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[b]atomic nucleus is created in a nuclear reaction that combines two other nuclei of unequal size[c] into one; roughly, the more unequal the two nuclei in terms ofmass, the greater the possibility that the two react.[20] 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.[21] 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.[21]
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.[21][22] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[21] 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.[d] 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.[21]
The resulting merger is anexcited state[25]—termed acompound nucleus—and thus it is very unstable.[21] To reach a more stable state, the temporary merger mayfission without formation of a more stable nucleus.[26] 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.[26] 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.[27][e]
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.[29] In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[f] 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.[29] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[32] The nucleus is recorded again once its decay is registered, and the location, theenergy, and the time of the decay are measured.[29]
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.[33] 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.[34][35] Superheavy nuclei are thus theoretically predicted[36] and have so far been observed[37] to predominantly decay via decay modes that are caused by such repulsion:alpha decay andspontaneous fission.[g] Almost all alpha emitters have over 210 nucleons,[39] and the lightest nuclide primarily undergoing spontaneous fission has 238.[40] In both decay modes, nuclei are inhibited from decaying by correspondingenergy barriers for each mode, but they can be tunneled through.[34][35]
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.[41]
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.[42] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[35] 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),[43] and by 30 orders of magnitude fromthorium (element 90) tofermium (element 100).[44] 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.[35][45] 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.[35][45] 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.[46] Experiments on lighter superheavy nuclei,[47] as well as those closer to the expected island,[43] have shown greater than previously anticipated stability against spontaneous fission, showing the importance of shell effects on nuclei.[h]
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.[i] (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.)[29] 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).[j] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[k]
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.[l]
UsingMendeleev's nomenclature for unnamed and undiscovered elements, meitnerium should be known aseka-iridium. In 1979, during theTransfermium Wars (but before the synthesis of meitnerium), IUPAC published recommendations according to which the element was to be calledunnilennium (with the corresponding symbol ofUne),[60] 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 either called it "element 109", with the symbol ofE109,(109) or even simply109, or used the proposed name "meitnerium".[6]
The naming of meitnerium was discussed in theelement naming controversy regarding the names of elements 104 to 109, butmeitnerium was the only proposal and thus was never disputed.[61][62] The namemeitnerium (Mt) was suggested by the GSI team in September 1992 in honor of the Austrian physicistLise Meitner, a co-discoverer ofprotactinium (withOtto Hahn),[63][64][65][66][67] and one of the discoverers ofnuclear fission.[68] In 1994 the name was recommended byIUPAC,[61] and was officially adopted in 1997.[62] It is thus the only element named specifically after a non-mythological woman (curium being named for bothPierre andMarie Curie).[69]
Meitnerium 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. Eight different isotopes of meitnerium have been reported withmass numbers 266, 268, 270, and 274–278, two of which, meitnerium-268 and meitnerium-270, have unconfirmedmetastable states. A ninth isotope with mass number 282 is unconfirmed. Most of these decay predominantly through alpha decay, although some undergo spontaneous fission.[70]
All meitnerium isotopes are extremely unstable and radioactive; in general, heavier isotopes are more stable than the lighter. The most stable known meitnerium isotope,278Mt, is also the heaviest known; it has a half-life of 4.5 seconds. The unconfirmed282Mt is even heavier and appears to have a longer half-life of 67 seconds. With a half-life of 0.8 seconds, the next most stable known isotope is270Mt.[3] The isotopes276Mt and274Mt have half-lives of 0.62 and 0.64 seconds respectively.[4]
The isotope277Mt, created as the final decay product of293Ts for the first time in 2012, was observed to undergospontaneous fission with a half-life of 5 milliseconds. Preliminary data analysis considered the possibility of this fission event instead originating from277Hs, for it also has a half-life of a few milliseconds, and could be populated following undetectedelectron capture somewhere along the decay chain.[72][73] This possibility was later deemed very unlikely based on observeddecay energies of281Ds and281Rg and the short half-life of277Mt, although there is still some uncertainty of the assignment.[73] Regardless, the rapid fission of277Mt and277Hs is strongly suggestive of a region of instability for superheavy nuclei withN = 168–170. The existence of this region, characterized by a decrease infission barrier height between the deformedshell closure atN = 162 and spherical shell closure atN = 184, is consistent with theoretical models.[72]
Other than nuclear properties, no properties of meitnerium or its compounds have been measured; this is due to its extremely limited and expensive production[o] and the fact that meitnerium and its parents decay very quickly. Properties of meitnerium metal remain unknown and only predictions are available.
Prediction of the probable chemical properties of meitnerium has not received much attention recently. Meitnerium is expected to be anoble metal. Thestandard electrode potential for the Mt3+/Mt couple is expected to be 0.8 V. Based on the most stable oxidation states of the lighter group 9 elements, the most stable oxidation states of meitnerium are predicted to be the +6, +3, and +1 states, with the +3 state being the most stable inaqueous solutions. In comparison, rhodium and iridium show a maximum oxidation state of +6, while the most stable states are +4 and +3 for iridium and +3 for rhodium.[6] The oxidation state +9, represented only by iridium in [IrO4]+, might be possible for its congener meitnerium in the nonafluoride (MtF9) and the [MtO4]+ cation, although [IrO4]+ is expected to be more stable than these meitnerium compounds.[75] The tetrahalides of meitnerium have also been predicted to have similar stabilities to those of iridium, thus also allowing a stable +4 state.[76] It is further expected that the maximum oxidation states of elements from bohrium (element 107) todarmstadtium (element 110) may be stable in the gas phase but not in aqueous solution.[6]
Meitnerium is expected to be a solid under normal conditions and assume aface-centered cubiccrystal structure, similarly to its lightercongener iridium.[8] It should be a very heavy metal with adensity of around 27–28 g/cm3, which would be among the highest of any of the 118 known elements.[9][10] Meitnerium is also predicted to beparamagnetic.[14]
Theoreticians have predicted the covalent radius of meitnerium to be 6 to 10 pm larger than that of iridium.[77] The atomic radius of meitnerium is expected to be around 128 pm.[12]
Meitnerium is the first element on the periodic table whose chemistry has not yet been investigated. Unambiguous determination of the chemical characteristics of meitnerium has yet to have been established[78][79] due to the short half-lives of meitnerium isotopes[6] and a limited number of likelyvolatile compounds that could be studied on a very small scale. One of the few meitnerium compounds that are likely to be sufficiently volatile is meitnerium hexafluoride (MtF 6), as its lighter homologueiridium hexafluoride (IrF 6) is volatile above 60 °C and therefore the analogous compound of meitnerium might also be sufficiently volatile;[66] a volatileoctafluoride (MtF 8) might also be possible.[6] 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.[66] Even though the half-life of278Mt, the most stable confirmed meitnerium isotope, is 4.5 seconds, long enough to perform chemical studies, another obstacle is the need to increase the rate of production of meitnerium 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 meitnerium isotopes and have automated systems experiment on the gas-phase and solution chemistry of meitnerium, as the yields for heavier elements are predicted to be smaller than those for lighter elements; some of the separation techniques used forbohrium andhassium could be reused. However, the experimental chemistry of meitnerium has not received as much attention as that of the heavier elements fromcopernicium tolivermorium.[6][78][80]
TheLawrence Berkeley National Laboratory attempted to synthesize the isotope271Mt in 2002–2003 for a possible chemical investigation of meitnerium, because it was expected that it might be more stable than nearby isotopes due to having 162neutrons, amagic number for deformed nuclei; its half-life was predicted to be a few seconds, long enough for a chemical investigation.[6][81][82] However, no atoms of271Mt were detected;[83] this isotope of meitnerium is currently unknown.[70]
An experiment determining the chemical properties of a transactinide would need to compare a compound of that transactinide with analogous compounds of some of its lighter homologues:[6] for example, in the chemical characterization of hassium, hassium tetroxide (HsO4) was compared with the analogousosmium compound,osmium tetroxide (OsO4).[84] In a preliminary step towards determining the chemical properties of meitnerium, the GSI attemptedsublimation of the rhodium compoundsrhodium(III) oxide (Rh2O3) andrhodium(III) chloride (RhCl3). However, macroscopic amounts of the oxide would not sublimate until 1000 °C and the chloride would not until 780 °C, and then only in the presence ofcarbon aerosol particles: these temperatures are far too high for such procedures to be used on meitnerium, as most of the current methods used for the investigation of the chemistry of superheavy elements do not work above 500 °C.[79]
Following the 2014 successful synthesis of seaborgium hexacarbonyl, Sg(CO)6,[85] studies were conducted with the stable transition metals of groups 7 through 9, suggesting that carbonyl formation could be extended to further probe the chemistries of the early 6d transition metals from rutherfordium to meitnerium inclusive.[86][87] Nevertheless, the challenges of low half-lives and difficult production reactions make meitnerium difficult to access for radiochemists, though the isotopes278Mt and276Mt are long-lived enough for chemical research and may be produced in the decay chains of294Ts and288Mc respectively.276Mt is likely more suitable, since producing tennessine requires a rare and rather short-livedberkelium target.[88] The isotope270Mt, observed in the decay chain of278Nh with a half-life of 0.69 seconds, may also be sufficiently long-lived for chemical investigations, though a direct synthesis route leading to this isotope and more precise measurements of its decay properties would be required.[82]
^The most stable isotope of meitnerium cannot be determined based on existing data due to uncertainty that arises from the low number of measurements. The half-life of278Mt corresponding to twostandard deviations is, based on existing data,4.5+7.0 −2.6 seconds[3], whereas that of274Mt is0.64+1.52 −0.46 seconds[4]; these measurements have overlappingconfidence intervals. It is also possible that the unconfirmed282Mt is more stable than both of these, with its half-life being 67 seconds.[5]
^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[15] or112;[16] sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypotheticalsuperactinide series).[17] 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.[18] 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.[19]
^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.[23]
^This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[28]
^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.[30] 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.[31]
^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.[43]
^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.[48] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[49] 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).[50]
^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).[39] 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,[51] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[52] 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.[28] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[51]
^For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics inStockholm,Stockholm County,Sweden.[53] 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.[54] 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.[54] JINR insisted that they were the first to create the element and suggested a name of their own for the new element,joliotium;[55] the Soviet name was also not accepted (JINR later referred to the naming of the element 102 as "hasty").[56] This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.[56] The name "nobelium" remained unchanged on account of its widespread usage.[57]
^Different sources give different values for half-lives; the most recently published values are listed.
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^abcdefghijklmHoffman, 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.
^Thierfelder, C.; Schwerdtfeger, P.; Heßberger, F. P.; Hofmann, S. (2008). "Dirac-Hartree-Fock studies of X-ray transitions in meitnerium".The European Physical Journal A.36 (2): 227.Bibcode:2008EPJA...36..227T.doi:10.1140/epja/i2008-10584-7.
^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.
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^"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.
^Münzenberg, G.; Armbruster, P.; Heßberger, F. P.; Hofmann, S.; Poppensieker, K.; Reisdorf, W.; Schneider, J. H. R.; Schneider, W. F. W.; Schmidt, K.-H.; Sahm, C.-C.; Vermeulen, D. (1982). "Observation of one correlated α-decay in the reaction58Fe on209Bi→267109".Zeitschrift für Physik A.309 (1): 89.Bibcode:1982ZPhyA.309...89M.doi:10.1007/BF01420157.S2CID120062541.
^Wiesner, Emilie; Settle, Frank A. (2001). "Politics, Chemistry, and the Discovery of Nuclear Fission".Journal of Chemical Education.78 (7): 889.Bibcode:2001JChEd..78..889W.doi:10.1021/ed078p889.
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