Americium physical properties

McWhan, D. B. Crystal Structure and Physical Properties of Americium... 

After the discovery of plutoninm and before elements 95 and 96 were discovered, their existence and properties were predicted. Additionally, chemical and physical properties were predicted to be homologous (similar) to europium (gjEu) and gadolinium ( Gd), located in the rare-earth lanthanide series just above americium (gjAm) and curium ((,jCm) on the periodic table. Once discovered, it was determined that curium is a silvery-white, heavy metal that is chemically more reactive than americium with properties similar to uranium and plutonium. Its melting point is 1,345°C, its boihng point is 1,300°C, and its density is 13.51g/cm. ... 

Figure 17 is a clear illustration of the Mott-Hubbard transition in the actinide series the 5f emission occurs, for a-Pu, at Ep, indicating a high 5f-density of states pinned at the Fermi-level, whereas the 5 f emission occurs at lower energy for americium metal. In this case, therefore, a theoretical concept deduced indirectly from the physical properties of the two metals, finds direct (one might even say photographic ) confirmation in the photoemission spectra. 

The transuranium elements such as neptunium, plutonium or americium form hexafluoride compounds at their highest valency state with physical properties close to those of UF6 but these compounds are not stable when the fluorine partial pressme decreases. Under such conditions they are converted into a lower valency state and remain as a soUd product. This means that the main fraction of these impurities can be collected as ash or dust if there is still a small proportion remaining in the liquid UFe, it can be removed by a special filter before the container is filled. The specified upper limit for residual transuranium activity in the reprocessed uranium amounts to 2.5Bq/gU, with Pu, Pu and Np as the guide isotopes. 

The actinoid series encompasses the fourteen chemical elements with atomic numbers from 90 to 103, thorium (Th) to lawrencium (Lr). The actinoid series derives its name from the group-IIla element actinium (Ac) which can be included in the series for the purpose of comparison. Only Th and uranium (U) occur in usable quantities in nature. The other actinoids are man-made elements. Pure Th is a silvery-white metal which is air-stable and retains its luster for several months. U exhibits three crystallographic modifications as follows a (688°C) —> P (776°C) —> U is a heavy, silvery-white metal. The luster of freshly prepared americium (Am) is white and more silvery than neptunium (Np) or plutonium (Pu) prepared in the same manner. All actinoid elements are radioactive. Table 2.113 sutnmarizes some physical properties of actinoid metals (Th, U and Am). 

Americium metal has been prepared by the following methods (1) reduction of AmF3 with barium (or lithium) metal (2) reduction of Am02 with lanthanum metal (3) bomb reduction of AmF4 with calcium metal (4) thermal decomposition of Pts Am. Lanthanum reduction of Am02 in tantalum equipment and subsequent distillation of the americium metal from the reaction mixture yields americium of very high (>99.9%) purity. There is about 10 -fold difference in americium-lanthanum volatility. Extensive application of this technique by the Euratom group has led to important new measurements of the physical properties and thermodynamic properties of americium metal [81,342], Rocky Flats workers have reported similar success with vacuum distillation [333]. 

These experts collectively have knowledge of americium s physical and chemical properties, toxicokinetics, key health end points, mechanisms of action, human and animal exposure, and quantification of risk to humans. All reviewers were selected in conformity with the conditions for peer review specified in Section 104(I)(13) of the Comprehensive Environmental Response, Compensation, and Liability Act, as amended. 

Physical and Chemical Properties of Americium and Selected Americium Compounds... 

Physical, Chemical, and Radiological Properties. Data on the physical and chemical properties of americium and americium compounds are available in the literature (Cotton and Wilkinson 1980 Lide 1994, 1998 Nenot and Stather 1980 Seaborg 1991). Data are also available on the radiological properties of americium isotopes (ICRP 1983 Lide 1998). 

In proceeding across the actinide series, two major and concomitant events occur (1) more 5f electrons are added, and (2) the 5f bands narrow. As 5f electrons are added, they remain in the valence band and hybridize strongly with the s-d electrons. In contrast to their importance in the rare earths, s-d electrons contribute little to the chemistry and physics of the early actinide metals compared to the enormous influence of the 5f electrons. Narrowing of these bands as one proceeds across the series results finally in the onset of f-electron localization. This incipient localization produces a bewildering array of temperature- and structure-dependent properties at plutonium. Localization is percipitous and complete at americium, driven finally by spin polarization. 

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