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Flashcards · 0625 · Topic 5
Nuclear physics flashcards
27 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
- Structure of an atom
- A small, positively charged nucleus of protons and neutrons, with negatively charged electrons in orbit around it.
- Proton number (Z) and nucleon number (A)
- Z: the number of protons. A: the number of protons + neutrons. Neutrons = A − Z.
- How many neutrons are in a nucleus of ²³⁸₉₂U?
- 238 − 92 = 146.
- Isotopes
- Atoms of the same element (same proton number) with different numbers of neutrons (different nucleon numbers).
- An atom of ²³₁₁Na loses one electron. Charge on the ion, and number of electrons?
- Charge +1; 10 electrons. (Losing electrons gives a positive ion; gaining gives a negative ion.)
- Relative charges of a proton, a neutron and an electron
- Proton +1, neutron 0, electron −1.
- Alpha-particle scattering: three observations and what each showsExtended
- Most pass straight through: the atom is mostly empty space. Some are deflected: the nucleus is positive. A few bounce back: the nucleus is tiny and holds most of the mass.
- Relative charge and relative mass of the nucleus of ⁵⁶₂₆FeExtended
- Relative charge +26 (= Z); relative mass 56 (= A).
- Nuclear fissionExtended
- A large nucleus splits into two smaller nuclei, usually after absorbing a neutron, releasing more neutrons and energy.
- Nuclear fusionExtended
- Two light nuclei join to form a heavier nucleus, releasing energy. In both fission and fusion the total mass of the products is slightly less than before.
- Check this fission equation balances: ²³⁵₉₂U + ¹₀n → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + 3 ¹₀nExtended
- Top: 235 + 1 = 236 and 144 + 89 + 3 = 236. Bottom: 92 + 0 = 92 and 56 + 36 + 0 = 92.
5.2 Radioactivity
- Four main sources of background radiation
- Radon gas in the air, rocks and buildings, food and drink, and cosmic rays.
- How is ionising radiation measured?
- With a detector connected to a counter. Count rate = counts per second or per minute.
- What are α, β and γ radiation?
- α: a helium nucleus (2 protons + 2 neutrons), charge +2. β: a fast electron from the nucleus, charge −1. γ: electromagnetic radiation, no charge.
- Ionising and penetrating abilities of α, β and γ
- α: most ionising; stopped by paper or a few cm of air. β: medium; stopped by a few mm of aluminium. γ: least ionising; reduced by thick lead or concrete.
- Radioactive decay
- A change in an unstable nucleus that can emit α or β particles and/or γ radiation. It is spontaneous and random. In α or β decay the nucleus becomes a different element.
- Half-life
- The time taken for half the nuclei of a particular isotope in any sample to decay.
- A count rate falls from 960 counts/s to 60 counts/s in 32 hours. Half-life?
- 960 → 480 → 240 → 120 → 60 is 4 half-lives. 32 ÷ 4 = 8.0 hours.
- Effects of ionising radiation on living things, and safe handling of sources
- Cell death, mutations and cancer. Use tongs, lead-lined boxes, point sources away from people, use for the shortest time, store locked and labelled.
- Corrected count rateExtended
- Measured count rate − background count rate (measure the background with the source removed).
- Changes to A and Z in α, β and γ emissionExtended
- α: A − 4, Z − 2. β: A unchanged, Z + 1 (a neutron → proton + electron). γ: no change; the nucleus loses energy.
- Write the β decay of ¹³¹₅₃I to xenon (Xe).Extended
- ¹³¹₅₃I → ¹³¹₅₄Xe + ⁰₋₁e
- Deflection of α, β and γ in electric and magnetic fieldsExtended
- α and β are deflected in opposite directions; β is deflected much more (it is far lighter). In an electric field α bends towards −, β towards +. γ is not deflected.
- Why is α radiation the most ionising?Extended
- It has the largest charge (+2) and a large kinetic energy, so it interacts strongly with the atoms it passes and loses its energy quickly.
- Which radiation for a smoke alarm, paper-thickness control and a medical tracer?Extended
- Smoke alarm: α, long half-life. Paper thickness: β (partly absorbed by paper). Tracer: γ, short half-life (detected outside the body, little lasting dose).
- Readings: 340, then 12 min later 100 counts/minute; background 20 counts/minute. Half-life?Extended
- Corrected: 320 and 80. 320 → 160 → 80 is 2 half-lives. 12 ÷ 2 = 6.0 minutes.
- Why are some isotopes radioactive?Extended
- They have too many neutrons and/or the nucleus is too heavy. Decay makes the nucleus more stable.