Edexcel IGCSE Physics: Radioactivity and Particles — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Radioactivity revision notes

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Questions

1. Complete the table for alpha, beta and gamma radiation: nature, charge, stopped by, ionising power. [6]

2. Explain the relationship between ionising power and penetrating power. [3]

3. Complete these nuclear equations, giving the mass and atomic numbers of the missing nuclide:

(a) ²³⁸₉₂U → ? + ⁴₂He [2] (b) ¹⁴₆C → ? + ⁰₋₁e [2]

4. Explain, in terms of the nucleus, why the atomic number increases in beta-minus decay while the mass number does not. [2]

5. A source has a half-life of 12 hours. Its initial count rate is 640 counts per minute. Background is 20 counts per minute.

(a) State the corrected initial count rate. [1] (b) Calculate the corrected count rate after 36 hours. [3] (c) State the measured reading the detector would show at that time. [1] (d) Explain why background must be subtracted. [2]

6. Choose the most suitable source for each use and justify:

(a) monitoring the thickness of aluminium foil [3] (b) a smoke detector [2] (c) a medical tracer [3]

7. State three ways of reducing exposure when handling radioactive sources. [3]

8. Describe nuclear fission, including how a chain reaction can occur. [3]

9. State the roles of the control rods and the moderator in a nuclear reactor. [2]

10. Describe nuclear fusion, and explain why it does not occur under everyday conditions on Earth. [3]

11. Distinguish between contamination and irradiation, including which is an ongoing risk after the source is removed. [3]

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Answers

1. Alpha — helium nucleus [1], charge +2, stopped by paper, strongly ionising [1]. Beta — fast electron [1], charge −1, stopped by ~3 mm aluminium, moderately ionising [1]. Gamma — electromagnetic wave [1], charge 0, reduced by thick lead, weakly ionising [1].

2. They are inversely related [1]. Strongly ionising radiation loses energy rapidly through many ionising interactions [1], so it cannot penetrate far; weakly ionising radiation interacts rarely and travels much further [1].

3. (a) ²³⁴₉₀Th — mass number 234 [1], atomic number 90 [1]. (b) ¹⁴₇N — mass number 14 [1], atomic number 7 [1].

4. A neutron changes into a proton and an electron [1]. The proton number rises by one while the total number of nucleons is unchanged, since a neutron has simply been replaced by a proton [1].

5. (a) 640 − 20 = 620 counts per minute [1]. (b) 36 ÷ 12 = 3 half-lives [1] 620 ÷ 2³ = 620 ÷ 8 [1] = 77.5 counts per minute [1]. (c) 77.5 + 20 = 97.5 counts per minute [1]. (d) Background radiation is always present from rocks, cosmic rays and food [1], so the measured reading includes it; only the corrected rate follows the exponential decay of the source [1].

6. (a) Beta [1]. Alpha would be completely absorbed by the foil and gamma would pass almost entirely through [1], so neither would respond to thickness changes; beta is partly absorbed, so the count rate varies with thickness [1]. (b) Alpha [1] — it is strongly ionising so it ionises the air in the detector, but is safely stopped within the device [1]. (c) Gamma, with a short half-life [1]. Gamma penetrates the body so it can be detected externally [1], and a short half-life ensures the activity falls quickly, limiting the patient’s exposure [1].

7. Any three: minimise the time of exposure [1]; maximise the distance from the source [1]; use shielding such as lead [1]; handle with tongs; store in a lead-lined container.

8. A heavy nucleus (e.g. uranium-235) splits into two smaller daughter nuclei plus several neutrons [1], releasing energy as kinetic energy of the products [1]. If the released neutrons strike further U-235 nuclei, a chain reaction results [1] — each fission event triggering further fission events in a self-sustaining process.

9. Control rods absorb excess neutrons to regulate the reaction rate [1]. The moderator slows fast neutrons to speeds more likely to cause further fission [1].

10. Fusion combines smaller nuclei into a larger one, with a loss of mass converted into a release of energy — the process powering stars, including the Sun [1] [1]. It requires extremely high temperature and pressure to overcome the electrostatic repulsion between positively charged nuclei, conditions not present on Earth under everyday circumstances [1].

11. Contamination is when radioactive material is transferred onto or into an object or person, remaining an ongoing risk even after the original source is removed [1]. Irradiation is exposure to radiation from an external source without any material being transferred [1] — the risk stops as soon as the source is removed or the person moves away [1].

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Where marks are usually lost

• Saying alpha is most penetrating because it is most ionising.
• Forgetting to subtract background before a half-life calculation.
• Not balancing both the mass number and the atomic number.
• Choosing a long half-life for a medical tracer.
• Confusing the roles of control rods (absorb neutrons) and the moderator (slows neutrons) in a reactor.
• Describing fusion without mentioning the need to overcome electrostatic repulsion between nuclei.
• Confusing contamination (material transferred, ongoing risk) with irradiation (external exposure, stops when the source is removed).