SEC G3 Combined Science Physics component K326/K327
CP16: Radioactivity
Compare emissions, interpret half-life, and balance uses against exposure risk.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Radioactivity begins with nuclide structure and then connects random nuclear decay to radiation properties, half-life evidence, applications, and exposure risk.
Atoms, isotopes, and nuclides
An atom has a compact positive nucleus containing protons and neutrons, with negative electrons outside it. Proton number Z identifies the element, while nucleon number A counts protons plus neutrons.
Write a nuclide as , where X is the element symbol, A at the upper left is the nucleon number, and Z at the lower left is the proton number. It contains Z protons and neutrons. Isotopes share proton number but differ in neutron number, so their nucleon numbers differ.
Decay, radiation, and background
An unstable nucleus can lose energy by spontaneous, random emission. Random means the time of one nucleus's decay cannot be predicted, even though a large sample follows a measurable pattern.
Alpha radiation consists of positively charged helium nuclei, beta-minus radiation consists of fast electrons, and gamma radiation is uncharged electromagnetic radiation. Alpha is strongly ionising and weakly penetrating, beta is intermediate, and gamma is weakly ionising but highly penetrating.
Background counts can come from cosmic rays, rocks and building materials, living matter, and medical or industrial activity. Measure or obtain the background count and subtract it when the task asks for a source's corrected count rate.
Half-life, uses, and hazards
Half-life is the time for the number of undecayed nuclei, or corrected count rate, to fall to half its value. Apply repeated halving to tables and decay curves.
Medical uses include tracers and treatment, while industrial uses include thickness monitoring and fault detection. Risk depends on ionising ability, penetration, activity, exposure time, distance, shielding, and whether radioactive material enters the body.
Formulae and relationships
| Relationship | Use |
|---|---|
| Model repeated half-lives in simple calculations. |
Worked examples
Example 1: A corrected count rate falls from 640 counts per minute to 80 counts per minute. How many half-lives pass?
- 640 becomes 320 after one half-life.
- 320 becomes 160 after two and 80 after three.
- Count the three halvings.
Answer: Three half-lives.
Example 2: Interpret the nuclide notation for a neutral atom.
- Read , so the atom has protons.
- Calculate neutrons as .
- A neutral atom has the same number of electrons as protons.
Answer: 13 protons, 14 neutrons, and 13 electrons.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Use proton number, mass number, and neutron count consistently when interpreting nuclide notation and isotopes.
- Distinguish alpha, beta, and gamma radiation by nature, penetration, and ionising effect.
- Correct for background, halve the remaining count each interval, and connect uses and hazards to radiation properties.
Official outcome coverage
K326 CP16: 8 mapped outcomes, references CP16(a), CP16(b), CP16(c), CP16(d), CP16(e), CP16(f), CP16(g), CP16(h). Check the official K326 syllabus.
K327 CP16: 8 mapped outcomes, references CP16(a), CP16(b), CP16(c), CP16(d), CP16(e), CP16(f), CP16(g), CP16(h). Check the official K327 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Interpret radioactive count data without handling an unapproved source. Correct for background, repeat counts over equal intervals, control time, distance and shielding, and explain why random decay causes scatter even when the method is unchanged.
Exam traps and retrieval check
Avoid these traps
- Using mass number as proton number.
- Saying random decay has no statistical pattern.
- Comparing raw count rates without subtracting background when required.
Check from memory
How is neutron number found?
A minus Z.
Which radiation penetrates furthest?
Gamma radiation.
What halves during one half-life?
The undecayed nuclei or corrected activity/count rate.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Atomic and nuclide structure, random decay, alpha, beta and gamma radiation, background, half-life, uses, and hazards.