THIRTEENTH ANNUAL MEETING 397

Honored by Seventh Annual Nuclear Pioneer Lecture, 13th Annual Meeting, Society of Nuclear Medicine

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By permissionof:The Max PlanckSociety,Cöttingen

PRoERssoH DR. DR.H.C.MULT. Orro HAHr@1

Göttingen Otto Hahn

A considerable majority of the pioneers, who were honored in this series of lectures before the Society of Nuclear Medicine, earned their merits in research work which originally did not have medicine as its goal. Hence the choice of Professor Otto Hahn as the hero of today's gathering fits well into this tradition. His great achievements are the discovery, mainly by chemical methods, of nu merous naturally occurring radioelements and of the revolutionary fission of the nucleus which subsequently led to important medical applications.

BIOGRAPHICAL DATA Otto Hahn was born in 1879, in Frankfurt, Main, where he attended high school. He studied first at the University of Marburg, then two semesters in Munich, and received his Dr. phil. at Marburg in 1901, on the basis of a thesis in organic chemistry performed under the direction of Professor Theodor Zincke. After one year of military service in Frankfurt and two years assistantship in Zincke's lectures, he planned to assume a position in the chemical industry. It happened that one of the neighboring firms was looking for a young organic chemist whom they wished to send abroad occasionally. Hahn appeared to be an appropriate candidate, especially as his parents were willing to finance his stay in England where he was supposed to improve his knowledge of the language. Sir William Ramsay agreed to let him work during this time at the University Col lege in London. When Hahn arrived there in 1904, Sir William asked him to separate radium from its mixture with barium in a preparation obtained from a mineral. Hahn found soon that this preparation produced not only radium emanation but also thorium emanation and that this was due to the presence of a new radioelement which he named radiothorium. The title of Hahn's autobiography, which ap peared in 1962, is “VomRadiothor zur Uranspaltung.― Ramsay was very impressed by this achievement and suggested to Emil Fischer, Director of the Chemical Laboratory at the University of Berlin, to let Hahn continue there his research work in radioactivity. Fischer approved this plan but Hahn thought that, before starting independent research work, it was advisable to learn more about the new field from its leader, Professor Ernest Rutherford at the McGill University in Montreal. Hahn spent the academic year 1905-6 in Canada and then the years 1906-12 in Fischer's institute. His activity during this time, as well as for many years later, was devoted nearly exclusively to research although he became Privatdocent at the University of Berlin in 1907, and received the title of Professor in 1910. In 1907, his investigations were joined by Dr. Lise Meitner who had just come from Vienna to extend her training as theoretical physicist by attending the lectures of Professor Max Planck. She already had done some experiments in radioactivity in Vienna. The Hahn-Meitner cooperation lasted more than thirty years and led to results of the greatest importance. 398 THIIITEENTH ANNUAL MEETING 399

At the end of 1912, the Research Institute for Chemistry of the new Kaiser Wilhelm Gesellschaft ( KWG ) was opened in Berlin-Dahlem and Hahn received there a Division of Radioactivity. Dr. Meitner was invited to work as a guest and in 1917 was put in charge of the Physical Section of this Division. In 1928, Hahn became Director of the whole Institute. In 1933, he was Baker Lecturer at Cornell University; his book “Applied Radiochemistry― contains his lectures delivered there and summarizes a part of his research work. The experimental cooperation with Dr. Meitner ended when she left Ger many in July 1938, to escape the consequences of Nazism. Hahn with several co workers continued the research in the KW Institute until it was bombed in March 1944. The usable apparatus were transferred to Wurttemberg and the re duced activity there came to a final end in April 1945, when Hahn and several of the leading German physicists were brought to England for security reasons. He returned to Germany at the beginning of 1946, and lives since that time in Got tingen. He became President of the KWG, the name of which was changed to Max Planck Gesellschaft. Since 1960, he is its Honorary President.

DISCOVERY OF NATURALLY OCCURRING RADIOELEMENTS At the beginning of his Faraday Lecture, delivered in England in 1956, Pro fessor Hahn modestly apologized for the subject of his research work being so “narrow. . . only radium and other radioactive substances.― Trying to summar ize here briefly the results of this work I also have to apologize: these results are so numerous and their consequences so important that one has to make a strict selection if one wishes to explain at least a small part of them. Hahn's first step in radioactivity, the finding of radiothorium, was followed by the discovery of a larger number of naturally occurring radioelements than any other investigator has to his credit. They are in chronological order of the discovery and with the presently used symbols : RdTh ( 1905), RdAc, ThC', MsTh1, MsTh2, AcC―, ThC―, RaG―, Pa (the last four with Meitner), and UZ (1921). The methods which led to the identification of these atomic species are anal ogous in some cases but differ considerably in others. Mesothorium was searched for, because the rate of growth of RdTh from thorium was found to be smaller than expected for a direct genealogical relation between them. The validity of the displacement laws, although not known at that time, helped to find the suspected intermediate substance. While RdTh and Th are chemically not separable from each other, the intermediate had to differ chemically from them both and it was not difficult to separate it. The discovery of protactinium was only partly analogous to that of meso thorium. The search for the mother substance of the relatively short-lived actin ium was already on for years. It followed from the displacement laws, as one of two possibilities, that it is an isotope of the element 91, of which only the short lived UX0 (Brevium) was known at the time. In fact, in 1918, Hahn and Meitner found this isotope after an appropriate chemical treatment of pinchblende. How 400 NUCLEARPIONEERLECFURE

ever, this was not as simple as it sounds now : in another laboratory, which had less experience and persistence, this isotope was not found and Soddy and Cran ston obtained it only in traces which did not allow a further investigation. Quite different was the method which led to the finding of ThC―and RaC―. In 1909, Hahn had shown that a puzzling behavior of the a-emitter RdAc was due to the transfer of its daughter substance AcX to a neighboring electrode: the formed atoms recoiled in a direction opposite to that of the s-particles. Hahn and Meitner applied this effect at once to other z-emitters, and thus introduced the simplest method of separating some pairs of radioelements. The proof of the existence of ThC' did not even require a separation from its mother substance ThC; it was sufficient to show that it emits the fastest a-par tides known among the naturally occurring radioelements. It was found later @ that this corresponds to a half-life of 3 x 10 sec, hence a separation was not possible. The versatility acquired in these and many other investigations, performed with his own hands, as well as an extreme self-criticism and precision, were the prerequisites for Hahn's greatest success : the recognition of the splitting of the uranium nucleus.

NUCLEAR FISSION

It is well known that it all started when Enrico Fermi et a!., bombarding uranium with neutrons, found in 1934 new radioelements for which only places beyond uranium seemed to be available in the periodic system. Hahn and Meit ner since 1935, together with F. Strassmann since 1937, and other investigators who repeated and extended these experiments agreed with Fermi's interpreta tion. Numerous transuranic radioelements seemed to have been produced until 1938, by bombarding uranium and thorium with neutrons. The chemical nature of some of these products was difficult to define and the clarification had not been reached until Hahn and Strassmann found among them species which fol lowed the reactions of barium and clearly showed the properties of an alkaline earth element. It appeared that they were isotopes of radium. The neutron source used in these experiments was rather weak and so was the activity of the barium preparations. In order to concentrate the alleged radium isotopes, the customary fractional crystallization was attempted but it failed completely: the activity re mained always with barium and not with added naturally occurring radium iso topes. Hence the radioactive alkaline earths produced by bombarding uranium with neutrons were isotopes not of radium but of barium which is far remote in the periodic system from the element of origin. This was contrary to all experi ences known earlier for natural or artificial nuclear changes. It is neither necessary nor possible even to indicate here the further growth of the magnificent building which has been erected on this corner stone. It ap pears, however, justifiable in honoring Otto Hahn to mention how he felt about the role of his main associate of many years, Lise Meitner. The paper which Hahn and Strassmann considered as the final proof of the formation of barium isotopes ends with the remark that the prompt identification of the numerous THIRTEENTH ANNUAL MEETING 401 radioelements they observed would not have been possible without the experi ence gained during their cooperation with Lise Meitner. It is a strange turn of fate that the communication in which Hahn and Strassmann discuss the ra dium isotopes for the first time was also the first which had been published with out Dr. Meitner after she had to give up the work in the laboratory. The spiritual contact remained, and when Hahn and Strassmann submitted to “DieNaturwiss enschaften― their finding that the alleged radium isotopes are barium isotopes, Hahn at once informed Dr. Meitner. She received the letter in a small resort place in Sweden at Christmas 1938 and together with her nephew 0. R. Frisch, who was visiting her, put forward considerations which Hahn designates in his autobiography as “thecorrect explanation.― Meitner and Frisch were also re sponsible for the term “nuclearfission―by adapting an originally biological term.

HONORS

It is customary to enumerate the honors conferred on a celebrity as expres sion of esteem for his person and accomplishments. Otto Hahn has half a dozen honorary doctorates, is an elected member or honorary member of numerous scientific societies, among them a dozen Academies of Sciences, and an honor ary citizen of Frankfurt, Main and Gottingen. He received three medals and has been decorated by three orders. He won the Nobel Prize in Chemistry in 1945, and a Hahn-Meitner Institute for Nuclear Research was created in Berlin in 1959. The preparation of this lecture has been an occasion for renewing my rela tions with our pioneer which started 55 years ago. I wish him warmly further years of good health and enjoyment of his great accomplishments.

KASIMIR FAJANS