Americium-241, atomic number 95, half-life of 432 years, is present as the oxide in household smoke detectors. A typical smoke detector source emits less than 35 kBq of alpha particles and low energy gamma rays. The alpha particles ionize air in a chamber fitted with two electrodes. When smoke enters the space between the electrodes, the alpha radiation is absorbed by smoke particles. The rate of ionization of the air is reduced, the electric current is reduced, and an alarm is triggered.

Plutonium is an alpha emitter, atomic number 94. When uranium-238 absorbs a neutron, it becomes uranium-239 which decays in several steps to plutonium-239. Between 1944 and 1988, the US is reported to have built and operated secret reactors that produced something like 100 metric tons of plutonium for nuclear weapons.

Thorium is a naturally-occurring radioactive alpha emitter, found in very small amounts in soil, rocks, and water. The most abundant isotope is thorium-232.

Uranium is present in trace amounts in virtually all rock, soil, ground water and sea water. Significant concentrations of uranium occur in some phosphate rocks, and minerals such as uraninite. The longer lived isotope, U-238 has a half-life 4.47x10⁹ years. On Earth, so far, only about half has decayed to lead.

Polonium-210

Polonium: discovered by the Curie's as a tiny fraction of uranium ore, before the isolation of radium; was named by them after their native Poland. Polonium is a pure alpha emitter that decays to lead with a half of life of 139 days. The alpha partices will not penetrate the skin, but if microgram amounts are ingested and distributed around the body, the intense alpha flux destroys the internal organs. Death follows. Polonium poisoning is not detected with the normal gamma ray detectors used in medical facilities.