RadioactivityCopy

O Level / IGCSE Physics Cheat Sheet – 5.2 Radioactivity

5.2.1 Detection of Radioactivity

Detection Methods:

• Alpha particles (α):
  • Cloud chamber: Shows visible tracks due to ionisation.
  • Spark counter: Sparks in gas indicate alpha radiation.
• Beta particles (β⁻) and Gamma rays (γ):
  • Geiger–Müller (GM) tube + counter: Detects ionisation and measures radiation via clicks or digital count.

Count Rate:

• Measured in counts per second (cps) or counts per minute (cpm) using a GM tube.

Background Radiation:

• Natural low-level radiation always present.
• Major sources:
  a) Radon gas in air (main source)
  b) Rocks and buildings
  c) Food and drink
  d) Cosmic rays from space

Corrected Count Rate:
Corrected Count Rate = Measured Count Rate − Background Count Rate

5.2.2 The Three Types of Emission

Nature of Radiation Emission:

• Random and spontaneous in time and direction.

Radiation Particles:

• Alpha (α): Helium nucleus → 2 protons + 2 neutrons → charge = +2
• Beta (β⁻): Fast-moving electron from neutron decay → charge = −1
• Gamma (γ): Electromagnetic wave → no mass or charge

Ionising Ability (strongest to weakest): α > β > γ
Penetration Power (weakest to strongest):

• α: Stopped by paper/skin
• β: Stopped by a few mm of aluminium
• γ: Partially stopped by thick lead/concrete

Deflection in Electric/Magnetic Fields:

• α: Deflected slightly (positive)
• β: Deflected strongly (negative)
• γ: Not deflected (no charge)

5.2.3 Radioactive Decay

Definition:

• Spontaneous, random process where unstable nuclei emit radiation to become more stable.

Decay Equations:

• Alpha decay:
  A Z X → A−4 Z−2 Y + ⁴₂He
• Beta decay (β⁻):
  A Z X → A Z+1 Y + ⁰₋₁e
• Gamma emission:
  • Usually follows α or β decay.
  • No change in A or Z.

5.2.4 Fission and Fusion

Nuclear Fusion:

• Two small nuclei combine to form a heavier nucleus.
• Mass is converted into a large amount of energy.
• Source of energy in stars and the Sun.

Nuclear Fission:

• Heavy nucleus (e.g., U-235) absorbs neutron → splits into two daughter nuclei + 2 or 3 neutrons + energy.

Chain Reaction in Reactors:

• Neutrons from one fission event cause further fissions.
• Controlled using:
  • Moderator: Slows down neutrons
  • Control rods: Absorb excess neutrons
  • Coolant: Removes heat

5.2.5 Half-Life

Definition:

• Time taken for half the radioactive nuclei in a sample to decay.

Use in Calculations:

• Apply to decay curves/tables to find activity or remaining nuclei.

Carbon Dating (¹⁴C):

• Used to date ancient organic materials.
• Half-life ≈ 5730 years.

Applications (based on type & half-life):
a) Smoke detectors – Americium-241 (α emitter, long half-life)
b) Food irradiation – γ rays kill bacteria
c) Sterilisation – γ rays for surgical instruments
d) Thickness control – β particles monitor paper/metal thickness
e) Cancer diagnosis/treatment – γ rays target tumours

5.2.6 Safety Precautions

Effects of Ionising Radiation:

• Can cause:
  • Cell damage or death
  • Gene mutations
  • Cancer

Safe Handling:
a) Reduce exposure time
b) Increase distance from source
c) Use shielding (lead, concrete)

• Store in lead-lined containers
• Handle using remote tongs or robotic arms
• Work in controlled radiation areas