AQA Physics 6.4.2.1
Radioactive decay and nuclear radiation
Unstable nuclei give out radiation at random. Sort alpha, beta and gamma by what they are, how far they go and how strongly they ionise, then pick the right one for a job.
Combined Science · Higher tier
Start here
A sheet of paper in the sun.
Hold a sheet of paper between your face and the sun, and the sunlight on your face is blocked.
If you had to guess, could one sheet of paper block every kind of radiation?
Unstable nuclei
Decay is random.
Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is radioactive decay, and it is random: you cannot predict when any one nucleus will decay.
Activity
The rate at which a source of unstable nuclei decays, measured in becquerel (Bq). 1 Bq is one decay per second.
Count-rate
The number of decays recorded each second by a detector, such as a Geiger–Müller tube. A detector records only some of the decays, so count-rate is not the activity.
What a nucleus can emit
Alpha particle (α)
Two neutrons and two protons: the same as a helium nucleus.
Beta particle (β)
A high-speed electron ejected from the nucleus as a neutron turns into a proton.
Gamma ray (γ)
Electromagnetic radiation from the nucleus.
Neutron (n)
Some nuclei emit a neutron.
Bench · identify three sources
Which absorber stops the count?
Each source sits a few centimetres from a Geiger–Müller tube. The background count, with no source, is about 20 counts per minute. Try absorbers, then name each source.
Source A · no absorber: 400 counts per minute
This source gives out
| Range in air | Stopped by | Ionising power | |
|---|---|---|---|
| Alpha | a few cm | a sheet of paper, or skin | strongest |
| Beta | up to about a metre | a few mm of aluminium | medium |
| Gamma | a very long way | reduced by thick lead or concrete | weakest |
Uses and hazards
The properties pick the job.
Smoke alarm · alpha
Alpha ionises the air in the alarm so a small current flows. Smoke absorbs the alpha, the current drops, and the alarm sounds. Its short range keeps it inside the alarm.
Paper thickness · beta
Beta partly passes through paper. If the count-rate falls, the paper is getting thicker, and the rollers are adjusted.
Sterilising · gamma
Gamma passes through sealed packaging and kills microbes on medical equipment. Irradiating an object does not make it radioactive.
Ionising radiation damages living cells. Alpha is most dangerous inside the body, where all its ionisation happens in nearby cells; outside, skin stops it. Gamma is the hazard from outside, because it reaches the organs. Keep your distance: alpha and beta only travel a short way in air, and gamma gets weaker further away. Use shielding, and keep exposure time short.
Think again · what the counter reads
“A Geiger counter reads the activity of a source.”
Your call
ChooseA Geiger–Müller tube near a source records 60 counts per second. Is the activity of the source 60 Bq?
Sort
Alpha, beta or gamma?
Place each statement.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Identify
- Use which absorber brings the count to background.
- Explain
- Link the choice to range, penetration or ionising power.
- Compare
- Give the same property for each radiation.
Key fact
Radioactive decay is random. Alpha is the most ionising and least penetrating; gamma is the least ionising and most penetrating; beta is between.
Key note · AQA 6.4.2.1
Radioactive decay and nuclear radiation
- Some nuclei are unstable and emit radiation as they change to become more stable. Decay is random.
- Activity: the rate of decay, in becquerel (Bq). Count-rate: decays recorded each second by a detector.
- Emissions: alpha (2 protons, 2 neutrons), beta (high-speed electron from the nucleus, as a neutron becomes a proton), gamma (EM radiation), neutron.
- Alpha: a few cm in air, stopped by paper, most ionising. Beta: up to about a metre, stopped by a few mm of aluminium. Gamma: very long range, reduced by thick lead, least ionising.
- Uses follow the properties: smoke alarms (alpha), paper thickness (beta), sterilising (gamma).
- Alpha is most dangerous inside the body; gamma from outside. Distance, shielding and short exposure reduce the risk.
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure which absorber stops which radiation?
Bench count-rates are round teaching figures, not measurements. Diagrams are schematic.