1. Introduction
Ever since the discovery of radioactivity by Becquerel [Bec96], chemical separation procedures have played an important part in isolating and studying radioactive isotopes of the elements. Radiochemical techniques provide an unambiguous way of identifying the atomic number of species produced in fission or nuclear reactions. As the need to isolate and study shorter and shorter half-lives has grown, new techniques have been developed, to the extent that today's separation techniques are challenging chemical reaction rates and the concepts of physical design. An example of the former is the development of continuous gas-phase chemical reactions, and an example of the latter is the development of the continuous-flow solvent extraction system, SISAK, that has revolutionized solvent extraction techniques by bringing them from batch processes that required several minutes to continuous SISAK separations that require only a few seconds.
The very first radiochemical work reported in the literature probably is that of the Curies [Cur98a, Cur98b]. Their work led to the identification and characterization of the elements polonium and radium. It was the separation of various radioactive species by chemical separation that led Rutherford and Soddy to conclude that radioactive decay is accompanied by a change of the parent atom [Rut03, Sod05]. If the half-life of the radioactive material is short, the chemical separation must be completed rapidly. Perhaps the earliest fast separation was performed by Rutherford [Rut00], who used gas-sweeping techniques in his experiments to identify emanation from thorium (55-s 220Rn) (see Sec. 5.1). To identify different radioactive decay products of uranium-x, a fast electroplating technique was developed. In this technique, a polished lead plate was immersed in a dilute acid solution of uranium-z for 1 min, and the deposit was shown to decay with a half-life of 1.15 min [Sod14].
Nuclear fission produces several hundreds of nuclides. In fact, it was the extremely careful and systematic radiochemical investigation of Hahn and Strassmann [Hah39] that established the nature of the fission process. During their studies, Hahn and Strassmann, in spite of the difficult conditions, developed fast chemical separation procedures and discovered short-lived nuclides such as 57-s 91Rb [Hah40]. As radiochemical separation techniques were developed, fission products became an abundant source of neutron-rich nuclides and allowed the study of the decay properties of the fission products.
The radiochemical procedures developed during the Manhattan Project were reported in a three-volume work entitled Radiochemical Studies: The Fission Products, edited by Coryell and Sugarman [Cor51a]. Fast, aqueous-phase chemical separation procedures were developed to isolate and study short-lived isotopes of bromine and iodine. An indication of the half-life that was accessible by these techniques is illustrated by their measurement of the half-life of 87Br as being 54 s, and that of 137I as being 24 s [Lev51]. Techniques were also devised to sweep gaseous fission products from uranium solution during irradiation and to determine half-lives of nuclides in the gas by collecting the daughter products from the flowing gas on a negatively charged wire [Ove51, Dil51]. Nuclides with half-lives as short as 0.8 s were identified using this technique.
The production of nuclides by charged-particle reactions provided another avenue for generating radioactive nuclides, especially neutron-deficient nuclides. A variety of chemical procedures, used in cyclotron bombardment experiments at the University of California at Berkeley (UCB), were compiled by Meinke [Mei49a]. The compilation included a number of rapid chemical procedures requiring less than 15 to 20 min for the isolation of elements from copper to protactinium. Meinke also developed a solvent extraction procedure for protactinium requiring about 2 min [Mei49b]. Hardy and coworkers used the reaction of recoiling silicon from a magnesium target with hydrogen gas in order to separate 2.2-s 26Si [Har75].
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.
Ideally, nuclear-reaction products should be subjected to mass and charge separation for the study of the decay characteristics of a nuclide. Westgaard and coworkers [Wes69, Wes91] investigated thermochromatographic separation as a possible technique for providing a chemically separated gaseous beam that can be subjected to mass separation. The first such separation was reported by Grapengiesser and Rudstam [Gra73, Rud73]. They attached a thermochromatographic separation apparatus to the isotope separator facility OSIRIS, and achieved separation of short-lived bromine and iodine isotopes. Rudstam and coworkers [Rud81] achieved separation of neutron-rich isotopes of zinc and cadmium using thermochromatography after a mass separation by OSIRIS. Progress in coupling mass separation and chemical separation is imperative to improve the unique elemental identification of very short-lived species, especially for far-from-stability mass chains where the half-lives of several isotopes in a particular mass chain are nearly identical.
In the early decades, separations requiring several hours were used routinely [Mei49a, Kle54]. For example, fission yields of lanthanide nuclides were determined using ion-exchange procedures that took hours [Ste61a, Iye63]. In the 1960s and early 1970s, nuclear chemists turned their attention to the study of nuclides with half-lives in the range of minutes to tens of minutes, and to utilization of such nuclides in neutron activation analysis. A fast separation procedure for lanthanides using a small ion-exchange column was reported that was able to separate any specific lanthanide within 20 to 25 minutes [Ren64]. Multistep solvent extraction procedures requiring several minutes were used for the study of tin and antimony isotopes formed in fission [Hag62, Ren66]. On the other hand, volatilization procedures of a few seconds were available for elements such as bromine [Nuh72]. As attention turned to shorter-lived nuclides, faster multistep procedures, capable of providing separated products within a few seconds, had to be developed. Unfortunately, manual procedures tended to be slow. For example, multistep solvent-extraction procedures developed for selenium [Ren68] and yttrium [Ren76] were accomplished in about 150 s and 100 s, respectively. Obviously, such techniques are not useful for studying nuclides with half-lives in the range of a few seconds.
The first step in developing rapid separation techniques was to automate many or all of the steps involved in a procedure. By using automation, Kratz and coworkers [Kra70] developed a 5-s procedure for selenium. A 10-s ion-exchange procedure-was used by Klein and coworkers [Kle75] to study short-lived yttrium isotopes. Several “autobatch” procedures were developed for the study of short-lived nuclides [Mey80, Ren86a]. The separation time varied from 1.6 s for antimony to nearly 11 min for samarium. When the half-lives of nuclides of interest are less than a few seconds, the efficiency of production, isolation, and study by autobatch techniques decreases compared to continuous production and isolation of nuclides [Ste78, Lie81]. Again taking selenium as an example, continuous, gas-phase separation of selenium from fission products has been achieved in less than 2 s [Zen80, Ren82a]. In the last decade, nuclear chemists have developed a number of continuous-separation procedures for a variety of elements. Most of the procedures are based on solvent extraction, gas-phase chemistry, or thermochromatography.
A number of collections of radiochemical procedures have appeared, as well as reviews of techniques used in radiochemical separations. The earliest collection of procedures was published by Meinke [Mei49a] and was a compilation of procedures used at Lawrence Berkeley Laboratory. Another collection was published by Coryell and Sugarman [Cor51b] entitled Radiochemical Studies: The Fission Products; book 3 of this series was a collection of procedures. Kleinberg 's report [Kle54] also provided a collection of radiochemical procedures. As part of the Radiochemistry monograph series, Kusaka and Meinke published a volume on Rapid Radio-chemical Separations [Kus61]. In the first section, they describe the general procedures used for the sample preparation; the second section reviews the techniques used in rapid radiochemical separations. The last section gives a summary of procedures arranged according to elements. Herrmann and coworkers from the University of Mainz have been publishing periodic reviews of rapid radiochemical methods [Her69, Tra76a, Her82]. Rapid automated separations and
