Editing Lise Meitner (section)

==Nuclear fission==
{{Main|Discovery of nuclear fission}}
[[File:Nuclear Fission Experimental Apparatus 1938 - Deutsches Museum - Munich.jpg|thumb|right|This was touted for many years as the table and experimental apparatus with which Otto Hahn discovered nuclear fission in 1938. The table and instruments are representative of the ones used, but not necessarily the originals, and would not have been together on the one table in the same room. Pressure from historians, scientists and feminists caused the [[Deutsches Museum]] to alter the display in 1988 to acknowledge the role of Meitner, Frisch and Strassmann.{{sfn|Sime|2010|pp=206–211}} ]]

Hahn and Strassmann isolated the three radium isotopes (verified by their half-lives) and used fractional crystallisation to separate it from its barium carrier by adding [[barium bromide]] crystals in four steps. Since radium precipitates preferentially in a solution of barium bromide, at each step the fraction drawn off would contain less radium than the one before. However, they found no difference between each of the fractions. In case their process was faulty in some way, they verified it with known isotopes of radium; the process was fine. On 19 December, Hahn wrote to Meitner, informing her that the radium isotopes behaved chemically like barium. Anxious to finish up before the Christmas break, Hahn and Strassmann submitted their findings to {{lang|de|Naturwissenschaften}} on 22 December without waiting for Meitner to reply.{{sfn|Sime|1996|pp=233–234}} Hahn concluded the paper with: "As chemists... we should substitute the symbols Ba, La, Ce for Ra, Ac, Th. As 'nuclear chemists' fairly close to physics we cannot yet bring ourselves to take this step which contradicts all previous experience in physics."<ref>{{cite journal |first1=O. |last1=Hahn |author-link1=Otto Hahn |first2=F. |last2=Strassmann |s2cid=5920336 |author-link2=Fritz Strassmann |title=Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle |language=de |trans-title=Concerning the existence of alkaline earth metals resulting from neutron irradiation of uranium |journal=Naturwissenschaften |issn=0028-1042 |volume=27 |issue=1 |pages=11–15 |date=6 January 1939 |doi=10.1007/BF01488241 |bibcode=1939NW.....27...11H }}</ref>

Frisch normally celebrated Christmas with Meitner in Berlin, but in 1938 she accepted an invitation from Eva von&nbsp;Bahr to spend it with her family at [[Kungälv]], and Meitner asked Frisch to join her there. Meitner received the letter from Hahn describing his chemical proof that some of the product of the bombardment of uranium with neutrons was barium. 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.{{sfn|Frisch|1979|pp=113–114}}{{sfn|Sime|1996|pp=235–239}} Nonetheless, she had immediately written back to Hahn to say: "At the moment the assumption 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.'"{{sfn|Sime|1996|p=235}}

[[File:Liquid drop model of nuclear fission.jpg|thumb|left|upright=1.5|[[Liquid-drop model]] of nuclear fission]]

Meitner dismissed the possibility that Hahn's identification of barium was in error; her faith in Hahn's expertise as a chemist was absolute. Meitner and Frisch then considered how it could be possible. Previous attempts at atom splitting had never had enough energy to chip away more than individual protons or alpha particles, but a barium nucleus was much larger. They considered the [[liquid-drop model]] of the nucleus that had been proposed by [[George Gamow]]: perhaps it was possible for a drop to become elongated and then divide itself in two.{{sfn|Frisch|1979|pp=115–116}}

Frisch later wrote:{{blockquote|text=<p>At that point we both sat down on a tree trunk (all that discussion had taken place while we walked through the wood in the snow, I with my skis on, Lise Meitner making good her claim that she could walk just as fast without), and started to calculate on scraps of paper. The charge of a uranium nucleus, we found, was indeed large enough to overcome the effect of the surface tension almost completely; so the uranium nucleus might indeed resemble a very wobbly unstable drop, ready to divide itself at the slightest provocation, such as the impact of a single neutron.</p><p>But there was another problem. After separation, the two drops would be driven apart by their mutual electric repulsion and would acquire high speed and hence a very large energy, about 200&nbsp;MeV in all; where could that energy come from? Fortunately Lise Meitner remembered the empirical formula for computing the masses of nuclei and worked out that the two nuclei formed by the division of a uranium nucleus together would be lighter than the original uranium nucleus by about one-fifth the mass of a proton. Now whenever mass disappears energy is created, according to [[Albert Einstein|Einstein]]'s formula ''E''&nbsp;= ''mc''<sup>2</sup>, and one-fifth of a proton mass was just equivalent to 200&nbsp;MeV. So here was the source for that energy; it all fitted!{{sfn|Frisch|1979|pp=115–116}}</p> }}

[[File:75th Anniversary Discovery of Nuclear Fission (01311665) (11049703086).jpg|thumb|right|Exhibition to mark the 75th anniversary of the discovery of nuclear fission, at the [[Vienna International Centre]] in 2013. The table (on loan from the Deutsches Museum Munich) is now described as a replica and images of Meitner and Strassmann are prominently displayed.]]

Meitner and Frisch had correctly interpreted Hahn's results to mean that the nucleus of uranium had split roughly in half. The first two reactions that the Berlin group had observed were light elements created by the breakup of uranium nuclei; the third, the 23-minute one, was a decay into the real element 93.{{sfn|Sime|1996|p=243}} On returning to Copenhagen, Frisch informed Bohr, who slapped his forehead and exclaimed "What idiots we have been!"{{sfn|Frisch|1979|p=116}} Bohr promised not to say anything until they had a paper ready for publication. To speed the process, they decided to submit a one-page note to ''Nature''. At this point, the only evidence that they had was the barium. Logically, if barium was formed, the other element must be [[krypton]],{{sfn|Sime|1996|p=246}} but Hahn had mistakenly believed that the [[atomic mass]]es had to add up to 239 rather than the [[atomic number]]s adding up to 92, and thought it was masurium ([[technetium]]), and so did not check for it:{{sfn|Sime|1996|pp=239, 456}}
: {{chem||92|U}} + n → {{chem||56|Ba}} + {{chem||36|Kr}} + some n{{Efn|Although Hahn and Strassmann believed that fission had occurred in the uranium-238 isotope, their observations indicate that they actually observed fission in uranium-235.{{sfn|Steinhauser|2016|pp=265–266}} }}   

Over a series of long-distance phone calls, Meitner and Frisch came up with a simple experiment to bolster their claim: to measure the recoil of the fission fragments, using a [[Geiger counter]] with the threshold set above that of the alpha particles. Frisch conducted the experiment on 13 January, and found the pulses caused by the reaction just as they had predicted.{{sfn|Sime|1996|p=246}} He decided he needed a name for the newly discovered nuclear process. He spoke to William A. Arnold, an American biologist working with [[George de Hevesy]], and asked him what biologists called the process by which living cells divided into two. Arnold told him that biologists called it [[fission (biology)|fission]]. Frisch then applied that name to the nuclear process in his paper.{{sfn|Rhodes|1986|p=263}} He mailed both papers to ''Nature'' on 16 January; the jointly-authored note appeared in print on 11 February and Frisch's paper on recoil on 18 February.<ref>{{cite journal |first1=L. |last1=Meitner |author-link1=Lise Meitner |first2=O. R. |last2=Frisch |s2cid=4113262 |author-link2=Otto Robert Frisch |doi=10.1038/143239a0 |title=Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction |year=1939 |page=239 |issue=3615 |volume=143 |journal=Nature |issn=0028-0836 |url=http://www.nature.com/physics/looking-back/meitner/index.html |bibcode=1939Natur.143..239M |access-date=11 March 2008 |archive-date=28 April 2019 |archive-url=https://web.archive.org/web/20190428141914/http://www.nature.com/physics/looking-back/meitner/index.html |url-status=live }}</ref><ref>{{cite journal |first=O. R. |last=Frisch |s2cid=4076376 |author-link=Otto Robert Frisch |doi=10.1038/143276a0 |title=Physical Evidence for the Division of Heavy Nuclei under Neutron Bombardment |year=1939 |page=276 |issue=3616 |volume=143 |journal=Nature |issn=0028-0836 |bibcode=1939Natur.143..276F |doi-access=free }}</ref>

These three reports, the first Hahn-Strassmann publications of 6 January and 10 February 1939, and the Frisch-Meitner publication of 11 February 1939, had electrifying effects on the scientific community.{{sfn|Stuewer|1985|pages=54–56}} In 1940 Frisch and [[Rudolf Peierls]] produced the [[Frisch–Peierls memorandum]], which established that an atomic explosion could be generated.{{sfn|Bernstein|2011|pp=441–446}}