Pearson Science Double Award Physics 7: Radioactivity and Particles
Pearson Science Double Award notes on radiation, half-life, nuclear equations, fission and fusion.
Unstable nuclei emit ionising radiation randomly, while large samples show predictable activity change. The full 4SD0 Topic 7 is included, from nuclear notation and detection through fission and fusion.
Nuclei and emissions
Atomic number is proton number. Mass number is protons plus neutrons. Isotopes have the same proton number but different neutron numbers. Nuclear notation places mass number above atomic number beside the element symbol.
Alpha radiation is a helium nucleus with two protons and two neutrons. It is strongly ionising but weakly penetrating and is stopped by paper or a short distance of air. Beta-minus radiation is a fast electron emitted when a neutron changes into a proton. It has intermediate ionisation and penetration and is reduced by thin aluminium. Gamma is electromagnetic radiation from the nucleus. It has no mass or charge, is weakly ionising and highly penetrating, so thick lead or concrete reduces it.
Radioactive decay is random for an individual nucleus. Temperature, pressure and chemical state do not control when it decays. A large sample nevertheless follows a predictable decay pattern.
In alpha emission, mass number decreases by 4 and atomic number by 2. In beta-minus emission, mass number stays unchanged and atomic number increases by 1. Gamma emission changes neither. Neutron emission reduces mass number by 1 without changing atomic number. Balance total mass and atomic numbers in nuclear equations.
Detection, background and half-life
Photographic film darkens after radiation exposure. A Geiger-Müller tube produces counts that can be recorded as count rate. Background radiation comes from cosmic rays, rocks and soil, radon gas, food, living organisms and medical or industrial sources.
Correct a measured count rate by subtracting background measured over a comparable time. Random count variation means readings fluctuate, so longer counting intervals and repeats improve the estimate.
Activity is decays per second and is measured in becquerels. Half-life is time for the activity or number of undecayed nuclei to fall to half its value. After half-lives, the remaining fraction is
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