Americium-241 (²⁴¹Am, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of 432.6 years. ²⁴¹Am is the most common isotope of americium as well as the most prevalent americium isotope in radioactive waste. It is used in ionization-type smoke detectors and is a potential fuel for long-lifetime radioisotope thermoelectric generators (RTGs). Its common parent nuclides are β⁻ from ²⁴¹Pu, EC from ²⁴¹Cm, and α from ²⁴⁵Bk. ²⁴¹Am is not fissile, but it is fissionable: the critical mass of a bare sphere is 57.6–75.6 kilograms (127.0–166.7 lb) and a sphere diameter of 19–21 centimetres (7.5–8.3 in). Americium-241 has a specific activity of 3.43 Ci/g (126.91 GBq/g). It is commonly found in the form of americium-241 dioxide (²⁴¹AmO₂). The presence of ²⁴¹Am in plutonium is determined by the original concentration of plutonium-241 (which decays to it) and its age. Older samples of plutonium containing ²⁴¹Pu build up ²⁴¹Am may require chemical removable of americium-241 (e.g., during reworking of plutonium's pits).

Americium-241 has been produced in small quantities in nuclear reactors for decades, and many kilograms of ²⁴¹Am have been accumulated by now. Nevertheless, since it was first offered for sale in 1962, its price, about US$1,500 per gram of ²⁴¹Am, remains almost unchanged owing to the very complex separation procedure.

Americium-241 is synthesized by three neutron captures on uranium-238 present in reactors:

The plutonium present in spent nuclear fuel contains about 12% of ²⁴¹Pu. Because it converts to ²⁴¹Am, ²⁴¹Pu can be extracted and may be used to generate further (isotopically pure) ²⁴¹Am. However, this process is rather slow: half of the original amount of ²⁴¹Pu decays to ²⁴¹Am after about 14 years, and the ²⁴¹Am amount reaches a maximum after 70 years.

The obtained ²⁴¹Am can be used for generating heavier americium isotopes by further neutron capture inside a nuclear reactor. In a light water reactor (LWR), 79% of neutron captures on ²⁴¹Am convert to ²⁴²Am and 10% to its nuclear isomer ^242mAm:

Americium has a lower valence and lower electronegativity than plutonium, neptunium or uranium, so in most nuclear reprocessing, americium tends to fractionate with the alkaline fission products – lanthanides, strontium, caesium, barium, yttrium – rather than with lighter actinides. Americium is therefore not recycled into new nuclear fuel unless special efforts are made.

In a thermal reactor, ²⁴¹Am captures a neutron to become americium-242, which quickly becomes curium-242 (or, 17.3% of the time, ²⁴²Pu) via beta decay. Both ²⁴²Cm and ²⁴²Pu are much less likely to absorb a neutron, and even less likely to fission; however, ²⁴²Cm is short-lived (half-life 160 days) and almost always undergoes alpha decay to ²³⁸Pu rather than capturing another neutron. In short, most ²⁴¹Am needs to absorb two neutrons before again becoming a fissile isotope, except that that becomes ^242mAm (fissile) or fissions directly.

Americium-241 decays by alpha emission, with a low-energy gamma ray byproduct. The α-decay is shown as follows: