OCR GCSE Physics A: Ionisation, half-life and penetrating power (J249) – Revision Notes

These notes condense statements P6.1h to P6.1l of the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0 (July 2026), for first assessment in 2018. Statements P6.1h, P6.1i, P6.1j and P6.1l are assessed in both tiers. P6.1k is in bold in the specification and is Higher Tier only. For full explanations and longer worked examples, read the study guide. Test yourself with the practice questions, and tick off topics on the printable checklist. The OCR GCSE Physics hub lists every unit, and the free diagnostics show where to start.

Electron energy levels and ionisation (P6.1h)

• Electrons are arranged at different distances from the nucleus. Each arrangement is an energy level.
• Absorb electromagnetic radiation: the electron gains energy and rises to a higher level. It is excited.
• Emit radiation: the electron loses energy, falls to a lower level, and the energy leaves as electromagnetic radiation.
• Bigger fall in energy gives radiation of higher frequency.
• Ionisation: loss of outer electrons. The atom becomes a positive ion.
• Excitation keeps the electron in the atom. Ionisation removes it.
• Fluorescence demonstration: ultraviolet from a black light is absorbed by tonic water, which then emits visible light.

Radiation from atoms and nuclei (P6.1i)

Where the change happens	Radiation
Outer electrons changing level	Visible, ultraviolet, infrared
Inner electrons changing level	Higher-frequency radiation, such as X-rays
Nucleus	Gamma rays

• Changes in atoms and nuclei can both generate and absorb radiation.
• The range is wide. Gamma rays are part of the electromagnetic spectrum.

Half-life and randomness (P6.1j)

• Decay is random: you cannot say when one nucleus will decay, and outside conditions do not change this.
• A large sample behaves predictably, so we can use half-life.
• Half-life: time for the number of undecayed nuclei, or the activity, to halve.
• Each half-life halves what is left. It does not subtract a fixed amount.
• Dice or coins can model it: remove the coins that land tails after each throw. Results scatter around the halving pattern, more so with small numbers.

Net decline after whole half-lives (P6.1k, Higher Tier only)

Half-lives, n	Remaining	Original : remaining	Decayed
1	1/2	2 : 1	1/2
2	1/4	4 : 1	3/4
3	1/8	8 : 1	7/8
4	1/16	16 : 1	15/16
5	1/32	32 : 1	31/32
6	1/64	64 : 1	63/64

Method in steps

1. n = elapsed time ÷ half-life (a whole number in this unit).
2. Work out 2ⁿ.
3. Remaining = start ÷ 2ⁿ. Ratio, original to remaining, is 2ⁿ : 1.
4. With a detector, subtract background first and add it back at the end.

Worked reminder. Activity 1600 Bq, half-life 5 minutes, after 20 minutes: n = 4, 2⁴ = 16, remaining = 1600 ÷ 16 = 100 Bq. Ratio 16 : 1.

Reading half-life from a graph or table. On a decay curve the activity falls steeply, then more slowly, and never reaches zero. Pick a value, find the time at which it has fallen to half, and repeat from a different starting point. If the intervals agree, that is the half-life.

Penetration (P6.1l)

Radiation	Stopped or reduced by	Ionising ability
Alpha	Paper, or a few centimetres of air	Strongest
Beta	A few millimetres of aluminium	Medium
Gamma	Intensity reduced by thick lead or very thick concrete	Weakest

• Penetration increases from alpha to beta to gamma.
• To test a source: record the background, then take readings with no absorber, paper, aluminium of increasing thickness, and lead. Subtract background from each.
• Gamma is reduced but never fully stopped.

More worked reminders

Nuclei count. 8.0 × 10⁷ undecayed nuclei, half-life 6 days, after 18 days. n = 18 ÷ 6 = 3, so divide by 8: 1.0 × 10⁷ remain. Decayed: 8.0 × 10⁷ − 1.0 × 10⁷ = 7.0 × 10⁷.

Background. Reading 220 counts per minute, background 30, half-life 10 minutes, wait 30 minutes. Source: 220 − 30 = 190. n = 3, so 190 ÷ 8 = 23.75. Add background: 53.75, which rounds to 54 counts per minute.

Reading a table. Activity falls 960, 480, 240 at 0, 15 and 30 minutes. The time to halve is 15 minutes each time, so the half-life is 15 minutes.

Explaining half-life in words

A full-mark explanation usually contains three separate ideas.

1. A definition: the time for the number of undecayed nuclei, or the activity, to halve.
2. The link to randomness: each nucleus has an unpredictable moment of decay.
3. The statistical result: in a large sample the overall pattern is regular, so the half-life is constant.

If a question gives a number of half-lives, show the number of half-lives explicitly before you divide. A line such as “30 ÷ 7.5 = 4 half-lives” makes your method clear.

Linking the ideas

• Radiation can ionise atoms. Ionisation removes outer electrons and leaves positive ions.
• A particle that ionises a lot loses energy quickly, which is why strongly ionising alpha particles have the shortest range.
• Gamma rays are produced by the nucleus and form part of the electromagnetic spectrum, so they are related to the radiation produced by electron changes, just at a higher frequency.
• Half-life depends on the nucleus, not on the amount of material. A small sample and a large sample of the same material have the same half-life, but not the same activity.

Describing a penetration experiment

A reliable answer sets out the steps.

1. Measure the background count rate with no source nearby.
2. Place the source a fixed distance from the Geiger-Müller tube and record the count rate.
3. Put paper between the source and the tube, then record again.
4. Replace the paper with aluminium plates of increasing thickness and record each count rate.
5. Finish with a thick sheet of lead.
6. Subtract background from every reading before drawing conclusions.
7. Interpret the pattern: a fall with paper means alpha; a fall with aluminium means beta; a reading reduced only by lead means gamma.

Keep the distance fixed and take readings over the same time period each time, so the comparison is fair. Counting for a longer time reduces the effect of random variation.

Must-know distinctions

Pair	Difference
Excitation and ionisation	Electron moves up a level, or electron leaves the atom
Random and unpredictable	One nucleus cannot be predicted, a large sample can
Half-life and half the lifetime	Half-life halves what remains every time
Source count and detector reading	Detector reading includes background
Irradiated and radioactive	Radiation passing through an object does not make it radioactive

Quick self-test

Questions 4 to 8 use P6.1k (Higher Tier only).

1. What word describes an electron that has absorbed energy and moved to a higher level?
2. What is the process of removing outer electrons called, and what charge does the atom then have?
3. Which radiation from the nucleus is part of the electromagnetic spectrum?
4. A source has a half-life of 2.5 days. How many half-lives pass in 20 days, and what fraction of the nuclei is left?
5. A source has activity 1600 Bq and a half-life of 4 h. What is its activity after 16 h?
6. A sample has 6.4 × 10⁶ undecayed nuclei. How many remain after 5 half-lives?
7. What is the ratio of original to remaining nuclei after 3 half-lives, and what fraction has decayed?
8. A detector reads 120 counts per minute. Background is 20 counts per minute and the half-life is 1 h. What does it read 2 h later?
9. Which radiation is stopped by paper?
10. Why can’t you predict when one particular nucleus will decay?

Answers

1. Excited.
2. Ionisation. The atom becomes a positive ion.
3. Gamma rays.
4. 20 ÷ 2.5 = 8 half-lives. Fraction left = 1/2⁸ = 1/256.
5. n = 4, so 1600 ÷ 16 = 100 Bq.
6. 6.4 × 10⁶ ÷ 32 = 2.0 × 10⁵.
7. 8 : 1. Seven eighths have decayed.
8. Source part: 100 counts per minute. After 2 half-lives: 100 ÷ 4 = 25. Add background: 45 counts per minute.
9. Alpha particles.
10. Decay is random. Each nucleus has a chance of decaying, and nothing fixes when it happens.

Where marks are usually lost

• Giving a half-life answer as a subtraction rather than repeated halving.
• Leaving out the unit: Bq, counts per minute, or number of nuclei.
• Forgetting to subtract background before halving.
• Writing a ratio without saying which quantity comes first.
• Saying excitation and ionisation are the same thing.
• Claiming that gamma rays are completely stopped by lead.
• Saying that decay can be sped up by heating.
• Reading half-life from a single interval on a graph without checking another.
• Describing the dice model without saying what each throw represents.
• Saying “radiation makes things radioactive” in a safety answer.

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.