Flashcards · 9203 · Topic 7

Nuclear physics flashcards

28 cards on Topic 7, subtopics 3.7.1 to 3.7.4. Pick a subtopic or study them all.

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3.7.1 Atomic structure

Size of an atom and its nucleus: Atom radius about 10⁻¹⁰ m. The nucleus is far smaller (about 10⁻¹⁵ m) and holds almost all the mass. Electrons surround it at various distances.
Relative charge and mass of a proton, neutron and electron: Proton: +1, mass 1. Neutron: 0, mass 1. Electron: −1, very small mass.
Atomic number and mass number. How many neutrons in ⁶⁰₂₇Co?: Atomic (proton) number: protons. Mass number: protons + neutrons. ⁶⁰₂₇Co: 60 − 27 = 33 neutrons.
Isotopes: Atoms with equal atomic numbers but different mass numbers, because their neutron numbers differ. Same chemistry, as they have the same number of electrons.
An atom of ⁵⁶₂₆Fe has 24 electrons. Its charge?: It is an ion. Charge = protons − electrons = 26 − 24 = +2. (Losing electrons gives a positive ion; the nucleus does not change.)
Alpha scattering: three observations and conclusions: Most go straight on: most of the atom is empty space. Some slightly deflected: repelled by a positive nucleus. Very few bounce back: a tiny nucleus holds the charge and the mass.

3.7.2 Ionizing radiation from the nucleus

Radioactive decay: what happens, and what affects it: An unstable nucleus emits alpha, beta, a neutron or gamma and becomes more stable. It is random and is not affected by chemical or physical conditions such as temperature and pressure.
What are alpha, beta and gamma radiation?: Alpha: 2 protons + 2 neutrons (a helium nucleus), charge +2. Beta: a fast electron from the nucleus (a neutron changes to a proton), charge −1. Gamma: electromagnetic radiation from the nucleus, no charge.
Ionising power, what stops each radiation, and range in air: Alpha: strongest; stopped by paper or skin; a few cm. Beta: moderate; a few mm of aluminium; tens of cm to a few m. Gamma: weakest; reduced by thick lead or concrete; very long range.
Ionising radiation, and why strong ionisers have a short range: Radiation that can remove electrons from atoms, making ions. A strongly ionising radiation uses up its energy quickly, so its range is short.
Changes in mass and atomic number. ²³⁰₉₀Th emits an alpha particle:: Alpha: −4, −2. Beta: no change, +1. Gamma: no change. ²³⁰₉₀Th → a nucleus with A = 226, Z = 88 (+ ⁴₂He).
2130 counts in 3 minutes with a source; background 30 per minute. Corrected count rate?: 2130 ÷ 3 = 710 per minute. Corrected = 710 − 30 = 680 per minute. Background comes from the environment (e.g. radon from rocks), space and devices such as X-ray machines.
Half-life (two definitions): The average time for the number of nuclei of the isotope in a sample to halve, or the time for the count rate from it to fall to half.
3200 nuclei, half-life 2 h. How many remain after 6 h?: 6 ÷ 2 = 3 half-lives: 3200 ÷ 8 = 400 nuclei.
A detector reads 150 counts/min (background 18). Half-life 4 min. Reading after 8 min?: Corrected 132; two half-lives: 132 ÷ 4 = 33; add background back: 51 counts per minute.
Contamination vs irradiation: Contamination: unwanted radioactive atoms on or in a material; it stays radioactive until they are removed or decay. Irradiation: exposure to radiation; the object does not become radioactive.
Why is an alpha source more dangerous inside the body than outside?: Inside, alpha is absorbed close by and is strongly ionising, so it is very damaging. Outside, it is stopped by the skin or air.
Choosing an isotope by half-life: Very short half-life: high activity at first, gone quickly (suits a medical tracer). Very long half-life: low activity lasting a very long time (suits a long-lived source, but needs safe storage).
Paper makes little difference to a count, but aluminium cuts it sharply. What is emitted?: Beta radiation is present (no significant alpha). If aluminium made little difference and thick lead cut the rate, it would be gamma.

3.7.3 Nuclear fission

Fissionable substances used in reactors: Uranium-235 and plutonium-239. Most reactors use uranium-235.
Describe the fission of a uranium-235 nucleus: It absorbs a neutron, becomes unstable and splits into two smaller nuclei, releasing two or three neutrons and energy (much more than a chemical reaction of similar mass).
Chain reaction: what happens when more than, exactly, or less than one neutron per fission causes another?: More than 1: fissions grow rapidly. Exactly 1: steady rate (normal reactor running). Less than 1: the reaction dies away.
What do control rods do?: They absorb neutrons so that on average only one neutron per fission causes another. Lowering them slows the reaction; raising them lets it speed up.
Why is nuclear waste a problem?: It is radioactive and may stay so for a long time, so its disposal must be managed carefully. This can influence whether nuclear power is used.

3.7.4 Nuclear fusion

Nuclear fusion: Two light nuclei join to form a heavier nucleus. Some of the mass of the smaller nuclei is converted into energy.
Why does fusion need very high temperature and pressure?: Both nuclei are positive and repel. High temperature makes them move fast and high pressure keeps them close, so the repulsion is overcome and they can fuse.
Where does fusion release energy naturally?: In stars.
Fission vs fusion: Fission: a large nucleus splits, after absorbing a neutron; used in reactors. Fusion: light nuclei join, once repulsion is overcome; happens in stars.