Hvor mange røykvarslere finnes det egentlig i Norden f eks?

Americium-241 is fissile (which is generally true of the isotopes of all elements starting with plutonium) and its critical mass is around 60–70 kg in its densest form. This amount of Am-241 has an activity level of about 200,000 Ci (Curies), since its specific activity is 3.43 Ci/g.

A typical ionization type smoke detector has 0.9 μCi (microcuries) of americium. So to get a critical mass you would need 200 billion of them to get a critical mass.