Glenn 1. Seaborg US. Atomic Energy Commission Washington, D.C.20545
Some Recollections of Early Nuclear Age Chemistry
Since we can think of the nuclear age as relating to the nuclear fission of uranium, my admittedly incomplete recollections will be concerned with the work on the nuclear fission of uranium and some of the work leading to this, as well as some of the work following the discovery of the nuclear fission reaction. This was all related to the periodic table and attempts to extend the Periodic Table beginning before World War 11. The work can be considered to begin with the search for the transuranium elements, that is, efforts to extend the Periodic Table beyond uranium. The Periodic Table before World War I1 placed the undiscovered transuranium elements in it in the manner shown in Figure 1. Thus, it can be seen that the first
Figure 1. Periodic table prior to World W a r 11, showing the tmnsuronium elemenh 09 homologr of the d shell tronrition series,
transuranium element, with the atomic number 93, was predicted to be a homolog of rhenium; element 94 was predicted to be a homolog of osmium; element 95 was predicted to be a homolog of iridium; 96, a homolog of platinum, and so forth. This is the way the Periodic Table was uniformly published before World War 11, and you will note that here thorium and protactinium and uranium appeared as homologs of (and hence under) hafnium and tantalum and tungsten. Inherent in such a periodic table is the assumption that the electrons for elements beyond uranium, that is, EDITOR'S NOTE:This paper by Dr. Seaborg and those immediately following by Drs. Allen, Hamilton, and Swartout are portions of the Symposium on Chemistry in the Nuclear Age sponsored jointly by the Division of Nuclear Chemistry and Technology and the Division of the History of Chemistry at the 154th Meeting of the American Chemical Society. Beeause of its considerable historical value to chemistry, Seaborg's paper has encouraged us to depart somewhat from the traditional format of THIS JOURNAL
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Figure 2. Periodic toble b y 1. L Quill showing the dements port 98 0s homdogr of the lanthonide series.
in the transuranium elements, are entering the 6d shell. There were, however, a number of suggestions that somewhere in the region of uranium, there should be another inner transition series like the rare earth e l e ments. I n the 14 rare earth elements the electrons successively enter the inner 4f electron shell. These considerations, therefore, suggested that there should be a second rare earth series in the region of uranium in which the 14 places in the 5f electron shell would be filled. One such Periodic Table was published by L. L. Quill in 1938 (Fig. 2). He felt that the electrons for elements beyond uranium would continue to be added to the 6d shell through element number 98. Then beginning with element 99, the electrons would be added to the 5f shell for the 14 following elements. Thus, there would be an element-by-element analogy between the two rare earth series so that element 99 would be a homolog of cerium, element 100 a homolog of praseodymium, and so on across to element 112,a homolog of lutecium. An earlier periodic table was published by Niels Bohr
Figure 3. Bohr's periodic toble showing the transuranium elemenh part 95 os forming o new series homologour to the hnthonide series.
in 1923 (Fig. 3). I n this, he suggested that the 14 member 5f electron shell would begin to be filled a t element 95 and would be completed a t element 108, as shown in Figure 3. Element 95 would thus be a homolog of cerium, 96 of praseodymium, and so across to element 108, a homolog of lutecium. These periodic classifications permitted the preparation of summaries that p r e dicted the physicaland chemicalproperties of the transuranium elements. These, of course, were all very speculative. The first serious attempt to actually produce elements with atomic numbers higher than uranium, atomic number 92, were those by Fermi, Amaldi, DlAgostino, Rasetti, and Segr6, working in Rome in 1934, as part of a broad program of irradiating elements throughout the Periodic Table with neutrons. The Fermi group irradiated uranium and obtained some very interesting results (Fig. 4). By using tracer techniques, they were able t o separate radioactive isotopes which appeared to be chemically similar to rhenium and manganese, as would be expected for the transuranium element with the atomic number 93, as you can see in Figure 1, representing the periodic table of that time. For example, when manganese dioxide was precipitated from the solution it carried a good fraction of these radioactivities, indicating that they might actually be due t o an isotope of element 93 that was chemically similar to manganese or rhenium. Similarly, chemical fractions which might be expected to contain uranium or protactinium or thorium did not contain these radioactivities, which were ascribed to the transuranium elements. Therefore, the conclusion was drawn that some of the radioactivities, notably two with half-lives of about 13 and 90 min were due to isotopes with atomic numbers larger than that of uranium, number 92. Hahn, Meitner, and Strassmann took up this work and werevery active in trying to establish by the tracer technique the chemical identity of these radioactivities. They irradiated uranium with slow neutrons and produced a number of radioactivities which they thought decayed as shown in Figures 5 and 6. In addition t o radioactivities which they attributed to ekarhenium, ekaosmium, eka-iridium, and eka-platinum, they observed an activity that decayed with a half-life of 23 rnin which they established as an isotope of uranium. For example, it could be carried in tracer amounts by the precipitate sodium uranylacetate and it appeared to be produced by slow neutrons only and with a cross section compatible with the
absorption of neutrons in uranium-238. We now know that this 23-min activity is due to the uranium isotope with the mass number 239. As a result of this work, Hahu, Meitner, and Strassmann published, in 1937, an extensive discussion of the chemical properties of the isotopes responsible for these radioactivities. This included a detailed comparison with the chemical properties of the supposedly homologous elements rhenium, osmium, iridium, and platinum. I recall that I read this paper at that time and considered myself already an expert on these "transuranium elements" and gave an hour-long talk a t the Chemistry Seminar at Berkeley, describing this work of Hahn, Meitner, and Strassmann. However, there were many questions that were difficult to answer a t that time. For example, it wasn't clear why there should be three isomers of uranium-239, nor was it clear how this isomery could be inheritable to give distinct isomeric chains of beta particle disintegrations. Curie and Savitch, working in Paris in 1937-38, found among the radioactivities that they investigated (formed as the result of the irradiation of uranium with neutrons) some beta particle radiation that decayed with a half-life of about 3.5 hr (Fig. 7). This activity was notable in that it precipitated with lanthanum as a carrier, suggesting to them that i t might be due to actinium. However, it was shown by a fractional precipitation of lanthanum oxalate to be more similar to lanthanum than to actinium. This put them very near to the discovery of fission; in fact, had they been able to arrive a t the definite conclusion that the 3.5 hr activity was due to lanthanum, they would have been the discoverers of nuclear fission. However, they were deterred by the fact that the 3.5-hr activity was somewhat confused with a similar activity due to an yttrium isotope and thus they weren't able to cleanly establish that this activity was due to lanthanum and hence was a fission product. Hahn and Strassmann, continuing the work in 1938, after Meitner had been forced to emigrate to Sweden, repeated and extended the Curie and Savitch experiments. They found among the products from the irradiation of uranium with neutrons some radioactivities that could be precipitated with barium carrier, as well as some that could be precipFigures 4-1 2. At lee are shown some of the historic reactions which led to the dixovery of fission. Tho fint reactions were carried out in on &tempt to produce tronrumnium elements. The subsequent erperimenh to determine the noture of the reaction prodush l o w e d thot some of them could only have been produced by fission of the uranium nucleus.
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Figure 13
Figure 14
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FISSION MASS DISTRIBUTION CURVE 1946 WITH 1940 POINTS ADDED AS X
The Discovery of Fission Otto Hahn, Lise Meitner, and F r i h Straumann did the preliminary work which led directly t o the discovery of fission by Hohn ond Strassmann in 1938. Lise Meitner, shown with Otto Hohn in their laboratory in the 1930's (Fig. 13, opposite poge, right), hod been compelled to leave Germany earlier in the year. Lord Rutherford, whose scattering experiments contributed to the early theories of the otom is shown (Fig. 14, opposite page, upper left) wearing H o h n i cuffs in his loboratory in Montreal in 1906. In 7966, Hohn, Strossmonn, and Meitner each wos presented the AEC's Enrico Fermi Aword. Miss Meitner received her award (Fig. 16, opposite page, below) from Dr. Glenn T. Seaborg, Chairman of the AEC, right) while her co-worker, O f t o Frisch looked on (left). Hahn (left) ond Strossmann (right) received their award from Dr. Seaborg in the Hofburg Palace (Fig. 15, below). At right (Fig. 17)is shown a mass distribution curve for all the fission products found by 1946.
1 110
140
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itated with lanthanum carrier material. They attributed the activities that were precipitated with barium carrier as due to isotopes of radium (Fig. 8). This would be the result of a rather peculiar n,2a reaction in order to produce radium from uranium. It certainly was difficult to explain how such a reaction could he induced by slow neutrons. Hahn and Strassmann therefore carried out elaborate tests to prove that the radioactivity was not due to isotopes of uranium or thorium or protactinium or actinium, or transuranium elements, and this work led them to the conclusion that they had produced four isotopes of radium and four concomitant daughter. isotopes of actinium, as shown in Figure 9. It was as a result of their attempt, in December 1938, to prove chemically that these radioactivities were indeed due to radium, and not due to barium, that fission was discovered (Fig. 10). They carried on experiments in order to separate barium and radium isotopes by fractional crystallization of such compounds as barium chloride precipitated from concentrated hydrochloric acid. As shown in Figure 11, they added as tracers for radium such isotopes as thorium X and mesothorium-1 and then attempted, by fractional crystallization, to show that the "radium IV" behaved just as the thorium X or as the mesothorium-1. They carried out fractional crystallization processes with such substances as barium chloride precipitated from concentrated HC1 and found to their immense surprise that although the barium chloride precipitate concentrated the radium isotopes, thorium X, and mesothorium-1, in the initial fraction, theL'radiumIV" (or300 hr activity) distributed uniformly between the barium in the barium chloride solution and in the precipitate. This forced them to the conclusion that barium had been produced as a result of the irradiation of uranium with neutrons, and this constituted the discovery of fission. Immediately following this, they found that the "actinium 11" of Figure 9 followed lanthanum and not actinium in a chemical separation process, thus in a sense confirming Curie and Savitch's work on their radioactivity of approximately 3.5 hr half-life, in which they had also had indications that this seemed to be due to lanthanum rather than actinium. The original work in which fission was thus discovered was published by Hahn and Strassmann in January 1939. In a second paper immediately following, Hahn and Strassmann showed beyond a doubt that "radium 111" as well as "radium IV" were actually due to isotopes of barium and they pointed out that some of these radioactivities which now, of course, must be identified as fission products, were due to previously known isotopes such as barium-139, which has a half-life of 86 min and would identify with their radium 111, and lanthanum-140, known to have a half-life of 3 1 4 6 hr and which would identify with their actinium IV (Fig. 12). Hahn and Strassmann also found a number of other radioactive fission products due to isotopes of such elements as strontium, yttrium, krypton, and xenon, and so forth, in the months following their discovery experiments. Figure 13shows a photograph of Hahn and Meitner in their laboratory sometime in the 1930's. Professor Hahn described his work on the discovery of fission, on the occasion of his receipt, with Strassmann, of the 282
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Atomic Energy Commission's Enrico Award for 1966 in Vienna: The Award is named after Enrico Fermi who was the first to use neutrons for producing artificial radioactivity in a great variety of chemical elements. I n ~articrtlar.Fermi concluded that by irradiating uranium with neutrons he had formed transuranicelements, that is, elements of higher atomic number than uranium. Miss Lise Meitner, Fritz Strassmann, and I decided to repeat and extend these very interesting experiments. We felt well qualified to do so. The physicist Lise Meitner and I had worked together on problems of radioactivity for over 30 years. Fritz S t r n r ~ ~ ~niy ~ ~frwun~d , p
