Americium

[Rennhack]

a silvery metal; it is a man-made element whose isotopes americium-237 through -246 are all radioactive. Americium-241 is formed spontaneously by the beta decay of plutonium-241. Trace quantities of americium are widely used in smoke detectors, and as neutron sources in neutron moisture gauges.

[Rennhack]

http://www.epa.gov/radiation/radionuclides/americium.htm