Nuclear physics flashcards

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Describe the structure of an atom.
A tiny, dense, positively charged nucleus (protons and neutrons) with negative electrons orbiting at relatively large distances. Most of the atom is empty space.
Give the relative charge and mass of a proton, a neutron and an electron.
Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass about 1/2000 (negligible).
What did the alpha-particle scattering experiment show?
Most alpha particles went straight through, so the atom is mostly empty space. A few were deflected through large angles, and a very few bounced back, so there is a small, dense, positive nucleus.
How did the scattering results disprove the plum-pudding model?
In that model positive charge was spread through the whole atom, so alpha particles would only be deflected slightly. Large-angle deflections need a concentrated positive charge.
Define proton number (atomic number) and nucleon number (mass number).
Proton number Z is the number of protons, which defines the element. Nucleon number A is the total number of protons plus neutrons.
How many protons, neutrons and electrons are in the neutral atom of carbon-14, written ¹⁴₆C?
Protons: 6. Neutrons: 14 − 6 = 8. Electrons: 6 (a neutral atom has equal numbers of electrons and protons).
What are isotopes?
Atoms of the same element (same number of protons) with different numbers of neutrons, so different nucleon numbers. They have the same chemical properties.
What is meant by radioactive decay and why is it described as random and spontaneous?
An unstable nucleus emits radiation to become more stable. It is spontaneous because it is not affected by external conditions, and random because we cannot predict which nucleus will decay or when.
What is background radiation and what are its main sources?
Ionising radiation always present around us. Sources include radon gas from rocks, cosmic rays, food and drink, and building materials, plus a small part from medical and man-made sources.
Name the three types of ionising radiation and what each is.
Alpha (α): a helium nucleus (2 protons, 2 neutrons). Beta (β⁻): a fast electron from the nucleus. Gamma (γ): high-energy electromagnetic radiation.
Compare the penetrating power of alpha, beta and gamma radiation.
Alpha is stopped by paper or a few cm of air. Beta passes through paper but is stopped by a few mm of aluminium. Gamma is only reduced by thick lead or concrete.
Compare the ionising power of alpha, beta and gamma radiation.
Alpha is the most strongly ionising, beta is moderate, and gamma is the least ionising. The more ionising, the less it penetrates.
How are alpha, beta and gamma deflected in electric and magnetic fields?
Alpha (positive) and beta (negative) bend in opposite directions; beta bends more because it is lighter. Gamma is uncharged and is not deflected.
How can radiation be detected?
A Geiger–Müller tube and counter give clicks or a count rate. Photographic film darkens when exposed. A cloud chamber shows tracks of ionisation.
What is meant by ionising radiation, and why is it harmful?
Radiation that knocks electrons off atoms, creating ions. In living cells it can damage DNA, causing mutations, cancer, or cell death.
(Extended) What happens to the nucleus in alpha decay? Complete: ²³⁸₉₂U → ? + ⁴₂He
The nucleus loses 2 protons and 2 neutrons, so A falls by 4 and Z by 2. ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.
(Extended) What happens to the nucleus in beta decay? Complete: ¹⁴₆C → ? + ⁰₋₁e
A neutron becomes a proton and an electron, which is emitted. A stays the same and Z rises by 1. ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.
What happens to the nucleus in gamma emission?
A nucleus with excess energy releases a gamma ray. The numbers of protons and neutrons do not change, so it is the same nuclide at a lower energy.
Define half-life.
The time taken for half the radioactive nuclei in a sample to decay, or equivalently for the count rate (corrected for background) to halve.
A sample of 80 g has a half-life of 3 days. How much remains after 12 days?
12 days is 4 half-lives. 80 → 40 → 20 → 10 → 5. So 5 g remains.
A source has a corrected count rate of 800 counts/min and a half-life of 6 hours. What is the rate after 18 hours?
18 h is 3 half-lives: 800 → 400 → 200 → 100 counts/min.
How do you determine half-life from a decay graph?
Read the time for the count rate (or number of nuclei) to fall from any value to half of it. Repeat for several starting points and average, after subtracting background.
Give uses of radioactive isotopes in medicine, industry and archaeology.
Medicine: gamma tracers and cancer treatment; sterilising equipment. Industry: thickness gauges (beta) and checking weld faults (gamma). Archaeology: carbon-14 dating.
Why are gamma emitters used as medical tracers and alpha emitters used in smoke detectors?
Gamma passes out of the body so it can be detected externally, with a short half-life to limit exposure. Alpha is absorbed by smoke particles, which reduces the detector's current and triggers the alarm.
What safety precautions are used when handling radioactive sources?
Keep sources in lead-lined containers, handle with tongs, keep a distance, limit exposure time, use shielding and never point them at people. These reduce the dose received.
What is nuclear fission and what does it release?
A large nucleus (such as uranium-235) splits into smaller nuclei when it absorbs a neutron, releasing energy and more neutrons, which can start a chain reaction. It is used in nuclear power stations.
How does the half-life of a source affect how it is used?
Short half-life sources (hours) are safe for tracers because they decay quickly. Long half-life sources (thousands of years) last for gauges, but waste needs long-term storage.

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