Americium

Americium is a radioactive transuranic chemical element with symbol Am and atomic number 95. This member of the actinide series is located in the periodic table under the lanthanide element europium, and thus by analogy was named after the Americas.

Americium was first produced in 1944 by the group of Glenn T. Seaborg from Berkeley, California, at the metallurgical laboratory of University of Chicago. Although it is the third element in the transuranic series, it was discovered fourth, after the heavier curium. The discovery was kept secret and only released to the public in November 1945. Most americium is produced by uranium or plutonium being bombarded with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains about 100 grams of americium. It is widely used in commercial ionization chamber smoke detectors, as well as in neutron sources and industrial gauges. Several unusual applications, such as a nuclear battery or fuel for space ships with nuclear propulsion, have been proposed for the isotope ^242mAm, but they are as yet hindered by the scarcity and high price of this nuclear isomer.

Americium-241

Americium-241 (²⁴¹Am) is an isotope of americium. Like all isotopes of americium, it is radioactive. ²⁴¹Am is the most common isotope of americium. It is the most prevalent isotope of americium in nuclear waste. Americium-241 has a half-life of 432.2 years. It is commonly found in ionization type smoke detectors. It 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 fissile and the critical mass of a bare sphere is 57.6-75.6 kilograms and a sphere diameter of 19–21 centimeters. Americium-241 has a specific activity of 3.43 Ci/g (Curies per gram or 117.29 Gigabequerels (GBq) per gram). It is commonly found in the form of americium-241 dioxide (²⁴¹AmO₂). This isotope also has one meta state; ^241mAm, with an exitation energy of 2.2 MeV, and a half-life of 1.23 μs. Its presence in plutonium is determined by the original concentration of plutonium-241 and the sample age. Because of the low penetration of alpha radiation, americium-241 only poses a health risk when ingested or inhaled. Older samples of plutonium containing plutonium-241 contain a buildup of ²⁴¹Am. A chemical removal of americium-241 from reworked plutonium (e.g. during reworking of plutonium pits) may be required in some cases.

Isotopes of americium

Americium (Am) is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no known stable isotopes. The first isotope to be synthesized was ²⁴¹Am in 1944. The artificial element decays into alpha particles. Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom).

Nineteen radioisotopes of americium have been characterized, with the most stable being ²⁴³Am with a half-life of 7,370 years, and ²⁴¹Am with a half-life of 432.2 years. All of the remaining radioactive isotopes have half-lives that are less than 51 hours, and the majority of these have half-lives that are less than 100 minutes. This element also has 8 meta states, with the most stable being ^242mAm (t_1/2 141 years). The isotopes of americium range in atomic weight from 231.046 u (²³¹Am) to 249.078 u (²⁴⁹Am).

Some notable isotopes

Americium-241

Americium-241 is the most prevalent isotope of americium in nuclear waste. It is the americium isotope used in an americium smoke detector based on an ionization chamber. It is a potential fuel for long-lifetime radioisotope thermoelectric generators.