Atomic Structure
What's covered
Key facts
Background radiation is the low-level ionising radiation that is always present around us, from natural and artificial sources.
Activity is measured in becquerels (Bq), where 1 Bq is one decay per second.
A chain reaction in a fission reactor is when released neutrons trigger further fissions, sustaining the reaction.
An alpha particle (helium nucleus) carries a charge of +2.
Alpha radiation is dangerous if inhaled or ingested because the alpha source sits inside the body where the strong ionisation directly damages cells.
When measuring a radioactive source, the background count rate must be subtracted from the total measured count to find the source's true activity.
Carbon-14 has a half-life of about 5,730 years; it is taken up by living organisms and decays predictably after death, allowing radiocarbon dating of once- living material.
Control rods absorb neutrons to control the rate of fission.
Alpha decay reduces the mass number by 4 and the atomic number by 2.
Alpha particles are large (2 protons + 2 neutrons), carry a 2+ charge, and are strongly ionising — they lose energy quickly to surrounding atoms, so they only travel a few cm in air and are stopped by paper or skin.
Sample questions
A taste of the 57 questions in this topic — answers marked. Sign up to practise the full set with spaced repetition.
List three sources of background radiation.
- •Burning fossil fuels (CO₂), car exhaust fumes (NOx), and mobile phone signals (microwaves)
- ✓Radon gas from rocks, cosmic rays from space, and medical X-ray imaging.
- •Sunlight (visible spectrum), wind energy (kinetic), and electrical appliances (low-voltage AC)
- •UV light (from the Sun), infrared heat (from radiators), and chemical reactions in the body (digestion)
A radioactive sample has an initial activity of 800 Bq and a half-life of 2 hours. What is the activity after 6 hours?
- ✓100 Bq
- •200 Bq
- •400 Bq
- •50 Bq
What is a chain reaction in a nuclear fission reactor?
- •Electrons released during fission flow through a circuit to generate power
- •Fusion of hydrogen nuclei triggered by the heat from fission
- •Neutrons released by fission decay into protons and electrons
- ✓Released neutrons trigger further fissions, sustaining the reaction
Which radiation is stopped by paper?
- ✓Alpha
- •Beta
- •Gamma
- •X-ray
Why is alpha radiation more ionising but less penetrating than gamma?
- •Alpha has a higher frequency than gamma but is absorbed by the atmosphere
- ✓Alpha is large, 2+ charged, and ionises rapidly, losing energy quickly
- •Alpha is more ionising because it travels faster than gamma rays do
- •Alpha is positively charged so it is attracted to negatively charged materials
What is the largest source of UK background radiation?
- •Cosmic rays from outer space that penetrate the atmosphere
- •Medical X-rays and radioactive tracers used in hospitals
- •Nuclear power stations releasing radiation during normal operation
- ✓Radon gas, which seeps from rocks and soil
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