Half-life — Explain

Explain how the type of radiation emitted and the half-life of an isotope determine which isotope is suitable for: (a) smoke alarms (b) irradiating food (c) sterilising equipment (d) measuring/controlling thickness (e) diagnosing and treating cancer.

(a) Smoke alarms use an alpha source with a long half-life. Alpha radiation ionises the air between two electrodes, allowing a small current to flow; smoke entering the detector absorbs the alpha particles (since alpha has a very short range), reducing the current and triggering the alarm. Alpha is safe here because it cannot travel far enough to leave the detector and reach people, and a long half-life means the source does not need frequent replacing. (b) Irradiating food to kill bacteria uses gamma radiation because it is highly penetrating, so it can pass through the food and kill bacteria/microorganisms throughout it, without making the food radioactive. (c) Sterilising equipment uses gamma rays because they are highly penetrating and can pass through packaging/equipment to kill bacteria without damaging the equipment or leaving any residue. (d) Measuring and controlling thickness requires radiation that is only partly absorbed by the material, so the count rate detected changes measurably with thickness. Beta radiation is used for thinner materials (e.g. paper): if the material gets thicker, more beta is absorbed and the count rate falls, which can be used to control the rollers. Gamma radiation is used for thicker/denser materials (e.g. metal sheet), since it can still penetrate through while remaining sensitive to thickness changes. Alpha is unsuitable as it would be completely absorbed by even a thin sheet and would not reach the detector at all. (e) Diagnosis and treatment of cancer uses gamma sources because gamma radiation can penetrate body tissue. For diagnosis, a gamma-emitting tracer with a short half-life is used so that it decays away quickly, minimising the radiation dose received by the patient, while still being detectable outside the body (e.g. with a gamma camera) to see how it moves through or is absorbed by the body. For treatment, a gamma source is directed at (or placed near) a tumour to kill the cancerous cells.