New superheavy element isotopes: ²⁴²Pu(⁴⁸Ca,5n) ²⁸⁵114

Spectroscopy along Flerovium Decay Chains: Discovery of ^{280}Ds and an Excited State in ^{282}Cn

A nuclear spectroscopy experiment was conducted to study α-decay chains stemming from isotopes of flerovium (element Z=114). An upgraded TASISpec decay station was placed behind the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. The fusion-evaporation reactions ^{48}Ca+^{242}Pu and ^{48}Ca+^{244}Pu provided a total of 32 flerovium-candidate decay chains, of which two and eleven were firmly assigned to ^{286}Fl and ^{288}Fl, respectively. A prompt coincidence between a 9.60(1)-MeV α particle event and a 0.36(1)-MeV conversion electron marked the first observation of an excited state in an even-even isotope of the heaviest man-made elements, namely ^{282}Cn. Spectroscopy of ^{288}Fl decay chains fixed Q_{α}=10.06(1)  MeV. In one case, a Q_{α}=9.46(1)-MeV decay from ^{284}Cn into ^{280}Ds was observed, with ^{280}Ds fissioning after only 518  μs. The impact of these findings, aggregated with existing data on decay chains of ^{286,288}Fl, on the size of an anticipated shell gap at proton number Z=114 is discussed in light of predictions from two beyond-mean-field calculations, which take into account triaxial deformation.

Synthesis and properties of isotopes of the transactinides

Isotopes of transactinide elements have to be synthesized in nuclear reactions with light or heavy beam particles. The efficient production by neutron capture and subsequent β − decay as it is used for the production of isotopes of actinide elements up to fermium is no longer possible due to the lack of suitable target material. The content of this article is about the synthesis and the study of the decay properties of nuclei to which atomic, respectively proton numbers from Z = 104 to 118 could be unambiguously assigned by physical means. The results identified the reaction products as isotopes of new elements beyond the actinides, the transactinides. As such the elements received names given by the discovers ranging from rutherfordium for element 104 to oganesson for element 118 which completes the 7th row of the Periodic Table of the Elements. Intensive heavy ion beams, sophisticated target technology, efficient electromagnetic ion separators, and sensitive detector arrays were the prerequisites for discovery of the elements from Z = 107 to 118 during the years from 1981 to 2013. The results and the techniques are described. Also given is a historical introduction into early experiments and the theoretical predictions for a possible existence of an island of stability located at the crossing of the next closed shells for the protons and neutrons beyond the doubly magic nucleus 208Pb. The experimental results are compared with recent theoretical calculations on cross-sections and decay modes of these superheavy nuclei, respectively isotopes of superheavy elements. An outlook is given on further improvement of experimental facilities which will be needed for exploration of the extension and structure of the island of superheavy nuclei, in particular for searching for isotopes with longer half-lives predicted to be located in the south east and for isotopes of further new elements expected in the north-east direction of the island at the upper end of the chart of nuclei.

Evolution of the periodic table through the synthesis of new elements

This brief introduction to the synthesis and chemistry of elements discovered since 1940 is focused primarily on Z=93–118. The goal of this work is not to simply catalogue the nuclear fusion reactions needed to prepare new elements, but rather to focus on the chemical and physical properties that these elements possess. These elements share a single common feature in that they all have large Z values, and thus have electronic structures that are significantly altered by both scalar relativistic effects and spin-orbit coupling. These effects scale nonlinearly with increasing Z and create unexpected deviations both across series and down groups of elements. The magnitude of these effects is large enough that orbital energies rearrange and mix in ways that complicate incomplete depictions of electronic structure that are based solely on electron repulsion. Thus, the primary aim of this review is to document the impact of relativistic effects on electronic structure and how this has altered not just our understanding of the chemistry of heavy elements, but has even created in the need to rearrange the Periodic Table itself.

First Direct Measurements of Superheavy-Element Mass Numbers

An experiment was performed at Lawrence Berkeley National Laboratory's 88-in. Cyclotron to determine the mass number of a superheavy element. The measurement resulted in the observation of two α-decay chains, produced via the ^{243}Am(^{48}Ca,xn)^{291-x}Mc reaction, that were separated by mass-to-charge ratio (A/q) and identified by the combined BGS+FIONA apparatus. One event occurred at A/q=284 and was assigned to ^{284}Nh (Z=113), the α-decay daughter of ^{288}Mc (Z=115), while the second occurred at A/q=288 and was assigned to ^{288}Mc. This experiment represents the first direct measurements of the mass numbers of superheavy elements, confirming previous (indirect) mass-number assignments.

Super-heavy element research

A review of the discovery and investigation of the 'island of stability' of super-heavy nuclei at the separator DGFRS (FLNR, JINR) in the fusion reactions of (48)Ca projectiles with target nuclei (238)U-(249)Cf is presented. The synthesis of the heaviest nuclei, their decay properties, and methods of identification are discussed. The role of shell effects in the stability of super-heavy nuclei is demonstrated by comparison of the experimental data and results of theoretical calculations. The radioactive properties of the new nuclei, the isotopes of elements 112-118 as well as of their decay products, give evidence of the significant increase of the stability of the heavy nuclei with rise of their neutron number and approaching magic number N = 184.

48Ca+249Bk fusion reaction leading to element Z = 117: long-lived α-decaying 270Db and discovery of 266Lr

The superheavy element with atomic number Z=117 was produced as an evaporation residue in the (48)Ca+(249)Bk fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their α-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-μs and a few days. Two decay chains comprising seven α decays and a spontaneous fission each were identified and are assigned to the isotope (294)117 and its decay products. A hitherto unknown α-decay branch in (270)Db (Z = 105) was observed, which populated the new isotope (266)Lr (Z = 103). The identification of the long-lived (T(1/2) = 1.0(-0.4)(+1.9) h) α-emitter (270)Db marks an important step towards the observation of even more long-lived nuclei of superheavy elements located on an "island of stability."

Superheavy element flerovium (element 114) is a volatile metal

The electron shell structure of superheavy elements, i.e., elements with atomic number Z ≥ 104, is influenced by strong relativistic effects caused by the high Z. Early atomic calculations on element 112 (copernicium, Cn) and element 114 (flerovium, Fl) having closed and quasi-closed electron shell configurations of 6d(10)7s(2) and 6d(10)7s(2)7p1/2(2), respectively, predicted them to be noble-gas-like due to very strong relativistic effects on the 7s and 7p1/2 valence orbitals. Recent fully relativistic calculations studying Cn and Fl in different environments suggest them to be less reactive compared to their lighter homologues in the groups, but still exhibiting a metallic character. Experimental gas-solid chromatography studies on Cn have, indeed, revealed a metal-metal bond formation with Au. In contrast to this, for Fl, the formation of a weak bond upon physisorption on a Au surface was inferred from first experiments. Here, we report on a gas-solid chromatography study of the adsorption of Fl on a Au surface. Fl was produced in the nuclear fusion reaction (244)Pu((48)Ca, 3-4n)(288,289)Fl and was isolated in-flight from the primary (48)Ca beam in a physical recoil separator. The adsorption behavior of Fl, its nuclear α-decay product Cn, their lighter homologues in groups 14 and 12, i.e., Pb and Hg, and the noble gas Rn were studied simultaneously by isothermal gas chromatography and thermochromatography. Two Fl atoms were detected. They adsorbed on a Au surface at room temperature in the first, isothermal part, but not as readily as Pb and Hg. The observed adsorption behavior of Fl points to a higher inertness compared to its nearest homologue in the group, Pb. However, the measured lower limit for the adsorption enthalpy of Fl on a Au surface points to the formation of a metal-metal bond of Fl with Au. Fl is the least reactive element in the group, but still a metal.

Spectroscopy of element 115 decay chains

A high-resolution α, x-ray, and γ-ray coincidence spectroscopy experiment was conducted at the GSI Helmholtzzentrum für Schwerionenforschung. Thirty correlated α-decay chains were detected following the fusion-evaporation reaction 48Ca + 243Am. The observations are consistent with previous assignments of similar decay chains to originate from element Z=115. For the first time, precise spectroscopy allows the derivation of excitation schemes of isotopes along the decay chains starting with elements Z>112. Comprehensive Monte Carlo simulations accompany the data analysis. Nuclear structure models provide a first level interpretation.

Production and decay of the heaviest nuclei (293,294)117 and (294)118

Two years after the discovery of element 117, we undertook a second campaign using the (249)Bk+(48)Ca reaction for further investigations of the production and decay properties of the isotopes of element 117 on a larger number of events. The experiments were started in the end of April 2012 and are still under way. This Letter presents the results obtained in 1200 hours of an experimental run with the beam dose of (48)Ca of about 1.5×10(19) particles. The (249)Bk target was irradiated at two energies of (48)Ca that correspond to the maximum probability of the reaction channels with evaporation of three and four neutrons from the excited (297)117. In this experiment, two decay chains of (294)117 (3n) and five decay chains of (293)117 (4n) were detected. In the course of the long-term work, (249)Cf-the product of decay of (249)Bk (330 d)-is being accumulated in the target. Consequently, in the present experiment, we also detected a single decay of the known isotope (294)118 that was produced during 2002-2005 in the reaction (249)Cf((48)Ca,3n)(294)118. The obtained results are compared with the data from previous experiments. The experiments are carried out in the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, using the heavy-ion cyclotron U400.

Nuclear structure and reaction studies near doubly magic 270Hs

Fast on-line gas chemical separations of Hs (hassium, element 108) in the form of HsO4 were applied to investigate the reactions 26Mg + 248Cm and 36S + 238U. In an experiment at the gas-filled separator DGFRS the reaction 48Ca + 226Ra was studied. In all cases the product of complete nuclear fusion is 274Hs*. For the first time, the new nuclide 270Hs was produced in the 4n evaporation channel and its decay properties investigated. The nuclide 270Hs was predicted by microscopic-macroscopic calculations to be a deformed doubly magic nucleus and its decay properties are therefore of special interest to theory. Also, much more detailed information was gained on the decay of 269Hs and its daughters, which led to a new assignment of decay properties of the daughter nuclides 265Sg and 261Rf. There is evidence fo r isomeric states in 265Sg and 261Rf, while 266Sg is not an alpha-particle emitter as believed previously, but decays by spontaneous fission (SF) with a rather short half-life. Also, interesting features of the used reaction 26Mg + 248Cm led to the discovery of the nucleus 271Hs in the same experiments. An investigation of the influence of the Q-value on the fusion reaction in relation to the location of the fusion barrier showed, that the high binding energy of 48Ca largely compensates for the lower fusion probability compared to more asymmetric reactions, while 36S is not as promising as a projectile.

Correlation between α-decay energies of superheavy nuclei involving the effects of symmetry energy

A formula for the relationship between the α-decay energies (Q values) of superheavy nuclei (SHN) is presented, which is composed of the effects of Coulomb energy and symmetry energy. It can be employed not only to validate the experimental observations and measurements to a large extent, but also to predict the Q values of heaviest SHN with a high accuracy generally which will be very useful for future experiments. Furthermore, the shell closures in superheavy region and the effect of the symmetry energy on the stability of SHN against α decay are discussed with the help of this formula.

Synthesis of superheavy elements by cold fusion

The new elements from Z = 107 to 112 were synthesized in cold fusion reactions based on targets of lead and bismuth. The principle physical concepts are presented which led to the application of this reaction type in search experiments for new elements. Described are the technical developments from early mechanical devices to experiments with recoil separators. An overview is given of present experiments which use cold fusion for systematic studies and synthesis of new isotopes. Perspectives are also presented for the application of cold fusion reactions in synthesis of elements beyond element 113, the so far heaviest element produced in a cold fusion reaction. Further, the transition of hot fusion to cold fusion is pointed out, which occurs in reactions for synthesis of elements near Z = 126 using actinide targets and beams of neutron rich isotopes of elements from iron to germanium.