Nuclear Physics

The One Thing

Nuclear physics explains how the structure and instability of atomic nuclei produce radiation, how radioactive decay can be represented and measured, and how radiation can be used safely. The nucleus is extremely small compared with the atom but contains nearly all of its mass and all of its positive charge.

Reactions, Processes and Experiments

Key Terms

• Atom: The smallest particle of an element that can take part in a chemical reaction; it contains a nucleus and electrons.
• Nucleus: The small, positively charged central part of an atom containing protons and neutrons.
• Proton: A positively charged particle in the nucleus with relative charge +1 and relative mass 1.
• Neutron: An uncharged particle in the nucleus with relative charge 0 and relative mass 1.
• Electron: A negatively charged particle outside the nucleus with very small mass compared with a proton or neutron.
• Atomic number: The number of protons in a nucleus, represented by Z; it identifies the element.
• Mass number: The total number of protons and neutrons in a nucleus, represented by A.
• Isotope: Atoms of the same element with the same number of protons but different numbers of neutrons.
• Nuclear notation: An atom or nucleus is written as ^A_Z X, where X is the element symbol, A is the mass number and Z is the atomic number.
• Radioactivity: The spontaneous and random emission of radiation from unstable atomic nuclei.
• Activity: The number of nuclear decays occurring per second, measured in becquerels (Bq).
• Alpha radiation: A stream of helium nuclei containing two protons and two neutrons; it is strongly ionising and has low penetration.
• Beta radiation: A stream of fast electrons emitted when a neutron changes into a proton; it has medium ionising power and penetration.
• Gamma radiation: High-frequency electromagnetic radiation emitted from an excited nucleus; it is weakly ionising but highly penetrating.
• Ionisation: The removal of electrons from atoms, producing ions.
• Half-life: The time required for the activity or number of undecayed nuclei in a sample to fall to half its initial value.
• Background radiation: Low-level radiation that is always present from natural sources and human activities.
• Nuclear decay equation: An equation showing that both mass number and atomic number are conserved during radioactive decay.
• Nuclear fission: The splitting of a large nucleus into smaller nuclei, releasing energy and usually extra neutrons.
• Nuclear fusion: The joining of small nuclei to form a larger nucleus, releasing energy at extremely high temperatures and pressures.
• Irradiation: Exposure of an object to radiation without making it radioactive.
• Contamination: The unwanted presence of radioactive material on or inside an object or person.

Easily Confused

• Atomic number and mass number: Atomic number, Z, is the number of protons; mass number, A, is the total number of protons and neutrons.
• Isotopes and ions: Isotopes differ in neutron number, whereas ions have gained or lost electrons.
• Alpha, beta and gamma radiation: Alpha is most ionising and least penetrating, beta has intermediate properties, and gamma is least ionising but most penetrating.
• Irradiation and contamination: Irradiation is exposure to radiation without transferring radioactive material; contamination is the unwanted presence of radioactive material on or inside an object or person.
• Random decay and half-life: The decay time of an individual nucleus is unpredictable, whereas the half-life of a large sample is predictable.
• Fission and fusion: Fission splits a large nucleus; fusion joins small nuclei. Both release energy because the products have greater binding energy per nucleon than the original nuclei.
• Alpha hazards inside the body and gamma hazards outside the body: Alpha sources are especially dangerous inside the body, while gamma sources are especially dangerous outside the body because gamma radiation penetrates deeply.

What Gets Asked

• Calculate the number of neutrons using A - Z, identify an element from its atomic number, and determine the number of electrons in a neutral atom. The common error is confusing atomic number with mass number.
• Complete or check nuclear notation in the form ^A_Z X. Marks are lost when the mass number and atomic number are placed in the wrong positions.
• Write and balance alpha, beta-minus and gamma decay equations. The key slips are reducing mass number by 4 and atomic number by 2 for alpha decay, increasing atomic number by 1 while keeping mass number unchanged for beta-minus decay, and changing neither number for gamma emission.
• Compare alpha, beta and gamma radiation in terms of ionising power, penetration and shielding. The required distinctions are paper or air for alpha, aluminium or plastic for beta, and thick lead or concrete for gamma.
• Use A = A_0(1/2)^(t/T) and half-life data to calculate remaining activity. The relevant sequence is 50% after one half-life, 25% after two, and 12.5% after three.
• Explain radiation risks and safety procedures, including exposure time, distance, shielding, handling with tongs or remote equipment, use of dosimeters, and the distinction between irradiation and contamination.