Berkelium availability

Americium, californium, and einsteinium oxides have been reduced by lanthanum metal, whereas thorium has been used as the reductant metal to prepare actinium, plutonium, and curium metals from their respective oxides. Berkelimn metal could also be prepared by Th reduction of Bk02 or Bk203, but the quantity of berkelium oxide available for reduction at one time has not been large enough to produce other than thin foils by this technique. Such a form of product metal can be very difficult to handle in subsequent experimentation. The rate and yield of Am from the reduction at 1525 K of americium dioxide with lanthanum metal are given in Fig. 2. 

Selected nuclear properties of the principal isotopes of berkelium are listed in Table I (6). In addition to these isotopes, ranging from mass numbers 240 to 251, there are spontaneously fissioning isomers known for berkelium mass numbers 242, 243, 244, and 245, all with half-lives of less than 1 /usee. Only 249Bk is available in bulk quantities for chemical studies, as a result of prolonged neutron irradiation of Pu, Am, or Cm (7). About 0.66 g of this isotope has been isolated from... 

In later work, the absorption spectra of Bk(III) and Bk(IV) were recorded in various media (95). New absorption bands were reported as the result of using larger quantities of berkelium-249 of higher purity than had been previously available. Observations of the spectrum of Bk(III) were extended further into the ultraviolet wavelength region (to 200 nm), and nine new absorption bands were reported (96). Later the absorption spectra of Bk(III) and Bk(IV) in 2 M perchloric and 0.5 M nitric acid solutions have been obtained (97). An interpretation of the low-energy bands in the solution absorption spectra of Bk(III) and Bk(IV) has been published (98). 

Later berkelium metal samples of up to 0.5 mg each have been prepared via the same chemical procedure (120). Elemental berkelium can also be prepared by reduction of BkF4 with lithium metal and by reduction of Bk02 with either thorium or lanthanum metal. The latter reduction process is better suited to the preparation of thin metal foils unless multimilligram quantities of berkelium are available. 

Earlier reviews of the physicochemical properties of berkelium are available in several new supplement series volumes of Gmelin Handbuch der Anorganischen Chemie (G. Koch, editor, Springer-Verlag, New York) and in The Chemistry of the Transuranium Elements (C. Keller, 1971, Verlag Chemie, Weinheim). 

This collection of the state-of-the-art papers emphasizes the continuing importance of industrial-scale production, separation, and recovery of transplutonium elements. Americium (At. No. 95) and curium (At. No. 96) were first isolated in weighable amounts during and immediately after World War II. Berkelium and californium were isolated in 1958 and einsteinium in 1961. These five man-made elements, in each case, subsequently became available in increasing quantities. 

Although the outermost electrons in the actinides are apparently more readily available for bonding than those in the lanthanides, even occurring, the preference for increases with atomic number. Redox potentials indicate that this trend is still more rapid than in the lanthanides. The fact that Cm is dominant (cf, gadolinium in the lanthanides), and that berkelium gives also Bk, lends support to the suggestion that these elements have a half-filled 5f shell with its characteristic stability. 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopically pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each berkelium, mendelevium, nobelium, and lawrencium, 10 each. There is frequendy a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availability have customarily been used. A list of these is provided in Table 1. 

Berkelium is currently available in microgram quantities—sufficient to determine struc- ... 

When these isotopes become available, chemical studies will be greatly simplified, and the complications introduced by the radioactivity of the actinide elements will be substantially minimized. The longest-lived isotopes of berkelium, californium, and einsteinium are still fairly short-lived substances, and macroscopic amounts have a tremendous associated radioactivity. Nevertheless, it should eventually be possible to prepare and study the solid halides of the actinide elements through the element einsteinium using weighable amounts of reactants. This remains for the future, however. The special experimental problems associated with highly radioactive substances are considered below. 

The quantity of information available for the berkelium oxygen system is far less than that for the previous actinides, due mainly to the scarcity and the shorter half-life (325 days) of the Bk-249 isotope used in solid-state studies. The sesquioxide and the dioxide are the two well-established oxides of Bk, and the dioxide is much more stable than the dioxides of Cm and Am. This stability can be attributed in part to the fact that the Bk(IV) state results in a half-filled 5f state, as found for TbOj. However, BkOj is much more stable and is formed more easily than is TbOj. 

Actinium and thorium have no / electrons and behave like transition metals with a body-centered cubic structure of thorium. Neptunium and plutonium have complex, low-symmetry, room-temperature crystal structures and exhibit multiple phase changes with increasing temperature due to their delocalized 5/ electrons. For plutonium metal, up to six crystalline modifications between room temperature and 915 K exist. The / electrons become localized for the heavier actinides. Americium, curium, berkelium, and californium all have room-temperature, double hexagonal, close-packed phases and high-temperature, face-centered cubic phases. Einsteinium, the heaviest actinide metal available in quantities sufficient for crystal structure studies on at least thin films, has a face-centered cubic structure as typical for a divalent metal. 

Although berkelium is available only in very small quantities, enough has been prepared to determine some stmctural parameters. 

The first structure determination of a compound of berkelium, the dioxide, was carried out in 1962 [5]. Four x-ray diffraction lines were obtained from 4 ng of Bk02 and indexed on the basis of a face-centered cubic structure with Oq = 0.533 0.001 nm. In the intervening 24 years, considerable information about the physicochemical properties of berkelium has been obtained in spite of the rather limited availability and short half-life of Bk, the only isotope available in bulk quantities. 

Also available are the results of relativistic relaxed-orbital ab initio calculations of L-shell Coster-Kronig transition energies for all possible transitions in berkelium atoms [75], relativistic relaxed-orbital Hartree-Fock-Slater calculations of the neutral-atom electron binding energies in berkelium [76], and... 

For basic studies on weighable quantities of californium, the Cf isotope is used. Its alpha half-life of 351 4 years [2,3] makes it suitable for chemical/physical experiments, where weighable quantities of californium are required. The Cf isotope is available as an isotopically pure material from the decay of Bk (beta emitter, half-life of 320 days), the latter being the major berkelium isotope obtained from reactors ( Bk is also formed, but it has a 3.5 h half-life). To obtain Cf free of other californium isotopes, it is first necessary to separate berkelium chemically from the californium produced in a reactor, and then permit the Bk to decay to Cf, which can subsequently be chemically separated from the berkelium. Currently, up to 60 mg per year of Bk are produced in the HFIR at ORNL, which is sufBdent to provide multi-milligram amounts of Cf [4]. The only other known production of Bk, and hence isotopically pure Cf (excluding the use of a mass separator), is in the USSR. The quantity of these materials available in the USSR is believed to be less than that produced by the HFIR. 

Desire, Hussonnois and GuUlaumont (1969) determined stability constants for the species AnOH + for the actinides, plutonium(III), americium(III), curium (III), berkelium(III) and californium (III) using a solvent extraction technique. The stability constants obtained for americium(III) and curium(III) are two orders of magnitude larger than other similar data available in the literature. The stability constants of the lanthanide(III) and actinide(III) ions are very difficult to obtain using solvent extraction due to problems associated with attainment of maximum extraction into the solvent phase before the narrow band of pH between the onset of hydrolysis reactions and the precipitation of solid hydroxide phases. Consequently, the data of Desire, Hussonnois and GuUlaumont (1969) are not retained in this review. 

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