IGCSE Physics: Nuclear Physics (Extended) — Practice Questions (Cambridge 0625)

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: every question here is marked (Extended): each one is on Supplement content of the 0625 syllabus — alpha-particle scattering (5.1.1 Supplement 3), fusion nuclide equations (5.1.2 Supplement 6), decay equations and the change in the nucleus in beta decay (5.2.3 Supplement 4–5), half-life with background radiation (5.2.4 Supplement 2), deflection of radiation in an electric field (5.2.2 Supplement 3) and choosing an isotope for a smoke alarm (5.2.4 Supplement 3). Core candidates do not need them.

Related: Core and Extended practice questions on nuclear physics, study guide and revision notes.

After each answer there is a common mistake to avoid.

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Questions

1. (Extended) A beam of alpha particles is fired at a very thin sheet of gold in a vacuum. State what each of these observations shows about the structure of an atom. (a) Almost all of the alpha particles pass straight through the sheet. (b) A very small number are deflected through large angles, and a few bounce almost straight back. [3]

2. (Extended) Strontium-90, ⁹⁰₃₈Sr, decays by emitting a beta particle and becomes an isotope of yttrium (Y). (a) Write the decay equation using nuclide notation. (b) State the change that happens inside the nucleus during beta decay. [3]

3. (Extended) A detector placed near a radioactive source records 830 counts per minute. Six hours later it records 230 counts per minute. When the source is taken away, the detector records a background count rate of 30 counts per minute. Calculate the half-life of the source. [3]

4. (Extended) Alpha particles, beta particles and gamma rays pass, in a vacuum, into the space between two parallel metal plates. One plate is positively charged and the other is negatively charged. Describe the path of each type of radiation between the plates. [3]

5. (Extended) A household smoke alarm contains a radioactive isotope that emits alpha particles and has a half-life of about 430 years. Explain why (a) an alpha emitter is suitable, and (b) a long half-life is needed. [3]

6. (Extended) In a fusion experiment, a nucleus of hydrogen-2 (²₁H) joins with a nucleus of hydrogen-3 (³₁H) to form a nucleus of helium-4 (⁴₂He) and one other particle. (a) Write the nuclide equation for this reaction, identifying the other particle. (b) Describe how the total mass of the products compares with the total mass of the two hydrogen nuclei, and state what this tells you about energy in the reaction. [3]

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Answers

1. (Extended) (a) The atom is mostly empty space, with a very small nucleus [1]. (b) The nucleus is positively charged, so it repels the positive alpha particles [1], and it contains most of the mass of the atom, so it can turn an alpha particle back [1].

Common mistake: saying the alpha particles “hit the electrons”. Electrons are far too light to deflect an alpha particle through a large angle; the large deflections come from the small, massive, positive nucleus.

2. (Extended) (a) ⁹⁰₃₈Sr → ⁹⁰₃₉Y [1] + ⁰₋₁e (or ⁰₋₁β) [1]. (b) A neutron changes into a proton and an electron, and the electron is emitted as the beta particle [1].

Common mistake: lowering the proton number. In beta decay the nucleon number stays the same and the proton number goes up by one.

3. (Extended) Corrected count rates: 830 − 30 = 800 and 230 − 30 = 200 counts per minute [1]. 800 → 400 → 200 is two half-lives [1], so half-life = 6 ÷ 2 = 3.0 hours [1].

Common mistake: using the readings without subtracting the background. 830 → 230 is not a whole number of halvings, and it leads to a wrong half-life.

4. (Extended) Alpha particles are deflected towards the negative plate, because they are positively charged [1]. Beta particles are deflected towards the positive plate, and more strongly than the alpha particles 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 more charge. Alpha particles are about 7000 times as massive as beta particles, so they are deflected much less.

5. (Extended) (a) Alpha particles are strongly ionising, so they ionise the air in the alarm and allow a small current; smoke absorbs the alpha particles, the current falls and the alarm sounds [1]. They have a very short range and are stopped by the casing, so they do not reach people in the room [1]. (b) With a long half-life the activity stays almost constant over the working life of the alarm, so the source does not need to be replaced [1].

Common mistake: choosing gamma “because it goes further”. Smoke would not stop gamma rays, so the alarm would never notice any smoke, and the radiation would escape into the room.

6. (Extended) (a) ²₁H + ³₁H → ⁴₂He + ¹₀n; the other particle is a neutron [1]. (b) The total mass of the products is slightly less than the total mass of the two hydrogen nuclei [1]; this missing mass has been released as energy (mostly the kinetic energy of the products) [1].

Common mistake: saying mass is “used up” and energy is made from nothing. Mass and energy are linked: the small decrease in mass matches the energy released.

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Where marks are usually lost

• Linking the large deflections in alpha scattering to electrons instead of the nucleus.
• Changing the proton number the wrong way in a beta-decay equation.
• Finding a half-life without first subtracting the background count rate.
• Saying alpha particles are deflected more than beta particles in an electric field.
• Choosing a radiation for an application without explaining both its range and its half-life.