☢️ CIE IGCSE Physics – Detection of Radioactivity

Radioactivity cannot be seen, heard, or felt directly.
It must be detected using special instruments.

This topic focuses on:

• Background radiation
• Sources of radiation
• Detecting radiation
• Corrected count rate

1️⃣ Background Radiation

🔑 Definition

Background radiation is:

The low level of ionising radiation that is always present in the environment.

It is present even when no radioactive source is nearby.

🔎 Important Point

When measuring radiation from a source:

• Background radiation must be measured first.
• It must be subtracted to get an accurate result.

2️⃣ Sources of Background Radiation

The main contributors are:

(a) Radon Gas (in the air)

• Radon is a radioactive gas.
• It comes from decay of uranium in rocks.
• It seeps out of the ground.
• It can accumulate in buildings.

🔹 In many countries, radon is the largest source of background radiation.

(b) Rocks and Buildings

• Many rocks contain small amounts of radioactive materials.
• Granite is a common example.
• Building materials made from rocks can emit radiation.

(c) Food and Drink

• Some foods contain small amounts of radioactive isotopes.
• For example, bananas contain potassium-40.
• The human body itself is slightly radioactive.

(d) Cosmic Rays

• High-energy radiation from space.
• Comes from the Sun and distant stars.
• Increases with altitude.
• Airplane passengers receive more cosmic radiation.

📊 Summary Table

Source	Description
Radon gas	From rocks underground
Rocks & buildings	Natural radioactive materials
Food & drink	Naturally occurring isotopes
Cosmic rays	From outer space

3️⃣ Detecting Ionising Radiation

Ionising radiation can be detected using:

🔬 Geiger–Müller (GM) Tube

Connected to:

• A counter
• A ratemeter

🔑 How It Works (Simple Explanation)

1. Radiation enters the GM tube.
2. It ionises gas inside.
3. A small current pulse is produced.
4. The counter records a “click”.
5. The number of clicks is counted.

📌 Important

The detector does NOT measure:

• Energy directly
• Type of radiation directly

It measures:

The number of ionising events.

4️⃣ Count Rate

🔑 Definition

Count rate is:

The number of radioactive counts detected per unit time.

Units

• Counts per second (counts/s)
• Counts per minute (counts/min)

📌 Example

If a detector records:

• 300 counts in 1 minute

Count rate:

5️⃣ Corrected Count Rate

Because background radiation is always present, we must subtract it.

🔎 Steps to Find Corrected Count Rate

1. Measure background count rate.
2. Measure total count rate (source + background).
3. Subtract background from total.

📌 Example 1

Background count rate = 20 counts/min
Measured count rate with source = 150 counts/min

Corrected count rate:

📌 Example 2 (in counts/s)

Background = 2 counts/s
Measured = 12 counts/s

Corrected:

🔥 Why This Is Important

Without correction:

• Results are inaccurate.
• Decay graphs would be wrong.

Examiners often test this calculation.

6️⃣ Safety and Distance Effect

Although not explicitly listed, remember:

• Count rate decreases as distance increases.
• Radiation spreads out.
• Further away → lower count rate.

🧠 Important Terms

Term	Meaning
Ionising radiation	Radiation that can remove electrons from atoms
Background radiation	Natural radiation always present
Count rate	Counts per unit time
GM tube	Device that detects radiation
Corrected count rate	Measured count – background

📝 Exam Tips

1️⃣ Always subtract background radiation.

2️⃣ Units Matter

Be clear whether answer is:

• counts/s
• counts/min

3️⃣ Common Mistakes

❌ Forgetting to subtract background
❌ Mixing units
❌ Saying GM tube measures energy

4️⃣ When Describing Detection

Say:

Ionising radiation ionises gas inside the GM tube, producing a current pulse that is counted.

📊 Typical 4–5 Mark Question

Often asks:

• Define background radiation
• List sources
• Calculate corrected count rate

Structure your answer clearly.

⭐ Quick Summary

• Background radiation is always present.
• Major sources: radon, rocks, food, cosmic rays.
• GM tube detects ionising radiation.
• Count rate measured in counts/s or counts/min.
• Corrected count rate = measured − background.

☢️ CIE IGCSE Physics – The Three Types of Nuclear Emission

Unstable nuclei release radiation to become more stable. This process is called radioactive decay.

The three types of nuclear emission are:

• Alpha (α)
• Beta (β⁻)
• Gamma (γ)

1️⃣ Emission from the Nucleus

🔑 Key Idea

Radioactive emission is:

Spontaneous – happens without external influence
Random in direction – emitted in all directions

🔎 Important Points

• You cannot predict when a single nucleus will decay.
• You can only predict probability (half-life).
• Emission comes from the nucleus, not electrons.

2️⃣ The Three Types of Radiation

🟡 A. Alpha (α) Radiation

🔬 Nature

Alpha particles are:

They are:

• 2 protons
• 2 neutrons
• Helium nucleus
• Charge = +2
• Relatively heavy

⚡ Ionising Power

• Very strongly ionising
• Cause many ionisations in a short distance

🛡️ Penetrating Power

• Very low penetration
• Stopped by:
  • Paper
  • Skin
  • A few cm of air

🔵 B. Beta (β⁻) Radiation

🔬 Nature

Beta particles are:

• Fast-moving electrons
• Emitted from nucleus
• Charge = –1
• Very small mass

⚡ Ionising Power

• Moderately ionising
• Less than alpha
• More than gamma

🛡️ Penetrating Power

• Medium penetration
• Stopped by:
  • Thin aluminium sheet (a few mm)

🟣 C. Gamma (γ) Radiation

🔬 Nature

Gamma rays are:

• High-energy electromagnetic waves
• No mass
• No charge

⚡ Ionising Power

• Weakly ionising
• Least ionising

🛡️ Penetrating Power

• Very high penetration
• Reduced by:
  • Thick lead
  • Thick concrete

📊 Comparison Table

Property	Alpha	Beta	Gamma
Nature	Helium nucleus	Electron	EM wave
Charge	+2	–1	0
Mass	Large	Very small	None
Ionising	Very strong	Medium	Weak
Penetration	Low	Medium	High

3️⃣ Deflection in Electric and Magnetic Fields

⚡ In Electric Fields

Remember:

• Positive particles move toward negative plate.
• Negative particles move toward positive plate.

Alpha (α)

• Positive charge (+2)
• Deflected toward negative plate
• Slight deflection (heavy mass)

Beta (β⁻)

• Negative charge (–1)
• Deflected toward positive plate
• Strong deflection (very light)

Gamma (γ)

• No charge
• Not deflected

🧲 In Magnetic Fields

Use left-hand rule for charged particles.

Alpha

• Deflected slightly
• Opposite direction to beta
• Small curve (heavy)

Beta

• Deflected strongly
• Opposite direction to alpha
• Large curve (light)

Gamma

• Not deflected

4️⃣ Why Ionising Effects Differ

Ionising effect depends on:

(a) Electric Charge

• Alpha has +2 → strong attraction to electrons → strong ionisation.
• Beta has –1 → moderate ionisation.
• Gamma has no charge → weak ionisation.

(b) Kinetic Energy

• Alpha particles are heavy and move slower.
• They transfer energy quickly in a short distance.
• Gamma spreads energy over large distance.

🔥 Summary of Ionising Ability

Most ionising → Least ionising:

🧠 Nuclear Equations (Examples)

Alpha Decay

• Proton number decreases by 2.
• Nucleon number decreases by 4.

Beta Decay (β⁻)

• Proton number increases by 1.
• Nucleon number unchanged.

Gamma Emission

• No change in proton or nucleon number.
• Just releases excess energy.

📝 Exam Tips

1️⃣ Always state:

Emission is spontaneous and random.

2️⃣ Remember order:

Ionising power:
α > β > γ

Penetration:
γ > β > α

3️⃣ Common Mistakes

❌ Saying gamma has mass
❌ Forgetting beta is negative
❌ Saying alpha travels furthest

4️⃣ Field Deflection Questions

✔ Alpha and beta deflect in opposite directions
✔ Gamma not deflected
✔ Beta deflects more than alpha

5️⃣ In Nuclear Equations

✔ Balance proton number
✔ Balance nucleon number

⚠️ Safety Note

• Alpha dangerous inside body.
• Gamma dangerous outside body.
• Beta can penetrate skin.

⭐ Quick Summary

• Radioactive decay is spontaneous and random.
• Alpha: heavy, +2, strong ionising, low penetration.
• Beta: light, –1, medium ionising, medium penetration.
• Gamma: EM wave, no charge, weak ionising, high penetration.
• Alpha and beta deflect in fields; gamma does not.
• Ionising power depends on charge and kinetic energy.

☢️ CIE IGCSE Physics – Radioactive Decay

Radioactive decay is the process by which an unstable nucleus becomes more stable by emitting radiation.

It is a fundamental concept in nuclear physics and radioactivity.

1️⃣ Nature of Radioactive Decay

🔑 Key Points

• Spontaneous: Happens naturally, without external influence.
• Random: Cannot predict when a single nucleus will decay.
• Emissions: Can include:
  • Alpha (α) particles
  • Beta (β⁻) particles
  • Gamma (γ) rays

The rate of decay of a large sample can be predicted statistically, but individual decays are random.

🔬 Example

• Uranium-238 emits an alpha particle spontaneously.

2️⃣ Decay Changes the Element

🔑 Key Idea

• Alpha decay: Decreases proton number by 2 → new element.
• Beta decay (β⁻): Converts neutron → proton → proton number increases by 1 → new element.

Radioactive decay can transform one element into another.

📌 Examples

Alpha Decay:

• Proton number decreases from 92 → 90.
• Nucleon number decreases 238 → 234.

Beta Decay:

• Neutron converts to proton + electron.
• Proton number increases 6 → 7.
• Nucleon number stays 14.

3️⃣ Why Isotopes Are Radioactive

• Some isotopes are unstable because:
  1. Too many neutrons → imbalance in nuclear forces.
  2. Too heavy → repulsive forces among protons overcome nuclear binding.

Example: Carbon-14 is radioactive because it has more neutrons than stable Carbon-12.

4️⃣ Effects on the Nucleus

Type of Emission	Effect on Nucleus	Resulting Stability
Alpha (α)	Loses 2 protons + 2 neutrons	Nucleus becomes lighter and more stable
Beta (β⁻)	Neutron → proton + electron; emits β⁻	Reduces neutron excess, increases stability
Gamma (γ)	Nucleus releases energy (no mass or charge)	Nucleus moves to lower energy state (more stable)

🔎 Important Concept

• Alpha & beta decay change the number of protons → new element.
• Gamma decay does not change the element; only reduces energy.
• Overall goal: Nucleus moves toward greater stability.

5️⃣ Nuclear Decay Equations (Nuclide Notation)

Alpha Decay (α)

• Proton number: 92 → 90
• Nucleon number: 238 → 234

Beta Decay (β⁻)

• Neutron → proton + electron
• Proton number: 6 → 7
• Nucleon number unchanged: 14

Gamma Decay (γ)

• Nucleus loses excess energy
• No change in proton or nucleon number

6️⃣ Special Notes on β-Decay

• β⁻ emission: Neutron transforms into a proton + electron.
• Electron (β⁻) is emitted, proton stays in nucleus.
• Reduces neutron excess → stabilizes nucleus.

7️⃣ Summary Table of Effects

Radiation	Proton Number	Neutron Number	Mass Number	Energy	Stability
Alpha (α)	–2	–2	–4	Moderate	More stable
Beta (β⁻)	+1	–1	0	Low	More stable
Gamma (γ)	0	0	0	Energy released	More stable

📝 Exam Tips

1. Always mention:
   • Decay is spontaneous and random
   • Type of particle emitted (α, β⁻, γ)
   • Effect on proton and nucleon number
2. For β-decay:
   • Emphasize neutron → proton + electron
3. For γ-decay:
   • Energy is released, no change in element
4. Common mistakes:
   • Confusing β⁻ with β⁺ (IGCSE uses β⁻)
   • Forgetting α decreases nucleon number
   • Forgetting γ does not change nucleus composition

⭐ Quick Summary

• Radioactive decay = unstable nucleus emits α, β⁻, or γ.
• Decay is spontaneous and random.
• α and β decay change element; γ does not.
• Isotopes are radioactive if: too many neutrons or too heavy.
• Decay increases stability.
• Use nuclide notation to write decay equations.

☢️ CIE IGCSE Physics – Half-Life

Half-life is a key concept in nuclear physics and radioactivity. It helps us understand how quickly radioactive isotopes decay and how they are used in practical applications.

1️⃣ Definition of Half-Life

🔑 Definition

Half-life (t₁/₂) is:

The time taken for half the nuclei in a sample of a radioactive isotope to decay.

🔎 Important Points

• Applies to a particular isotope.
• Decay is spontaneous and random, so half-life is a statistical average.
• After one half-life: 50% of nuclei remain.
• After two half-lives: 25% remain.
• After three half-lives: 12.5% remain, etc.

Visual Representation (Decay Curve)

• Count rate vs time graph is exponential.
• Each half-life, the count rate halves.
• Count rate is proportional to number of undecayed nuclei.

2️⃣ Calculating Half-Life

📌 From Data

1. Identify initial count rate or number of nuclei.
2. Find the time at which count rate halves.
3. That is one half-life.

Example 1 – Table Data

Time (days)	Count Rate (counts/s)
0	200
2	100
4	50
6	25

• Initial = 200 counts/s
• Half of 200 = 100 → occurs at 2 days
• Half of 100 = 50 → occurs at 4 days

✅ Half-life = 2 days

Example 2 – Decay Curve

• Count rate halves from 800 → 400 at t = 3 hours.
• Count rate halves again 400 → 200 at t = 6 hours.
• Half-life = 3 hours.

Even if background radiation is present, you use the measured values directly for simple IGCSE calculations.

3️⃣ Applications of Radioactive Isotopes

The type of radiation and half-life determine practical use.

(a) Household Smoke Alarms

• Use alpha emitters (e.g., Americium-241).
• α radiation ionises air, allowing current to flow.
• Smoke blocks α particles → triggers alarm.
• Half-life must be long enough to last years.

(b) Irradiating Food

• Use gamma emitters (e.g., Cobalt-60).
• Gamma rays penetrate food → kill bacteria.
• Half-life must be long enough for storage, but safe.

(c) Sterilisation of Equipment

• Gamma radiation used.
• Penetrates through packaging.
• Kills microorganisms without heating.

(d) Measuring / Controlling Thickness

• Example: Paper or metal sheet production.
• Beta or gamma radiation used depending on material.
• Radiation passes through material → measured on detector.
• Penetration and absorption determines type of radiation:
  • β for thin/light materials
  • γ for thick/heavy materials

(e) Diagnosis and Treatment of Cancer

• Gamma rays used:
  • Penetrate tissue → target tumor.
  • Half-life must be long enough for treatment, but not so long that material remains radioactive afterward.

4️⃣ Key Points on Half-Life & Radiation

Factor	Effect on Application
Half-life	Must match intended use (long-lived or short-lived)
Type of radiation	Determines penetration & ionising ability
Alpha (α)	Strong ionisation, low penetration – smoke alarms
Beta (β)	Moderate penetration – thickness control
Gamma (γ)	High penetration – sterilisation, cancer treatment

5️⃣ Typical Calculations

Example 1 – Simple Decay

Initial nuclei: 1600
Half-life = 2 hours

Time	Nuclei Remaining
0h	1600
2h	800
4h	400
6h	200

Example 2 – From Count Rate

• Count rate drops from 1200 → 600 → 300 counts/s.
• If 1200 → 600 occurs at 5 hours: half-life = 5 hours

6️⃣ Exam Tips

1. Always check units: seconds, minutes, hours, days.
2. Use count rate directly for half-life calculations.
3. Draw decay curves if needed; each half-life is a halving of count rate.
4. Remember applications link to:
   • Radiation type
   • Penetration ability
   • Half-life

7️⃣ Common Mistakes

❌ Confusing half-life with time for all nuclei to decay – it’s only half.
❌ Forgetting to consider type of radiation for application questions.
❌ Using incorrect time units in calculations.
❌ Assuming exponential decay is linear (it halves each interval).

⭐ Quick Summary

• Half-life (t₁/₂): time for half the nuclei to decay.
• Count rate halves every half-life.
• Can calculate from tables or decay curves.
• Applications depend on radiation type & half-life:
  • α: smoke alarms
  • β: thickness control
  • γ: sterilisation, food irradiation, cancer treatment

☢️ CIE IGCSE Physics – Safety Precautions for Ionising Radiation

Ionising radiation (α, β, γ) can damage living tissue. Understanding effects, handling, and safety is crucial.

1️⃣ Effects of Ionising Radiation on Living Things

Ionising radiation can remove electrons from atoms and molecules in cells, causing damage.

🔹 Short-term Effects

• Cell death – tissues with rapidly dividing cells (like skin or bone marrow) are most affected.
• Radiation sickness – nausea, vomiting, hair loss, fatigue at very high doses.

🔹 Long-term Effects

• Mutations – changes in DNA, may pass to future generations.
• Cancer – damaged cells may divide uncontrollably.
• Genetic defects – in offspring if reproductive cells are affected.

Alpha particles are the most ionising but cannot penetrate skin.
Beta particles can penetrate skin and cause surface damage.
Gamma rays penetrate deep tissues → greatest overall hazard externally.

2️⃣ Safe Handling, Transport, and Storage of Radioactive Materials

🔹 General Principles

1. Use small quantities whenever possible.
2. Keep sources in secure containers (lead-lined for γ sources).
3. Move sources using tools or tongs to avoid direct contact.
4. Store sources in shielded, labeled, and locked areas.
5. Limit time near the source to reduce exposure.

🔹 Examples

Material	Use	Safety Measures
Alpha source (e.g., Americium-241)	Smoke alarms	Encased, never handled directly
Beta source (e.g., Strontium-90)	Thickness gauges	Shielded container, remote handling
Gamma source (e.g., Cobalt-60)	Sterilisation, cancer treatment	Lead containers, controlled rooms, limited access

3️⃣ Radiation Safety Principles

The three main strategies to reduce exposure:

(a) Reduce Exposure Time

• Explanation: The less time spent near a source, the lower the dose received.
• Example: Technicians using radioactive tracers move quickly and leave the room.

(b) Increase Distance

• Explanation: Radiation intensity decreases with distance (Inverse Square Law for point sources).
• Example: Stand further from a gamma source when using measuring instruments.

(c) Use Shielding

• Explanation: Materials absorb or block radiation.
• Alpha: Paper, clothing, or air is sufficient.
• Beta: Thin metal (aluminium) absorbs β⁻.
• Gamma: Dense materials like lead or thick concrete.

Shielding depends on type and energy of radiation.

🔹 Combined Safety Measures

• Wear lab coats, gloves, and goggles for α and β sources.
• Use lead screens or lead-lined rooms for γ sources.
• Always label radioactive materials clearly.
• Monitor exposure with dosimeters for staff working with radiation.

🧠 Key Terms

Term	Meaning
Ionising radiation	Radiation that can remove electrons from atoms
Dosimeter	Device to measure accumulated radiation dose
Shielding	Material used to absorb radiation
Alpha particle (α)	Helium nucleus, strongly ionising, low penetration
Beta particle (β⁻)	Electron, moderately ionising, medium penetration
Gamma ray (γ)	Electromagnetic radiation, weakly ionising, high penetration
Exposure time	Duration near radiation source
Inverse Square Law	Intensity ∝ 1/distance²

📝 Exam Tips

1. Always mention α, β, γ when discussing radiation type.
2. When asked about protection, structure your answer using time, distance, shielding.
3. Examples strengthen answers – e.g., smoke alarms, Cobalt-60, Strontium-90.
4. Do not confuse external vs internal hazards:
   • Alpha is safe outside the body but dangerous if ingested or inhaled.
   • Gamma can penetrate outside tissues → external hazard.

⚠️ Safety Summary

Effects: Cell death, mutations, cancer.
Handling: Small quantities, tongs/tools, shielded storage, limit time.
Protection:

1. Reduce exposure time
2. Increase distance
3. Use shielding

Principle: “Time, Distance, Shielding” – always remember for exam answers.