LEARNING OBJECTIVES
What you will be able to do
- describe the development and structure of the atom
- compare alpha, beta and gamma radiation
- use half-life graphs and calculations
- explain radiation safety, fission and fusion
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Use atomic and nuclear models to explain radiation, decay, fission and fusion.
Nuclear physics deals with changes in unstable nuclei. Radioactive decay is random for a single nucleus but statistically predictable for a very large sample.
Ionising radiation has valuable medical and industrial uses, but risk depends on penetration, ionising ability, activity, exposure time and whether the source is inside or outside the body.
SECTION 01
Atomic and nuclear structure
Rutherford scattering showed that most of an atom is empty space with mass and positive charge concentrated in a tiny nucleus. The nucleus contains protons and neutrons; electrons occupy the surrounding space.
Nuclide notation gives mass number A above proton number Z. In nuclear equations, total mass number and total proton number are conserved.
Completing an alpha-decay equation
- An alpha particle has mass number 4 and proton number 2.
- Subtract 4 from the parent mass number and 2 from its proton number.
- Use the new proton number to identify the daughter element.
Answer: Alpha decay decreases A by 4 and Z by 2.
SECTION 02
Radioactive emissions and detection
Alpha is a helium nucleus, beta-minus is a fast electron emitted when a neutron changes to a proton, and gamma is high-frequency electromagnetic radiation. Alpha is strongly ionising but weakly penetrating; gamma is weakly ionising but highly penetrating.
A Geiger–Müller tube detects ionising events. Background count should be measured and subtracted. Paper stops alpha, thin aluminium reduces beta and thick lead or concrete reduces gamma.
| Radiation | Charge | Penetration | Ionisation |
|---|---|---|---|
| alpha | +2 | low | very high |
| beta-minus | −1 | medium | medium |
| gamma | 0 | high | low |
SECTION 03
Activity and half-life
Activity is decays per second and is measured in becquerels. Count rate is a detector reading and may include background radiation from rocks, space, medical sources and other surroundings.
After each half-life, half the nuclei remaining decay; the decrease is exponential, not linear. A short half-life gives high initial activity but rapid decay.
Three half-lives
- A corrected count rate starts at 640 counts/min.
- After one half-life: 320; after two: 160; after three: 80.
- If this took 18 h, one half-life = 18 ÷ 3.
Answer: Half-life = 6 h and final corrected count rate = 80 counts/min.
SECTION 04
Uses, fission and fusion
A tracer needs detectable radiation and a suitable half-life; smoke alarms use ionisation; radiotherapy uses controlled radiation to damage cancer cells; sterilisation kills microorganisms. Benefit must be balanced against cell damage and mutation risk.
Fission splits a heavy nucleus after neutron absorption, releasing energy and more neutrons that can form a chain reaction. A reactor uses fuel rods, a moderator, control rods, coolant and shielding. Fusion joins light nuclei and powers stars, but requires extreme temperature and confinement.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Nuclear equations conserve mass and proton numbers.
- Radiations differ in nature, penetration and ionisation.
- Half-life is constant for a nuclide and decay is exponential.
- Radiation use requires justified benefits and controlled exposure.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- describe the development and structure of the atom
- compare alpha, beta and gamma radiation
- use half-life graphs and calculations
- explain radiation safety, fission and fusion