The center of this coin-sized aluminum button holds a 141 ng speck of americium-241 dioxide. Inside a household smoke detector, 241AmO2 provides a tiny but measureable stream of gamma radiation that ionizes the air between two electrodes, allowing current to flow. When smoke gets in the way, it intercepts that radiation and stops the current, triggering an alarm. In addition to emitting gamma radiation, 241Am also emits α particles, aka helium nuclei, and morphs into neptunium-237. 241Am is the product of β− decay of plutonium-241.
Credit: Andrew Magill/Wikimedia
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