Applications of radiation: smoke detectors use alpha, thickness gauges use beta

Nuclear Structure & Radiation — AQA A-Level Physics

Smoke detectors (alpha)

Contain Americium-241, a weak alpha source with a half-lifeThe time taken for half the number of radioactive nuclei in a sample to decay, or for the activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. to halve. of 460 years.
Alpha particles ionise nitrogen and oxygen molecules in air, enabling a small currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). to flow.
Smoke absorbs the alpha particles, reducing the currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). and triggering the alarm.

Thickness monitoring (beta)

Beta radiation passes through thin sheets of metal. A Geiger counter measures the transmitted count rate.
Changes in thickness alter the count rate, allowing automatic roller pressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻². adjustment.
Alpha would be stopped entirely. Gamma would pass through without detectable change. Only beta is sensitive to small thickness variations.
A source with a long half-lifeThe time taken for half the number of radioactive nuclei in a sample to decay, or for the activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. to halve. is chosen so it does not need frequent replacement.

Medical tracers (gamma)

Technetium-99m: gamma emitter, 6-hour half-lifeThe time taken for half the number of radioactive nuclei in a sample to decay, or for the activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. to halve., ideal energyThe capacity to do work. Measured in joules (J). (~140 keV) for external detection.
Short half-life minimises patient dose. Easily prepared at hospitals.
Iodine-131: beta emitter taken up by the thyroid gland, 8-day half-life. Used for thyroid monitoring.