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Flashcards · 4PH1 · Topic 7
Radioactivity and particles flashcards
24 cards on Topic 7, subtopics 7a to 7c. Pick a subtopic or study them all.
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7a Units
- Unit of activity
- The becquerel (Bq): one decay per second.
- Units for half-life
- Any unit of time: seconds (s), minutes (min) or hours (h), chosen to suit the isotope.
- Which unit is used for how far radiation travels through air?
- Centimetres (cm). For example, alpha particles travel only a few cm in air.
7b Radioactivity
- Atomic number and mass number
- Atomic number: number of protons. Mass number: protons + neutrons. Nuclide notation shows the mass number above and the atomic number below the symbol.
- Isotopes
- Atoms with the same number of protons but different numbers of neutrons: identical chemistry, different nuclear stability.
- Alpha radiation
- A helium nucleus, charge +2. Stopped by paper. Strongly ionising.
- Beta radiation
- A fast electron, charge −1. Stopped by about 3 mm of aluminium. Moderately ionising.
- Gamma radiation
- An electromagnetic wave, no charge. Reduced by thick lead. Weakly ionising.
- Effect on mass number and atomic number of α, β, γ and neutron emission
- α: mass −4, atomic −2. β: mass unchanged, atomic +1 (a neutron becomes a proton + electron). γ: no change. Neutron: mass −1, atomic unchanged.
- How is ionising radiation detected?
- With photographic film (it darkens) or a Geiger–Müller detector (clicks or counts).
- Sources of background radiation
- Rocks (notably radon gas), cosmic rays, food, and medical sources. Subtract it from measured count rates before half-life calculations.
- Half-life
- The time for half the undecayed nuclei in a sample to decay, or for the activity (count rate) to halve. Decay is random, so it is an average; it is not affected by temperature or pressure.
- A source has activity 800 Bq and a half-life of 2 hours. Activity after 6 hours?
- 6 ÷ 2 = 3 half-lives: 800 → 400 → 200 → 100 Bq.
- Monitoring foil thickness
- Beta. Alpha would be completely absorbed and gamma would pass almost entirely through, so neither would respond to changes in thickness; beta is partly absorbed, so the count rate varies as the foil thickness changes.
- Smoke detector
- Alpha. It strongly ionises the air in the detector but is safely stopped within the device itself.
- Medical tracer
- Gamma, with a short half-life. Gamma penetrates the body so it can be detected externally, and a short half-life means activity falls quickly, limiting the patient's overall exposure.
- Contamination vs irradiation
- Contamination: radioactive material gets onto or into an object or person, so the risk continues after the source is removed. Irradiation: exposure to an external source; the risk stops when the source is removed.
- Dangers of ionising radiation and how to reduce exposure
- Mutation and damage to cells and tissue; radioactive waste must be disposed of safely. Reduce exposure by shielding (lead, concrete), distance (tongs) and limiting time.
7c Fission and fusion
- Fission of uranium-235
- A U-235 nucleus absorbs a neutron, becomes unstable and splits into two daughter nuclei, releasing energy and two or three neutrons.
- Chain reaction
- Neutrons released by one fission cause further fissions, which release more neutrons, and so on.
- Roles of control rods, the moderator and shielding in a reactor
- Control rods absorb neutrons to control the rate of fission. The moderator slows neutrons so they can cause fission. Thick shielding absorbs radiation escaping from the core.
- Nuclear fusion
- Small (light) nuclei join to form a larger nucleus, with a loss of mass and a release of energy. Fusion is the energy source of stars.
- Why is fusion hard to achieve on Earth?
- Both nuclei are positively charged and repel. Overcoming this electrostatic repulsion needs extremely high temperature and pressure, which are hard to contain.
- Fission vs fusion
- Fission splits a large nucleus into smaller ones; fusion joins small nuclei into a larger one. Both release energy.