Actinide targets for the synthesis of super-heavy elements☆
Introduction
Actinides are radioactive elements ranging from atomic number 89 through 103. First identified as a new row in the Periodic Table by Seaborg in the 1940s [1], [2], most actinides are highly reactive metals with a wide range of valence states and physical properties. Thorium and uranium are the only actinides that occur in substantial quantities in nature. Actinium and protactinium were discovered around 1900 as decay products in uranium ores [3].
All actinides heavier than uranium were discovered using other actinides as targets. Neptunium (1940), plutonium (1941), americium (1944), curium (1944), berkelium (1949), and californium (1950) were first produced at the University of California, Berkeley using accelerator-based nuclear reactions with neutrons, deuterons, and alpha particles on actinide targets [4]. Einsteinium and fermium were first isolated at Berkeley in 1952 from the products of neutron irradiation of uranium in a nuclear explosion [5]. Mendelevium (1955), nobelium (1965), and lawrencium (1961–1965) were first produced in accelerators at Berkeley [6], the Joint Institute for Nuclear Research, Dubna [7], [8], and Berkeley [9] and Dubna [10], respectively, all using actinide targets. Synthetic production in nuclear reactors is required to accumulate more than trace quantities of actinides heavier than uranium, with only neptunium, plutonium, americium, curium, berkelium, and californium available in the quantities () needed for targets for super-heavy element synthesis.
Super-heavy elements are elements with atomic numbers of 104 or greater. Nine of the 15 super-heavy elements synthesized to date have been produced using actinide targets. The first super-heavy element, rutherfordium (), was originally observed [11] at Dubna in 1964 using accelerator irradiation of a plutonium target with neon ions, with further work [12], [13] at Dubna and Berkeley leading to formal acceptance as a new element. Dubnium () was produced [14], [15] contemporaneously at Berkeley and Dubna in 1970, from nitrogen on californium and neon on americium, respectively. Berkeley used oxygen beams on californium to produce seaborgium () [16] in 1974 with contemporaneous work at Dubna [17]. From 1981–1996, super-heavy elements were first produced at GSI, Darmstadt using non-actinide lead and bismuth targets in cold fusion reactions [18], [19], [20], [21], [22], [23].
Since 2000, six new elements with atomic numbers have been produced [24], [25], [26], [27], [28] using the “hot fusion” technique [29], where neutron-rich actinide targets are bombarded by neutron-rich 48Ca ions, as summarized in Table 1. These elements were first produced at Dubna, with later hot fusion experiments at Berkeley confirming element 114 [30] and GSI, Darmstadt confirming elements 114 [31], 115 [32], 116 [33] and 117 [34]. Successfully pioneered and developed [29] at Dubna, hot fusion takes advantage of the increasing fission barrier height and reduced neutron separation energy as the predicted closed shell at is approached. This allows the compound nucleus to stabilize by shedding 2–4 neutrons, resulting in increased survivability in comparison to compound nuclei from cold fusion reactions for or greater. The use of actinide + 48Ca reactions has increased the production rate for super-heavy elements with or greater by one or more orders of magnitude, reducing required accelerator times from years to months and enabling the discovery of elements up to and possibly higher. Element 113 has also been produced using a non-actinide target, in a multi-year experiment using a zinc beam on a bismuth target at RIKEN, Wako [35]. Additional information on the observation and identification of isotopes with can be found in [36].
Section snippets
Production and availability of actinides
Actinide targets for super-heavy element synthesis using 48Ca beams have included 238U, 237Np, 242,244Pu, 243Am, 245,248Cm, 249Bk, and 249Cf, with all target materials heavier than uranium produced in nuclear reactors. Many of these actinide materials have been obtained from Oak Ridge National Laboratory (ORNL) and from the Research Institute for Advanced Reactors (RIAR) at Dmitrovgrad. At ORNL, the combination of the High Flux Isotope Reactor (HFIR) and the Radiochemical Engineering
Plutonium, americium, and curium
Transuranic actinides, including Pu, Am, and Cm, have been used for discovery research on elements 104 and 105 using actinide + 22Ne reactions [11], [15] and elements 114, 115, and 116 using actinide + 48Ca [24], [25], [26]. These actinide isotopes generally have long half-lives (18.1y for 244Cm, hundreds of years and greater for the others) and can be recovered and recycled after completion of an experiment. Inventories at ORNL (Table 2) are substantially greater than the tens of milligrams
Deposition of actinides
The first actinide target for nuclear studies was ammonium diurante, prepared by precipitation [56]. This target was used by Lise Meitner, Otto Hahn, and Fritz Strassmann in the 1930s to investigate the irradiation of uranium with neutrons. The science and engineering of fabricating actinide targets for the synthesis of super heavy elements (SHE) has progressed significantly since then both in variety and complexity.
There are several important characteristics to consider when selecting
Summary
Over the past 70 years, the Periodic Table has been expanded by more than 25%, as 26 new, artificially created elements have been observed. Some of these new elements, such as plutonium and californium, have enabled important technological advances, while others have contributed to significant scientific progress. All have been made possible by continued advances in target materials and technology, with actinide targets contributing to 20 of these discoveries, including 9 of 15 super-heavy
Acknowledgements
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, Isotope Development and Production for Research and Applications Program, under contract DE-AC05-00OR22725 with UT–Battelle, LLC. We are grateful to the staffs of the ORNL Radiochemical Engineering Development Center and the ORNL High Flux Isotope Reactor, a DOE Office of Science, Office of Basic Energy Sciences User Facility, for their support in the production and
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