Edexcel IGCSE Physics 7(b) Radioactivity — Notes
Written for Pearson Edexcel International GCSE Physics (4PH1), Science Double Award (4SD0) and Science Single Award (4SS0), linear and modular. This page also covers the 7(a) units for Topic 7.
Which parts do I need?
Single Award candidates can skip every section tagged Double · Physics. On this page that is: how each type of radiation changes the atomic and mass numbers (7.7), balancing nuclear equations (7.8), detecting radiation with photographic film or a Geiger–Müller tube (7.9), activity and the becquerel (7.11), and half-life calculations and graphs (7.13). Single Award still needs the definition of half-life and the idea that it differs between isotopes (7.12). Nothing on this page is Physics-only.
What this page covers
| Spec | Modular no. | You need to be able to… | Route |
|---|---|---|---|
| 7.1 | 8.1 | use Bq, cm, h, min and s | Single · Double · Physics |
| 7.2 | 8.2 | describe an atom as protons and neutrons in a nucleus with electrons around it, and read and write nuclide symbols | Single · Double · Physics |
| 7.3 | 8.3 | know what atomic number, mass number and isotope mean | Single · Double · Physics |
| 7.4 | 8.4 | know that unstable nuclei emit alpha, beta and gamma radiation at random, and that all three ionise | Single · Double · Physics |
| 7.5 | 8.5 | describe what alpha, beta and gamma are and compare how far they penetrate and how strongly they ionise | Single · Double · Physics |
| 7.6 | 8.6 | investigate penetrating power with real sources or a simulation (practical) | Single · Double · Physics |
| 7.7 | 8.7 | describe how alpha, beta, gamma and neutron emission change the atomic and mass numbers | Double · Physics |
| 7.8 | 8.8 | balance nuclear equations for mass and charge | Double · Physics |
| 7.9 | 8.9 | know that photographic film and a Geiger–Müller tube detect ionising radiation | Double · Physics |
| 7.10 | 8.10 | explain where background radiation comes from, on Earth and from space | Single · Double · Physics |
| 7.11 | 8.11 | know that a source's activity falls with time and is measured in becquerels | Double · Physics |
| 7.12 | 8.12 | define half-life and know that each isotope has its own value | Single · Double · Physics |
| 7.13 | 8.13 | do simple half-life calculations on activity, including from a graph | Double · Physics |
| 7.14 | 8.14 | describe medical and industrial uses of radioactivity | Single · Double · Physics |
| 7.15 | 8.15 | describe how contamination differs from irradiation | Single · Double · Physics |
| 7.16 | 8.16 | describe the dangers of ionising radiation: mutations, cell and tissue damage, and the disposal of radioactive waste | Single · Double · Physics |
Modular candidates: Topic 7 is Topic 8 in the modular specifications. The modular number is the same in 4XPH1 Unit 2 (4WPH2) and 4XSD1 Physics Unit 6 (4WSD6).
In one minute
- An atom has a tiny nucleus of protons and neutrons, with electrons around it. In \({}^{14}_{6}\text{C}\), 14 is the mass number and 6 the atomic number, so there are 8 neutrons.
- Isotopes of an element have the same number of protons but different numbers of neutrons.
- Unstable nuclei emit alpha (a helium nucleus), beta (a fast electron) or gamma (an electromagnetic wave) at random. Alpha ionises most and penetrates least; gamma ionises least and penetrates most.
- Half-life is the time for half the unstable nuclei in a sample to decay; each isotope has its own half-life.
- Alpha emission lowers the mass number by 4 and the atomic number by 2; beta emission raises the atomic number by 1. Every nuclear equation balances top and bottom. Double · Physics
- Ionising radiation can damage cells and cause mutations. Contamination means the radioactive material is on or in you; irradiation means you are only exposed to its radiation.
Which units do I need for Topic 7? Single · Double · Physics
| Quantity | Unit | Symbol | Route |
|---|---|---|---|
| activity of a source | becquerel | Bq | All |
| range of radiation, thickness of an absorber | centimetre | cm | All |
| time, half-life | second, minute, hour | s, min, h | All |
- 1 Bq is one nucleus decaying per second.
- Half-lives range from fractions of a second to billions of years, so they are often given in min, h, days or years. Keep the time and the half-life in the same unit before you divide one by the other: 1 h = 60 min = 3600 s.
- A Geiger–Müller counter usually gives a count rate in counts per second or counts per minute. A count rate is not the same as the activity, because the detector catches only some of the radiation emitted.
What is inside an atom, and what do nuclide symbols mean? Single · Double · Physics
An atom has a very small, dense, positively charged nucleus at its centre, containing protons (positive) and neutrons (no charge). Negatively charged electrons move around the nucleus. A neutral atom has the same number of electrons as protons.
- Number of neutrons = mass number − atomic number = \(A-Z\).
- The atomic number fixes which element it is: every carbon nucleus has 6 protons. \({}^{12}_{6}\text{C}\) and \({}^{14}_{6}\text{C}\) are isotopes of carbon.
- Isotopes have the same chemistry, but some are unstable (radioactive) while others are stable.
- An isotope is often named by its mass number alone: carbon-14 or C-14.
Cobalt has two isotopes, \({}^{59}_{27}\text{Co}\) and \({}^{60}_{27}\text{Co}\).
(a) State the number of protons and the number of neutrons in each nucleus. (b) State the number of electrons in a neutral atom of cobalt-60. (c) Explain why these are isotopes of the same element.
(a) Both have 27 protons. Cobalt-59 has 59 − 27 = 32 neutrons; cobalt-60 has 60 − 27 = 33 neutrons.
(b) 27 electrons, the same as the number of protons.
(c) They have the same number of protons (so the same element) but a different number of neutrons.
What are alpha, beta and gamma radiation? Single · Double · Physics
Some nuclei are unstable. To become more stable they emit radiation; this is radioactive decay. Decay is random: you cannot predict which nucleus will decay next, or when. You can only say what fraction of a large number of nuclei will decay in a given time. Because of this, repeated counts from the same source vary.
Alpha, beta and gamma are all ionising radiations: they knock electrons out of the atoms they pass, turning them into ions.
| Alpha (α) | Beta (β⁻) | Gamma (γ) | |
|---|---|---|---|
| What it is | a helium nucleus: 2 protons + 2 neutrons | a high-speed electron emitted from the nucleus | a high-frequency electromagnetic wave |
| Charge | +2 | −1 | 0 |
| Mass | large (4 nucleons) | very small | none |
| Ionising ability | strongly ionising | moderately ionising | weakly ionising |
| Penetrating power | low: stopped by a sheet of paper or a few cm of air | medium: stopped by a few mm of aluminium | high: reduced by several cm of lead or thick concrete |
- Ionising and penetrating are linked: alpha ionises strongly, so it loses its energy very quickly and travels only a short distance. Gamma ionises weakly, so it loses energy slowly and travels much further.
- Alpha is strongly ionising because it is large and has a charge of +2, so it interacts with many atoms along its path.
- A beta particle is an electron, but it comes from the nucleus, not from the electrons around it.
Practical: how do you investigate the penetrating power of radiation? Single · Double · Physics
The specification lets you carry out this practical with real sealed sources (normally a teacher demonstration) or with a simulation. Questions ask you to describe the method, identify the radiation from results, and explain the safety precautions.
Identifying the radiation from a source by what stops it
- Aim: find which absorber stops the radiation from a source, and so identify the type of radiation it emits.
- Apparatus: sealed radioactive source in a holder, a Geiger–Müller tube and counter (the detector), a stopwatch or timer, sheets of paper, aluminium sheets about 1–5 mm thick, lead sheets, a metre rule, long-handled tongs. (Or a simulation with the same choices.)
- Independent variable: the absorber material (none, paper, aluminium, lead), and its thickness.
- Dependent variable: the count rate measured by the detector.
- Control variables: the distance from the source to the detector, the same source, the same counting time.
Method
- With no source in the room, count for 5 minutes and work out the background count rate in counts per minute.
- Place the source a fixed short distance (for example 3 cm) from the detector, with no absorber. Measure the count rate.
- Place a sheet of paper between the source and the detector. Measure the count rate again.
- Replace the paper with 3 mm of aluminium, then with 2 cm of lead, measuring the count rate each time.
- Repeat each count and take a mean, because decay is random. Subtract the background count rate from every reading to get the corrected count rate.
Processing — a sample set of results
| Absorber | Count rate / counts per min | Corrected count rate / counts per min |
|---|---|---|
| none | 1825 | 1800 |
| paper | 1830 | 1805 |
| 3 mm aluminium | 26 | 1 |
| 2 cm lead | 24 | −1 |
Background = 25 counts per min. Paper makes no real difference (1800 and 1805 differ only by random variation), so there is no alpha. Aluminium reduces the count to background, so the source emits beta. If gamma were present, the count with aluminium would stay well above background.
Errors and improvements
- Decay is random, so short counts vary a lot. Count for longer and repeat, then take a mean.
- Alpha has a range of only a few cm in air; keep the source close to the detector, or alpha could be stopped by the air and wrongly appear absent.
- A small corrected count (like −1 above) is just random variation about zero — treat it as zero.
- Safety: handle the source only with long tongs, never with hands; hold it at arm's length and point it away from people; keep exposure time short; return it to its lead-lined box straight after use; wash hands afterwards. With a simulation there is no hazard, but you should still be able to state these precautions.
Single Award: you do not need to learn about Geiger–Müller tubes as a separate statement (7.9 is Double · Physics), but you should know that a detector and counter measure the count rate in this practical.
How does radioactive decay change a nucleus, and how do you balance nuclear equations? Double · Physics
Not required for Single Award (4SS0).
When a nucleus emits a particle, its atomic number and mass number may change — and if the atomic number changes, it becomes a different element. Gamma emission and neutron emission do not change the element.
| Emission | Symbol | Mass number A | Atomic number Z | New element? |
|---|---|---|---|---|
| alpha (α) | \({}^{4}_{2}\text{He}\) or \({}^{4}_{2}\alpha\) | falls by 4 | falls by 2 | yes |
| beta (β⁻) | \({}^{\ 0}_{-1}\text{e}\) or \({}^{\ 0}_{-1}\beta\) | unchanged | rises by 1 | yes |
| gamma (γ) | \({}^{0}_{0}\gamma\) | unchanged | unchanged | no |
| neutron (n) | \({}^{1}_{0}\text{n}\) | falls by 1 | unchanged | no — a different isotope of the same element |
- In beta decay a neutron in the nucleus changes into a proton and an electron; the electron is emitted. That is why Z rises by 1 while A stays the same.
- Gamma is often emitted straight after an alpha or beta decay, as the new nucleus loses extra energy.
- Balancing rule: the total of the top numbers (mass numbers) is the same on both sides of the arrow, and so is the total of the bottom numbers (charge numbers). The electron's −1 counts as a bottom number.
Complete each nuclear equation and name the radiation emitted.
(a) \({}^{238}_{\ 92}\text{U}\rightarrow{}^{a}_{b}\text{Th}+{}^{4}_{2}\text{He}\) (b) \({}^{90}_{38}\text{Sr}\rightarrow{}^{c}_{d}\text{Y}+{}^{\ 0}_{-1}\text{e}\)
(c) \({}^{13}_{\ 4}\text{Be}\rightarrow{}^{12}_{\ 4}\text{Be}+X\)
(d) \({}^{99}_{43}\text{Tc}\rightarrow{}^{99}_{43}\text{Tc}+Y\) (the first Tc nucleus has extra energy)
(e) Bismuth-212 (\({}^{212}_{\ 83}\text{Bi}\)) emits a beta particle, and the new nucleus then emits an alpha particle. Deduce the atomic number and mass number of the final nucleus.
(a) top: 238 = a + 4, so a = 234; bottom: 92 = b + 2, so b = 90. \({}^{234}_{\ 90}\text{Th}\); alpha decay.
(b) top: 90 = c + 0, so c = 90; bottom: 38 = d − 1, so d = 39. \({}^{90}_{39}\text{Y}\); beta decay.
(c) A falls by 1 and Z is unchanged, so X = \({}^{1}_{0}\text{n}\), a neutron.
(d) Nothing changes, so Y = \({}^{0}_{0}\gamma\), a gamma ray.
(e) Beta: Z 83 → 84, A stays 212. Alpha: Z 84 → 82, A 212 → 208. Final nucleus: Z = 82, A = 208 (an isotope of lead).
Check every answer: add the top numbers on each side, then the bottom numbers. Both totals must match.
How can ionising radiation be detected? Double · Physics
Not required for Single Award (4SS0).
- Geiger–Müller (GM) tube: radiation entering the tube ionises the gas inside, producing a pulse of current. A counter connected to it counts the pulses, giving a count or count rate.
- Photographic film: radiation darkens (fogs) the film when it is developed. The more radiation, the darker the film. Workers who handle radioactive materials can wear film badges; small windows of different materials over the film show which types of radiation reached them, and how much.
Where does background radiation come from? Single · Double · Physics
| Source | Why it gives off radiation |
|---|---|
| radon gas | a radioactive gas released from rocks and soil (especially granite); it collects in buildings and is breathed in |
| rocks, soil and building materials | contain radioactive isotopes such as uranium, thorium and potassium-40 |
| food and drink | contain small amounts of radioactive isotopes (for example potassium-40 and carbon-14), which end up inside our bodies |
| cosmic rays (space) | high-energy radiation from the Sun and from outer space; the atmosphere absorbs much of it, so the dose is greater at high altitude and when flying |
| artificial sources | medical X-rays and tracers, and small amounts from nuclear power and past weapons tests |
- Most background radiation is natural, and radon is usually the largest single part.
- The amount varies from place to place, depending on the local rocks and the altitude.
- In any experiment, measure the background count rate first and subtract it from each reading.
What is half-life? Single · Double · Physics
- Every radioactive isotope has its own half-life, from a tiny fraction of a second to billions of years. It cannot be changed by heating or by chemical reactions.
- Decay of any one nucleus is random, but with billions of nuclei the half-life is predictable.
- An isotope with a short half-life decays quickly and is very active at first, but soon becomes much less active. One with a long half-life stays radioactive for a long time.
- Be precise in a definition: “half the nuclei decay” or “the activity halves”. “Half the mass” or “half the substance disappears” is not credited — the decayed nuclei turn into other nuclei, so the mass hardly changes.
Activity Double · Physics
- As nuclei decay, fewer unstable nuclei are left, so the activity of a source decreases over time.
- Each half-life the activity halves: after 1, 2, 3, 4 half-lives it is ½, ¼, ⅛, 1/16 of the starting activity.
How do you do half-life calculations and use decay graphs? Double · Physics
Not required for Single Award (4SS0).
- Halving method: keep halving the activity, one half-life per step, and count the steps. For example, 1200 Bq → 600 → 300 → 150 Bq in 24 h is 3 half-lives, so the half-life is 24 ÷ 3 = 8.0 h. Write the steps out — the working can still earn credit.
- From a graph: read the starting activity, find half of it on the vertical axis, go across to the curve and down to the time axis. Repeat from a different starting point and take an average for a more reliable value.
- Count rates: subtract the background count rate before halving, then add it back if the question asks for the measured reading.
| Number of half-lives | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|
| Fraction of original activity left | 1 | ½ | ¼ | ⅛ | 1/16 |
The graph shows how the activity of a radioactive sample changes with time.
(a) Use the graph to determine the half-life. (b) Calculate the activity after 150 minutes. (c) Determine how long it takes for the activity to fall to 100 Bq.
(a) 800 Bq → 400 Bq takes 30 min; 400 Bq → 200 Bq takes another 30 min. Half-life = 30 min.
(b) 150 ÷ 30 = 5 half-lives: 800 → 400 → 200 → 100 → 50 → 25 Bq (that is 1/32 of the original).
(c) 800 → 400 → 200 → 100 is 3 half-lives: 3 × 30 = 90 min.
A student measures the count rate near a source as 990 counts per minute. The background count rate is 30 counts per minute. The isotope has a half-life of 2.5 hours.
(a) Calculate the count rate the student would measure 10 hours later. (b) Calculate the time taken for the corrected count rate to fall to 120 counts per minute. (c) Explain why the count rate measured after 10 hours is not 1/16 of 990 counts per minute.
(a) Corrected count rate = 990 − 30 = 960 counts per minute. 10 h ÷ 2.5 h = 4 half-lives: 960 → 480 → 240 → 120 → 60.
Measured count rate = 60 + 30 = 90 counts per minute
(b) 960 → 480 → 240 → 120 is 3 half-lives: 3 × 2.5 = 7.5 h
(c) Only the radiation from the source halves each half-life. The background (30 counts per minute) stays the same, so it must be removed before halving and added back afterwards.
Common error: halving 990 four times to get 62 counts per minute, which ignores the background.
How is radioactivity used in medicine and industry? Single · Double · Physics
To choose an isotope for a use, ask two questions: which radiation does it need to emit (how far must it penetrate?) and what half-life does it need (how long must it stay active?).
| Use | How it works | Radiation and half-life |
|---|---|---|
| medical tracer | a small amount is injected or swallowed; a detector outside the body tracks where it goes, showing how an organ is working | gamma (passes out of the body, weakly ionising); half-life of a few hours, so it lasts long enough for the scan but soon decays |
| radiotherapy | beams of gamma rays from several directions meet at a tumour and kill the cancer cells | gamma; a long half-life, so the source does not need replacing often |
| sterilising medical equipment and food | gamma rays kill bacteria, even through sealed packaging | gamma; long half-life |
| thickness control (paper, foil) | the amount of beta passing through the sheet is measured; if it rises, the sheet is too thin and the rollers are adjusted | beta (partly absorbed, so small changes in thickness change the count); long half-life so the count does not drop on its own |
| leak detection in pipes | a tracer is added to the liquid; a high count rate above the ground shows where it is leaking out | gamma (passes through soil); short half-life |
| smoke detectors | alpha ionises air so a small current flows; smoke absorbs the alpha, the current falls and the alarm sounds | alpha; long half-life (years) |
A factory makes cardboard sheets about 2 mm thick. A source and a detector are placed on opposite sides of the sheet to monitor its thickness. Three sources are available: an alpha emitter with a half-life of 430 years, a beta emitter with a half-life of 29 years, and a gamma emitter with a half-life of 6 hours.
Explain which source should be used.
Use the beta emitter.
- Alpha would be stopped completely by the cardboard, so the count would be zero whatever the thickness.
- Gamma would pass through almost unchanged, so the count would hardly change when the thickness changes.
- Beta is partly absorbed, so the count rate changes noticeably when the thickness changes.
- A long half-life (29 years) means the activity stays almost constant over months, so any change in count rate is caused by the thickness, not by decay.
What is the difference between contamination and irradiation, and why is radiation dangerous? Single · Double · Physics
| Irradiation | Contamination | |
|---|---|---|
| Where is the source? | outside the body | on the skin or inside the body (breathed in, swallowed) |
| When does exposure stop? | as soon as you move away or the source is shielded | it continues until the material is removed or decays |
| Does the person become radioactive? | no | yes — they carry radioactive material |
| Most dangerous radiation | gamma and beta (they reach the body); alpha is stopped by air and the outer skin | alpha — inside the body it ionises the cells right next to it |
Dangers of ionising radiation
- Cell and tissue damage: ionisation damages molecules in cells. A large dose can kill cells and cause burns or radiation sickness.
- Mutations: radiation can damage DNA, causing mutations. These can make cells divide uncontrollably (cancer), and mutations in sex cells can be passed to offspring.
- Reducing the risk to people: keep exposure time short, keep your distance (use tongs), use shielding (lead, concrete), and wear a film badge to monitor the dose.
Radioactive waste
- Waste from power stations, hospitals and industry can stay dangerously radioactive for thousands of years because some isotopes have very long half-lives.
- It must not leak into water supplies or the food chain, and it must be kept secure.
- Reducing the risks: seal it in thick containers (for example in glass and steel, inside concrete), store it in secure, monitored sites, and bury high-level waste deep underground in geologically stable rock. Low-level waste with short half-lives can be stored until its activity has fallen to a safe level.
Exam tips for 7(b)
When you complete a nuclear equation, check the mass numbers and the atomic numbers separately. Many candidates remember that alpha lowers Z by 2 but forget the top number, or write the beta particle without its −1.
The definition needs “half the unstable nuclei decay” or “activity falls to half”, plus “the time taken”. Saying “half the substance” or “half the mass” does not earn the mark.
When a question tells you a tracer has a shorter half-life, do not just repeat that fact. Say what it means: it stays active in the patient for less time, so the patient receives a smaller dose.
“Which source and why?” questions normally carry one mark for the type of radiation (linked to penetration) and one for the half-life (linked to how long it must last). Give both, each with its reason.
If a background count is given, subtract it before you halve and add it back only if the question asks for the measured reading.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| After two half-lives the sample has completely decayed. | Each half-life halves what is left, not the original amount. | After two half-lives, ¼ of the unstable nuclei remain. |
| A beta particle is one of the atom's orbiting electrons. | Beta is created in the nucleus when a neutron changes into a proton. | Beta is a fast electron emitted from the nucleus. |
| Being irradiated makes you radioactive. | Irradiation leaves no radioactive material behind. | Only contamination makes a person or object radioactive. |
| Alpha is the most dangerous radiation in every situation. | Outside the body it is stopped by air or skin. | Alpha is most dangerous inside the body; gamma and beta are the danger from outside. |
| Thick lead stops all gamma radiation. | Gamma intensity is only reduced by absorbers. | Lead or thick concrete greatly reduces gamma but never removes it completely. |
Key facts
| Fact | Route |
|---|---|
| number of neutrons = mass number − atomic number | All |
| isotopes: same proton number, different neutron number | All |
| ionising: alpha > beta > gamma; penetrating: gamma > beta > alpha | All |
| half-life: time for half the unstable nuclei to decay (activity to halve) | All |
| alpha: A − 4, Z − 2; beta: Z + 1; gamma: no change; neutron: A − 1 | Double · Physics |
| activity in Bq = decays per second; it falls over time | Double · Physics |
| after n half-lives the activity is the starting value halved n times | Double · Physics |
There are no Topic 7 relationships on the Equation List — half-life calculations use repeated halving.
Check your understanding
1. State the number of protons, neutrons and electrons in a neutral atom of \({}^{235}_{\ 92}\text{U}\). All
92 protons, 235 − 92 = 143 neutrons, 92 electrons.
2. Which radiation is stopped by a sheet of paper? Explain why it has such a short range. All
Alpha. It is strongly ionising (large, charge +2), so it loses its energy quickly as it ionises the atoms it passes.
3. A patient is given a gamma-emitting tracer by injection. A nurse stands near the patient. Explain which of them is contaminated and which is irradiated. All
The patient is contaminated — the radioactive material is inside their body. The nurse is irradiated — exposed to the gamma rays but has no radioactive material on or in them.
4. Polonium-210 (\({}^{210}_{\ 84}\text{Po}\)) decays by alpha emission to lead (Pb). Write the balanced equation. Double · Physics
\({}^{210}_{\ 84}\text{Po}\rightarrow{}^{206}_{\ 82}\text{Pb}+{}^{4}_{2}\text{He}\). Check: 210 = 206 + 4 and 84 = 82 + 2.
5. A source has an activity of 640 Bq and a half-life of 3.0 days. Calculate its activity after 12 days. Double · Physics
12 ÷ 3.0 = 4 half-lives: 640 → 320 → 160 → 80 → 40 Bq.
6. Cobalt-60 has a half-life of 5.3 years. How long does it take for the activity of a cobalt-60 source to fall to ⅛ of its starting value? Double · Physics
⅛ = three halvings, so 3 × 5.3 = 15.9 years (about 16 years).
7. Suggest why a medical tracer should have a half-life of a few hours rather than a few seconds or many years. All
A few seconds: it would decay before the scan could be done. Many years: it would stay active in the body for a long time, giving the patient an unnecessary dose of radiation. A few hours is long enough for the scan, then it decays quickly.
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