SISAK 3 – An Improved System for Rapid Radiochemical Separations by Solvent Extraction

The research reactor TRIGA Mainz – a strong and versatile neutron source for science and education

The TRIGA Mark II-reactor at the Johannes Gutenberg University Mainz (JGU) is one of three research reactors in Germany. The TRIGA Mainz became first critical on August 3rd, 1965. It can be operated in the steady state mode with a maximum power of 100 kWth and in the pulse mode with a peak power of 250 MWth and a pulse length of 30 ms. The TRIGA Mainz is equipped with a central thimble, a rotary specimen rack, three pneumatic transfer systems, four beam tubes, and a graphite thermal column. The TRIGA Mainz is intensively used both for basic and applied research in nuclear chemistry and nuclear physics. Two sources for ultra-cold neutrons (UCN) are operational at two beam ports. At a third beam port a Penning-Trap for highly precise mass measurements of exotic nuclides is installed. Education and training is another main field of activity. Here, various courses in nuclear and radiochemistry, reactor operation and reactor physics are held for scientists, advanced students, engineers, and technicians utilizing the TRIGA Mainz reactor.

Development of nuclear chemistry at Mainz and Darmstadt

This review describes some key accomplishments of Günter Herrmann such as the establishment of the TRIGA Mark II research reactor at Mainz University, the identification of a large number of very neutron-rich fission products by fast, automated chemical separations, the study of their nuclear structure by spectroscopy with modern detection techniques, and the measurement of fission yields. After getting the nuclear chemistry group, the target laboratory, and the mass separator group established at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, a number of large international collaborations were organized exploring the mechanism of deeply inelastic multi-nucleon transfer reactions in collisions of Xe and U ions with U targets, Ca and U ions with Cm targets, and the search for superheavy elements with chemical separations after these bombardments. After the Chernobyl accident, together with members of the Institute of Physics, a powerful laser technique, the resonance ionization mass spectometry (RIMS) was established for the ultra-trace detection of actinides and long-lived fission products in environmental samples. RIMS was also applied to determine with high precision the first ionization potentials of actinides all the way up to einsteinium. In the late 1980ies, high interest arose in results obtained in fusion-evaporation reactions between light projectiles and heavy actinide targets investigating the chemical properties of transactinide elements (Z≥104). Remarkable was the observation, that their chemical properties deviated from those of their lighter homologs in the Periodic Table because their valence electrons are increasingly influenced by relativistic effects. These chemical results could be reproduced with relativistic quantum-chemical calculations. The present review is selecting and describing examples for fast chemical separations that were successful at the TRIGA Mainz and heavy-ion reaction studies at GSI Darmstadt.

Fast chemical separations and laser mass spectrometry – tools for nuclear research

Fast chemical separation procedures applied in nuclear research require dedicated experimental techniques. Rapid discontinuous separation procedures are illustrated by an example to isolate technetium from fission products. The use of a gas jet and its combination with a thermochromatographic separation and with the continuous solvent extraction technique SISAK is described and examples are given for the investigation of short-lived fission products. The potential of resonance ionization mass spectrometry (RIMS) as a highly sensitive technique using different experimental systems is outlined for ultra trace analysis of long-lived plutonium and neptunium isotopes, including isotope ratio measurements of the plutonium isotopes. In addition, the precise determination of the first ionization potentials (IP) of ten actinide elements up to einsteinium and of technetium carried out by using the photoionization threshold method and requiring sample sizes of ∼ 1012atoms is presented.

Aqueous-phase chemistry of the transactinides

The experimental techniques developed to perform rapid chemical separations of the heaviest elements in the aqueous phase are presented. In general, these include transport of the nuclear reaction products to a separation device by the gas-jet technique and dissolution in an aqueous solution containing inorganic ligands for complex formation. The complexes are chemically characterized by a partition method which can be liquid–liquid extraction, ion-exchange- or reversed-phase extraction chromatography. The separated fractions are quickly evaporated to dryness for the preparation of samples for α-particle spectroscopy. Comments are given on the special situation in which chemistry has to be studied with single atoms. Theoretical predictions of chemical properties are compared to the presently known chemical behaviour of rutherfordium, Rf (element 104), dubnium, Db (element 105), seaborgium, Sg (element 106), and hassium, Hs (element 108) and to that of their lighter homologs in the Periodic Table in order to assess the role of relativistic effects in the chemistry of the heaviest elements.

Development of a SISAK extraction system for chemical studies of element 108, hassium

A liquid–liquid extraction system suitable for studies of chemical properties of Hs (element 108), in the form of HsO4, was developed using γ-emitting isotopes of its homologue Os. The system is targeted for the fast on-line extraction system SISAK, which operates in a continuous manner and is suitable for liquid-phase studies of transactinide elements. The distribution of OsO4 between various dilute NaOH solutions and toluene was studied. Both batch and SISAK on-line experiments were performed to develop an appropriate system. From analysis of the extraction curves equilibrium constants for the formation of the presumed complexes, Na[OsO4(OH)] and Na2[OsO4(OH)2], were obtained: K 1=(1±0.5)×104 and K 2=12±8, respectively. The SISAK system includes a liquid-scintillation detection system for α measurements. Due to quenching effects it is not possible to perform direct measurement of the aqueous phase α´s. Therefore, a two-stage extraction method that provides an indirect measurement of the activity in the aqueous phase was developed as part of the proposed system for Hs: Acidification of the raffinate from the first stage result in recovery of OsO4, which is highly extractable into toluene. The yield of extraction in the second step, from 0.01 M NaOH solution after acidification with H2SO4 solution, was (90±3) %.

Extraction of Nb and Ta, homolgues of Db, from sulphuric acid solutions with TOA in toluene using SISAK

Extractions of niobium and tantalum as homologues of dubnium (element 105) into tri-octylamine (TOA) in toluene from H2SO4and H2SO4/K2SO4solutions were performed to explore the possibilities of applying the automated on-line liquid–liquid extraction system SISAK to study chemical properties of dubnium in solutions. The results show that the extraction systems TOA/H2SO4and TOA/H2SO4+K2SO4are promising with respect to performing a study of Db with the SISAK system. Suggestions for performing a SISAK Db-experiment are presented. In batch experiments, Pa has also been investigated.

A comparision of the extraction of carrier-free 176,177W and 99Mo with that of U, Th, Am, Cm, La, Ce, Tm, Yb, Lu and Hf into Aliquat 336 in toluene from nitric, phosphoric, and sulphuric acid media

Carrier-free 176,177W and 99Mo have been extracted from 0.1-5.0M nitric, phosphoric and sulphuric acids into 0.22M Aliquat 336, CH3NR3Cl, in toluene. Their extraction properties have been compared to hafnium, to the lanthanides lanthanum, cerium, thulium, ytterbium and lutetium, and to the actinides americium, curium, thorium and uranium. From nitric acid solutions the extracted species of tungsten and molybdenum at low concentrations are probably CH3NR3· HMO4, where M=Mo or W. At higher nitric acid concentrations the extracted species are most likely similar to those of uranium, i.e. (CH3NR3)2·MO2(NO3)4 and possibly CH3NR3·MO2(NO3)3. For the extraction from phosphoric acid solutions the major extracted complex is most likely CH3NR3·MO2(H2PO4)3, while the minor extracted complex may be (CH3NR3)2·MO2(H2PO4)4 or even the more hydrolysed species CH3NR3·HMO4. From sulphuric acid solutions the extraction of molybdenum and tungsten has similarity with the extraction of uranium and thorium, although the decrease with acid concentration is slower.

A comparison of the extraction of carrier-free176,177W and99Mo with that of U, Th, Am, Cm, La, Ce, Tm, Yb, Lu, and Hf into tri-n-octyl amine in toluene from nitric, phosphoric, and sulphuric acid media

The extraction of carrier-free tungsten and molybdenum from nitric, phosphoric and sulphuric acid with 0.05 M TOA (tri-n-octyl amine, R3N) in toluene and 2.5% by volume 1-dodecanol has been studied. Their extraction behaviour has also been compared to hafnium, some trivalent lanthanides (lanthanum, cerium, thulium, ytterbium and lutetium) and some actinides (thorium, uranium, americium and curium). The extraction of these metal ions decreases in the order M4+>MO22+>M3+from >1 M nitric acid solutions, if hafnium, which is on the form HfO2+, is disregarded. At low nitric acid concentrations tungsten and molybdenum are most likely extracted as tungstate or molybdate ions (R3NH)2·MO4, while at higher concentrations they are most likely extracted similar to uranium, i.e. on the form R3NH·MO2(NO3)3. Further, the extraction from phosphoric acid solutions decreases in the order M4+>MO22+>M3+, if the elements which are easily hydrolysed, hafnium, molybdenum and tungsten, are disregarded. The major extracted complex of tungsten contains one TOA and the minor two TOA. The extracted complex with one TOA is probably (R3NH)·WO2(H2PO4)3or R3N·WO2(H2PO4)2, while that with two TOA more likely is (R3N)2H·WO2(H2PO4)3or even the more hydrolysed species of tungsten (R3NH)2·WO4. For molybdenum, the extracted complexes seem reasonably similar. Disregarding HfO2+the extraction decreases in the order MO22+>M4+>M3+for the chosen range of sulphuric acid concentration.

Aqueous chemistry of transactinides

The aqueous chemistry of the first three transactinide elements is briefly reviewed with special emphasis given to recent experimental results. Short introductory remarks are discussing the atom-at-a-time situation of transactinide chemistry as a result of low production cross-sections and short half-lives. In general, on-line experimental techniques and, more specifically, the Automated Rapid Chemistry Apparatus, ARCA, are presented. Present and future developments of experimental techniques and resulting perspectives are outlined at the end. The central part is mainly focussing on hydrolysis and complex formation aspects of the superheavy group 4, 5, and 6 transition metals with F-and Cl-anions. Experimental results are compared with the behaviour of lighter homologuous elements and with relativistic calculations. It will be shown that the chemical behaviour of the first superheavy elements is already strongly influenced by relativistic effects. While it is justified to place rutherfordium, dubnium and seaborgium in the Periodic Table of the Elements into group 4, 5 and 6, respectively, it is no more possible to deduce from this position in detail the chemical properties of these transactinide or superheavy elements.

Chemistry of Superheavy Elements

The number of chemical elements has increased considerably in the last few decades. Most excitingly, these heaviest, man-made elements at the far-end of the Periodic Table are located in the area of the long-awaited superheavy elements. While physical techniques currently play a leading role in these discoveries, the chemistry of superheavy elements is now beginning to be developed. Advanced and very sensitive techniques allow the chemical properties of these elusive elements to be probed. Often, less than ten short-lived atoms, chemically separated one-atom-at-a-time, provide crucial information on basic chemical properties. These results place the architecture of the far-end of the Periodic Table on the test bench and probe the increasingly strong relativistic effects that influence the chemical properties there. This review is focused mainly on the experimental work on superheavy element chemistry. It contains a short contribution on relativistic theory, and some important historical and nuclear aspects.