Editing Nuclear fission (section)

===Discovery of nuclear fission===

{{main|Discovery of nuclear fission}}
[[File:Hahn and Meitner in 1912.jpg|thumb|373x373px|[[Otto Hahn]] and [[Lise Meitner]] in 1912]]
The discovery of nuclear fission occurred in 1938 in the buildings of the [[Kaiser Wilhelm Society]] for Chemistry, today part of the [[Free University of Berlin]], following over four decades of work on the science of [[radioactivity]] and the elaboration of new nuclear physics that described the components of atoms. In 1911, [[Ernest Rutherford]] proposed a model of the atom in which a very small, dense and positively charged nucleus of protons was surrounded by orbiting, negatively charged electrons (the [[Rutherford model]]).<ref>{{cite journal |author=E. Rutherford|year=1911|title=The scattering of α and β particles by matter and the structure of the atom|journal=Philosophical Magazine|volume= 21|pages=669–688|url=http://web.ihep.su/dbserv/compas/src/rutherford11/eng.pdf|bibcode=2012PMag...92..379R|doi=10.1080/14786435.2011.617037|issue=4|s2cid=126189920}}</ref> [[Niels Bohr]] improved upon this in 1913 by reconciling the quantum behavior of electrons (the [[Bohr model]]). In 1928, [[George Gamow]] proposed the [[Liquid drop model]], which became essential to understanding the physics 
of fission.<ref name=rr/>{{rp|49–51,70–77,228}}<ref name=ww/>{{rp|6–7}}

In 1896, [[Henri Becquerel]] had found, and [[Marie Curie]] named, radioactivity. In 1900, Rutherford and [[Frederick Soddy]], investigating the radioactive gas emanating from [[thorium]], "conveyed the tremendous and  inevitable conclusion that the element thorium was slowly and spontaneously [[nuclear transmutation|transmuting]] itself into argon gas!"<ref name=rr/>{{rp|41–43}}

In 1919, following up on an earlier anomaly [[Ernest Marsden]] noted in 1915, Rutherford attempted to "break up the atom." Rutherford was able to accomplish the first artificial transmutation of [[nitrogen]] into [[oxygen]], using alpha particles directed at nitrogen <sup>14</sup>N + α → <sup>17</sup>O + p.&nbsp; Rutherford stated, "...we must conclude that the nitrogen atom is disintegrated," while the newspapers stated he had ''split the atom''. This was the first observation of a nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. It also offered a new way to study the nucleus. Rutherford and [[James Chadwick]] then used alpha particles to "disintegrate" boron, fluorine, sodium, aluminum, and phosphorus before reaching a limitation associated with the energy of his alpha particle source.<ref name="rr">{{cite book |last1=Rhodes |first1=Richard |title=The Making of the Atomic Bomb |date=1986 |publisher=Simon & Schuster Paperbacks |location=New York |isbn=9781451677614 |pages=135–138}}</ref> Eventually, in 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleagues [[Ernest Walton]] and [[John Cockcroft]], who used artificially accelerated protons against lithium-7, to split this nucleus into two alpha particles. The feat was popularly known as "splitting the atom", and would win them the 1951 Nobel Prize in Physics for ''"Transmutation of atomic nuclei by artificially accelerated atomic particles"'', although it was not the nuclear fission reaction later discovered in heavy elements.<ref>{{cite web |url=http://www-outreach.phy.cam.ac.uk/camphy/cockcroftwalton/cockcroftwalton9_1.htm |title=Cockcroft and Walton split lithium with high energy protons April 1932 |publisher=Outreach.phy.cam.ac.uk |date=1932-04-14 |access-date=2013-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20120902195556/http://www-outreach.phy.cam.ac.uk/camphy/cockcroftwalton/cockcroftwalton9_1.htm |archive-date=2012-09-02 }}</ref><ref>{{Cite web |title=Trump claims Manchester atom split as US achievement |url=https://www.bbc.com/news/articles/cg451wx2n63o |access-date=2025-01-21 |website=www.bbc.com |date=21 January 2025 |language=en-GB}}</ref><ref>{{Cite magazine |last=AP |first=Charlotte Graham-McLay / |date=2025-01-21 |title=New Zealanders Balk at Trump Claim That Americans Split Atom |url=https://time.com/7208628/trump-america-split-atom-new-zealand-fact-check/ |access-date=2025-01-21 |magazine=TIME |language=en}}</ref>

English physicist [[James Chadwick]] discovered the neutron in 1932.<ref>{{cite journal |author=J. Chadwick|doi=10.1038/129312a0|title=Possible Existence of a Neutron|year=1932|page=312|issue=3252|volume=129|journal=Nature|url=http://web.mit.edu/22.54/resources/Chadwick.pdf|bibcode= 1932Natur.129Q.312C|s2cid=4076465|doi-access=free}}</ref> Chadwick used an [[ionization chamber]] to observe protons knocked out of several elements 
by beryllium radiation, following up on earlier observations made by [[Irène Joliot-Curie|Joliot-Curies]]. In Chadwick's words, "...In order to explain the great penetrating power of the radiation we must further assume that the particle has no net charge..." The existence of the neutron was first postulated by Rutherford in 1920, and in the words of Chadwick, "...how on earth were you going to build up a big nucleus with a large positive charge? And the answer was a neutral particle."<ref name=rr/>{{rp|153–165}} Subsequently, he communicated his findings in more detail.<ref>{{cite journal |doi=10.1098/rspa.1932.0112|author=Chadwick, J.|year=1932|title=The existence of a neutron|journal=Proceedings of the Royal Society A|volume=136|issue=830|pages=692–708|url=http://www.chemteam.info/Chem-History/Chadwick-1932/Chadwick-neutron.html|bibcode= 1932RSPSA.136..692C|doi-access=free}} and {{cite journal |doi=10.1098/rspa.1933.0152|author=Chadwick, J.|year=1933|title=The Bakerian Lecture: The neutron|journal=Proceedings of the Royal Society A|volume=142|issue=846 |pages=1–25|bibcode= 1933RSPSA.142....1C|doi-access=free}}</ref>

In the words of [[Richard Rhodes]], referring to the neutron, "It would therefore serve as a new nuclear probe of surpassing power of penetration." [[Philip Morrison]] stated, "A beam of [[thermal neutron]]s moving at about the speed of sound...produces nuclear reactions in many materials much more easily than a beam of protons...traveling thousands of times faster."  
According to Rhodes, "Slowing down a neutron gave it more time in the vicinity of the nucleus, and that gave it more time to be captured." Fermi's team, studying radiative capture which is the emission of gamma radiation after the nucleus captures a neutron, studied sixty elements, inducing radioactivity in forty. In the process, they discovered the ability of hydrogen to slow down the neutrons.<ref name=rr/>{{rp|165,216–220}}

[[Enrico Fermi]] and his colleagues in [[Rome]] studied the results of bombarding uranium with neutrons in 1934.<ref>E. Fermi, E. Amaldi, O. D'Agostino, F. Rasetti, and E. Segrè (1934) "Radioattività provocata da bombardamento di neutroni III", ''La Ricerca Scientifica'', vol.&nbsp;5, no.&nbsp;1, pages&nbsp;452–453.</ref> Fermi concluded that his experiments had created new elements with 93 and 94 protons, which the group dubbed [[ausenium and hesperium]]. However, not all were convinced by Fermi's analysis of his results, though he would win the 1938  [[Nobel Prize in Physics]] for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". The German chemist [[Ida Noddack]] notably suggested in 1934 that instead of creating a new, heavier element 93, that "it is conceivable that the nucleus breaks up into several large fragments."<ref>{{cite journal |author=Ida Noddack|year=1934|page=653|issue=37|title=Über das Element 93|volume=47|journal=Zeitschrift für Angewandte Chemie|url=http://www.chemteam.info/Chem-History/Noddack-1934.html|doi=10.1002/ange.19340473707|bibcode=1934AngCh..47..653N}}</ref> However, the quoted objection comes some distance down, and was but one of several gaps she noted in Fermi's claim. Although Noddack was a renowned analytical chemist, she lacked the background in physics to appreciate the enormity of what she was proposing.<ref>{{cite book |last=Hook |first=Ernest B. |editor-last=Hook |editor-first=Ernest B. |title=Prematurity in Scientific Discovery: On Resistance and Neglect |contribution=Interdisciplinary Dissonance and Prematurity: Ida Noddack’s Suggestion of Nuclear Fission |pages=124–148 |publisher=University of California Press |location=Berkeley and Los Angeles |date=2002 |isbn=978-0-520-23106-1 |oclc=883986381 }}</ref>

[[File:Nuclear Fission Experimental Apparatus 1938 - Deutsches Museum - Munich.jpg|thumb|left|The nuclear fission display at the [[Deutsches Museum]] in [[Munich]]. The table and instruments are originals,<ref>{{cite web | url=https://digital.deutsches-museum.de/de/digital-catalogue/collection-object/71930/ | title=Originalgeräte zur Entdeckung der Kernspaltung, "Hahn-Meitner-Straßmann-Tisch" }}</ref><ref>{{cite web | url=https://www.youtube.com/watch?v=ww8rqqVCBxo | title=Entdeckung der Kernspaltung 1938, Versuchsaufbau, Deutsches Museum München &#124; Faszination Museum | website=[[YouTube]] | date=7 July 2015 }}</ref> but would not have been together in the same room.]]

After the Fermi publication, [[Otto Hahn]], [[Lise Meitner]], and [[Fritz Strassmann]] began performing similar experiments in [[Berlin]]. Meitner, an Austrian Jew, lost her Austrian citizenship with the ''[[Anschluss]]'', the union of Austria with Germany in March 1938, but she fled in July 1938 to Sweden and started a correspondence by mail with Hahn in Berlin. By coincidence, her nephew [[Otto Robert Frisch]], also a refugee, was also in Sweden when Meitner received a letter from Hahn dated 19 December describing his chemical proof that some of the product of the bombardment of uranium with neutrons was [[barium]]. Hahn suggested a ''bursting'' of the nucleus, but he was unsure of what the physical basis for the results were. Barium had an atomic mass 40% less than uranium, and no previously known methods of radioactive decay could account for such a large difference in the mass of the nucleus. Frisch was skeptical, but Meitner trusted Hahn's ability as a chemist. Marie Curie had been separating barium from radium for many years, and the techniques were well known. Meitner and Frisch then correctly interpreted Hahn's results to mean that the nucleus of uranium had split roughly in half. Frisch suggested the process be named "nuclear fission", by analogy to the process of living cell division into two cells, which was then called [[fission (biology)|binary fission]]. Just as the term nuclear "chain reaction" would later be borrowed from chemistry, so the term "fission" was borrowed from biology.<ref>{{cite book |last=Frisch |first=Otto Robert |title=What Little I Remember |year=1980 |publisher=Cambridge University Press |isbn=0-52-128010-9 |pages=114–117 |quote=The paper was composed by several long-distance telephone calls, Lise Meitner having returned to Stockholm in the meantime. I asked an American biologist who was working with Hevesy what they call the process by which single cells divide in two; 'fission', he said, so I used the term 'nuclear fission' in that paper. Placzek was sceptical; couldn’t I do some experiments to show the existence of those fast-moving fragments of the uranium nucleus? Oddly enough that thought hadn’t occurred to me, but now I quickly set to work, and the experiment (which was really very easy) was done in two days, and a short note about it was sent off to Nature together with the other note I had composed over the telephone with Lise Meitner.}}</ref>

News spread quickly of the new discovery, which was correctly seen as an entirely novel physical effect with great scientific—and potentially practical—possibilities. Meitner's and Frisch's interpretation of the discovery of Hahn and Strassmann crossed the Atlantic Ocean with Niels Bohr, who was to lecture at [[Princeton University]]. [[Isidor Isaac Rabi|I.I. Rabi]] and [[Willis Lamb]], two [[Columbia University]] physicists working at Princeton, heard the news and carried it back to Columbia. Rabi said he told Enrico Fermi; Fermi gave credit to Lamb. Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found [[Herbert L. Anderson]]. Bohr grabbed him by the shoulder and said: "Young man, let me explain to you about something new and exciting in physics."<ref>Richard Rhodes. (1986) ''The Making of the Atomic Bomb'', Simon and Schuster, p. 268, {{ISBN|0-671-44133-7}}.</ref>

It was clear to a number of scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University team conducted the first nuclear fission experiment in the United States,<ref>{{cite journal |author1=H. L. Anderson |author2=E. T. Booth |author3=J. R. Dunning |author4=E. Fermi |author5=G. N. Glasoe |author6=F. G. Slack  |name-list-style=amp |title=The Fission of Uranium|journal=Physical Review|volume=55|issue=5|page=511|year=1939|doi=10.1103/PhysRev.55.511.2|bibcode= 1939PhRv...55..511A}}</ref> which was done in the basement of [[Pupin Hall]]. The experiment involved placing uranium oxide inside of an ionization chamber and irradiating it with neutrons, and measuring the energy thus released. The results confirmed that fission was occurring and hinted strongly that it was the isotope [[uranium 235]] in particular that was fissioning. The next day, the fifth [[Washington Conference on Theoretical Physics]] began in [[Washington, D.C.]] under the joint auspices of the George Washington University and the [[Carnegie Institution of Washington]]. There, the news on nuclear fission was spread even further, which fostered many more experimental demonstrations.<ref>Richard Rhodes (1986). ''The Making of the Atomic Bomb'', Simon and Schuster, pp. 267–270, {{ISBN|0-671-44133-7}}.</ref>
The 6 January 1939 Hahn and Strassman paper announced the discover of fission. In their second publication on nuclear fission in February 1939, Hahn and Strassmann used the term ''Uranspaltung'' (uranium fission) for the first time, and predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction.<ref>{{cite journal |last1=Hahn |first1=O. |last2=Strassmann |first2=F. |author-link2=Fritz Strassmann |title=Nachweis der Entstehung aktiver Bariumisotope aus Uran und Thorium durch Neutronenbestrahlung; Nachweis weiterer aktiver Bruchstücke bei der Uranspaltung |journal=Naturwissenschaften |volume=27 |issue=6 |pages=89–95 |date=February 1939 |doi=10.1007/BF01488988 |bibcode=1939NW.....27...89H |s2cid=33512939 }}</ref> The 11 February 1939 paper by Meitner and Frisch compared the process to the division of a liquid drop and estimated the energy released at 200 MeV.<ref>{{cite journal |last1=Meitner |first1=Lisa |last2=Frisch |first2=O.R. |title=Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction |url=https://www.nature.com/articles/143239a0 |journal=Nature |access-date=20 September 2023 |date=1939|volume=143 |issue=3615 |pages=239–240 |doi=10.1038/143239a0 |bibcode=1939Natur.143..239M |s2cid=4113262 }}</ref>  The 1 September 1939 paper by Bohr and Wheeler used this liquid drop model to quantify fission details, including the energy released, estimated the cross section for neutron-induced fission, and deduced {{chem|235|U}} was the major contributor to that cross section and slow-neutron fission.<ref>{{cite journal |last1=Bohr |first1=Niels |last2=Wheeler |first2=John |title=The Mechanism of Nuclear Fission |journal=Physical Review |date=1939|volume=56 |issue=5 |pages=426–450 |doi=10.1103/PhysRev.56.426 |bibcode=1939PhRv...56..426B |doi-access=free }}</ref><ref name=rr/>{{rp|262,311}}<ref name=ww/>{{rp|9–13}}