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ULTRAFAST CHEMICAL SEPARATIONS
Just as the fission process was discovered by careful chemical separation studies, unambiguous evidence for the production of the first man-made element was obtained by McMillan and Abelson [MCM40]. Similarly, chemical identification obtained by means of separation procedures established the production of element 94 by bombardment of uranium with deuterons [Sea46a, Sea46b, Sea82]. The ion-exchange elution characteristics of actinides were used to identify many of the actinides [Sea58]. For example, the element mendelevium was discovered with the help of ion-exchange chromatography [Ghi55]. More and more rapid chemical separation procedures have been developed and used for the separation and identification of higher-atomic-number transactinides with shorter and shorter half-lives. Zvara and coworkers (see [Zva66 –85] inclusive) have done pioneering work in the utilization of thermochromatographic techniques for the separation and identification of transplutonium elements. They identified and studied a number of isotopes of elements with Z ≥ 104 using thermochromatography.
Many of the heaviest elements have been identified through isotopes with short half-lives. Weighable quantities of the elements are not available for elements beyond fermium, so chemical studies have been performed with very few atoms. A number of ingenious methods have been developed for “atom-at-a-time” radiochemical separations. Silva and coworkers [Sil70a] used a solvent extraction procedure and a semi-automated, cation-exchange separation procedure [Sil70b] that required approximately 60 s to identify lawrencium and rutherfordium and to establish their position in the periodic table. The international collaborative efforts to study the properties of mendelevium, lawrencium, and element 105 illustrate the applications of rapid radiochemical separations [Br ü88, Hof88, Jos88, Sch88a, Kra89]. Keller and Seaborg [Kel77], Herrmann and Trautmann [Her82], Hulet [Hul83], and Keller [Kel84] have reviewed the applications of fast separation techniques for the study of man-made elements.
Fast radiochemical separations have also been used in the unsuccessful searches for super-heavy elements. Armbruster and coworkers, in their exhaustive search for the formation of superheavy elements by fusion of 48Ca with 248Cm, used a variety of rapid separations to look for lead-like, radon-like, and platinum-like species [Arm85]. They used on-line chemical separations based on gas chromatography, ion-exchange, and cryogenics. In works to date, the absence of any identifiable species after appropriate chemical separations has led them to conclude that superheavy elements with half-lives in the range from 1µs to 10 yr were not produced with cross section greater than 10−34 to 10−35 cm2.
Radiochemical separations provide high-resolution charge separation for the products of nuclear reactions. As seen from the previous paragraphs, radiochemical separations have been used widely in different branches of nuclear and radiochemistry. A large number of fast separation procedures have been developed for the study of short-lived radioactive materials.
We note that the technique of on-line isotope separation, or ISOL, isolates products as isobars. In this case, ions of nuclear reaction products are isolated by a mass separator. In recent years, progress has been made on the use of chemistry within the ion source in order to provide atomic number (chemical) selectivity through the use of differing types of ion sources and conditions. On-line mass separation was first demonstrated by Kofoed-Hansen and Nielsen [Kof51], using krypton isotopes produced in uranium fission. During the last three decades, a number of on-line mass separations have been developed for the study of short-lived nuclides produced by charged-particle reactions as well as by nuclear fission. CERN's ISOLDE-2, Oak Ridge National Laboratory's UNISOR, the University of Maine's HELIOS, Orsay's ISOCELE, and Brookhaven National Laboratory 's TRISTAN-2 are a few examples of such on-line mass separators. Several authors have reviewed on-line mass separation techniques and their potential [Kla69, Kla74, Mac74a, Han79, Rav79, Dau86, All86, Gil86, Har86, Roe86, Tal86, Tal87]. The detailed reviews by Hansen [Han79] and Ravn [Rav79] discuss mass separation techniques and their application to the nuclear spectroscopy of nuclei far from stability.