OCR GCSE Physics A: Ionisation, half-life and penetrating power (J249) – 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 – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions follow the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0 (July 2026), for first assessment in 2018. They cover statements P6.1h to P6.1l. Statement P6.1k is Higher Tier only, so questions 6, 7, 8 and 12 are marked (Higher Tier only); the rest are assessed in both tiers. Study the study guide first or use the revision notes to refresh. Hub: OCR GCSE Physics. Checklist: printable checklist. Find weak spots with the free diagnostics.

Questions

1. An atom absorbs ultraviolet radiation and one electron moves to a higher energy level. Describe what happens to the electron, and what happens when the electron later loses that energy. [3]

2. Ionisation can happen to an atom.

(a) State what is meant by ionisation. [1] (b) State the overall charge on the particle that remains, and explain why it has that charge. [2]

3. A teacher shines a black light lamp on a glass of tonic water, and the liquid glows with visible light. Explain this in terms of electrons. [3]

4. Radiation is produced by changes in atoms and in nuclei.

(a) State the part of the electromagnetic spectrum that is produced by changes in a nucleus. [1] (b) State one other part of the spectrum that changes in atoms can produce or absorb. [1]

5. (a) Explain what is meant by the half-life of a radioactive isotope. [2] (b) A sample is heated from 20 °C to 400 °C. State and explain what happens to its half-life. [1]

6. (Higher Tier only) A source has an activity of 2880 Bq. Its half-life is 12 minutes.

(a) Calculate the activity after 1 hour. [3] (b) State the ratio of the original activity to the final activity. [1]

7. (Higher Tier only) A sample contains 5.6 × 10⁸ undecayed nuclei. Its half-life is 40 hours. Calculate (a) the number of undecayed nuclei after 120 hours, and (b) the fraction of the original nuclei that have decayed. [4]

8. (Higher Tier only) A detector measures a source of half-life 10 minutes. The background count rate is 30 counts per minute.

Time / min	0	10	20	30	40
Reading / counts per min	510	270	150	90	60

(a) Calculate the count rate due to the source alone at each time. [2] (b) Show from your results that the half-life is 10 minutes. [2] (c) Predict the reading at 50 minutes. [2]

9. Three unknown sources P, Q and R are tested with a Geiger-Müller tube. P is detected with no absorber at 1 cm but not with paper in place. Q is detected through paper but not through 5 mm of aluminium. R is still detected, at a lower rate, behind 2 cm of lead.

(a) Identify the radiation from each of P, Q and R. [3] (b) State which source is the most strongly ionising. [1] (c) Suggest why P might not be detected at all if the tube is moved 20 cm away. [1]

10. A student models decay with 128 coins. Each throw, all remaining coins are thrown and those landing tails are removed. Each throw represents one half-life.

(a) Predict the number of coins left after 4 throws. [2] (b) The student’s results are 128, 61, 33, 14, 9. Explain why they differ from the prediction. [2] (c) Explain what this shows about decay in a very small sample. [2]

11. A source gives out two kinds of radiation. A tube is used with aluminium plates between it and the source. The background is 30 counts per minute.

Aluminium thickness / mm	0	1	2	3	4	6
Reading / counts per min	1190	710	470	350	290	245

(a) Calculate the corrected count rates. [2] (b) Use your results to explain which two radiations are present. [3] (c) Estimate the count rate due to the gamma rays. [1] (d) Describe a further test to confirm the gamma rays. [2]

12. (Higher Tier only) A source and detector give a reading of 1030 counts per minute. The background is 30 counts per minute and the half-life of the source is 18 minutes.

(a) Calculate the number of half-lives in 54 minutes. [1] (b) Calculate the count rate from the source after 54 minutes. [2] (c) Calculate the reading on the detector at that time. [1] (d) A student says the source will have no activity after 36 minutes, because that is two half-lives. Explain the mistake and give the activity then. [2] (e) State the ratio of the count rate from the source after 54 minutes to the original. [1]

Answers

1. The electron absorbs energy and moves to a higher level [1]. It is excited but stays in the atom [1]. When it loses the energy it falls to a lower level and the energy is emitted as electromagnetic radiation [1]. [3] Examiner insight: Answers that mention both the rise and the emission of radiation tend to cover the full sequence; saying the electron leaves the atom describes ionisation instead.

2. (a) Ionisation is the loss of outer electrons from an atom [1]. (b) The particle has a positive charge [1] because it has lost negative electrons while the nuclear charge is unchanged [1]. [3] Examiner insight: A reason that mentions lost electrons tends to earn more than a bare charge; “it gained protons” is a common wrong reason.

3. The electrons absorb ultraviolet radiation from the lamp [1]. They are excited to a higher energy level [1]. When they fall back, they emit the energy as visible light [1]. [3] Examiner insight: Naming ultraviolet as absorbed and visible as emitted shows the change in the radiation; vague statements about “light going in” are less secure.

4. (a) Gamma rays [1]. (b) Any one of visible light, ultraviolet, infrared or X-rays [1]. [2] Examiner insight: Naming a particular part of the spectrum is clearer than writing “waves”; avoid listing alpha or beta, which are particles.

5. (a) Half-life is the time taken for the number of undecayed nuclei to halve [1], or equivalently the time for the activity to halve [1]. (b) It stays the same, because decay is random and not changed by temperature [1]. [3] Examiner insight: A definition that says “half of the sample decays” without a time loses clarity; include the time taken and what halves.

6. (a) Number of half-lives = 60 ÷ 12 = 5 [1]. 2⁵ = 32 [1]. Activity = 2880 ÷ 32 = 90 Bq [1]. (b) 2880 : 90 = 32 : 1 [1]. [4] Examiner insight: Showing the count of half-lives before dividing makes the working easy to follow; include the unit Bq.

7. (a) Number of half-lives = 120 ÷ 40 = 3 [1]. Nuclei left = 5.6 × 10⁸ ÷ 8 = 7.0 × 10⁷ [1]. (b) Fraction remaining is 1/8 [1], so the fraction decayed is 7/8 [1]. [4] Examiner insight: Keep to the quantity asked: the decayed fraction is one minus the remaining fraction, not the remaining fraction itself.

8. (a) Subtract 30 from each reading: 480, 240, 120, 60, 30 counts per minute [2]. (b) 480 falls to 240 in 10 minutes [1]. 240 falls to 120 in the next 10 minutes, so the time to halve is constant at 10 minutes [1]. (c) At 50 minutes, n = 5 and 480 ÷ 32 = 15 [1]. Add background: 45 counts per minute [1]. [6] Examiner insight: A “show that” answer tends to be strongest when it uses at least two intervals; finish by adding the background back for a prediction of a detector reading.

9. (a) P is alpha [1], Q is beta [1], R is gamma [1]. (b) P (alpha) [1]. (c) Alpha particles are absorbed by a few centimetres of air [1]. [5] Examiner insight: Link each absorber result to a radiation; saying “P is weakest” does not name the radiation.

10. (a) Each throw halves the number: 128, 64, 32, 16, 8 [1]. After 4 throws there are 8 coins [1]. (b) Each coin has a random chance of being removed [1], so the number removed in any throw varies about the expected half [1]. (c) With few nuclei or coins the random scatter is large compared with the total [1], so the pattern is a poor predictor and the half-life is less reliable [1]. [6] Examiner insight: Explanations that use the word random together with a reason tend to carry more weight than “human error”, which is not the cause of the scatter here.

11. (a) Subtract 30: 1160, 680, 440, 320, 260, 215 counts per minute [2]. (b) The corrected rate falls steeply at first, with the fall halving every millimetre (480, 240, 120, 60), which is typical of beta particles stopped by aluminium [1]. The rate then levels off near 200 rather than falling to zero [1], so a second radiation is present that aluminium does not stop: gamma rays [1]. (c) About 200 counts per minute (accept values close to the levelled-off figure) [1]. (d) Place a thick lead sheet in front of the tube [1]. The remaining count rate should fall, but not to background, showing that gamma rays are reduced by lead [1]. [8] Examiner insight: Strong answers quote figures from the table, such as the differences between readings, rather than writing only “it goes down then flattens”.

12. (a) 54 ÷ 18 = 3 [1]. (b) Source rate = 1030 − 30 = 1000 [1]. After 3 half-lives, 1000 ÷ 8 = 125 counts per minute [1]. (c) 125 + 30 = 155 counts per minute [1]. (d) After one half-life the count halves again, so after two half-lives one quarter remains [1]. Source rate = 1000 ÷ 4 = 250 counts per minute [1]. (e) 125 : 1000 = 1 : 8 [1]. [7] Examiner insight: Subtract background before halving and add it back at the end; the ratio asked in part (e) is source count to original source count, in that order.

Where marks are usually lost

• Dividing the whole detector reading by 2ⁿ, including the background.
• Treating the decayed fraction and the remaining fraction as the same thing.
• Writing a ratio the wrong way round.
• Omitting units such as Bq or counts per minute.
• Describing ionisation as an electron changing level.
• Saying heat or pressure affects the half-life.
• Saying lead stops gamma rays completely.
• Explaining scatter in a coin or dice model by “mistakes” instead of random behaviour.
• Reading half-life from only one interval on a table or graph.

Next steps

• Revision notes
• Study guide
• OCR GCSE Physics hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class: trial

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018. Statements P6.1h to P6.1l.