Americium is alkaline metal element with the symbol Am and atomic number 95. It is above uranium in the periodic table and is referred to as a transuranic element. Silvery in color, Americium is a soft radioactive metal. It was first synthesized in 1944 by Glenn T. Seaborg at the University of California, Berkeley. The name was taken from America.
Americium oxidizes easily and dissolves well in acids. It forms compounds with halogen gases, sulfur., selenium, phosphorus, arsenic, antimony, bismuth and silicon. The metallic form is malleable and has a relatively high melting point of 1173 C°
Americium has 19 isotopes, all of them radioactive. The atomic weights vary from Am-231 to Am-249. The half-lives vary from Am-239m at 163 billionths of a second to Am-243 at 7,370 years. Americium is produced by the bombardment of uranium, plutonium or existing Americium with slow neutrons. A tiny amount is produced in nuclear reactors and in nuclear explosions. Am-241 with a half-life of 432 years is the most plentiful isotope in radioactive waste. It decays by alpha emission with gamma ray production.
Commercial production of americium consists of dissolving the uranium and plutonium out of spent mixed oxide nuclear fuel. This leaves a mixture of oxides of actinide and lanthanide isotopes. Using various solvents along with chromatography and centrifuges, the Am-241 is extracted. The Am-241 can then be used to create other isotopes of americium if desired.
The most common use of americium is in home smoke detectors. It is a source of ionizing radiation in the form of alpha particles but it emits much less gamma radiation and is preferred over radium-228. About one quarter of a millionth of a gram of americium is used in a typical smoke detector. The americium ionizes the air between two electrodes allowing a small current to flow. When smoke enters the alarm, it reduces the ionization which lowers the current and triggers the alarm. These alarms are more sensitive but more prone to false alarms than optical smoke detectors.
Americium combined with beryllium is used as a source of neutrons in devices that measure the water content of soil, neutron radiography, tomography, and other applications that require neutrons.
Use of americium in radioisotope thermoelectric generators has been suggested. It has five times the half life of the plutonium which is currently used in such generators. Americium may also find use as part of a propulsion system for spacecraft. The fission products of americium could be used to directly drive the craft or it could be used to heat a thrusting gas or liquid. Efficient nuclear batteries based on the charge properties of americium have also been proposed. The main impediment to these new applications lies in the high cost and low availability of americium.
As with other transuranics, americium has no biological function. It mainly emits alpha particles which can be easily blocked but are dangerous if inhaled or consumed. Inside the body, the americium finds it way to the bones, the liver and the testicles or ovaries and can cause cancer. Daughter products from americium decay produce dangerous gamma rays and neutrons. There are few regulations about disposure of smoke detectors which is the main way that americium enters the environment.
Inside a smoke detector:
