Edexcel IGCSE Physics: Radioactivity and Particles — Revision Notes

Condensed for the final weeks. For the full explanation, use the Radioactivity and Particles study guide.

Atomic structure

Protons and neutrons in the nucleus; electrons in shells. Atomic number = protons; mass number = protons + neutrons.

Isotopes — same protons, different neutrons. Identical chemistry, different nuclear stability.

Background — beyond the specification, not examinable (4PH1 does not name Rutherford, alpha scattering or the nuclear model of the atom): Rutherford’s alpha-scattering experiment, with each observation tied to its conclusion:

1. Most alpha particles passed straight through → the atom is mostly empty space.
2. Some were deflected → the nucleus is positively charged.
3. A very few bounced back → the nucleus is tiny but holds most of the mass.

Types of radiation

	Nature	Charge	Penetration	Ionising
Alpha	Helium nucleus	+2	Stopped by paper	Strongly
Beta	Fast electron	−1	Stopped by ~3 mm aluminium	Moderately
Gamma	EM wave	0	Reduced by thick lead	Weakly
Neutron	Neutron	0	Reduced by water or paraffin wax (hydrogen-rich material)	Weakly

Penetration and ionisation are inversely related. Alpha ionises strongly, so it loses energy quickly and cannot penetrate far. Gamma barely interacts, so it penetrates deeply. Understanding that trade-off answers most application questions.

Which source for which use follows directly:

• Thickness monitoring of paper or foil → beta, because alpha would be stopped entirely and gamma would pass straight through, so neither would respond to thickness changes.
• Smoke detector → alpha, because it ionises air strongly but is safely stopped inside the device.
• Medical tracer → gamma, because it penetrates the body to be detected outside.

Nuclear equations

alpha decay:     mass number -4,  atomic number -2
beta decay:      mass number unchanged,  atomic number +1
gamma:           no change to either
neutron emission: mass number -1,  atomic number unchanged

In beta decay a neutron becomes a proton plus an electron, which is why the atomic number rises while the mass number does not.

Both mass number and atomic number must balance on each side of a nuclear equation — always check both.

Half-life

The average time for half the undecayed nuclei in a sample to decay, or for the count rate to halve.

Radioactive decay is random and spontaneous — you cannot predict when a given nucleus will decay, which is why half-life is a statistical average. It is unaffected by temperature, pressure or chemical state.

Method: after n half-lives, the activity is the original divided by 2ⁿ. Count the halvings rather than trying to use a formula.

Fission and fusion

Fission — a large nucleus splits after absorbing a neutron, releasing energy and more neutrons, which can trigger a chain reaction. Controlled in a reactor by control rods absorbing neutrons and a moderator slowing them.

Fusion — small nuclei join to form a larger one, releasing energy. It powers stars.

Fusion is hard to achieve on Earth because both nuclei are positively charged and repel; overcoming that repulsion needs extremely high temperature and pressure, which are difficult to contain.

Safety

Reduce exposure by shielding (lead, concrete), distance, and limiting time. Handle sources with tongs and store them in lead-lined containers.

Background radiation comes from rocks (notably radon), cosmic rays, food, and medical sources. It must be subtracted from measured count rates before doing half-life calculations.

Contamination is when radioactive material is transferred onto or into an object or person, remaining an ongoing risk even after the original source is removed. Irradiation is exposure to radiation from an external source without any material being transferred — the risk stops as soon as the source is removed.

Choosing a source for a job — reasoning, not just a fact:

• Monitoring foil thickness: beta. Alpha would be completely absorbed and gamma would pass almost entirely through, so neither would respond to changes in thickness; beta is partly absorbed, so the count rate varies as the foil thickness changes.
• Smoke detector: alpha. It strongly ionises the air in the detector but is safely stopped within the device itself.
• Medical tracer: gamma, with a short half-life. Gamma penetrates the body so it can be detected externally, and a short half-life means activity falls quickly, limiting the patient’s overall exposure.

Exam traps

• Saying alpha is the most penetrating because it is most ionising.
• Choosing the wrong source for an application.
• Forgetting to balance both numbers in a nuclear equation.
• Not subtracting background count.
• Saying half-life depends on temperature.
• Confusing contamination with irradiation — only contamination remains a risk once the source is removed.
• Choosing gamma for foil-thickness monitoring — it passes through almost unaffected, so the reading would barely change with thickness.

Self-test

1. (Background, not examinable.) Give the three alpha-scattering observations and what each shows.
2. Why is beta used for thickness monitoring rather than alpha or gamma?
3. What happens to mass number and atomic number in beta decay, and why?
4. Define half-life, and say why it is an average.
5. Why is fusion difficult to achieve on Earth?

Answers: 1. (Background, not examinable.) Most passed through — the atom is mostly empty space; some deflected — the nucleus is positively charged; a few rebounded — the nucleus is very small and contains most of the mass. 2. Alpha would be completely absorbed and gamma would pass through almost unaffected, so neither would vary with thickness; beta is partly absorbed, so the count rate responds to thickness changes. 3. Mass number is unchanged and atomic number increases by one, because a neutron converts into a proton and an emitted electron. 4. The average time for half the undecayed nuclei in a sample to decay; decay is random, so only the average behaviour of a large number of nuclei is predictable. 5. Both nuclei are positively charged and repel each other, so extremely high temperatures and pressures are needed to bring them close enough to fuse.