Synthesis of the Heaviest Chemical Elements—Results and Perspectives

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Abstract
All 17 artificially produced elements heavier than uranium were discovered by nuclear‐chemical synthesis. The three heaviest ones–elements 107, 108 and 109–were synthesized at the heavy‐ion accelerator UNILAC in Darmstadt by nuclear fusion from the heaviest stable nuclei, lead‐208 and bismuth‐209, and the most neutron‐rich stable isotopes of chromium and iron: element 107 from bismuth‐209 (atomic number Z = 83) and chromium‐54 (Z = 24), element 108 from lead‐208 (Z = 82) and iron‐58 (Z = 26), and element 109 from bismuth‐209 and iron‐58. The first isotopes detected were those with mass numbers 262 (Z = 107), 265 (Z = 108) and 266 (Z = 109); these nuclei are short‐lived α‐emitters with half‐lives of 8.2 ms, 1.8 ms and 3.4 ms, respectively. The yields of these reactions are extremely small; only three atoms of element 109 have ever been observed. Experiments to synthesize element 110 have given ambiguous results. All attempts to detect the “superheavy” elements with proton numbers near Z = 114 and neutron numbers near N = 184 have thus far failed. These elements have been predicted theoretically, and many attempts to synthesize them have been made, e.g. at UNILAC by fusion of calcium‐48 (Z = 20) with curium‐248 (Z = 96), or by transference of protons in the collision of two very heavy nuclei such as uranium‐238 (Z = 92). Surprisingly, the heaviest known nuclei are far more stable toward spontaneous fission into two fragments than was expected, but their synthesis is strongly hindered–much more than initially anticipated. It is this hindrance rather than the decreasing nuclear stability which seems to presently limit the extent of the periodic table: even heavier elements should be able to exist, but no way has yet been found to produce them.