Nuclear physics
🎯What you need to be able to do
- Describe the nuclear model of the atom and the evidence for it.
- Use nuclide notation, and explain what isotopes are.
- Describe background radiation and where it comes from.
- Compare alpha, beta and gamma radiation: nature, penetration, ionising ability and deflection.
- Explain that decay is random, and use half-life.
- Describe safety precautions for handling and storing radioactive materials.
- EXTENDEDWrite balanced nuclear decay equations.
📚The physics
The nuclear model. An atom is a tiny, dense, positively charged nucleus surrounded by electrons, and it is mostly empty space. The evidence is the alpha-particle scattering experiment: most alpha particles passed straight through a thin gold foil, a few were deflected, and a very small number bounced back. Each observation gives a conclusion — mostly empty space, a concentrated positive charge, and a nucleus that is both very small and very massive. Learn them as three pairs, because that is how the question is asked.
Inside the nucleus are protons and neutrons, together called nucleons. In the notation \( ^{A}_{Z}\mathrm{X} \), \(Z\) is the proton number (which decides the element) and \(A\) is the nucleon number. The number of neutrons is \( A - Z \). Isotopes are atoms of the same element with different numbers of neutrons — same \(Z\), different \(A\). They behave identically in chemical reactions and can behave very differently in nuclear ones.
Background radiation is around us all the time and must be subtracted before you use any count-rate measurement. Most of it is natural: radon gas seeping from the ground, rocks and soil, cosmic rays from space, and radioactive isotopes inside our own bodies. A smaller part is artificial, mainly medical.
The three radiations. This is worth learning until it is automatic.
- Alpha (α) — a helium nucleus, 2 protons and 2 neutrons, charge +2. Strongly ionising, so it is stopped by paper or a few centimetres of air. Deflected slightly by electric and magnetic fields.
- Beta (β) — a fast electron from the nucleus, charge −1. Moderately ionising, stopped by a few millimetres of aluminium. Deflected strongly, and in the opposite direction to alpha because its charge is opposite.
- Gamma (γ) — a high-energy electromagnetic wave, no charge, no mass. Weakly ionising, so it is very penetrating — reduced by thick lead or concrete but never completely stopped. Not deflected at all.
EXTENDED Decay equations must balance the top numbers and the bottom numbers separately. Alpha decay reduces \(A\) by 4 and \(Z\) by 2. Beta decay leaves \(A\) unchanged and increases \(Z\) by 1, because a neutron turns into a proton and an electron. Gamma emission changes neither.
Decay is random. We cannot predict when any individual nucleus will decay, and nothing we do to it — heating, cooling, chemical reaction — changes the odds. What is predictable is the behaviour of very large numbers.
Half-life is the time taken for half the undecayed nuclei in a sample to decay, or equally for the count rate to halve. Each isotope has its own fixed half-life, ranging from fractions of a second to billions of years.
Safety. Keep sources in lead-lined containers, handle them with tongs rather than fingers, keep them at arm’s length and pointed away from people, minimise the time you are exposed, and never bring a source near your eyes.
✏️Worked example
(a) What is the corrected count rate after 24 hours? 24 hours is 4 half-lives, so halve four times: 800 → 400 → 200 → 100 → 50 counts per minute.
(b) The background count is 20 counts per minute. What will the detector actually read after 24 hours? \( 50 + 20 = 70 \) counts per minute. The detector always reads source plus background — the halving applies only to the source.
(c) After how long will the corrected rate fall to 25 counts per minute? That is five halvings, so \( 5 \times 6 = 30 \) hours.
🔭See it happen
Roll a hundred dice and remove every six. Roll the rest, remove the sixes again, and keep going, recording how many are left each round. Plot the numbers. You get a decay curve, and you get it from a genuinely random process where no individual die can be predicted — which is exactly the point about radioactive decay.
📝Practise
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- BBC Bitesize — Radioactivity
- The Physics Classroom — Nuclear Chemistry