# Radioactivity

> Edexcel International GCSE Physics · 4PH1 (2017)
> Source: https://www.owlsprep.com/study/edexcel-igcse-physics-s7-radioactivity/

This guide covers all core Edexcel IGCSE Physics 4PH1 radioactivity content, including atomic structure, radiation properties, nuclear equations, half-life calculations, practical skills, uses and safety rules.

**Prerequisites:** Basic atomic structure knowledge from Key Stage 3 Science; Basic graph reading and arithmetic skills

## Learning objectives

- Describe atomic structure and use nuclide notation to represent isotopes
- Distinguish alpha, beta-minus, gamma and neutron radiation by ionisation power, penetration and deflection
- Balance nuclear decay equations using conservation of mass and charge
- Calculate half-life using successive halving and decay graph analysis
- Explain background radiation sources, real-world uses of radioactivity and associated safety risks
- Differentiate between contamination and irradiation in exam scenarios

## Atomic Structure, Nuclide Notation and Isotopes

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.

**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 $^{15}_{7}N$
3. Isotopes have the same atomic number and different mass number, so nitrogen-15 is an isotope of nitrogen-14

**Check your understanding**

1. An atom has atomic number 92 and mass number 238. How many neutrons does it contain?

   - 92
   - 146
   - 238
   - 330

   *Why:* Number of neutrons = mass number - atomic number = 238 - 92 = 146

## Types of Ionising Radiation: Properties and Detection

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 |

> **mnemonic**
>
> PENetration order: **P**aper stops Alpha, **A**luminium stops Beta, **L**ead stops Gamma = PAL

**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.

*Calculator:* allowed

## Nuclear Decay Equations

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

1. An alpha particle is represented as $^{4}_{2}He$
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: $^{238}_{92}U \rightarrow ^{234}_{90}Th + ^{4}_{2}He$

> **Exam tip**
>
> Always double check that the sum of mass numbers and sum of atomic numbers are identical on both sides of the decay equation; this is the easiest way to catch mistakes in beta decay questions, where atomic number increases by 1.

## Half-Life: Concepts and Calculations

**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

**Check your understanding**

1. 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

   *Why:* Number of halvings: 800 → 400 → 200 → 100 = 3 half-lives. 3 half-lives = 24 years, so 1 half-life = 24 / 3 = 8 years.

*Calculator:* allowed

## Radioactivity: Background Sources, Uses and Safety

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.

## Common pitfalls

- **Wrong:** Swapping mass and atomic number position in nuclide notation, putting atomic number on top
  - Why it fails: Nuclide notation convention strictly places mass number (A) top-left and atomic number (Z) bottom-left; examiners penalise this heavily in equation questions
  - Correct: Always write nuclides as $^A_Z X$, and verify that total A and Z values balance on both sides of decay equations
- **Wrong:** Assuming beta-minus decay reduces atomic number by 1
  - Why it fails: 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: Remember the beta particle is $^0_{-1}e$, so to balance charge the product atomic number is original Z + 1
- **Wrong:** Using exponential decay formulas for half-life calculations
  - Why it fails: Exponential decay equations are out of scope for Edexcel IGCSE Physics, and all questions are designed to be solved with successive halving
  - Correct: 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:** Stating that irradiation makes an object radioactive
  - Why it fails: Irradiation only exposes an object to external radiation, with no transfer of radioactive material, so the object does not become radioactive
  - Correct: Only contamination (physical presence of radioactive material on/in an object) makes it radioactive; always check for contact with the source in question
- **Wrong:** Forgetting to subtract background count from measured activity
  - Why it fails: 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: First measure background count with no source present, then subtract this value from all source activity measurements
- **Wrong:** Stating alpha radiation is the most dangerous in all scenarios
  - Why it fails: 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: Contextualise your answer: alpha is most dangerous inside the body, gamma is most dangerous for external exposure

## Cheatsheet

| Key Concept | Quick Facts | Exam Check |
| --- | --- | --- |
| Nuclide Notation | $^A_Z X$: 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 |

## 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.

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/edexcel-igcse-physics-s7-radioactivity/