucleus is much more massive than an alpha particle.

10. (Extended) (a) Relative charge +84 (equal to the proton number) [1]; relative mass 210 (equal to the nucleon number) [1]. (b) The nucleus is too heavy (it has too many nucleons to be stable) [1]. (c) ²¹⁰₈₄Po → ²⁰⁶₈₂Pb [1] + ⁴₂He (or ⁴₂α) [1]. Check: top 210 = 206 + 4; bottom 84 = 82 + 2. (d) The nucleus becomes more stable [1].

Common mistake: taking away 2 from the nucleon number and 4 from the proton number. An alpha particle carries away 4 nucleons and 2 protons, so A falls by 4 and Z falls by 2.

11. (Extended) (a) ⁶⁰₂₇Co → ⁶⁰₂₈Ni [1] + ⁰₋₁e (or ⁰₋₁β) [1]. Check: top 60 = 60 + 0; bottom 27 = 28 + (−1). (b) A neutron changes into a proton and an electron, and the electron is emitted as the beta particle [1]. This reduces the number of excess neutrons, so the nucleus is more stable [1]. (c) The nucleon number and proton number are both unchanged, and the nucleus loses energy [1].

Common mistake: lowering the proton number in beta decay. A neutron becomes a proton, so Z goes up by 1 and A stays the same.

12. (Extended) (a) Nuclear fission [1]. (b) Nucleon numbers: 235 + 1 = 141 + 92 + x, so x = 236 − 233 = 3 neutrons [1]. ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n [1]. Check: top 235 + 1 = 236 and 141 + 92 + 3 = 236; bottom 92 + 0 = 92 and 56 + 36 + 0 = 92. (c) The total mass of the products is slightly less than the total mass before [1]; the missing mass has been released as energy (mostly kinetic energy of the products) [1]. (d) Fission is the splitting of a large nucleus into smaller nuclei; fusion is the joining of light nuclei to form a heavier nucleus [1].

Common mistake: forgetting the neutron that is absorbed. Count it on the left-hand side, or the number of neutrons released comes out one too few.

13. (Extended) (a) An alpha particle has a larger charge (+2, against −1 for a beta particle), so it exerts larger forces on the electrons of the atoms it passes [1]. It also has a large kinetic energy and, being much more massive, moves more slowly, so it interacts with many atoms along a short path and transfers its kinetic energy by ionising them [1]. (b) Alpha and beta particles are deflected in opposite directions, because they have opposite charges [1]. Beta particles are deflected much more (their path curves more tightly), because their mass is much smaller [1]. Gamma rays pass straight through undeflected, because they have no charge [1].

Common mistake: saying alpha particles are deflected more because they have twice the charge. Their much larger mass matters more, so they are deflected less than beta particles.

14. (Extended) (a) 250 − 10 = 240 counts per minute [1]. (b) Corrected count rates: 240, 120, 60 and 30 counts per minute [1]. The corrected rate halves every 4.0 hours, so half-life = 4.0 hours [1]. (c) About 10 counts per minute [1]: the source’s activity becomes very small, so only the background radiation is still being detected [1].

Common mistake: using the readings as recorded. 250 to 130 is not a halving, and 130 to 70 is not either, so the uncorrected data give no consistent half-life.

15. (Extended) (a) B [1]. Beta radiation is partly absorbed by paper, so the count rate changes when the thickness changes (alpha would be stopped completely; gamma would pass through almost unchanged) [1]. Its long half-life means the count rate stays almost constant from day to day, so a change is due to the thickness, and the source does not need replacing often [1]. (b) C [1]. Gamma radiation penetrates the body, so it can be detected outside the patient [1]. The short half-life means the activity falls quickly, so the patient is exposed for only a short time, but it lasts long enough to carry out the test [1]. (c) D [1]. Gamma radiation penetrates the packaging and the food to kill bacteria; or its long half-life means the source lasts for years without frequent replacement [1]. (d) A [1] (alpha ionises the air in the detector but cannot escape the casing; the long half-life keeps the activity steady).

Common mistake: justifying a choice with only the type of radiation. Name the type of radiation and the half-life, and link each to the use.

16. (Extended) (a) It increases the distance between the source and the technician’s hands and body [1]. (b) The lead shields the technician by absorbing the radiation [1]. (c) It reduces the exposure time [1].

Common mistake: writing “lead stops all the radiation”. Thick lead absorbs most of the gamma radiation; shielding reduces the dose rather than removing it.

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Where marks are usually lost

• Defining isotopes by protons or electrons instead of neutrons.
• Putting the proton number at the top in nuclide notation.
• Dividing by the wrong quantity in a count-rate calculation.
• Halving the time instead of the count rate in a half-life problem.
• Changing the proton number the wrong way in a decay equation (Extended).
• Leaving background in the readings when finding a half-life (Extended).
• Saying alpha is deflected more than beta in a field (Extended).
• Choosing an isotope for a use from the radiation type alone, without the half-life (Extended).

Next steps

• Nuclear physics study guide and revision notes.
• Extended practice set on nuclear physics for six more Supplement questions.
• Free 0625 Core diagnostic or 0625 Extended diagnostic.
• Next topic: Space physics study guide.

Official syllabus

Cambridge International, Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028 (Version 2), Topic 5 Nuclear physics (sections 5.1–5.2).