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Flashcards · 5054 · Topic 5
Nuclear physics flashcards
15 cards on Topic 5, subtopics 5.1 to 5.2. Pick a subtopic or study them all.
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5.1 The nuclear model of the atom
- Proton number, nucleon number and isotopes
- Z = number of protons; A = protons + neutrons; neutrons = A − Z. Isotopes: same element (same Z), different A, so different numbers of neutrons.
- Alpha scattering: what each observation shows
- Most pass straight through: mostly empty space. A few deflect a lot: a small, positive nucleus holding most of the mass. A very few bounce back: repelled by a heavy, positive nucleus.
- Structure of the atom
- A small, positively charged nucleus of protons (positive) and neutrons (no charge), with negatively charged electrons in orbit around it.
- How do atoms form ions?
- A positive ion forms when an atom loses electrons; a negative ion forms when it gains electrons. The nucleus does not change.
- A nuclide is written ²³⁵₉₂U. How many protons, neutrons and nucleons?
- Proton number Z = 92 and nucleon number A = 235, so neutrons = 235 − 92 = 143 and there are 235 nucleons.
5.2 Radioactivity
- Alpha, beta and gamma: nature and penetration
- α: helium nucleus (2p + 2n), stopped by paper or a few cm of air. β: electron from the nucleus, stopped by a few mm of aluminium. γ: very high frequency EM wave, reduced by thick lead or concrete.
- Ionising power, and deflection in an electric field
- Ionising: α strongest, β medium, γ weakest (the reverse of penetrating power). Electric field: α bends slightly to the negative plate, β bends a lot to the positive plate, γ is not deflected.
- Background radiation sources, and corrected count rate
- Cosmic rays, radon gas, rocks and buildings, food and drink. Corrected count rate = count rate with the source − background count rate.
- Changes in A and Z for α, β and γ emission
- α: A − 4, Z − 2. β: A unchanged, Z + 1. γ: no change. e.g. ²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He.
- Half-life. 720 counts/min, half-life 5 min: count rate after 20 min?
- The time for half the nuclei of an isotope in any sample to decay (decay is random). 20 ÷ 5 = 4 half-lives: 720 ÷ 16 = 45 counts/min.
- Fission of uranium-235, and a chain reaction
- U-235 absorbs a neutron and splits into two daughter nuclei, releasing energy and two or more neutrons. In a chain reaction these neutrons cause further fissions.
- Jobs of the moderator, control rods and coolant in a reactor
- Moderator: slows neutrons so they can be absorbed. Control rods: absorb neutrons (in to reduce power). Coolant: carries thermal energy out of the core to make steam.
- Carbon-14 dating: a sample has one quarter of the activity of living material. Age? (half-life 5730 years)
- One quarter = two half-lives, so 2 × 5730 = 11 460 years. Living things keep a constant proportion of carbon-14; after death intake stops and it decays.
- Radiation for a smoke alarm, thin-sheet thickness control and a medical tracer
- Smoke alarm: alpha (ionises air, easily stopped; long half-life). Thin sheet: beta (partly absorbed). Tracer: gamma (detected outside the body) with a short half-life.
- Effects of ionising radiation, and the three ways to reduce exposure
- Cell death, mutations, cancer. Reduce exposure time, increase distance (use tongs), use shielding (lead-lined containers).