Oscar Lanzi answered a question I’d had for ages:

The usage in ionizing smoke detectors requires a radioactive isotope to work. In addition to a sufficient half-life to make a smoke detector with a suitable service life, Am-241 emits mostly alpha radiation (which is used for the ionizing mechanism) with relatively little of the more hazardous and useless gamma radiation.

He linked to Wikipedia, which states:

The radiation passes through an ionization chamber, an air-filled space between two electrodes, and permits a small, constant current between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and affects this current, triggering the alarm.

Today I learned. And while we’re here, here’s another article on Wikipedia, found by clicking random article:

Apiotoma tibiaformis is a species of sea snail, a marine gastropod mollusk in the family Cochlespiridae.

Geshunteit. And let’s try another:

The radiation passes through an ionization chamber, an air-filled space between two electrodes, and permits a small, constant current between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and affects this current, triggering the alarm.

Wait a minute.