Editing Otto Hahn (section)

===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}}