Nuclear physics
The nuclear atom, isotopes, background radiation, alpha, beta and gamma radiation, half-life, fission and fusion.
📌 Key points
- An atom has a tiny, dense, positively charged nucleus of protons and neutrons, with electrons around it. Alpha-particle scattering showed that most of the atom is empty space.
- The proton number Z is the number of protons; the nucleon number A is the number of protons + neutrons. Isotopes have the same number of protons but different numbers of neutrons.
- Background radiation comes from radon gas, rocks and buildings, food and drink, cosmic rays, and medical sources. Subtract it from measured count rates.
- Alpha (α): a helium nucleus (2 protons + 2 neutrons); most ionising; stopped by paper or a few cm of air.
- Beta (β): a fast electron emitted from the nucleus; stopped by a few mm of aluminium. Gamma (γ): an electromagnetic wave; least ionising; reduced by thick lead or concrete.
- Alpha decay: nucleon number falls by 4 and proton number by 2. Beta decay: a neutron becomes a proton and an electron, so the proton number rises by 1 and the nucleon number stays the same. Gamma emission changes neither.
- Radioactive decay is random and spontaneous. The half-life is the time for half the nuclei in a sample to decay (or for the count rate to halve).
- Uses: alpha in smoke alarms, beta for monitoring the thickness of paper, gamma for sterilising, and isotopes such as carbon-14 for dating.
- Safety: keep exposure time short, keep your distance (e.g. tongs) and use shielding.
- Fission: a large nucleus (e.g. uranium-235) absorbs a neutron and splits, releasing energy and more neutrons. Fusion: light nuclei join together, e.g. hydrogen into helium in stars.
📖 Key terms
- Isotopes
- Atoms of the same element with the same number of protons but different numbers of neutrons.
- Proton number
- The number of protons in the nucleus (Z).
- Nucleon number
- The total number of protons and neutrons in the nucleus (A).
- Ionising radiation
- Radiation that can remove electrons from atoms, making ions.
- Half-life
- The time taken for half the nuclei of a radioactive isotope in a sample to decay.
- Background radiation
- Low-level ionising radiation that is around us all the time.
- Nuclear fission
- The splitting of a large nucleus into two smaller nuclei, releasing energy.
- Nuclear fusion
- The joining of two light nuclei to form a heavier nucleus, releasing energy.
✏️ Worked example
A sample has a count rate of 640 counts/s (background already subtracted). Its half-life is 3.0 hours. What will the count rate be after 12 hours?
- Number of half-lives = 12 ÷ 3.0 = 4.
- Halve the count rate 4 times: 640 → 320 → 160 → 80 → 40.
- Answer: 40 counts/s.
Always subtract the background count rate before working out a half-life from measured readings.
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