Americium
Americium was first produced in 1944 by the group of Glenn T. Seaborg at the University of California, Berkeley. 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 bombarding uranium or plutonium 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 is a relatively soft radioactive metal with silvery appearance. Its most common isotopes are 241Am and 243Am. In chemical compounds, they usually assume the oxidation state +3, especially in solutions. Several other oxidation states are known, which range from +2 to +7 and can be identified by their characteristic optical absorption spectra. The crystal lattice of solid americium and its compounds contains intrinsic defects, which are induced by self-irradiation with alpha particles and accumulate with time; this results in a drift of some material properties.
Occurrence
The longest-lived and most common isotopes of americium, 241Am and 243Am, have half-lives of 432.2 and 7,370 years, respectively. Therefore, all primordial americium (americium that was present on Earth during its formation) should have decayed by now.
Existing americium is concentrated in the areas used for the atmospheric nuclear weapons tests conducted between 1945 and 1980, as well as at the sites of nuclear incidents, such as the Chernobyl disaster. For example, the analysis of the debris at the testing site of the first U.S. hydrogen bomb, Ivy Mike, (1 November 1952, Enewetak Atoll), revealed high concentrations of various actinides including americium; due to military secrecy, this result was published only in 1956. Trinitite, the glassy residue left on the desert floor near Alamogordo, New Mexico, after the plutonium-based Trinity nuclear bomb test on 16 July 1945, contains traces of americium-241. Elevated levels of americium were also detected at the crash site of a US B-52 bomber, which carried four hydrogen bombs, in 1968 in Greenland.
In other regions, the average radioactivity of surface soil due to residual americium is only about 0.01 picocuries/g (0.37 mBq/g). Atmospheric americium compounds are poorly soluble in common solvents and mostly adhere to soil particles. Soil analysis revealed about 1,900 higher concentration of americium inside sandy soil particles than in the water present in the soil pores; an even higher ratio was measured in loam soils.
Americium is produced mostly artificially in small quantities, for research purposes. A tonne of spent nuclear fuel contains about 100 grams of various americium isotopes, mostly 241Am and 243Am. Their prolonged radioactivity is undesirable for the disposal, and therefore americium, together with other long-lived actinides, have to be neutralized. The associated procedure may involve several steps, where americium is first separated and then converted by neutron bombardment in special reactors to short-lived nuclides. This procedure is well known as nuclear transmutation, but it is still being developed for americium.
A few atoms of americium can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits.
| Symbol | Am | |
| Atomic Number | 95 | |
| Atomic Weight | 243.0614 | |
| Oxidation States | +3, +4, +5, +6 | |
| Electronegativity, Pauling | 1.3 | |
| State at RT | Solid, Metal | |
| Melting Point, K | 1267 | |
| Boiling Point, K | 2880 |
Appearance and Characteristics of Americium
Harmful effects:
Americium is harmful due to its radioactivity.
Characteristics:
- Americium is a silvery-white highly radioactive metal that has a density similar to lead.
- It tarnishes slowly in dry air at room temperature.
- Isotope 241Am, the most common isotope, decays to 237Np, emitting alpha and gamma radiation.
Uses of Americium
- Isotope 241Am is used (in the form of americium dioxide) in very small amounts in ‘ionization chamber’ smoke detectors. One gram of americium dioxide provides enough active material for more than three million household smoke detectors.
- Americium is used as a portable source of gamma rays and alpha particles for use in medicine, science and industry.
- It is also used as a target material in nuclear research to make even heavier elements.
Americium-241 radiation emissions (preceded by Pitchblend ore ) displayed in a cloud chamber
