A graphic depiction of anuclear fusion reaction. Two nuclei fuse into one, emitting aneutron. Reactions that created new elements to this moment were similar, with the only possible difference that several singular neutrons sometimes were released, or none at all.
A superheavy[a]atomic nucleus is created in a nuclear reaction that combines two other nuclei of unequal size[b] into one; roughly, the more unequal the two nuclei in terms ofmass, the greater the possibility that the two react.[16] The material made of the heavier nuclei is made into a target, which is then bombarded by thebeam of lighter nuclei. Two nuclei can onlyfuse into one if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due toelectrostatic repulsion. Thestrong interaction can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatlyaccelerated in order to make such repulsion insignificant compared to the velocity of the beam nucleus.[17] The energy applied to the beam nuclei to accelerate them can cause them to reach speeds as high as one-tenth of thespeed of light. However, if too much energy is applied, the beam nucleus can fall apart.[17]
Coming close enough alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for about 10−20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.[17][18] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[17] Each pair of a target and a beam is characterized by itscross section—the probability that fusion will occur if two nuclei approach one another expressed in terms of the transverse area that the incident particle must hit in order for the fusion to occur.[c] This fusion may occur as a result of the quantum effect in which nuclei cantunnel through electrostatic repulsion. If the two nuclei can stay close past that phase, multiple nuclear interactions result in redistribution of energy and an energy equilibrium.[17]
The resulting merger is anexcited state[21]—termed acompound nucleus—and thus it is very unstable.[17] To reach a more stable state, the temporary merger mayfission without formation of a more stable nucleus.[22] Alternatively, the compound nucleus may eject a fewneutrons, which would carry away the excitation energy; if the latter is not sufficient for a neutron expulsion, the merger would produce agamma ray. This happens in about 10−16 seconds after the initial nuclear collision and results in creation of a more stable nucleus.[22] The definition by theIUPAC/IUPAP Joint Working Party (JWP) states that achemical element can only be recognized as discovered if a nucleus of it has notdecayed within 10−14 seconds. This value was chosen as an estimate of how long it takes a nucleus to acquireelectrons and thus display its chemical properties.[23][d]
The beam passes through the target and reaches the next chamber, the separator; if a new nucleus is produced, it is carried with this beam.[25] In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[e] and transferred to asurface-barrier detector, which stops the nucleus. The exact location of the upcoming impact on the detector is marked; also marked are its energy and the time of the arrival.[25] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[28] The nucleus is recorded again once its decay is registered, and the location, theenergy, and the time of the decay are measured.[25]
Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, its influence on the outermostnucleons (protons and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, and its range is not limited.[29] Totalbinding energy provided by the strong interaction increases linearly with the number of nucleons, whereas electrostatic repulsion increases with the square of the atomic number, i.e. the latter grows faster and becomes increasingly important for heavy and superheavy nuclei.[30][31] Superheavy nuclei are thus theoretically predicted[32] and have so far been observed[33] to predominantly decay via decay modes that are caused by such repulsion:alpha decay andspontaneous fission.[f] Almost all alpha emitters have over 210 nucleons,[35] and the lightest nuclide primarily undergoing spontaneous fission has 238.[36] In both decay modes, nuclei are inhibited from decaying by correspondingenergy barriers for each mode, but they can be tunneled through.[30][31]
Scheme of an apparatus for creation of superheavy elements, based on the Dubna Gas-Filled Recoil Separator set up in theFlerov Laboratory of Nuclear Reactions in JINR. The trajectory within the detector and the beam focusing apparatus changes because of adipole magnet in the former andquadrupole magnets in the latter.[37]
Alpha particles are commonly produced in radioactive decays because the mass of an alpha particle per nucleon is small enough to leave some energy for the alpha particle to be used as kinetic energy to leave the nucleus.[38] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[31] As the atomic number increases, spontaneous fission rapidly becomes more important: spontaneous fission partial half-lives decrease by 23 orders of magnitude fromuranium (element 92) tonobelium (element 102),[39] and by 30 orders of magnitude fromthorium (element 90) tofermium (element 100).[40] The earlierliquid drop model thus suggested that spontaneous fission would occur nearly instantly due to disappearance of thefission barrier for nuclei with about 280 nucleons.[31][41] The laternuclear shell model suggested that nuclei with about 300 nucleons would form anisland of stability in which nuclei will be more resistant to spontaneous fission and will primarily undergo alpha decay with longer half-lives.[31][41] Subsequent discoveries suggested that the predicted island might be further than originally anticipated; they also showed that nuclei intermediate between the long-lived actinides and the predicted island are deformed, and gain additional stability from shell effects.[42] Experiments on lighter superheavy nuclei,[43] as well as those closer to the expected island,[39] have shown greater than previously anticipated stability against spontaneous fission, showing the importance of shell effects on nuclei.[g]
Alpha decays are registered by the emitted alpha particles, and the decay products are easy to determine before the actual decay; if such a decay or a series of consecutive decays produces a known nucleus, the original product of a reaction can be easily determined.[h] (That all decays within a decay chain were indeed related to each other is established by the location of these decays, which must be in the same place.)[25] The known nucleus can be recognized by the specific characteristics of decay it undergoes such as decay energy (or more specifically, thekinetic energy of the emitted particle).[i] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[j]
The information available to physicists aiming to synthesize a superheavy element is thus the information collected at the detectors: location, energy, and time of arrival of a particle to the detector, and those of its decay. The physicists analyze this data and seek to conclude that it was indeed caused by a new element and could not have been caused by a different nuclide than the one claimed. Often, provided data is insufficient for a conclusion that a new element was definitely created and there is no other explanation for the observed effects; errors in interpreting data have been made.[k]
Two more atoms followed on November 12 and 17.[54] (Yet another was originally reported to have been found on November 11, but it turned out to be based on data fabricated byVictor Ninov, and was later retracted.)[55]
In the same series of experiments, the same team also carried out the reaction using heavier nickel-64 ions. During two runs, 9 atoms of271 Ds were convincingly detected by correlation with known daughter decay properties:[56]
208 82Pb +64 28Ni →271 110Ds +1 0n
Prior to this, there had been failed synthesis attempts in 1986–87 at theJoint Institute for Nuclear Research inDubna (then in theSoviet Union) and in 1990 at the GSI. A 1995 attempt at theLawrence Berkeley National Laboratory resulted in signs suggesting but not pointing conclusively at the discovery of a new isotope267 Ds formed in the bombardment of209 Bi with59 Co, and a similarly inconclusive 1994 attempt at the JINR showed signs of273 Ds being produced from244 Pu and34 S. Each team proposed its own name for element 110: the American team proposedhahnium afterOtto Hahn in an attempt to resolve the controversy of namingelement 105 (which they had long been suggesting this name for), the Russian team proposedbecquerelium afterHenri Becquerel, and the German team proposeddarmstadtium after Darmstadt, the location of their institute.[57] TheIUPAC/IUPAP Joint Working Party (JWP) recognised the GSI team as discoverers in their 2001 report, giving them the right to suggest a name for the element.[58]
UsingMendeleev's nomenclature for unnamed and undiscovered elements, darmstadtium should be known aseka-platinum. In 1979, IUPAC published recommendations according to which the element was to be calledununnilium (with the corresponding symbol ofUun),[59] asystematic element name as aplaceholder, until the element was discovered (and the discovery then confirmed) and a permanent name was decided on. Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations were mostly ignored among scientists in the field, who called it "element 110", with the symbol ofE110,(110) or even simply110.[3]
In 1996, the Russian team proposed the namebecquerelium afterHenri Becquerel.[60] The American team in 1997 proposed the namehahnium[61] afterOtto Hahn (previously this name had been used forelement 105).
The namedarmstadtium (Ds) was suggested by the GSI team in honor of the city of Darmstadt, where the element was discovered.[62][63] The GSI team originally also considered naming the elementwixhausium, after the suburb of Darmstadt known asWixhausen where the element was discovered, but eventually decided ondarmstadtium.[64]Policium had also been proposed as a joke due to theemergency telephone number in Germany being 1–1–0.[65] The new namedarmstadtium was officially recommended byIUPAC on August 16, 2003.[62]
Darmstadtium has no stable or naturally occurring isotopes. Several radioactive isotopes have been synthesized in the laboratory, either by fusing two atoms or by observing the decay of heavier elements. Eleven different isotopes of darmstadtium have been reported with atomic masses 267, 269–271, 273, 275–277, and 279–281, although darmstadtium-267 is unconfirmed. Three darmstadtium isotopes, darmstadtium-270, darmstadtium-271, and darmstadtium-281, have knownmetastable states, although that of darmstadtium-281 is unconfirmed.[78] Most of these decay predominantly through alpha decay, but some undergo spontaneous fission.[79]
This chart of decay modes according to the model of theJapan Atomic Energy Agency predicts several superheavy nuclides within theisland of stability having total half-lives exceeding one year (circled) and undergoing primarily alpha decay, peaking at294Ds with an estimated half-life of 300 years.[80]
All darmstadtium isotopes are extremely unstable and radioactive; in general, the heavier isotopes are more stable than the lighter. The most stable known darmstadtium isotope,281Ds, is also the heaviest known darmstadtium isotope; it has a half-life of 14 seconds. The isotope279Ds has a half-life of 0.18 seconds, while the unconfirmed281mDs has a half-life of 0.9 seconds. The remaining isotopes and metastable states have half-lives between 1 microsecond and 70 milliseconds.[79] Some unknown darmstadtium isotopes may have longer half-lives, however.[81]
Theoretical calculation in a quantum tunneling model reproduces the experimental alpha decay half-life data for the known darmstadtium isotopes.[82][83] It also predicts that the undiscovered isotope294Ds, which has amagic number ofneutrons (184),[3] would have an alpha decay half-life on the order of 311 years; exactly the same approach predicts a ~350-year alpha half-life for the non-magic293Ds isotope, however.[81][84]
Other than nuclear properties, no properties of darmstadtium or its compounds have been measured; this is due to its extremely limited and expensive production[85] and the fact that darmstadtium (and its parents) decays very quickly. Properties of darmstadtium metal remain unknown and only predictions are available.
Prediction of the probable chemical properties of darmstadtium has not received much attention recently. Darmstadtium should be a verynoble metal. The predictedstandard reduction potential for the Ds2+/Ds couple is 1.7 V.[3] Based on the most stable oxidation states of the lighter group 10 elements, the most stable oxidation states of darmstadtium are predicted to be the +6, +4, and +2 states; however, the neutral state is predicted to be the most stable inaqueous solutions. In comparison, only platinum is known to show the maximum oxidation state in the group, +6, while the most stable state is +2 for both nickel and palladium. It is further expected that the maximum oxidation states of elements frombohrium (element 107) to darmstadtium (element 110) may be stable in the gas phase but not in aqueous solution.[3] Darmstadtium hexafluoride (DsF6) is predicted to have very similar properties to its lighter homologueplatinum hexafluoride (PtF6), having very similar electronic structures and ionization potentials.[3][86][87] It is also expected to have the sameoctahedral molecular geometry as PtF6.[88] Other predicted darmstadtium compounds are darmstadtium carbide (DsC) and darmstadtium tetrachloride (DsCl4), both of which are expected to behave like their lighter homologues.[88] Unlike platinum, which preferentially forms acyanidecomplex in its +2 oxidation state, Pt(CN)2, darmstadtium is expected to preferentially remain in its neutral state and formDs(CN)2− 2 instead, forming a strong Ds–C bond with some multiple bond character.[89]
Darmstadtium is expected to be a solid under normal conditions and to crystallize in thebody-centered cubic structure, unlike its lightercongeners which crystallize in theface-centered cubic structure, because it is expected to have different electron charge densities from them.[4] It should be a very heavy metal with adensity of around 26–27 g/cm3. In comparison, the densest known element that has had its density measured,osmium, has a density of only 22.61 g/cm3.[5][6]
The outerelectron configuration of darmstadtium is calculated to be 6d8 7s2, which obeys theAufbau principle and does not follow platinum's outer electron configuration of 5d9 6s1. This is due to the relativistic stabilization of the 7s2 electron pair over the whole seventh period, so that none of the elements from 104 to 112 are expected to have electron configurations violating the Aufbau principle. The atomic radius of darmstadtium is expected to be around 132 pm.[3]
Unambiguous determination of the chemical characteristics of darmstadtium has yet to have been established[90] due to the short half-lives of darmstadtium isotopes and a limited number of likely volatile compounds that could be studied on a very small scale. One of the few darmstadtium compounds that are likely to be sufficiently volatile is darmstadtium hexafluoride (DsF 6), as its lighter homologue platinum hexafluoride (PtF 6) is volatile above 60 °C and therefore the analogous compound of darmstadtium might also be sufficiently volatile;[63] a volatileoctafluoride (DsF 8) might also be possible.[3] For chemical studies to be carried out on atransactinide, at least four atoms must be produced, the half-life of the isotope used must be at least 1 second, and the rate of production must be at least one atom per week.[63] Even though the half-life of281Ds, the most stable confirmed darmstadtium isotope, is 14 seconds, long enough to perform chemical studies, another obstacle is the need to increase the rate of production of darmstadtium isotopes and allow experiments to carry on for weeks or months so that statistically significant results can be obtained. Separation and detection must be carried out continuously to separate out the darmstadtium isotopes and have automated systems experiment on the gas-phase and solution chemistry of darmstadtium, as the yields for heavier elements are predicted to be smaller than those for lighter elements; some of the separation techniques used for bohrium andhassium could be reused. However, the experimental chemistry of darmstadtium has not received as much attention as that of the heavier elements fromcopernicium tolivermorium.[3][90][91]
The moreneutron-rich darmstadtium isotopes are the most stable[79] and are thus more promising for chemical studies.[3][63] However, they can only be produced indirectly from the alpha decay of heavier elements,[92][93][94] and indirect synthesis methods are not as favourable for chemical studies as direct synthesis methods.[3] The more neutron-rich isotopes276Ds and277Ds might be produced directly in the reaction betweenthorium-232 andcalcium-48, but the yield was expected to be low.[3][95][96] Following several unsuccessful attempts,276Ds was produced in this reaction in 2022 and observed to have a half-life less than a millisecond and a low yield, in agreement with predictions.[70] Additionally,277Ds was successfully synthesized using indirect methods (as a granddaughter of285Fl) and found to have a short half-life of 3.5 ms, not long enough to perform chemical studies.[71][93] The only known darmstadtium isotope with a half-life long enough for chemical research is281Ds, which would have to be produced as the granddaughter of289Fl.[97]
^Innuclear physics, an element is calledheavy if its atomic number is high;lead (element 82) is one example of such a heavy element. The term "superheavy elements" typically refers to elements with atomic number greater than103 (although there are other definitions, such as atomic number greater than100[11] or112;[12] sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypotheticalsuperactinide series).[13] Terms "heavy isotopes" (of a given element) and "heavy nuclei" mean what could be understood in the common language—isotopes of high mass (for the given element) and nuclei of high mass, respectively.
^In 2009, a team at the JINR led by Oganessian published results of their attempt to create hassium in a symmetric136Xe + 136Xe reaction. They failed to observe a single atom in such a reaction, putting the upper limit on the cross section, the measure of probability of a nuclear reaction, as 2.5 pb.[14] In comparison, the reaction that resulted in hassium discovery,208Pb +58Fe, had a cross section of ~20 pb (more specifically, 19+19 -11 pb), as estimated by the discoverers.[15]
^The amount of energy applied to the beam particle to accelerate it can also influence the value of cross section. For example, in the28 14Si +1 0n →28 13Al +1 1p reaction, cross section changes smoothly from 370 mb at 12.3 MeV to 160 mb at 18.3 MeV, with a broad peak at 13.5 MeV with the maximum value of 380 mb.[19]
^This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[24]
^This separation is based on that the resulting nuclei move past the target more slowly then the unreacted beam nuclei. The separator contains electric and magnetic fields whose effects on a moving particle cancel out for a specific velocity of a particle.[26] Such separation can also be aided by atime-of-flight measurement and a recoil energy measurement; a combination of the two may allow to estimate the mass of a nucleus.[27]
^It was already known by the 1960s that ground states of nuclei differed in energy and shape as well as that certain magic numbers of nucleons corresponded to greater stability of a nucleus. However, it was assumed that there was no nuclear structure in superheavy nuclei as they were too deformed to form one.[39]
^Since mass of a nucleus is not measured directly but is rather calculated from that of another nucleus, such measurement is called indirect. Direct measurements are also possible, but for the most part they have remained unavailable for superheavy nuclei.[44] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[45] Mass was determined from the location of a nucleus after the transfer (the location helps determine its trajectory, which is linked to the mass-to-charge ratio of the nucleus, since the transfer was done in presence of a magnet).[46]
^If the decay occurred in a vacuum, then since total momentum of an isolated system before and after the decaymust be preserved, the daughter nucleus would also receive a small velocity. The ratio of the two velocities, and accordingly the ratio of the kinetic energies, would thus be inverse to the ratio of the two masses. The decay energy equals the sum of the known kinetic energy of the alpha particle and that of the daughter nucleus (an exact fraction of the former).[35] The calculations hold for an experiment as well, but the difference is that the nucleus does not move after the decay because it is tied to the detector.
^Spontaneous fission was discovered by Soviet physicistGeorgy Flerov,[47] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[48] In contrast, the LBL scientists believed fission information was not sufficient for a claim of synthesis of an element. They believed spontaneous fission had not been studied enough to use it for identification of a new element, since there was a difficulty of establishing that a compound nucleus had only ejected neutrons and not charged particles like protons or alpha particles.[24] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[47]
^For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics inStockholm,Stockholm County,Sweden.[49] There were no earlier definitive claims of creation of this element, and the element was assigned a name by its Swedish, American, and British discoverers,nobelium. It was later shown that the identification was incorrect.[50] The following year, RL was unable to reproduce the Swedish results and announced instead their synthesis of the element; that claim was also disproved later.[50] JINR insisted that they were the first to create the element and suggested a name of their own for the new element,joliotium;[51] the Soviet name was also not accepted (JINR later referred to the naming of the element 102 as "hasty").[52] This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.[52] The name "nobelium" remained unchanged on account of its widespread usage.[53]
^Different sources give different values for half-lives; the most recently published values are listed.
^abcdefghijklmnopqHoffman, Darleane C.; Lee, Diana M.; Pershina, Valeria (2006). "Transactinides and the future elements". In Morss; Edelstein, Norman M.; Fuger, Jean (eds.).The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Dordrecht, The Netherlands:Springer Science+Business Media.ISBN978-1-4020-3555-5.
^abGyanchandani, Jyoti; Sikka, S. K. (May 10, 2011). "Physical properties of the 6 d -series elements from density functional theory: Close similarity to lighter transition metals".Physical Review B.83 (17) 172101.Bibcode:2011PhRvB..83q2101G.doi:10.1103/PhysRevB.83.172101.
^abKratz; Lieser (2013).Nuclear and Radiochemistry: Fundamentals and Applications (3rd ed.). p. 631.
^"Популярная библиотека химических элементов. Сиборгий (экавольфрам)" [Popular library of chemical elements. Seaborgium (eka-tungsten)].n-t.ru (in Russian). RetrievedJanuary 7, 2020. Reprinted from"Экавольфрам" [Eka-tungsten].Популярная библиотека химических элементов. Серебро – Нильсборий и далее [Popular library of chemical elements. Silver through nielsbohrium and beyond] (in Russian).Nauka. 1977.
^abUtyonkov, V. K.; Brewer, N. T.; Oganessian, Yu. Ts.; Rykaczewski, K. P.; Abdullin, F. Sh.; Dimitriev, S. N.; Grzywacz, R. K.; Itkis, M. G.; Miernik, K.; Polyakov, A. N.; Roberto, J. B.; Sagaidak, R. N.; Shirokovsky, I. V.; Shumeiko, M. V.; Tsyganov, Yu. S.; Voinov, A. A.; Subbotin, V. G.; Sukhov, A. M.; Karpov, A. V.; Popeko, A. G.; Sabel'nikov, A. V.; Svirikhin, A. I.; Vostokin, G. K.; Hamilton, J. H.; Kovrinzhykh, N. D.; Schlattauer, L.; Stoyer, M. A.; Gan, Z.; Huang, W. X.; Ma, L. (January 30, 2018)."Neutron-deficient superheavy nuclei obtained in the240Pu+48Ca reaction".Physical Review C.97 (14320) 014320.Bibcode:2018PhRvC..97a4320U.doi:10.1103/PhysRevC.97.014320.
^Hofmann, S.; Heinz, S.; Mann, R.; Maurer, J.; Khuyagbaatar, J.; Ackermann, D.; Antalic, S.; Barth, W.; Block, M.; Burkhard, H. G.; Comas, V. F.; Dahl, L.; Eberhardt, K.; Gostic, J.; Henderson, R. A.; Heredia, J. A.; Heßberger, F. P.; Kenneally, J. M.; Kindler, B.; Kojouharov, I.; Kratz, J. V.; Lang, R.; Leino, M.; Lommel, B.; Moody, K. J.; Münzenberg, G.; Nelson, S. L.; Nishio, K.; Popeko, A. G.; et al. (2012). "The reaction48Ca +248Cm →296116* studied at the GSI-SHIP".The European Physical Journal A.48 (5): 62.Bibcode:2012EPJA...48...62H.doi:10.1140/epja/i2012-12062-1.S2CID121930293.
^abcSonzogni, Alejandro."Interactive Chart of Nuclides". National Nuclear Data Center: Brookhaven National Laboratory. Archived fromthe original on August 1, 2020. RetrievedJune 6, 2008.
^Waber, J. T.; Averill, F. W. (1974). "Molecular orbitals of PtF6 and E110 F6 calculated by the self-consistent multiple scattering Xα method".J. Chem. Phys.60 (11):4460–70.Bibcode:1974JChPh..60.4466W.doi:10.1063/1.1680924.
^abThayer, John S. (2010), "Relativistic Effects and the Chemistry of the Heavier Main Group Elements",Relativistic Methods for Chemists, Challenges and Advances in Computational Chemistry and Physics, vol. 10, p. 82,doi:10.1007/978-1-4020-9975-5_2,ISBN978-1-4020-9974-8
^abDüllmann, Christoph E. (2012). "Superheavy elements at GSI: a broad research program with element 114 in the focus of physics and chemistry".Radiochimica Acta.100 (2):67–74.doi:10.1524/ract.2011.1842.S2CID100778491.
^Moody, Ken (November 30, 2013). "Synthesis of Superheavy Elements". In Schädel, Matthias; Shaughnessy, Dawn (eds.).The Chemistry of Superheavy Elements (2nd ed.). Springer Science & Business Media. pp. 24–8.ISBN978-3-642-37466-1.
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