Editing Otto Hahn (section)

==Chemical Institute in Berlin==
===Discovery of mesothorium I===
[[File:Otto Hahn und Lise Meitner.jpg|thumb|left|upright|Hahn and Meitner, 1913, in the chemical laboratory of the [[Kaiser Wilhelm Institute for Chemistry]]. When a colleague she did not recognise said that they had met before, Meitner replied: "You probably mistake me for Professor Hahn."{{sfn|Hahn|1966|p=66}}]]
In 1906, Hahn returned to Germany, where Fischer placed at his disposal a former woodworking shop (''Holzwerkstatt'') in the basement of the Chemical Institute to use as a laboratory. Hahn equipped it with [[electroscope]]s to measure alpha and [[beta particle]]s and [[gamma rays]]. In Montreal these had been made from discarded coffee tins; Hahn made the ones in Berlin from brass, with aluminium strips insulated with amber. These were charged with hard rubber sticks that he rubbed against the sleeves of his suit.{{sfn|Hahn|1966|pp=37–38}} It was not possible to conduct research in the wood shop, but [[Alfred Stock]], the head of the inorganic chemistry department, let Hahn use a space in one of his two private laboratories.{{sfn|Hahn|1966|p=52}} Hahn purchased two milligrams of radium from [[Friedrich Oskar Giesel]], the discoverer of [[emanium]] (radon), for 100 marks a milligram ({{Inflation|DE|100|1906|fmt=eq|cursign=€|r=-2}}),{{sfn|Hahn|1966|pp=37–38}} and obtained thorium for free from Otto Knöfler, whose Berlin firm was a major producer of thorium products.{{sfn|Hahn|1966|pp=39–40}}

In the space of a few months Hahn discovered [[mesothorium]] I (radium-228), mesothorium II (actinium-228), and – independently from Boltwood – the mother substance of radium, ionium (later identified as [[Isotopes of thorium#Thorium-230|thorium-230]]). In subsequent years, mesothorium I  assumed great importance because, like radium-226 (discovered by [[Pierre Curie|Pierre]] and [[Marie Curie]]), it was ideally suited for use in medical radiation treatment, but cost only half as much to manufacture. Along the way, Hahn determined that just as he was unable to separate thorium from radiothorium, so he could not separate mesothorium I from radium.{{sfn|Hahn|1966|pp=40–50}}<ref>{{Cite journal |date=December 1945 |title=Nobel Prize for Chemistry for 1944: Prof. Otto Hahn |journal=Nature |volume=156 |issue=3970 |pages=657 |doi=10.1038/156657b0 |bibcode=1945Natur.156R.657. |issn=0028-0836 |doi-access=free}}</ref>

In Canada there had been no requirement to be circumspect when addressing the egalitarian New Zealander Rutherford, but many people in Germany found his manner off-putting, and characterised him as an "Anglicised Berliner".{{sfn|Hahn|1966|p=68}} Hahn completed his [[Habilitation#Germany|habilitation]] in early 1907, and became a ''Privatdozent''. A thesis was not required; the Chemical Institute accepted one of his publications on radioactivity instead.{{sfn|Stolz|1989|p=20}} Most of the organic chemists at the Chemical Institute did not regard Hahn's work as real chemistry.{{sfn|Hahn|1966|p=50}} Fischer objected to Hahn's contention in his habilitation [[seminar|colloquium]] that many radioactive substances existed in such tiny amounts that they could only be detected by their radioactivity, venturing that he had always been able to detect substances with his keen sense of smell, but soon gave in.{{sfn|Hahn|1966|p=52}} One department head remarked: "it is incredible what one gets to be a ''Privatdozent'' these days!"{{sfn|Hahn|1966|p=50}}

[[File:Berliner Physiker u Chemiker 1920.jpg|thumb|right|Physicists and chemists in Berlin in 1920. Front row, left to right: [[Hertha Sponer]], [[Albert Einstein]], Ingrid Franck, [[James Franck]], [[Lise Meitner]], [[Fritz Haber]], and Otto Hahn. Back row, left to right: [[Walter Grotrian]], [[Wilhelm Westphal]],
{{ill|Otto von Baeyer|de}}, {{ill|Peter Pringsheim|de}} and [[Gustav Hertz]] ]]
Physicists were more accepting of Hahn's work, and he began attending a colloquium at the Physics Institute conducted by [[Heinrich Rubens]]. It was at one of these colloquia where, on 28 September 1907, he made the acquaintance of the Austrian physicist [[Lise Meitner]]. Almost the same age as himself, she was only the second woman to receive a doctorate from the [[University of Vienna]], and had already published two papers on radioactivity. Rubens suggested her as a possible collaborator. So began the thirty-year collaboration and lifelong close friendship between the two scientists.{{sfn|Hahn|1966|p=50}}{{sfn|Hahn|1966|p=65}}

In Montreal, Hahn had worked with physicists including at least one woman, [[Harriet Brooks]], but it was difficult for Meitner at first. Women were not yet admitted to universities in [[Prussia]]. Meitner was allowed to work in the wood shop, which had its own external entrance, but could not enter the rest of the institute, including Hahn's laboratory space upstairs. If she wanted to go to the toilet, she had to use one at the restaurant down the street. The following year, women were admitted to universities, and Fischer lifted the restrictions and had women's toilets installed in the building.{{sfn|Sime|1996|pp=28–29}}

===Discovery of radioactive recoil===
Harriet Brooks observed a [[atomic recoil|radioactive recoil]] in 1904, but interpreted it wrongly. Hahn and Meitner succeeded in demonstrating the radioactive recoil incident to [[alpha particle]] emission and interpreted it correctly. Hahn pursued a report by [[Stefan Meyer (physicist)|Stefan Meyer]] and [[Egon Schweidler]] of a decay product of actinium with a half-life of about 11.8 days. Hahn determined that it was actinium X ([[radium-223]]). He also discovered that at the moment when a radioactinium (thorium-227) atom emits an alpha particle, it does so with great force, and the actinium X experiences a recoil. This is enough to free it from chemical bonds, and it has a positive charge, and can be collected at a negative electrode.{{sfn|Hahn|1966|pp=58–64}} 

Hahn was thinking only of actinium, but on reading his paper, Meitner told him that he had found a new way of detecting radioactive substances. They set up some tests, and soon found actinium C{{'}}{{'}} (thallium-207) and thorium C{{'}}{{'}} (thallium-208).{{sfn|Hahn|1966|pp=58–64}} The physicist [[Walther Gerlach]] described radioactive recoil as "a profoundly significant discovery in physics with far-reaching consequences".{{sfn|Gerlach|Hahn|1984|p=39}}