edited by Leonard W. Fine and Eric S. horkauer
Kurt Starke Phiiipps University D-3550 Marburg, Germany
The Detours Leading to the Discovery of Nuclear Fission
The discovery of nuclear fission has amost interesting, and for radiochemists even fascinating, history. There was no straight path to success. The process was not seriously predicted and, consequently, never looked for. On the contrary, the experiments leading to the fragmentation of uranium were aimed originally a t building-up a heavier transuranic element. With respect to fission, detours in research were made right from the beginning. The False Transuranic Elements Producing an isotope of element 93 became pwsible in 1934, when Fermi and his group discovered the absorption of thermalized neutrons (I).With most elements a beta-active product was formed which decayed to a n isotope of the next element in the periodic system. Uranium, with the highest atomic number then known, yielded artificial radionuclides which Fermi arranged in a decay series r 0r dI+93+94+95-
(1)
However, research did not proceed for much more than one year. The group had fallen apart, for external reasons, as Mrs. Fermi stated in her memoirs (2).Later, Fermi also admitted: "We did not know enough chemistry to separate the products of uranium disintegration from one another" (3).At any rate, the transuranic elements were open to investigation by other groups. Uranium and its natural decay products had been arranged in two series of alpha- and beta-emitters. Similarities to stable elements had helped to determine their position in the peritxlic system. T h e same atomic numher had heen assigned to r;idionuclides with r~rilc~icallv i d e n r i d rhrmical properties. the isotopes. Most of this woik had been finished-some time before 1934, but a t least two laboratories were still qualified to examine the radioactive and chemical properties of Fermi's artificial products. I t seemed to he a task no more difficult than with the natural series. In France, attempts to identify the transuranics were undertaken by Irene and Fr6d6ric Joliot-Curie, both physicists were experienced in radiochemistry, as shown by their discoverv of artificial radioactivitv. In Germanv. Otto Hahn was ,he lmdinr rudiochemist. ~ n , ~ 1 9 to 0 519116;he had idrntified n number 4,t radioactive iubstnnces in Hutherford's I a I ~ o r a t ~ , r ~ in Montreal. Since 1907 in Berlin, Hahn had found, besides other nuclides, the long-lived protactinium and the first nuclear isomer in cooperation with his colleague Lise Meitner who also had distinguished herself by work on alpha- and beta-decay and radioactive recoil. In 1934, Hahn and Meitner decided to study the supposed transuranic elements. They were soon joined by the chemist Fritz Strassmann. Up to 1938, it was mainly the German and French groups who reported falsely the isotopes of the elements following uranium (4). Hahn (5) as well as Meitner (6) gave accounts of this period of research. All claims concerning transuranics were incorrect for reasons which became obvious later when some radiochemical separations were repeated (7).
Nuclear Physics In the Thirties The errors in early transuranic research are better understood if one recalls the general views on nuclear physics and chemistry during these years. The binding energy per nucleon was known to increase all the way from uranium to medium weieht elements. However. since uranium decaved in nature only as far as stable lead, one did not expect the liberation of all the nuclear energy hound in it. At any rate, no mechanism was imaginable, though the liquid drop model was available as was the idea of the c o m ~ o u n dnucleus. De-excitation hv gamma-radiation was the most likely process to follow neutron ahsor~tionsince it had been observed with the other elements. At mist, nuclear physicists were willing to expect the emission of heavv. particles, thouah so far slow neutrons had knocked . thcie out onlv from light nuclr~.At any rate, an alpha particle u,us the heaviest m a s imaginable lc~avinrthe putenrial well of Gamow's theory. That the whole well couldfall apart was not foreseen. One could hardly anticipate that adding just one neutron might make all the difference. In all nuclear processes so far the atomic number had changed by not more than two units. The only scientist not taking this for granted was the chemist Ida Noddack, who had discovered rhenium, the element supposedly rescml~llngelement 93 She declared that she was not convmced ot the existmce or rransuranlc elements a s lone as the irradiation ~ r o d u c t sof uranium had not been shown-to be non-isotopic with all known elements (8)."One could imaaine" she added "that when heavv nuclei are hombarded with neutrons, these nuclei break apart into several large fragments, which are indeed isotopes of known elements, but not neighbors of the irradiated elements." Ida Noddack made no experiments to support her statement, while the incredulous nuclear physicists regarded it as useless to prove her wrong. As for the experimental situation, any process in uranium, normal or unexpected, was difficult to investigate a t that time. The neutron fluxes of the radium-herylliu~sourceswere so ~
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Typical equipmen! used in the dscovery of fission Photo courtesy of Deutsch M U ~ ~ U IMunich. TI.
Volume 56, Number 12, December 1979 / 771
Irene and Frederic Jaiiat-Curie in lheir laboratory in the 1930's. Courtesy of the French Embassy Press and information Division and the AIP Nieis Bahr Library.
low that thin targets would show few events. Therefore, as many neutrons as possible had to be taken advantage of by surrounding the source with uranium. A small maximum of artificial radionuclides was obtained, but again very few ~xirticles,elnitted or recoiling, would penetrate the target surface to he registered. Though the uranium was freed from it-. natural daughter products just hefure irradiation, decay cuntinued steadily, interfering increasingly with the measurement of the artificial products. As long as the short nuclear ahsorption process itself was hard tu observe only longer-lived rarlioactiw products cuuld provide a clue to it. Radiochemical mvthods of isdating, purifying, and identifving the few radi#,active;itmns had to be relied on before higher fluxes from neutron generators became available. Radiochemistry and the Periodic System in the Thirties Radiochemical procedures had been developed reasonably well during the past. They must not be equated with ordinary analytical separations of weighable quantities of stable elements. The traces of radioisotopes of different elements, often decaying rapidly, must be separated from each orher and precipitated with the help of a carrier element. Decay rate and the half-life tvuical of the unweiphahle nuclide can then be determined i i t h a counter. But it takes the experience of a radiochemist to distinguish between a precipitate on which the few radioactive ions are adsorbed on the surface unspecificallv and one in which they are part of the crystal lattice. Only in the last case does the carrier element provide certain information on the atomic number of the radionuclide. Elements that are neighbors in a horizontal period or a vertical group can frequently substitute for each other in crystals because of the equal charge of their ions and only a small difference in their ionic radii. This allows specific isolation of a radionuclide and a t least an estimate of its atomic number.. 0
I
I1
I11
IV
v
VI VII
. . . . . . . ,,.-.
,,Kr ,,Rb ,,Sr ,,Y ,,Zr ,,Nb , P o ,,Xe ,,Cs ,,La ,,Ce
VIII
,,Ru:,,Rh
,.Pd
-
to
,,Rn.,
,,Lu ,,Hf ,,Ta ,,W ,,Re ,,Os ,,Ir ,,U v ,,
,,Ra.,,Ac,,Th,,,Pa
d t ' 9&
'
Figure 1. Section ot periodic system as accepted during the midies. Elements in vertical groups have similar properties, tor instance the alkaline eanhs (ii) and the rare earths (ill). Elements 9 0 to 103 would now be arranged vertically below ssAc in group ill.
772 1 Jouml of Chemical Education
Enrica Fermi (1901-54). extreme right, with his group. Rome 1934. Photo courtesy of the American institute of Physics.
Otto Hahn in 1940 after his discovery of fission Photo courtesy of Hans Gotte.
But since the radii differ somewhat, recrystallization may change the elemental ratio, that is, radioactive crystals will not retain their radioactivitv "Der . unit of weieht. - . the soecific activity. A classic example is Marie Curie's isolation of radium from barium as carrier.. a Drocedure with which Otto Hahn had . hecome quire familiar during his career. In the cnse where n nudide is a radioisutooe ot'the carrier. the ionic radii are so much alike that the specific activity wili not change when fractional crystallization is attempted. The atomic number of the radionuclide is that of the carrier element. Choosing the right carrier for an unknown transuranic element was decisive. Predicting it depended on the periodic system as it was accepted in the thirties (see Fig. 1).Thorium, protactinium, and uranium were arranged below hafnium, tantalum and wolfram, and, consequently, the transuranic, elements below the transition elements rhenium to platinum, which therefore were regarded as suitable carriers. The similarity of tetravalent uranium to thorium had struck a few chemists (9,10) and a geologist (11). But this was scant evidence for the beginning of a long series analogous to the rare earth elements. Ienorine this faint oossibilitv cost the erouos .. . looking fm transuranirs several years fruitless research. The sulfides of rhenium tu i~larinumdid indeed carrv radionuclides, many by adsorption, but left others in solution that would have been isolated earlier. for instance bv usine an alkaline or rare earth.
Ernest Rutherlard 11871-1937).first row. extreme r ~ g h t .w t h his group. Ono Hahn, second row in the middie. Montreal 1905. Photo Courtesy of MacDonald Physics Laboratory, McGill University.
Ida Noddack (1896-1979).From a paper by (1971). courtesy of Professor Hans Meier.
Habashi. Chemistry. 44, [PI. 14
Growing Doubts about the Transuranic Elements
For some time, there had been reasons to become uneasy nhout the transuranics. So manv had been claimed that they became difficult to arrange. ~a;allelseries of beta-decaying isomers had to he set un in order to avoid unreasonably. high . atomic numbers in one series. The gradually embarrassing increase would have been reversed by. alpha-decay, which, . however, had been searrhvd ftn in vain i n Liw Meitncr'; nuclear . nhvsicj cnn1r1(12j. Inability todemmstrate delayed or . . prompt alpha-emission had not discouraged assigning atomic numbers lower than that of the target element. Meitner, Strassmann, and Hahn (13) had thought that they had produced radium by knocking out alpha-particles from thorium $zTh(n,a)'8'88Ra
(2)
Naturally, barium had served as carrier. Wrong as the assumption of a (n,a)-reaction was, the readiness to consider it with uranium led research into a new direction. A decisive stimulus to choose rare and alkaline earth carriers again came from the Paris group. Curie and Savitch (14) reported the striking activity of a new nuclide. Surprisingly, it was precipitated with lanthanum, that is, it behaved like a rare earth and definitely not like a transition element. Fractional
crystallization of lanthanum oxalate containing the artificial nuclide and actinium as tracer was carried out. Since both showed different reactions the atomic number of actinium was ruled out. The artificial product could be separated from thorium and also, so it seemed, from lanthanum. Somewhat reluctantly, the authors took it for a transuranium element, a hypothesis which they found difficult to interpret in view of its rare earth properties. At this time, the German scientists were shocked by the emigration of Lise Meitner, forced upon her by the loss of her citizenshin after the occunation of her native Austria. The group was therefore reduced to the two chemists. Though sceptical, Hahn and Strassmann hastened to cheek the results obtained in Paris. Though still maintaining the integrity of their transuranics Hahn and Strassmaun 115) confirmed the French group's nuclide with rare earth properties; hut did not regard it as a transuranic element. They claimed instead the existence of radium isotopes decaying into those of the rare earth actinium, for instance
Since Hahn and Strassmann directed their attention mainly to the chemical identification of radium, research was, fortunately, not held hack by futile attempts made to detect alpha-particles. Discovery of Barium and other Light Fragments
Oddly enough, the handicap of low activity of the samples was to bring about the unexpected and most exciting discovery after so many years of vain search for transuranic elements. Samples of harium hromide and chromate were to he enriched in radium by fractional crystallization. However, no change in snecific activitv in the fractious was observed. Therefore. natiral radium was added deliberately to check the procedure: When the tracer behaved as expected. the artificial irradiation prducr could no longer be regarded as an isurope ot'radi~~m. I n foll
