Introduction to superheavy elements: Difference between revisions

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{{external media|width=230px|float=left|video1=[https://www.youtube.com/watch?v=YovAFlzFtzg Visualization] of unsuccessful nuclear fusion, based on calculations by the [[Australian National University]]<ref>{{Cite journal|last=Wakhle|first=A.|last2=Simenel|first2=C.|last3=Hinde|first3=D. J.|display-authors=3|last4=Dasgupta|first4=M.|last5=Evers|first5=M.|last6=Luong|first6=D. H.|last7=du Rietz|first7=R.|date=2015|editor-last=Simenel|editor-first=C.|editor2-last=Gomes|editor2-first=P. R. S.|editor3-last=Hinde|editor3-first=D. J.|display-editors=3|editor4-last=Madhavan|editor4-first=N.|editor5-last=Navin|editor5-first=A.|editor6-last=Rehm|editor6-first=K. E.|title=Comparing Experimental and Theoretical Quasifission Mass Angle Distributions|journal=[[European Physical Journal WOC|European Physical Journal Web of Conferences]]|volume=86|pages=00061|doi=10.1051/epjconf/20158600061|bibcode=2015EPJWC..8600061W|issn=2100-014X|doi-access=free}}</ref>}}

A superheavy{{efn|In [[nuclear physics]], an element is called [[heavy element|heavy]] if its atomic number is high; [[lead]] (element&nbsp;82) is one example of such a heavy element. The term "superheavy elements" typically refers to elements with atomic number greater than [[lawrencium|103]] (although there are other definitions, such as atomic number greater than 100<ref>{{Cite web|url=https://www.chemistryworld.com/news/explainer-superheavy-elements/1010345.article|title=Explainer: superheavy elements|last=Krämer|first=K.|date=2016|website=[[Chemistry World]]|language=en|access-date=2020-03-15}}</ref> or 112;<ref>{{Cite web|archive-url=https://web.archive.org/web/20150911081623/https://pls.llnl.gov/research-and-development/nuclear-science/project-highlights/livermorium/elements-113-and-115|url=https://pls.llnl.gov/research-and-development/nuclear-science/project-highlights/livermorium/elements-113-and-115|title=Discovery of Elements 113 and 115|publisher=[[Lawrence Livermore National Laboratory]]|archive-date=2015-09-11|access-date=2020-03-15}}</ref> sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypothetical [[superactinide]] series).<ref>{{cite encyclopedia|last=Eliav|first=E.|title=Electronic Structure of the Transactinide Atoms|date=2018|encyclopedia=Encyclopedia of Inorganic and Bioinorganic Chemistry|pages=1–16|editor-last=Scott|editor-first=R. A.|publisher=[[John Wiley & Sons]]|language=en|doi=10.1002/9781119951438.eibc2632|isbn=978-1-119-95143-8|last2=Kaldor|first2=U.|last3=Borschevsky|first3=A.}}</ref> 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.}} [[atomic nucleus]] is created in a nuclear reaction that combines two other nuclei of unequal size{{Efn|In 2009, a team at JINR led by Oganessian published results of their attempt to create hassium in a symmetric ¹³⁶Xe&nbsp;+&nbsp;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&nbsp;[[picobarn|pb]].<ref>{{Cite journal|last=Oganessian|first=Yu. Ts.|author-link=Yuri Oganessian|last2=Dmitriev|first2=S. N.|last3=Yeremin|first3=A. V.|last4=Aksenov|first4=N. V.|last5=Bozhikov|first5=G. A.|last6=Chepigin|first6=V. I.|last7=Chelnokov|first7=M. L.|last8=Lebedev|first8=V. Ya.|last9=Malyshev|first9=O. N.|last10=Petrushkin|first10=O. V.|last11=Shishkin|first11=S. V.|display-authors=3|date=2009|title=Attempt to produce the isotopes of element 108 in the fusion reaction ¹³⁶Xe + ¹³⁶Xe |journal=[[Physical Review C]]|language=en|volume=79|issue=2|pages=024608|doi=10.1103/PhysRevC.79.024608|issn=0556-2813}}</ref> In comparison, the reaction that resulted in hassium discovery, ²⁰⁸Pb + ⁵⁸Fe, had a cross section of ~20&nbsp;pb (more specifically, 19{{su|p=+19|b=−11}}&nbsp;pb), as estimated by the discoverers.<ref name="84Mu01">{{cite journal|last1=Münzenberg|first1=G.|author-link=Gottfried Münzenberg|last2=Armbruster|first2=P.|author-link2=Peter Armbruster|last3=Folger|first3=H.|last4=Heßberger|first4=F. P.|last5=Hofmann|first5=S.|last6=Keller|first6=J.|last7=Poppensieker|first7=K.|last8=Reisdorf|first8=W.|last9=Schmidt|first9=K.-H.|display-authors=3|date=1984|title=The identification of element 108|url=http://www.gsi-heavy-ion-researchcenter.org/forschung/kp/kp2/ship/108-discovery.pdf|url-status=dead|journal=Zeitschrift für Physik A|volume=317|issue=2|pages=235–236|bibcode=1984ZPhyA.317..235M|doi=10.1007/BF01421260|archiveurlarchive-url=https://web.archive.org/web/20150607124040/http://www.gsi-heavy-ion-researchcenter.org/forschung/kp/kp2/ship/108-discovery.pdf|archivedatearchive-date=7 June 2015|accessdateaccess-date=20 October 2012|first10=H.-J.|last10=Schött|first11=M. E.|last11=Leino|first12=R.|last12=Hingmann}}</ref>}} into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react.<ref name="Bloomberg">{{Cite web|last=Subramanian|first=S.|author-link=Samanth Subramanian|url=https://www.bloomberg.com/news/features/2019-08-28/making-new-elements-doesn-t-pay-just-ask-this-berkeley-scientist|title=Making New Elements Doesn't Pay. Just Ask This Berkeley Scientist|website=[[Bloomberg Businessweek]]|access-date=2020-01-18}}</ref> The material made of the heavier nuclei is made into a target, which is then bombarded by the [[Particle beam|beam]] of lighter nuclei. Two nuclei can only [[nuclear fusion|fuse]] into one if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due to [[Coulomb's law|electrostatic repulsion]]. The [[strong interaction]] can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatly [[particle accelerator|accelerated]] in order to make such repulsion insignificant compared to the velocity of the beam nucleus.<ref name="n+1">{{Cite web|url=https://nplus1.ru/material/2019/03/25/120-element|title=Сверхтяжелые шаги в неизвестное|last=Ivanov|first=D.|date=2019|website=nplus1.ru|language=ru|trans-title=Superheavy steps into the unknown|url-status=live|access-date=2020-02-02}}</ref> Coming close alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for approximately 10⁻²⁰&nbsp;seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.<ref name="n+1" /><ref>{{Cite web|url=http://theconversation.com/something-new-and-superheavy-at-the-periodic-table-26286|title=Something new and superheavy at the periodic table|last=Hinde|first=D.|date=2017|website=[[The Conversation]]|language=en|access-date=2020-01-30}}</ref> If fusion does occur, the temporary merger—termed a [[compound nucleus]]—is an [[excited state]]. To lose its excitation energy and reach a more stable state, a compound nucleus either [[Nuclear fission|fissions]] or [[Spallation#Nuclear spallation|ejects]] one or several [[neutron]]s,{{Efn|The greater the excitation energy, the more neutrons are ejected. If the excitation energy is lower than energy binding each neutron to the rest of the nucleus, neutrons are not emitted; instead, the compound nucleus de-excites by emitting a [[gamma ray]].<ref name=CzechNuclear/>}} which carry away the energy. This occurs in approximately 10⁻¹⁶&nbsp;seconds after the initial collision.<ref name="CzechNuclear">{{cite web|url=https://pdfs.semanticscholar.org/ba08/30dcab221b45ca5bcc3cfa8ae82558d624e7.pdf|title=Neutron Sources for ADS|last=Krása|first=A.|date=2010|work=Faculty of Nuclear Sciences and Physical Engineering|publisher=[[Czech Technical University in Prague]]|pages=4–8|urlaccess-status=live|accessdatedate=October 20, 2019}}</ref>{{efn|The definition by the [[IUPAC/IUPAP Joint Working Party]] states that a [[chemical element]] can only be recognized as discovered if a nucleus of it has not [[Radioactive decay|decayed]] within 10⁻¹⁴ seconds. This value was chosen as an estimate of how long it takes a nucleus to acquire its outer [[electron]]s and thus display its chemical properties.<ref>{{Cite journal|last=Wapstra|first=A. H.|author-link=Aaldert Wapstra|date=1991|title=Criteria that must be satisfied for the discovery of a new chemical element to be recognized|url=http://publications.iupac.org/pac/pdf/1991/pdf/6306x0879.pdf|journal=[[Pure and Applied Chemistry]]|volume=63|issue=6|page=883|doi=10.1351/pac199163060879|issn=1365-3075}}</ref> This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.<ref name=BerkeleyNoSF/>}}

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.<ref name="SHEhowvideo">{{Cite web|url=https://www.scientificamerican.com/article/how-to-make-superheavy-elements-and-finish-the-periodic-table-video/|title=How to Make Superheavy Elements and Finish the Periodic Table [Video]|author=Chemistry World|date=2016|website=[[Scientific American]]|language=en|url-status=live|access-date=2020-01-27}}</ref> In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products){{Efn|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.{{sfn|Hoffman|2000|p=334}} Such separation can also be aided by a [[Time-of-flight mass spectrometry|time-of-flight measurement]] and a recoil energy measurement; a combination of the two may allow to estimate the mass of a nucleus.{{sfn|Hoffman|2000|p=335}}}} and transferred to a [[Semiconductor detector|surface-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.<ref name="SHEhowvideo" /> The transfer takes about 10⁻⁶&nbsp;seconds; in order to be detected, the nucleus must survive this long.{{sfn|Zagrebaev|2013|page=3}} The nucleus is recorded again once its decay is registered, and the ___location, the [[Decay energy|energy]], and the time of the decay are measured.<ref name="SHEhowvideo" />

Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, its influence on the outermost [[nucleon]]s ([[proton]]s and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, as it has unlimited range.{{sfn|Beiser|2003|p=432}} Superheavy nuclei are thus theoretically predicted<ref>{{Cite journal|last=Staszczak|first=A.|last2=Baran|first2=A.|last3=Nazarewicz|first3=W.|date=2013|title=Spontaneous fission modes and lifetimes of superheavy elements in the nuclear density functional theory|url=|journal=Physical Review C|volume=87|issue=2|pages=024320–1|doi=10.1103/physrevc.87.024320|arxiv=1208.1215|bibcode=2013PhRvC..87b4320S|issn=0556-2813}}</ref> and have so far been observed{{sfn|Audi|2017|pp=030001-129–030001-138}} to predominantly decay via decay modes that are caused by such repulsion: [[alpha decay]] and [[spontaneous fission]].{{efn|Not all decay modes are caused by electrostatic repulsion. For example, [[beta decay]] is caused by the [[weak interaction]].{{sfn|Beiser|2003|p=439}}}} Alpha decays are registered by the emitted [[alpha particle]]s, 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.{{efn|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.<ref>{{Cite journal|last=Oganessian|first=Yu. Ts.|last2=Rykaczewski|first2=K. P.|date=2015|title=A beachhead on the island of stability|journal=[[Physics Today]]|volume=68|issue=8|pages=32–38|doi=10.1063/PT.3.2880|bibcode=2015PhT....68h..32O|osti=1337838|issn=0031-9228|url=https://www.osti.gov/biblio/1337838}}</ref> The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.<ref>{{Cite journal|last=Grant |first=A.|date=2018|title=Weighing the heaviest elements|journal=Physics Today|language=EN|doi=10.1063/PT.6.1.20181113a}}</ref> 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).<ref name="C&EN">{{Cite web|url=https://cen.acs.org/physical-chemistry/periodic-table/IYPT-Exploring-the-superheavy-elements-at-the-end-of-the-periodic-table/97/i21|title=Exploring the superheavy elements at the end of the periodic table|last=Howes|first=L.|date=2019|website=[[Chemical & Engineering News]]|language=en|url-status=live|archive-url=|archive-date=|access-date=2020-01-27}}</ref>}} Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.{{Efn|Spontaneous fission was discovered by Soviet physicist [[Georgy Flerov]],<ref name=Distillations>{{Cite journal|last=Robinson|first=A. E.|url=https://www.sciencehistory.org/distillations/the-transfermium-wars-scientific-brawling-and-name-calling-during-the-cold-war|title=The Transfermium Wars: Scientific Brawling and Name-Calling during the Cold War|date=2019|journal=[[Distillations (magazine)|Distillations]]|language=en|access-date=2020-02-22}}</ref> a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.<ref name="coldfusion77">{{Cite web|url=http://n-t.ru/ri/ps/pb106.htm|title=Популярная библиотека химических элементов. Сиборгий (экавольфрам)|trans-title=Popular library of chemical elements. Seaborgium (eka-tungsten)|language=ru|website=n-t.ru|access-date=2020-01-07}} Reprinted from {{cite book|author=<!--none-->|date=1977|title=Популярная библиотека химических элементов. Серебро — Нильсборий и далее|chapter=Экавольфрам|trans-title=Popular library of chemical elements. Silver through nielsbohrium and beyond|trans-chapter=Eka-tungsten|language=ru|publisher=[[Nauka (publisher)|Nauka]]}}</ref> 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.<ref name=BerkeleyNoSF>{{Cite journal|last=Hyde|first=E. K.|last2=Hoffman|first2=D. C.|author-link2=Darleane C. Hoffman|last3=Keller|first3=O. L.|date=1987|title=A History and Analysis of the Discovery of Elements 104 and 105|journal=Radiochimica Acta|volume=42|issue=2|doi=10.1524/ract.1987.42.2.57|issn=2193-3405|pages=67–68|url=http://www.escholarship.org/uc/item/05x8w9h7}}</ref> They thus preferred to link new isotopes to the already known ones by successive alpha decays.<ref name=Distillations/>}}

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.{{Efn|For instance, element&nbsp;102 was mistakenly identified in 1957 at the Nobel Institute of Physics in [[Stockholm]], [[Stockholm County]], [[Sweden]].<ref name=RSC>{{Cite web|url=https://www.rsc.org/periodic-table/element/102/nobelium|title=Nobelium - Element information, properties and uses {{!}} Periodic Table|website=[[Royal Society of Chemistry]]|access-date=2020-03-01}}</ref> 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.{{sfn|Kragh|2018|pp=38–39}} 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.{{sfn|Kragh|2018|pp=38–39}} JINR insisted that they were the first to create the element and suggested a name of their own for the new element, ''joliotium'';{{sfn|Kragh|2018|p=40}} the Soviet name was also not accepted (JINR later referred to the naming of element&nbsp;102 as "hasty").<ref name="1993 responses">{{Cite journal|year=1993|title=Responses on the report 'Discovery of the Transfermium elements' followed by reply to the responses by Transfermium Working Group|url=https://www.iupac.org/publications/pac/1993/pdf/6508x1815.pdf|url-status=live|journal=Pure and Applied Chemistry|volume=65|issue=8|pages=1815–1824|doi=10.1351/pac199365081815|archiveurlarchive-url=https://web.archive.org/web/20131125223512/http://www.iupac.org/publications/pac/1993/pdf/6508x1815.pdf|archivedatearchive-date=25 November 2013|access-date=7 September 2016|last1=Ghiorso|first1=A.|last2=Seaborg|first2=G. T.|author-link2=Glenn T. Seaborg|last3=Oganessian|first3=Yu. Ts.|last4=Zvara|first4=I|last5=Armbruster|first5=P|last6=Hessberger|first6=F. P|last7=Hofmann|first7=S|last8=Leino|first8=M|last9=Munzenberg|first9=G|last10=Reisdorf|first10=W|last11=Schmidt|first11=K.-H|display-authors=3}}</ref> The name "nobelium" remained unchanged on account of its widespread usage.<ref name=IUPAC97>{{Cite journal|doi=10.1351/pac199769122471|title=Names and symbols of transfermium elements (IUPAC Recommendations 1997)|date=1997|journal=Pure and Applied Chemistry|volume=69|pages=2471–2474|issue=12|author=Commission on Nomenclature of Inorganic Chemistry|url=http://publications.iupac.org/pac/pdf/1997/pdf/6912x2471.pdf}}</ref>}}</onlyinclude>

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