Otto Hahn (1952) - Radiochemistry and the Fission of Uranium (German Presentation)

My most esteemed ladies and gentlemen, Firstly, I believe I may be permitted at this point to offer a word of cordial thanks on behalf of the other laureates for the very kind words of the Count, as well as the warm words of the Minister of Cultural Affairs and the gentleman from the Parliament of the Federal Republic of Germany. We scientists have been especially pleased, of course, by what we have heard today, and Count Bernadotte perhaps believed at the outset here that we feel morally obliged to come here because we have been invited. Ladies and gentlemen, we do not feel morally obliged, but instead were as excited as children when we received the invitation to this wonderful city. Excited to have a pretext to come to this wonderful city and, it is hoped, to be permitted to repeat this visit often. And to Minister Schwalber from Cultural Affairs - unfortunately, he had to depart - we would like to express our appreciation, in the name of German chemists, that the university chair held by Nobel laureates Adolf von Baeyer, Richard Willstätter, and Heinrich Wieland will now finally be established again in a manner that will permit the renown it achieved thanks to German chemistry in Munich to be equalled again in the future, that it will again be achieved. The last years have been very difficult. However, I do not wish to say much more at this point, but instead to express my heartfelt thanks, and to thank you for the invitation as well, that I myself have been permitted to come here to the group of laureates in chemistry. And as described in the introduction - as the main idea of this conference - besides exchanging thoughts and establishing new connections between domestic and international specialists, the opportunity to give a large auditorium of listeners the chance to hear about contemporary research results straight from the horse's mouth. Lectures are therefore meant to be given here from this admirable perspective. And so I must immediately disclose to you now, with a somewhat guilty conscience, that I am unable to report on new research results from the recent period of time, since I have no longer been able to work in a scientific capacity for about the last seven or eight years, but instead have been more of an organiser or, or an administrative professor. But perhaps I may be allowed to draw on something from the past today indeed for that very reason, and that it seems appropriate to discuss in somewhat greater detail the work which is the reason for my presence here today, namely, the splitting of uranium. But it would perhaps be quite instructive as well to roughly sketch out the path that Mr. Straßmann and I proceeded along after many wrong turns, and that finally forced us to claim to have split uranium in this reaction, which was so very peculiar then. I would therefore like to go back to some much older research work in our institute and of my own research that proved to be important later on in this new work on uranium. And we will see, at the same time, from this research how work that has been done without any intention of commercial success can indeed be of broad and considerable significance in the end. Therefore, I will begin somewhat historically and provide you with some details from earlier research which proved useful to us later during our subsequent work. This began in 1904 to 1905 when I went to England as a young organic chemist and had the opportunity to work for Sir William Ramsey for half a year at his institute in London. At that time, Sir William Ramsey gave me a quantity of radium, which he believed to be an impure mixture of radium and barium, with the task of producing radium from it using the fractionating procedure developed by Madame Curie, and then to determine the atomic weight. And strangely, I commenced this work and completely by accident discovered what is known as a radioactive element in it. It was determined later to be exact that this radium sample provided by Ramsey, an impure radium preparation he had obtained from a mineral, had originated not from a sample of a pure uranium mineral in which radium occurs, but rather from a thorium-laden uranium mineral. And thus I had found by chance - I could not have done otherwise - what was a nuclear transmutation product of thorium. And this accidental discovery occasioned my decision, at the suggestion of Sir William Ramsey, to leave organic chemistry and continue working with radium. And then I went to Montreal for advanced instruction, to Professor Rutherford, where I became acquainted with the new area in somewhat greater detail, and had the good fortune there to find two new radioelements, as they were referred to then, one being called radioactinum. But more important than that was my acquaintance then with the outstanding American radiochemist, Professor Boltwood, Bertram B. Boltwood. The two of us actually had had a disagreement during our correspondence. I maintained that the radiothorium I had found decayed or decreased its activity at some specific rate. Boltwood had thorium salts and did not detect the increase and said: "Hahn must not have been working properly", while I said: "Boltwood must have measured incorrectly." At Rutherford's suggestion, we had a meeting and discussed the matter, but both of us maintained that we were right. Then I came up with the hypothesis that "we could perhaps settle our row with one another, if I were to present you with a hypothesis that there is one more as-yet-unknown, long-lived intermediate substance between thorium and radiothorium, that was just not known about yet at this point." And Boltwood said to me: "Yes, if you were to come up with that, then that would be it." And then I went back to Berlin after a few months and had the good fortune to actually find this unknown intermediate substance and named it mesothorium. I am explaining all this because it has a relationship to the later work. Because the production and enrichment of mesothorium was carried out according to the usual methods and soon it became apparent that I could not separate this mesothorium from radium. I just couldn't do it. Despite exhaustive attempts. In the minerals from which it was produced, there was radium as well as ... there were uranium and thorium minerals, so there were nuclear transmutation products of the uranium as well as those of thorium in it, thus I found this mesothorium and could not separate it from radium. And as I had previously found radiothorium in my work for Ramsey, it followed then, it was explained by the thorium in reality not having been split off from thorium at all, but instead from its antecedent, mesothorium. I believe that Professor Soddy will go into greater detail about the history of his wonderful discovery of isotopy. Not only were we unsuccessful in separating mesothorium from radium, I was unsuccessful in separating thorium from radiothorium. I was unsuccessful in separating thorium from the ionium samples received in the meantime from Boltwood and from the ionium proven to be in our own thorium minerals as well. And it was most peculiar that we had all the various groups of elements without rightly knowing how they could be sorted out from one another. I personally always believed in small differences, similar to the differences between rare earths, which are so very difficult to separate, but did not have the courage to assert something different. And then Frederick Soddy took the decisive step with his assertion of the isotopy of the chemical elements. Whereby all the difficulties were remedied in a single stroke by classifying the many - nearly 40 at the time - naturally occurring radioactive nuclear transmutation products into the system of elements, into the periodic system. After all, we had acquired all kinds of experience at my institute with how to work with small samples of substances in the chemical procedures for separating all of these different radioactive substances; the purification of one or the other preparation had taught us how to avoid impurities, how to prevent undesirable adsorption by precipitates with high surface areas or how to produce them, and so forth. And we thereby had experience indeed with all of the radioactive substances then known, we discovered some of them ourselves, we had considerable experience with the enrichment, dilution, crystallisation, etc. of radioactive substances. We were able ... of false conclusions. And these were procedures that were references for us, that dated back as much as thirty years. And what we did with the uranium at that time, I do not need to mention, we also verified with thorium during that same month in 1939. Thorium was also split, those are the two elements that are still available even today for these processes. We, neither Mr. Straßmann nor I, had absolutely any suspicion then that splitting uranium and thorium would prove to have such fateful and momentous consequences. And we would neither have believed nor wished that these consequences had arisen as a result. Namely, that the exploitation of the binding energies of the atomic nuclei would produce such drastic proof of their feasibility, as demonstrated by the atomic bombs on Nagasaki and Hiroshima, in just a few years' time. And I believe that all of humanity and all peoples of good will maintain the hope that the beneficent effects of nuclear energy in a regulated and controlled machine may triumph over the horror of uncontrolled reactions that lead to destruction, to the bombs.

Otto Hahn (1952)

Radiochemistry and the Fission of Uranium (German Presentation)

Otto Hahn (1952)

Radiochemistry and the Fission of Uranium (German Presentation)

Comment

The recording presented here is a short and incomplete part of the first of five lectures given by Otto Hahn in Lindau. All five lectures have been printed in a special publication of the journal Naturwissenschaftliche Rundschau from 1981. The present recording is only 12 minutes and 5 seconds, but it still has two very interesting components. The first one appears in the introductory comments given by Hahn. They were not printed and were probably spontaneous. It seems that Count Lennart Bernadotte had expressed the view, that the invited Nobel Laureates were morally obliged to accepting the invitation. Hahn counters by expressing his and the other Laureates joy in being in Lindau, comparing it to the joy of school children on an outing. In fact, Otto Hahn became extremely fond of the Lindau Meetings and after having been invited for the first time in 1952, didn’t miss a single meeting until the year he died (in July, 1968). All in all, this amounts to 16 meetings in a row, a number that during the 20th Century was only topped by another German chemist, Ernst Otto Fischer, who participated in 27 meetings in a row! The second interesting component is that the letter of invitation apparently had asked the Nobel Laureates to speak about their most recent research. In 1952, like in 1951, the audience mainly consisted of mature practitioners with academic degrees and not, as today, of students and young researchers. Hahn is aware of the letter-of-invitation, but since he has not been given the possibility to perform any research recently, he has decided to tell an educative story from the past: How the development of radiochemistry led to the splitting of the uranium nucleus. Interestingly enough, in the audience is Frederick Soddy, Nobel Laureate in Chemistry 1921, who with his discovery of isotopes at once cleared up most of the problems that radiochemistry had encountered before 1920. After Hahn’s introductory comments, he starts to deliver the printed lecture, about one page of which can be heard here. The continuation of this printed version of the lecture takes up the research work done by Hahn together with the physicist Lise Meitner and later on also with the chemist Fritz Strassmann. It appears that Lise Meitner, though not a Nobel Laureate, was invited to Lindau at least once. On a beautiful colour photograph from the middle of the 1950’s, she and Hahn are discussing with physics Nobel Laureates Werner Heisenberg and his former mentor and co-quantum-mechanics inventor Max Born. A historic photograph published in the book Nobelpreisträger in Lindau from 1963!

Anders Bárány

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