Radioactivity

Edexcel International GCSE Physics· Section 7.2–7.16 (4PH1 2017 Specification)· 25 min read

1. Atomic Structure, Nuclide Notation and Isotopes★★☆☆☆⏱ 5 min

Atoms consist of a central dense nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons in orbiting shells. The overall charge of a neutral atom is zero, as the number of protons equals the number of electrons.

📘 Definition

Isotope

Atoms of the same element with identical atomic (proton) numbers but different mass (nucleon) numbers, due to a different number of neutrons in the nucleus.

Example:

Carbon-12 and carbon-14 are isotopes of carbon, both with 6 protons but 6 and 8 neutrons respectively.

📐 Worked Example

Write the nuclide notation for a nitrogen atom with 7 protons and 8 neutrons. State if it is an isotope of nitrogen-14, which has 7 protons and 7 neutrons.

1
Atomic number (Z) = number of protons = 7, mass number (A) = protons + neutrons = 7 + 8 = 15
2
Nuclide notation is $_{7}^{15} N$
3
Isotopes have the same atomic number and different mass number, so nitrogen-15 is an isotope of nitrogen-14

✓ Quick check

An atom has atomic number 92 and mass number 238. How many neutrons does it contain?
- 92
- 146
- 238
- 330
Reveal answer
146 —
Number of neutrons = mass number - atomic number = 238 - 92 = 146

2. Types of Ionising Radiation: Properties and Detection★★★☆☆⏱ 6 min

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Unstable nuclei emit ionising radiation spontaneously and randomly, meaning you cannot predict which nucleus will decay next, and decay is unaffected by temperature, pressure or chemical state. Four main types of radiation are emitted: alpha, beta-minus, gamma and neutron.

Radiation Type	Nature	Ionisation Power	Penetration Power	Electric Field Deflection
Alpha (α)	Helium nucleus (2 protons, 2 neutrons)	Highest	Stopped by paper/skin/few cm of air	Deflected towards negative plate
Beta-minus (β⁻)	High-speed electron emitted when a neutron decays to a proton	Medium	Stopped by 2-3 mm of aluminium	Deflected towards positive plate
Gamma (γ)	High-frequency electromagnetic wave	Lowest	Reduced by thick lead or concrete	No deflection
Neutron	Neutral subatomic particle	Medium	Stopped by thick water or concrete	No deflection

📘 Definition

Becquerel (Bq)

Unit of radioactive activity, where 1 Bq = 1 nuclear decay per second.

📐 Worked Example

A student tests three unknown radioactive sources: Source A is stopped by a sheet of paper, Source B is stopped by 3mm aluminium but not paper, Source C is only partially blocked by 5cm of lead. Identify the radiation emitted by each source.

1
Source A is stopped by paper = alpha radiation
2
Source B is stopped by aluminium = beta-minus radiation
3
Source C is only reduced by thick lead = gamma radiation

Ionising radiation is detected using two common methods: photographic film (which darkens when exposed to radiation, used in worker dose badges) and Geiger-Müller (GM) detectors, which count individual decays and output activity in counts per second or minute.

3. Nuclear Decay Equations★★★★☆⏱ 6 min

When a nucleus emits radiation, its atomic and mass number change, but the total mass number and total atomic number are always conserved across the decay equation. The effect of each emission on the parent nucleus is:

Alpha emission: Mass number -4, atomic number -2
Beta-minus emission: Mass number unchanged, atomic number +1
Gamma emission: No change to mass or atomic number (only energy is lost)
Neutron emission: Mass number -1, atomic number unchanged

📐 Worked Example

Uranium-238 ( $_{92}^{238} U$ ) emits an alpha particle to form thorium (Th). Write the balanced nuclear equation for this decay.

1
An alpha particle is represented as $_{2}^{4} H e$
2
Conserve mass number: 238 = A(Th) + 4 → A(Th) = 234
3
Conserve atomic number: 92 = Z(Th) + 2 → Z(Th) = 90
4
Balanced equation: $_{92}^{238} U \to_{90}^{234} T h +_{2}^{4} H e$

4. Half-Life: Concepts and Calculations★★★☆☆⏱ 5 min

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📘 Definition

Half-life

The time taken for the activity of a radioactive sample to fall to half its original value, or for half of the unstable nuclei in a sample to decay. Half-life is unique to each radioactive isotope.

No decay formulas are provided for this qualification: all half-life calculations are done using successive halving of activity, or by reading values from a decay graph. Always subtract background count from measured source activity before doing calculations.

📐 Worked Example

A radioactive sample has an initial activity of 1600 Bq, and a half-life of 30 minutes. Calculate the activity of the sample after 2 hours.

1
Calculate number of half-lives passed: 2 hours = 120 minutes, so 120 / 30 = 4 half-lives
2
After 1 half-life: 1600 / 2 = 800 Bq
3
After 2 half-lives: 800 / 2 = 400 Bq
4
After 3 half-lives: 400 / 2 = 200 Bq
5
After 4 half-lives: 200 / 2 = 100 Bq
6
Final activity after 2 hours = 100 Bq

✓ Quick check

A sample's activity falls from 800 Bq to 100 Bq in 24 years. What is its half-life?
- 3 years
- 6 years
- 8 years
- 12 years
Reveal answer
8 years —
Number of halvings: 800 → 400 → 200 → 100 = 3 half-lives. 3 half-lives = 24 years, so 1 half-life = 24 / 3 = 8 years.

5. Radioactivity: Background Sources, Uses and Safety★★☆☆☆⏱ 4 min

Background radiation is low-level ionising radiation present everywhere on Earth, from both natural and man-made sources. Natural sources include radon gas from rocks, cosmic rays from space, and radioactive isotopes in soil, rocks and food. Man-made sources include medical x-rays, nuclear waste and fallout from nuclear weapons testing.

Industrial uses: Beta radiation for thickness gauging in paper/metal manufacturing, gamma radiation to detect leaks in underground pipes
Medical uses: Gamma-emitting tracers for diagnosis of organ function, gamma radiation for radiotherapy to kill cancer cells, gamma radiation to sterilise medical equipment

📐 Worked Example

A patient is given a drink containing a gamma-emitting tracer to test kidney function. Explain why gamma radiation is used for this purpose, and state if the patient is contaminated or irradiated.

1
Gamma radiation is weakly ionising, so it causes minimal cell damage to the patient
2
Gamma is highly penetrating, so it can be detected outside the patient's body using a GM detector
3
The tracer has a short half-life, so it decays quickly after the test, reducing long-term exposure risk
4
The patient is contaminated, as radioactive material is present inside their body

Ionising radiation causes damage to living cells: high doses kill cells immediately, while lower doses can cause DNA mutations leading to cancer. Radioactive waste is dangerous because many isotopes have very long half-lives; waste is stored in sealed, shielded containers deep underground to prevent leakage into the environment. Exposure risk is reduced by using lead shielding, keeping maximum distance from sources, using tongs to handle sources, and limiting exposure time.

6. Common Pitfalls

Wrong move:

Swapping mass and atomic number position in nuclide notation, putting atomic number on top

Why:

Nuclide notation convention strictly places mass number (A) top-left and atomic number (Z) bottom-left; examiners penalise this heavily in equation questions

Correct move:

Always write nuclides as $_{Z}^{A} X$ , and verify that total A and Z values balance on both sides of decay equations

Wrong move:

Assuming beta-minus decay reduces atomic number by 1

Why:

Beta-minus decay occurs when a neutron turns into a proton and emits an electron, so the number of protons (atomic number) increases by 1

Correct move:

Remember the beta particle is $_{- 1}^{0} e$ , so to balance charge the product atomic number is original Z + 1

Wrong move:

Using exponential decay formulas for half-life calculations

Why:

Exponential decay equations are out of scope for Edexcel IGCSE Physics, and all questions are designed to be solved with successive halving

Correct move:

Count the number of half-lives passed first, then divide the original activity by 2 once per half-life, or read values directly from the provided decay graph

Wrong move:

Stating that irradiation makes an object radioactive

Why:

Irradiation only exposes an object to external radiation, with no transfer of radioactive material, so the object does not become radioactive

Correct move:

Only contamination (physical presence of radioactive material on/in an object) makes it radioactive; always check for contact with the source in question

Wrong move:

Forgetting to subtract background count from measured activity

Why:

Background radiation contributes to the count measured by a GM detector, so unadjusted values give incorrect source activity leading to wrong half-life calculations

Correct move:

First measure background count with no source present, then subtract this value from all source activity measurements

Wrong move:

Stating alpha radiation is the most dangerous in all scenarios

Why:

Alpha radiation cannot penetrate skin, so it is only dangerous if ingested, inhaled or enters the body through a wound; gamma is more dangerous for external exposure

Correct move:

Contextualise your answer: alpha is most dangerous inside the body, gamma is most dangerous for external exposure

7. Quick Reference Cheatsheet

Key Concept	Quick Facts	Exam Check
Nuclide Notation	$_{Z}^{A} X$ : A = protons + neutrons, Z = protons; Isotopes = same Z, different A	Check A and Z totals balance on both sides of decay equations
Radiation Properties	α: most ionising, stopped by paper; β: medium, stopped by Al; γ: least ionising, stopped by lead	Ionisation power is reverse of penetration power
Decay Effects	α: A-4, Z-2; β⁻: A same, Z+1; γ: no change; n: A-1, Z same	Write all particle symbols correctly when balancing equations
Half-Life	Time for activity to halve; calculated via successive halving or graph reading	No exponential formulas allowed, count half-lives first
Safety	Contamination = radioactive material present; Irradiation = external radiation exposure	Mention both when explaining radiation risk questions
Units	Activity = Bq (1 decay/s), time = s/min/h, distance = cm	Always include correct units in final answers

8. Frequently Asked

What is the difference between atomic and mass number?

Atomic (proton) number is the number of protons in a nucleus, equal to the positive charge of the atom. Mass (nucleon) number is the total number of protons plus neutrons in the nucleus.

Do I get a decay formula on the Edexcel IGCSE Physics formula sheet?

No formula sheet is provided for Edexcel IGCSE Physics. All half-life calculations are done using successive halving of activity or reading decay graphs; exponential decay equations are out of scope for this qualification.

Does irradiation make an object radioactive?

No, irradiation only exposes an object to radiation from an external source, with no transfer of radioactive material. Only contamination (physical presence of radioactive material on/in an object) will make it radioactive.

Which radiation type is most dangerous?

Alpha radiation is most dangerous if ingested or inhaled, as it is highly ionising and damages nearby cells heavily. Gamma radiation is most dangerous for external exposure, as it can penetrate deep into the body through skin.

Going deeper

official_documentEdexcel 4PH1 2017 Specification
simulationPhET Radioactive Dating Game
worksheetRadioactivity Half-Life Practice Questions

What's Next

You have now mastered all core radioactivity content for Edexcel IGCSE Physics 4PH1, which makes up 8-10% of total exam marks, assessed in both Paper 1 and Paper 2. This content is frequently tested in practical questions on radiation penetration, data analysis questions on half-life graphs, and extended response questions on safety and uses. Next, you should move on to the second sub-topic in the Radioactivity and particles unit: fission, fusion and nuclear reactions, which builds on your understanding of nuclear structure. Be sure to practice past paper questions to reinforce your knowledge, and pay close attention to command terms like 'explain' which require structured, detailed answers to gain full marks.