Learning outcomes
- Explain alpha use in an ionisation smoke alarm.
- Explain gamma use in food irradiation and sterilisation.
- Choose radiation for thickness monitoring.
- Link radiation type and half-life to each application.
11.1 Choosing a radioisotope
An application requires a suitable emission type and half-life. The radiation must have enough penetration for the task but should not create unnecessary exposure. The half-life must be long enough for useful operation but not so long that disposal creates avoidable long-term activity.
Activity also matters. A tiny sealed source may be sufficient for detection or control. The design must keep the radioactive material contained and shield users from unnecessary radiation.
11.2 Ionisation smoke alarms
A small alpha source ionises air between electrodes, allowing a tiny steady current. Smoke particles enter the chamber, capture ions or interfere with their movement, reducing the current. The electronic circuit detects the change and sounds the alarm.
Alpha is suitable because it ionises strongly but has very low penetration, so the sealed source gives little external exposure in normal use. A reasonably long half-life allows the alarm to operate for years.
11.3 Irradiating food
Gamma radiation can pass through packaging and into food, killing bacteria, moulds or pests and slowing spoilage. The food does not become radioactive simply because gamma radiation passes through it; irradiation is not the same as contamination by radioactive material.
The source is housed in a shielded industrial facility and exposed remotely. High penetration is essential, and the process must deliver a controlled dose.
11.4 Sterilising medical equipment
Gamma radiation sterilises sealed equipment such as syringes or dressings by killing microorganisms after packaging. This avoids opening the sterile pack and can treat heat-sensitive items that might be damaged by high-temperature sterilisation.
A penetrating emission and suitable source life are required. Equipment becomes irradiated during treatment but does not normally contain the radioactive source.
11.5 Thickness control
A source and detector are placed on opposite sides of moving sheet material. If the sheet becomes thicker, more radiation is absorbed and detector count falls. A control system adjusts rollers to restore the target thickness.
Beta is suitable for paper, plastic or thin metal because it is partly absorbed by these thicknesses. Gamma may be used for thicker or denser material. Alpha would be absorbed too easily and would not pass through ordinary sheet material.
Worked examples
Choosing for paper
Beta is preferred because paper thickness changes produce measurable changes in transmission; gamma would pass too easily and alpha would be almost completely absorbed.
Smoke alarm choice
Alpha provides strong ionisation in the chamber and is contained easily because of its short range.
Practical focus
Investigation
Analyse transmission data for several materials and select a source type that gives a detector rate neither almost zero nor almost unchanged. Explain why a useful control system needs sensitivity to thickness changes.
Examination guidance
- Always link the chosen radiation to penetration and ionisation.
- Irradiated food is not made radioactive by ordinary gamma treatment.
- A thickness gauge uses changes in detector count as feedback.
- Alpha is suitable for smoke alarms because of strong ionisation and short range.
- Discuss half-life as well as radiation type when asked about source choice.
Check your understanding
- Why is alpha suitable inside a smoke alarm?
- Why is gamma used to sterilise sealed equipment?
- Why is beta often used for paper thickness control?
Answers
- It strongly ionises the chamber air but is easily contained because it has low penetration.
- It penetrates the packaging and equipment to kill microorganisms.
- Paper partly absorbs beta, so count rate changes measurably with thickness.
