Americium (Am) and Neptunian (Np) are both radioactive transuranic elements. With enough time, both eventually degrade to lead. Americium has a half life of 432 years, meaning that after 19 years, about 3% of its atoms have decayed by radiating an alpha particle, or the equivalent of a helium nucleus. This turns Americium into Neptunian.

Neptunian was the first element discovered by nuclear bombardment of uranium. Since uranium was named after the planet Uranus, the new element was named after Neptune. Americium is the actinide equivalent to the lanthanide Europium, and was named after the Americas.

Alpha radiation is very easily absorbed by even the thinnest of surfaces, including those of smoke particles. This is the inner working of most home smoke alarms today. That is also the only place to find both of these synthetic elements.

Rather than detail out the process of dissecting a smoke alarm, I will point you to an existing tutorial.

The consistent supply of alpha radiation from Americium can be used to make yourself a true random number generator.