5.2 Radioactivity
Radioactivity
Background Radiation
Definition: Background radiation is the natural nuclear radiation present in the environment at all times, even without any artificial radioactive source nearby.
Sources of background radiation include:
Radon gas: radioactive gas seeping from rocks into the air (largest contributor).
Rocks and buildings: some rocks contain uranium and other radioactive isotopes.
Food and drink: naturally contain isotopes such as potassium-40 and carbon-14.
Cosmic rays: high-energy particles from outer space.
Detection of Radiation
Radiation can be detected using a Geiger–Müller (GM) tube connected to a counter.
The GM tube detects individual particles entering it. The counter displays the count rate in counts per second or counts per minute.
Corrected Count Rate
The actual activity of a source is obtained by subtracting the background count from the measured count.
Worked Example: Measured count = 200 counts/min
Background count = 50 counts/min
-> Corrected count = 200 − 50 = 150 counts/min
The Three Types of Nuclear Emission
Nuclear radiation is emitted spontaneously and randomly from unstable nuclei. The main types are
alpha (α),
beta (β), and
gamma (γ).
Type | Nature | Mass/Charge | Ionising Power | Penetration Ability |
Alpha (α) | 2 protons + 2 neutrons | Heavy, +2 | Very strong | Stopped by paper |
Beta (β-) | High-speed electron | Light, -1 | Moderate | Stopped by thin aluminium |
Gamma (γ) | Electromagnetic wave | No mass/charge | Weak | Stopped by thick lead |
Worked Question: Which type of radiation is most dangerous inside the body, and why?
Answer: Alpha radiation, because it is highly ionising and causes severe local damage inside tissue.
Deflection in Fields
Charged particles are deflected in electric and magnetic fields. Gamma rays are not affected.
Alpha: positive and heavy → slight deflection towards the negative plate.
Beta: negative and light → strong deflection towards the positive plate.
Gamma: no charge → not deflected.
Worked Question: Why is β radiation deflected more than α in a magnetic field?
Answer: Because β particles are lighter and are affected more by the same magnetic force.
Radioactive Decay
Radioactive decay is a spontaneous and random change in an unstable nucleus.
It cannot be controlled or predicted for a single nucleus.
Each unstable nucleus has the same chance of decaying at any time.
Gamma radiation often accompanies alpha or beta decay to release excess energy.
Effects on the Nucleus:
Alpha decay: mass number decreases by 4, proton number decreases by 2.
Beta decay: neutron changes into a proton and an electron; proton number increases by 1.
Gamma emission: no change in mass or proton number.
Decay Equations:
Alpha: U-238 → Th-234 + α
Beta: C-14 → N-14 + β
Gamma: Co-60 → Co-60 + γ
Worked Question: Polonium-210 emits an alpha particle. Po-210 → Pb-206 + α
Half-life:
Definition: Half-life is the time taken for half the radioactive nuclei in a sample to decay.
A sample never reaches zero activity; it continues decreasing.
Long half-life isotopes remain radioactive for thousands of years.
Short half-life isotopes are useful in medicine.
Worked Example:
Initial activity = 640 counts/s
After 1 half-life → 320
After 2 half-lives → 160
After 3 half-lives → 80 counts/s
Applications of Radioactivity
Smoke alarms: Americium-241 (alpha emitter) ionizes air. Smoke reduces ionization and triggers the alarm.
Food irradiation: Gamma rays kill bacteria.
Sterilization: Gamma rays are used to sterilize medical equipment.
Thickness control: Beta radiation monitors paper or metal thickness.
Cancer treatment: Gamma rays are used in radiotherapy.
Application | Isotope | Radiation | Reason |
Smoke alarm | Am-241 | α | Ionizes air, long half-life |
Food irradiation | Co-60 | γ | Kills bacteria, penetrates food |
Sterilization | Co-60 | γ | Destroys microbes |
Thickness control | β source | β | Absorbed by material, control thickness |
Cancer treatment | Co-60 | γ | Destroys cancer cells |
Medical Applications:
Radioactive tracers (short half-life, gamma emitters) are used in diagnosis (e.g., iodine-131).
Radiotherapy uses strong gamma rays to destroy cancer cells.
Industrial Applications:
Thickness control using beta radiation.
Sterilization using gamma rays.
Smoke detectors use alpha radiation.
Safety Precautions
Radiation can damage living tissue and cause mutations or cancer.
Precautions:
Minimise exposure time.
Maximise distance from the source.
Use lead shielding or thick concrete.
Store in lead-lined containers.
Handle with tongs and wear protective clothing.
Always subtract background radiation from the measured count before calculations.
Extra Practice Questions
A Geiger counter measures 150 counts/min. Background = 30 counts/min. Find the corrected count rate.
Which radiation is most strongly deflected in a magnetic field? Explain.
A radioactive isotope has a half-life of 2 days. Initial activity = 800 counts/min. What is the activity after 6 days?
Why is gamma radiation used to sterilize medical equipment instead of alpha or beta?
Describe two safety precautions when using radioactive isotopes in hospitals.
Made by Hiba Shakeel
Curated by Yassein Abdoun