Atomic and Nuclear Physics for Singapore O-Level Physics (6091): the nuclear model of the atom, radioactivity and the types of emission, half-life and decay, and the uses and hazards of radiation

What this module covers

Atomic and Nuclear Physics closes O-Level Physics (SEAB 6091) by looking inside the atom. The module sets out the nuclear model, describes the three types of radioactive emission and their properties, develops half-life as the constant timescale of decay, and weighs the uses of radiation against its hazards.

It reuses ideas from across the course: gamma radiation is part of the electromagnetic spectrum, and decay involves conservation of charge and nucleon number. Each dot point below has full worked answers and practice questions.

The nuclear model of the atom

The nuclear model of the atom describes the atom as a tiny, dense, positively charged nucleus of protons and neutrons surrounded by electrons. The proton (atomic) number identifies the element and the nucleon (mass) number is the total number of protons and neutrons. Isotopes are atoms of the same element with different numbers of neutrons.

Almost all the mass sits in the nucleus, while the atom as a whole is mostly empty space, a result first shown by alpha-scattering.

Radioactivity and types of emission

Radioactivity and types of emission introduces the three emissions from unstable nuclei. Their penetrating power increases from alpha to gamma, while their ionising power decreases:

• Alpha ($\alpha$): a helium nucleus, most ionising, stopped by paper.
• Beta ($\beta$): a fast electron, moderately penetrating, stopped by a few millimetres of aluminium.
• Gamma ($\gamma$): a high-energy electromagnetic wave, least ionising, reduced only by thick lead or concrete.

Half-life and decay

Half-life and decay defines half-life as the constant time for half the radioactive nuclei (or the activity) to decay, and treats decay as a random process that is predictable only on average. After $n$ half-lives the fraction remaining is

Uses and hazards of radiation

Uses and hazards of radiation balances benefit against risk. Uses include sterilising equipment, treating cancer, tracing fluids and dating with carbon-14. The hazard is that ionising radiation damages living cells and can cause mutations, so safe practice means limiting time of exposure, keeping a safe distance, and using shielding such as lead.

How this module is examined

• Identify radiation by its properties. Use penetration and ionisation, or the stopping material, to name alpha, beta or gamma.
• Halve, do not subtract, for half-life. After each half-life the activity halves; count whole half-lives and halve repeatedly.
• State decay as random but predictable on average. Individual decays cannot be predicted, but a large sample decays at a fixed rate.

Check your knowledge

Recall and calculation questions across the module. Try them, then check the worked solutions.

1. State what the nucleus of an atom contains. (2 marks)
2. State which type of radiation is the most penetrating. (1 mark)
3. Define the half-life of a radioactive source. (2 marks)
4. A source has an activity of $640\ \text{Bq}$ and a half-life of $2\ \text{hours}$. Calculate its activity after $6\ \text{hours}$. (2 marks)
5. State two precautions when handling radioactive sources. (2 marks)