Editing Otto Hahn (section)

===Discovery of nuclear fission===
{{main|Discovery of nuclear fission}}
[[File:Nuclear Fission Experimental Apparatus 1938 - Deutsches Museum - Munich.jpg|thumb|right| This set up is on display in the [[Deutsches Museum]] in [[Munich]]. The table and instruments are original, but the instruments would not have been together on the one table in the same room.<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' |trans-title=Original equipment for the discovery of nuclear fission, 'Hahn-Meitner-Straßmann table' |lang=de |publisher=Deutsches Museum |access-date=8 October 2024 }}</ref> Pressure from historians, scientists and feminists caused the museum to alter the display in 1988 to acknowledge [[Lise Meitner]], [[Otto Frisch]] and [[Fritz Strassmann]].{{sfn|Sime|2010|pages=206–210}} ]]
After [[James Chadwick]] discovered the [[neutron]] in 1932,{{sfn|Rhodes|1986|pp=39, 160–167, 793}} [[Irène Curie]] and [[Frédéric Joliot]] irradiated aluminium foil with alpha particles. They found that this results in a short-lived radioactive [[isotopes of phosphorus|isotope of phosphorus]]. They noted that [[positron emission]] continued after the neutron emissions ceased. Not only had they discovered a new form of radioactive decay, they had transmuted an element into a hitherto unknown radioactive isotope of another, thereby inducing radioactivity where there had been none before. Radiochemistry was now no longer confined to certain heavy elements, but extended to the entire periodic table.{{sfn|Rhodes|1986|pp=200–201}}{{sfn|Sime|1996|pp=161–162}} Chadwick noted that being electrically neutral, neutrons could penetrate the [[atomic nucleus]] more easily than protons or alpha particles.{{sfn|Fergusson|2011|pp=150–151}} [[Enrico Fermi]] and his colleagues in Rome picked up on this idea,{{sfn|Rhodes|1986|pp=210–211}} and began irradiating elements with neutrons.<ref name="Segrè-1989" />

The radioactive displacement law of Fajans and Soddy said that beta decay causes isotopes to move one element up on the periodic table, and alpha decay causes them to move two down. When Fermi's group bombarded uranium atoms with neutrons, they found a complex mix of half-lives. Fermi therefore concluded that the new elements with atomic numbers greater than 92 (known as [[transuranium elements]]) had been created.<ref name="Segrè-1989">{{cite journal |title=Discovery of Nuclear Fission |first=Emilio G. |last=Segrè |author-link=Emilio Segrè |journal=Physics Today |issn=0031-9228 |volume=42 |issue=7 |date=July 1989 |pages=38–43 |doi=10.1063/1.881174 |bibcode=1989PhT....42g..38S }}</ref> Meitner and Hahn had not collaborated for many years, but Meitner was eager to investigate Fermi's results. Hahn, initially, was not, but he changed his mind when [[Aristid von Grosse]] suggested that what Fermi had found was an isotope of protactinium.{{sfn|Sime|1996|pp=164–165}} They set out to determine whether or not the 13-minute isotope was indeed an isotope of protactinium.{{sfn|Hahn|1966|pp=140–141}}

Between 1934 and 1938, Hahn, Meitner and Strassmann found a great number of radioactive transmutation products, all of which they regarded as transuranic.<ref>{{cite magazine|last1=Hahn |first1=O. |title=The Discovery of Fission |doi= 10.1038/scientificamerican0258-76 |magazine=Scientific American |volume=198 |issue=2 |pages=76–84 |year=1958 |bibcode=1958SciAm.198b..76H}}</ref> At that time, the existence of [[actinide]]s was not yet established, and uranium was wrongly believed to be a [[group 6 element]] similar to [[tungsten]]. It followed that the first transuranic elements would be similar to group 7 to 10 elements, i.e. [[rhenium]] and [[platinoid]]s. They established the presence of multiple isotopes of at least four such elements, and (mistakenly) identified them as elements with atomic numbers 93 through 96. They were the first scientists to measure the 23-minute half-life of uranium-239 and to establish chemically that it was an isotope of uranium, but were unable to continue this work to its logical conclusion and identify the real element 93.{{sfn|Sime|1996|pp=170–172}} They identified ten different half-lives, with varying degrees of certainty. To account for them, Meitner had to hypothesise a new class of reaction and the alpha decay of uranium, neither of which had ever been reported before, and for which physical evidence was lacking. Hahn and Strassmann refined their chemical procedures, while Meitner devised new experiments to shine more light on the reaction processes.{{sfn|Sime|1996|pp=170–172}}
[[File:Otto Hahn's notebook 1938 - Deutsches Museum - Munich.jpg|thumb|left|Otto Hahn's notebook]]
In May 1937, they issued parallel reports, one in the ''[[Zeitschrift für Physik]]'' with Meitner as the principal author, and one in the ''[[Chemische Berichte]]'' with Hahn as the principal author.{{sfn|Sime|1996|pp=170–172}}<ref name="L.-1937">{{cite journal |first1=Meitner |last1=L. |author-link1=Lise Meitner |first2=Hahn |last2=O. |first3=F. |last3=Strassmann |author-link3=Fritz Strassmann |title=Über die Umwandlungsreihen des Urans, die durch Neutronenbestrahlung erzeugt werden |trans-title=On the series of transformations of uranium that are generated by neutron radiation |language=de |journal=Zeitschrift für Physik |issn=0939-7922 |volume=106 |issue=3–4 |pages=249–270 |date=May 1937 |doi=10.1007/BF01340321 |bibcode=1937ZPhy..106..249M |s2cid=122830315 }}</ref><ref name="O.-1937">{{cite journal |first1=Hahn |last1=O.  |first2=Meitner |last2=L. |author-link2=Lise Meitner |first3=F. |last3=Strassmann |author-link3=Fritz Strassmann |title=Über die Trans-Urane und ihr chemisches Verhalten |trans-title=On the transuranes and their chemical behaviour |lang=de |journal=Berichte der Deutschen Chemischen Gesellschaft |issn=0365-9496 |date=9 June 1937 |volume=70 |issue=6 |pages=1374–1392 |doi=10.1002/cber.19370700634 }}</ref> Hahn concluded his by stating emphatically: {{lang|de|Vor allem steht ihre chemische Verschiedenheit von allen bisher bekannten Elementen außerhalb jeder Diskussion}} ("Above all, their chemical distinction from all previously known elements needs no further discussion").<ref name="O.-1937"/> Meitner, however, was increasingly uncertain. She considered the possibility that the reactions were from different isotopes of uranium; three were known: uranium-238, uranium-235 and uranium-234. However, when she calculated the [[neutron cross section]], it was too large to be anything other than the most abundant isotope, uranium-238. She concluded that it must be another case of the nuclear isomerism that Hahn had discovered in protactinium. She therefore ended her report on a very different note to Hahn,{{sfn|Sime|1996|p=177}} reporting that: {{lang|de|Also müssen die Prozesse Einfangprozesse des Uran 238 sein, was zu drei isomeren Kernen Uran 239 führt. Dieses Ergebnis ist mit den bisherigen Kernvorstellungen sehr schwer in Übereinstimmung zu bringen}} ("The processes must be neutron capture by uranium-238, which leads to three isomeric nuclei of uranium-239. This result is very difficult to reconcile with current concepts of the nucleus.")<ref name="L.-1937" />

With the ''[[Anschluss]]'', Germany's annexation of Austria on 12 March 1938, Meitner lost her Austrian citizenship,{{sfn|Sime|1996|pp=184–185}} and fled to Sweden. She carried only a little money, but before she left, Hahn gave her a diamond ring he had inherited from his mother.{{sfn|Sime|1996|pp=200–207}} Meitner continued to correspond with Hahn by mail. In late 1938 Hahn and Strassmann found evidence of isotopes of an alkaline earth metal in their sample. Finding a group 2 metal was problematic, because it did not logically fit with the other elements found thus far. Hahn initially suspected it to be radium, produced by splitting off two alpha-particles from the uranium nucleus, but chipping off two alpha particles via this process was unlikely. The idea of turning uranium into [[barium]] (by removing around 100 nucleons) was seen as preposterous.{{sfn|Sime|1996|pp=227–230}}

During a visit to Copenhagen on 10 November, Hahn discussed these results with [[Niels Bohr]], Meitner, and [[Otto Robert Frisch]].{{sfn|Sime|1996|pp=227–230}} Further refinements of the technique, leading to the decisive experiment on 16–17 December 1938, produced puzzling results: the three isotopes consistently behaved not as radium, but as barium. Hahn, who did not inform the physicists in his Institute, described the results exclusively in a letter to Meitner on 19 December:
{{blockquote|We are more and more coming to the awful conclusion that our Ra isotopes behave not like Ra, but like Ba... Perhaps you can come up with some fantastic explanation. We ourselves realize that it ''can't'' actually burst apart into Ba. Now we want to test whether the Ac-isotopes derived from the "Ra" behave not like Ac but like La.{{sfn|Sime|1996|p=233}}}}

[[File:Dahlem Thielallee Hahn-Meitner-Bau-2.JPG|thumb|right|upright|Plaque commemorating Hahn and Strassmann's discovery of fission in Berlin (unveiled in 1956)]]
In her reply, Meitner concurred. "At the moment, the interpretation of such a thoroughgoing breakup seems very difficult to me, but in nuclear physics we have experienced so many surprises, that one cannot unconditionally say: 'it is impossible'." On 22 December 1938, Hahn sent a manuscript to ''[[Die Naturwissenschaften|Naturwissenschaften]]'' reporting their radiochemical results, which were published on 6 January 1939.<ref>{{cite journal |last1= Hahn |first1= O. |author-link= Otto Hahn |last2= Strassmann |first2= F. |author-link2=Fritz Strassmann |doi= 10.1007/BF01488241 |title=Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle |language=de |journal= Die Naturwissenschaften |issn=0028-1042 |volume=27 |issue=1 |pages=11–15 |year=1939 |bibcode=1939NW.....27...11H |s2cid= 5920336 |trans-title=On the detection and characteristics of the alkaline earth metals formed by irradiation of uranium with neutrons}}</ref> On 27 December, Hahn telephoned the editor of the ''Naturwissenschaften'' and requested an addition to the article, speculating that some [[platinum group]] elements previously observed in irradiated uranium, which were originally interpreted as transuranium elements, could in fact be [[technetium]] (then called "masurium"), mistakenly believing that the [[atomic mass]]es had to add up rather than the [[atomic number]]s. By January 1939, he was sufficiently convinced of the formation of light elements that he published a new revision of the article, retracting former claims of observing transuranic elements and neighbours of uranium.{{sfn|Sime|1996|pp=248–249}}

As a chemist, Hahn was reluctant to propose a revolutionary discovery in physics, but Meitner and Frisch worked out a theoretical interpretation of [[nuclear fission]], a term appropriated by Frisch from biology. In January and February they published two articles discussing and experimentally confirming their theory.{{sfn|Frisch|1979|pp=115–116}}<ref>{{cite journal |last1= Meitner |first1= L. |author-link1= Lise Meitner |last2= Frisch |first2= O. R. |author-link2= Otto Robert Frisch |doi= 10.1038/143239a0 |title= Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction |journal= Nature |issn=0028-0836 |volume=143 |issue=3615 |page=239 |date=January 1939 |bibcode= 1939Natur.143..239M|s2cid= 4113262 }}</ref><ref>{{cite journal |last1=Frisch |first1=O. R. |author-link1=Otto Robert Frisch |title=Physical Evidence for the Division of Heavy Nuclei Under Neutron Bombardment |doi=10.1038/143276a0 |journal=Nature |issn=0028-0836 |volume=143 |issue=3616 |page=276 |date=February 1939 |bibcode=1939Natur.143..276F|s2cid= 4076376 |doi-access= free }}</ref> In their second publication on nuclear fission, 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 |trans-title=Evidence of the Formation of Active Barium Isotopes From Uranium and Thorium by Neutron Irradiation; Evidence of Further Active Fragments During Uranium Fission |lang=de |journal=Naturwissenschaften |issn=0028-1042 |volume=27 |issue=6 |pages=89–95 |date=February 1939 |doi=10.1007/BF01488988 |bibcode=1939NW.....27...89H |s2cid=33512939 }}</ref> This was shown to be the case by Frédéric Joliot and his team in March 1939.<ref>{{cite journal|last1=Von Halban |first1=H. |author-link=Hans von Halban |last2=Joliot |first2=F. |author-link2=Frédéric Joliot |last3=Kowarski |first3=L. |author-link3=Lew Kowarski |title=Number of Neutrons Liberated in the Nuclear Fission of Uranium|journal=Nature  |issn=0028-0836 |volume=143|issue=3625|date=22 April 1939|pages=680 |doi=10.1038/143680a0|bibcode=1939Natur.143..680V |s2cid=4089039 |doi-access=free }}</ref> [[Edwin McMillan]] and [[Philip Abelson]] used the [[cyclotron]] at the [[Berkeley Radiation Laboratory]] to bombard uranium with neutrons, and were able to identify an isotope with a 23-minute half-life that was the daughter of uranium-239, and therefore the real element 93, which they named [[neptunium]].{{sfn|Walker|1993|pp=22–23}} "There goes a Nobel Prize", Hahn remarked.{{sfn|Hoffmann|2001|p=150}}

At the KWIC, [[Kurt Starke]] independently produced element 93, using only the weak neutron sources available there. Hahn and Strassmann then began researching its chemical properties.{{sfn|Hahn|1966|pp=175–177}} They knew that it should decay into the [[plutonium|real element 94]], which according to the latest version of the [[liquid drop model]] of the nucleus propounded by Bohr and [[John Archibald Wheeler]], would be even more [[fissile]] than uranium-235, but were unable to detect its radioactive decay. They concluded that it must have an extremely long half-life, perhaps millions of years.{{sfn|Walker|1993|pp=22–23}} Part of the problem was that they still believed that element 94 was a platinoid, which confounded their attempts at chemical separation.{{sfn|Hahn|1966|pp=175–177}}