4.1. CHEMICAL IDENTITY

Americium is a human-made actinide element (atomic number 95) and has no stable isotopes. It was discovered by Glen Seaborg, Leon Morgan, Ralph James, and Albert Ghiorso in 1944 and isolated by B.B. Cunningham as the isotope ²⁴¹Am in Am(OH)₃ in the fall of 1945. It was named after the Americas (Seaborg 1991; Seaborg and Loveland 1990). Actinides are the 15 elements starting with actinium, atomic number 89, and extending to lawrencium, atomic number 103. All of the isotopes of these elements are radioactive. Of the 15 americium isotopes and isomers currently identified, the longest-lived is ²⁴³Am.

4.2. PHYSICAL, CHEMICAL, AND RADIOLOGICAL PROPERTIES

Americium is a silvery, ductile, very maleable, non-magnetic metal. Americium melts at 1,176 °C, boils at 2,011 °C, and has an electron configuration of 5f⁷7s². The Chemical Abstract Service (CAS) registry numbers, decay modes, half-lives, and specific activities of the four principal americium isotopes and isomers, ²⁴¹Am,^242mAm, ²⁴²Am, and ²⁴³Am, are presented in Table 4-1. ²⁴¹Am and ²⁴³Am decay by alpha emission forming neptunium (atomic number 93), ²³⁷Np (t_½=2.14×10⁶ years) and ²³⁹Np (t_½=2.355 days) as the respective products. ²³⁹Np subsequently decays to ²³⁹Pu (t_½=2.411×10⁴ years), and then to ²³⁵U, which is also a naturally-occurring isotope of uranium. The decay of ²⁴¹Am to ²³⁷Np is accompanied by a predominant gamma ray photon of 59.54 keV. ^242mAm is an isomer (long-lived excited state of the nucleus) of ²⁴²Am and has a half-life of 141 years; 95.5% of ^242mAm undergoes an isomeric transition to ²⁴²Am (t_½=16.02 hours) with the emission of a 48.6 keV gamma ray, and 0.5% decays by alpha emission to ²³⁸Np. ²⁴²Am primarily (87%) undergoes beta decay to curium 242 (atomic number 96) (t_½=162.8 days), which decays to ²³⁸Pu (t_½=87.74 years) and then to ²³⁴U (t_½=2.45×10⁵ years) (another naturally-occurring uranium isotope); 17% undergoes electron capture to ²⁴²Pu (t_½=3.76×10⁵ years).

²⁴¹Am has a high specific activity of 3.428 Ci/g (0.1268 TBq/g), emitting ~7×10⁹ alpha particles/mg/minute. ²⁴³Am has a specific alpha activity about 17 times lower than ²⁴¹Am and is therefore more attractive for chemical investigations of the element (ICRP 1983; Lide 1998; O’Neil 2001; Seaborg 1991).

Table 4-1

Principal Amencium Isotopes.

The known oxidation states of americium are +2, +3, +4, +5, and +6. However, the stable oxidation states are +3 and +4; the common oxidation state is +3, in which state, the behavior of americium and other actinides is similar to the lanthanides. The trivalent state is the only state of importance in biological systems. The +2 oxidation state is very unstable and has only been produced in solid compounds. The stability of the americium oxidation states higher than +3 is less than that of uranium, neptunium, and plutonium (Cotton and Wilkinson 1980; Nenot and Stather 1979; Seaborg 1991). Am⁺³ hydrolyzes and forms weak complexes with serum proteins and other ligands. The physical and chemical properties of americium and selected americium compounds are shown in Table 4-2. Properties of some americium ions are shown in Table 4-3. The decay schemes for ²⁴¹Am and ²⁴³Am are shown in Tables 4-4 and 4-5.

Table 4-2

Physical and Chemical Properties of Amencium and Selected Americium Compounds.

Table 4-3

Properties of Some Americium Ions.

Table 4-4

²⁴¹Am Decay Scheme.

Table 4-5

²⁴³Am Decay Scheme.