Politics, Chemistry, and the Discovery of Nuclear Fission - Journal of

Chemistry for Everyone

Politics, Chemistry, and the Discovery of Nuclear Fission† Emilie Wiesner and Frank Settle Jr.* Department of Chemistry, Washington and Lee University, Lexington, VA 24450; *[email protected]

Nineteen thirty-eight is an important year in the history of science. It marks the discovery of nuclear fission, the culmination of a four-year collaborative effort among the scientists Lise Meitner, Otto Hahn, and Fritz Strassmann that changed the world. In addition to revolutionizing the concept of the nucleus and opening new areas of research, it also had a dramatic impact on society, unleashing a new and powerful energy source that would be used for both destructive and progressive ends. While fission is a testament to the power of scientific investigation, the portrayal of its discovery and the subsequent recognition for that discovery provide examples of human fallibility. The political environment in Nazi Germany that surrounded the discovery, personal prejudices among scientists, and the power of the discovery itself brought fame to Otto Hahn while relegating his collaborators to relative obscurity. The neglect of Lise Meitner, in particular, reveals shortcomings of judgment in the scientific community. Although she was a driving force in nuclear physics and intimately involved in the discovery of fission, her contributions were almost lost in the aftermath of World War II when the Nobel Prize in Chemistry was awarded in 1945 to Hahn (1) for the year 1944. Hahn and Meitner’s Early Days in Berlin Otto Hahn and Lise Meitner (Fig. 1) first met in 1907 when Meitner, having recently completed her Ph.D. in physics at the University of Vienna, came to Berlin to attend lectures by the famous physicist Max Planck. Meitner found she had time to pursue experimental work and sought a collaborator (2, Chapter 2). She found one in Hahn, who had received his Ph.D. in organic chemistry several years earlier from the University of Marburg. Before obtaining his current position at the Chemical Institute of the University of Berlin, he had worked in radiochemistry with such notables as William Ramsay in London and Ernest Rutherford in Montreal (3, Chapter 2). Thus Hahn was a natural choice, given that Meitner’s most recent work had also been in radioactivity, a subject that integrated physics with chemistry. Hahn agreed to a joint scientific effort, and their collaboration, as well as a lifelong friendship, began. Their initial investigations of β emissions led to the theory of radioactive recoil,1 an important concept in nuclear physics. However, their greatest achievement in this early period was the discovery of a new element, protactinium, in 1918 (3, p 91). These and other scientific successes brought Meitner and Hahn recognition and advancement. †

This publication is based on a paper written by Emilie Wiesner for an honor’s seminar on the atomic bomb at Washington and Lee University, winter 2000. Her current address is Department of Mathematics, University of Wisconsin, Madison, WI 53706. Those wishing further information on the issues presented in this paper should consult annotated references in the Alsos Digital Library for Nuclear Issues at http://alsos.wlu.edu.

Figure 1. Lise Meitner and Otto Hahn. (Otto Hahn, A Scientific Autobiography; Charles Scribner’s Sons: New York, 1966; courtesy AIP Emilio Segrè Visual Archives.)

In 1912, both scientists moved to the newly opened Kaiser Wilhelm Institute (KWI) for Chemistry, just outside Berlin, and by 1919 both had become professors (a prestigious title in German academe) with solid reputations. In 1917, Meitner was asked to form a separate physics section while Hahn headed a small, independent department for the study of radioactivity. They now had the excellent experimental resources of the KWI at their disposal. Although their joint efforts had been fruitful, both Hahn and Meitner were ready to pursue independent work. Although their initial collaboration was productive and satisfying for both Meitner and Hahn, it foreshadowed the injustice Meitner would later experience. Although she had earned her Ph.D. before coming to the University of Berlin, the doors remained closed to her because she was female.2 At the University her talents went largely unrecognized, and her initial position at the KWI came through the special efforts of friends.3 Furthermore, Meitner did not receive full credit for her scientific contributions. Hahn was listed as the senior author in all papers they published on protactinium even though Meitner did the majority of the work during his absence in World War I.4 Later, Hahn was honored for the discovery of this new element while Meitner’s contribution was largely ignored. Meitner herself felt that scientists of the day naturally put her in Hahn’s shadow (2, Chapter 3). Divergent Paths In the 1920s, that shadow began to recede as Hahn and Meitner’s interests diverged. Meitner’s work included the spectra of β emissions, an explanation of the “Auger effect”, and the radiationless transitions between nuclei and electrons, demonstrating that orbital electrons are capable of penetrating the nucleus. Between 1920 and 1933, Meitner published 69 scientific articles either alone or with coauthors (4, pp 381– 390). Her circle of scientific colleagues was impressive, with

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particularly strong links to Niels Bohr. Meitner received (with Hahn) nominations for the Nobel Prize in Chemistry in 1924, 1925, 1930, 1933, and 1934. During this time, she also received other honors, including an award from the American Association to Aid Women in Science, the Leibniz Prize silver medal from the Berlin Academy of Science, and the Lieben Prize from the Vienna Academy of Science (4, pp 362–365). Meanwhile, Hahn had his own successes. In 1924 he became director of the KWI for Chemistry and focused much of his scientific effort on industrial applications of radiochemistry. Hahn also continued some of the work begun with Meitner, identifying the radioactivities of various heavy-element isotopes and discovering nuclear isomerism,5 a concept that would play an important role in the discovery of fission. The Discovery of the Neutron and Consequent Research In 1934, the research efforts of both Meitner and Hahn underwent striking shifts as a result of a series of revolutionary advances in nuclear physics (5, p 147). These developments began in 1932 with the discovery of the neutron by James Chadwick. Early in 1934, the Joliot-Curies, working in Paris, reported another significant advance when they produced the first artificially radioactive substance, phosphorus-30, by bombarding aluminum with α particles (6, pp 200–202). In Rome, Enrico Fermi immediately extended the work of Chadwick and the Joliot-Curies. Realizing that the neutron could easily penetrate the positively charged nucleus,6 in the winter of 1934 he undertook a systematic neutron bombardment of all the known elements starting with hydrogen (6, pp 209–213). Fermi’s experiments produced new substances that captivated the scientific community. The results from neutron bombardment of the heaviest element, uranium, created the greatest interest. Irradiated uranium produced β emissions analogous to those obtained from bombardment of its heavy element neighbors6 and led to the conclusion that a new element heavier than uranium had been produced (7 ). The Rome group supported their claim by verifying that the radioactivities of the products were distinct from those they observed as a result of neutron bombardment of the elements close to uranium (lead through protactinium). Hahn and Meitner Unite with Strassmann Meitner, having followed Fermi’s progress through the periodic table, was intrigued by the uranium results (2, pp 163– 164). Realizing the importance of chemical analysis in these experiments, she saw that further investigation into this area was not work for a physicist alone.7 Therefore she approached Hahn about renewing their collaboration. Although Hahn was reluctant at first, an article by a former student proposing that Fermi’s “new” element was in fact protactinium piqued his interest. By October 1934, Hahn and Meitner had begun to irradiate uranium with neutrons. Fritz Strassmann, an analytical chemist, joined them at the KWI in early 1935 to provide the skills that complemented Hahn’s radiochemistry and Meitner’s physics (5, p 137). The team was now complete. Hahn, Meitner, and Strassmann would spend the next four years trying to untangle the behavior of the uranium nucleus. The groups in Rome, Paris (the Joliot-Curies), and Berlin found that neutron bombardment of uranium produced a 890

complex mixture of radioactive substances and corresponding half-lives (8). They all set about cataloging these half-lives and trying to explain their existence. The group in Berlin believed they had discovered a system of three decay sequences (as shown below), resulting from a triple isomerism of uranium-239 (9). Note that each sequence begins with neutron capture by a uranium-238 nucleus to form uranium-239. It is also important to note that these isomers only reveal themselves in successive generations as daughters of the parent nuclide. 1)

92U

+n

92(U

+ n)

β 10 sek.

β

Eka·Re 93

β

2)

3)

92U

+n

92U

+n

92(U

+ n)

92(U

+ n)

β 40 sek.

β 23 Min.

β

Eka·Re 93

16 Min.

96

β

Eka·Os

95

Eka·Au?

2,5 Std.

94

Eka·Ir

59 Min.

94

Eka·Pt

66 Std.

β

Eka·Os

2,2 Min.

β 5,7 Std.

97

Eka·Ir? 95

Eka·Re? 93

This explanation was far from sound. Its most troubling problem, the proposed triple isomerism, contradicted the current theory explaining this phenomenon, which allowed for only two isomers (10, pp 99–101). The inheritance of isomer characteristics also contradicted the thought of the day. Despite these problems, the Berlin team stood behind their results. Moreover, the physics community at large accepted this explanation for the behavior of irradiated uranium.8 Other possible explanations, namely α decay, had been discounted. Their precise measurements confirming the last decay process appeared to confirm the production of element atomic #97, EkaAu (10, p 101).9 Furthermore, the experiments of the Berlin group had been reproduced in several other laboratories.10 More importantly, though, scientists simply could not conceive of an alternative explanation for the behavior of uranium when bombarded by neutrons. Two established hypotheses prevented progress and delayed the discovery of fission. One was the current model of the nucleus as a collection of particles bound together in a potential-energy well. An incoming neutron might provide the necessary energy for lighter α and β particles to escape the nucleus. However, this energy is not large enough to allow particles heavier than an α to escape. Thus scientists ignored the possibility of a nuclear disintegration, in both their theories and their interpretation of experimental results.11 In 1934, the German chemist Ida Noddack (11) suggested that neutron bombardment of uranium might cause it to split into several large fragments, which would be isotopes of known elements but would not be neighbors of uranium (12). However, physicists rejected such a possibility without much consideration and Noddack did not follow up with definitive experiments. The second costly assumption made by scientists fell within the realm of chemistry. The last row of the 1934 periodic chart was incomplete (Fig. 2). Scientists believed that any new elements would complete this row, which at the time ended with uranium. Thus it was assumed that element 93 (known briefly as eka-rhenium) would behave in a way chemically similar to rhenium, the element directly above the 93 position. The existence of the rare earth elements (atomic numbers 58–71) at the bottom of the periodic chart suggested

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Figure 2. The periodic chart of 1934 (10, p 101). Originally found in Lise Meitner, Scientia, International Review of Scientific Synthesis (Via Guastalla 9, Milano, Italy), Annus XXXII, 1938, p 13.

an alternative placement of uranium and its neighboring elements that was not considered at the time. As time would reveal, thorium, protactinium, uranium, and the transuranic elements would form a second rare earth series, found on modern periodic charts. However, in this instance, scientists observed not what was actually there but what they thought was there. Because fission produces many different lighter elements, scientists did find radioactive materials with chemical properties similar to rhenium and neighboring elements in the sixth period. The radioactivities of these substances were then shown to be distinct from those of the elements immediately preceding uranium (atomic numbers 82–91). This left only the space beyond uranium as a possibility for the “new” elements. This search for the “new” transuranic elements also caused the Berlin team and others to ignore many of the other fission products, because they resided in the seemingly unimportant waste from the chemical separations. Meitner’s Departure from Berlin While Hahn, Meitner, and Strassmann pursued the erroneous characterization of uranium isomers German politics took a turn for the worse (4, pp 364–369). By 1933 Hitler had completed his takeover of the German government. One of the many “reforms” enacted that year was the law for the Reestablishment of the Professional Civil Service Act. This law removed Jews from any government-related jobs and resulted in the decimation of German academe. It also struck close to

home in the KWI, where Lise Meitner, who was one-quarter Jewish, became subject to this new policy. As an Austrian citizen she received a temporary respite. However, in March 1938, the Anschluss (Germany’s Annexation of Austria) made Meitner a citizen of the German Reich and many of her colleagues feared for her safety. In July 1938, with the help of friends, Meitner escaped from Germany to Stockholm, Sweden, where she was given a position in Manne Siegbahn’s Physics Institute. Hahn’s ambiguous role in Meitner’s escape reflects his later attitudes toward her role in the discovery of fission. After the Anschluss, Hahn became unnerved by Meitner’s presence at the KWI and its possible implications for the Institute. He discussed the situation with one of the KWI’s sponsors, thereby bringing Meitner’s situation to the attention of the Ministry of Education. However, on a personal level, Hahn provided Meitner with a great deal of support. When Meitner left Germany quickly with little preparation, Hahn took care of her belongings and other personal affairs (2, pp 214–215). Thus while Hahn cared for Meitner, he lacked the integrity to take any risks on her behalf. The Discovery of Fission Meitner’s departure coincided with dramatic developments in the study of irradiated uranium. In France in 1937, Irene Curie and her collaborator Pavel Savitch had taken up uranium again, with startling results. Instead of precipitating out the


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supposed transuranic elements, as the Berlin team had done, the French group decided to study the irradiated mass as a whole. Using this new approach, they found a new 3.5-h halflife apparently associated with an isotope of thorium. Thorium has atomic number 90 (two less than uranium’s 92), implying that irradiated uranium undergoes an α decay, a reaction that the Berlin group had ruled out. When the scientists in Berlin challenged these findings, Curie reported even more astonishing results; the new 3.5-h half-life was chemically similar not to thorium, but to lanthanum, atomic number 57, an “impossible” distance from uranium! Curie could offer no theoretical explanation but stood behind her experimental results (13). The results from the French group caused Hahn and Strassmann, now working without Meitner, to reexamine their work. They found several previously undetected radioactive nuclides whose existence required an explanation even stranger than their theory of triple isomerism in uranium. One of the nuclides appeared to be an isotope of radium (atomic number 88), which they thought was produced when an irradiated uranium nucleus emitted two α particles (14 ). This isotope of radium, like uranium-239, was characterized by triple isomerism, an explanation that again violated existing nuclear theory. Meanwhile, Meitner maintained contact with Berlin through a steady correspondence with Hahn and, although she was not physically participating in the experimental work at the KWI, her role in the team’s operations had not changed much. Strassmann later reflected on Meitner’s involvement in the last months before the discovery of fission (2, p 241): What difference did it make that Lise Meitner did not directly participate in the “discovery”?? … she was bound to us intellectually from Sweden [through] correspondence Hahn-Meitner. … [She] was the intellectual leader of our team.

Werner Heisenberg, a preeminent German physicist, would later comment on Meitner’s contribution (5, p 139): She not only asked “What but also why”. She wanted to understand…, she wanted to trace the laws of nature that were at work in that new field. Consequently her strength lay in the asking of questions and in the interpretation of experiment. We may suppose that also in their later joint work Lise Meitner exercised a strong influence on the asking of questions and that Hahn mainly felt responsible for the thoroughness and accuracy of the experiments.

Meitner’s probing nature is evident throughout the letters that traveled between Stockholm and Berlin. Reading about Hahn and Strassmann’s “radium” isomers, she realized that a neutron did not bring enough energy into the nucleus to produce this mode of radioactive decay and instructed her colleagues to take another look. Meitner, now supported by Niels Bohr, urged Hahn to test for radium again at a meeting the three had in Copenhagen early in November 1938 (2, pp 224–228). Early in December 1938, Hahn and Strassmann did indeed take another look at their “radium”. Their results seemed to verify the products observed by the Joliot-Curies. Presumably, isotopes of radium produced by the initial neutron bombardment of uranium subsequently underwent β decay to actinium and thorium.

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Figure 3. Hahn and Strassmann. Chemical discovery of fission, Berlin, December 1938.

To be absolutely certain, Hahn and Strassmann decided to identify the “radium” isotopes by chemical means. The chemical separation is shown in Figure 3 with the suspected radioisotope of radium shown as ‘Ra’. Since radium and barium are group IIA elements with similar chemical properties, barium was added as a carrier to facilitate the chemical isolation of the small amounts of the suspected radium. The final step in the isolation of radium was a fractional crystallization to separate the barium carrier from the minute amount of radioactive radium. Hahn and Strassmann, both experienced radiochemists, were unable to separate the radioactivity of the ‘Ra’ from the barium fractions and thus confirmed that one of the products of neutron bombardment of uranium was indeed “distance” barium, not neighboring radium. Could Ida Noddack’s hypothesis be correct? Had the uranium atoms split into fragments of approximately equal mass? Hahn and Strassmann repeated the experiment numerous times but were never able to isolate the radioactive “radium” from barium. They reported their results as follows (15): As chemists, we must actually say the new particles do not behave like radium, but in fact, like barium; as nuclear physicists, we cannot make this conclusion, which is in conflict with all experience in nuclear physics.

Hahn the chemist was reluctant to go against the ideas of respected nuclear physicists, despite clear chemical evidence for barium. Unable to explain the presence of barium, Hahn again turned to Meitner. In a letter he asks, “So please think about whether there is any possibility—perhaps a barium isotope with much higher atomic weight than 137?” (2, p 234).



Meitner and Frisch Explain Fission While Hahn and Strassmann prepared their new finding for publication, Meitner thought about the presence of barium. Within a short time she had found an explanation: fission. Meitner learned of Hahn and Strassmann’s latest experiments verifying the barium just before Christmas 1938 while she was on a holiday in Sweden with her nephew Otto R. Frisch, also a physicist. As she discussed her work with him, Meitner suddenly had a new vision of the uranium nucleus, based on Bohr’s liquid-drop model.12 Under certain circumstances, the uranium nucleus, when struck by a neutron, would begin to wobble and elongate like a water droplet. This distortion could lower the strong nuclear force that held the nucleus together, allowing the repulsive electrical force to dominate and blow the nucleus apart. Applying this model, Frisch and Meitner calculated the energy13 that would be released in such a process, based on the difference between the mass of the uranium nucleus and the total masses of expected fission products such as barium and krypton (16 ). Shortly after, Frisch (17 ) and others (18) conducted follow-up experiments to obtain the recoil energies of the fission products and thus experimentally verified their theory of fission. The discovery of fission was soon revealed to the world, in papers published separately by Hahn and Strassmann (15) and Meitner and Frisch (16 ) in early 1939. Niels Bohr learned of the discovery from Frisch and immediately embraced it. Bohr then personally carried this exciting news to the physics community in the United States, where it ultimately contributed to the development of the atomic bomb in 1945. The use of these bombs by the United States against Japan brought notoriety to nuclear physics. As a principal contributor to the discovery of fission and a victim of the Nazi regime, Lise Meitner became the perfect celebrity. Some members of the press portrayed her as “the fleeing Jewess, the woman scientist who had snatched the secret of the bomb from Hitler and delivered it to the Allies” (2, p 315). A more believable account of the discovery of fission appeared in Life magazine (19): The German group was puzzled by the appearance of barium, an element that is much lighter than uranium. One of them, Dr. Lise Meitner, was exiled from Germany because of her Jewish ancestry. In Copenhagen, working with Dr. O. R. Frisch, she came to the conclusion that the neutron was not producing a new element at all, but that it was splitting the uranium nucleus in two, producing barium and other elements.

Among the other inaccuracies here, there is no mention of contributions made by Hahn and Strassmann, the “Aryan” members of the Berlin team. Hahn Receives Nobel Prize for Fission While Meitner received attention from the press and credit for the discovery of fission, Otto Hahn found himself detained by British authorities just outside Cambridge, England, along with nine other German scientists involved in the German atomic energy project. Undoubtedly, news of Meitner’s portrayal in the American press left Hahn bemused (20, p 97).

The tables would soon be turned, however. In late 1945, Hahn learned that he had received the Nobel Prize in Chemistry for the year 1944 “for his discovery of the fission of heavy nuclei”. Neither Meitner, Strassmann, nor Frisch was included in the award. Thus, the Nobel Prize conferred the discovery of fission to Hahn alone. This turn of events was not a reflection of the actual details of the discovery but rather the result of prejudice and misunderstanding. In many respects, the odds were stacked against Meitner even before the Nobel Committee began its deliberations. While her contributions to the discovery of fission were real and substantial, Meitner’s absence from Berlin during the actual discovery presented a different picture. Meitner herself noted, “much as these results [about the fission of uranium] make me happy for Hahn … many people here [Berlin] must think I contributed absolutely nothing to it” (2, p 255). Owing to the Nazi domination of German science, Hahn and Strassmann could not admit to any collaborative work with an exiled Jewish scientist. Although Hahn included a reference to Meitner and Frisch’s interpretation of fission in his February 1939 article on the verification of barium and other radioactive products from the neutron irradiation of uranium, he worded it in such a way as to minimize its importance. With the passage of time, Hahn himself seemed to forget Meitner’s contribution. In a letter written to Meitner shortly after the discovery, he claims, “In all our work we absolutely never touched upon physics, instead we only did chemical separations over and over again. We know our limits and of course we also know that in this particular case it was useful to do only chemistry” (2, p 256). Hahn was able to convince himself of this statement because of both a sense of selfpreservation and a lack of understanding. Certainly Hahn felt the pressure of the Nazi establishment and had an ability to see things as he wanted them to be.14 Although he acknowledges the contribution of Meitner and Frisch in his autobiography (3, p 158), there are also indications that he simply did not understand Meitner’s later theoretical contributions and therefore felt she had contributed little to the discovery (2, p 263). Although the Nazi threat disappeared with the end of the war, many problems remained for Hahn and German science. Speaking to his fellow German scientists while still in England, Hahn warned, “the outlook for the future is very dark for us” (20, p 51). He understood the blow German science had received during the war and its impact on postwar progress. Hahn used “his” discovery to help reestablish the prominence of science in Germany. Again, Meitner became expendable, as evidenced by the following statement from the German scientists being detained in England (20, pp 105–106): The fission of the atomic nucleus in uranium was discovered by Hahn and Strassmann in the Kaiser Wilhelm Institute for Chemistry. … The Hahn discovery was checked in many laboratories, particularly in the United States, shortly after publication. Various research workers, Meitner and Frisch were probably the first, pointed out the enormous energies which were released by the fission of uranium. On the other hand, Meitner had left Berlin six months before the discovery and was not concerned herself in the discovery.


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The Politics of the Prize Meitner’s position after leaving Berlin in 1938 did not enhance her scientific image. She went to Stockholm to work in Manne Siegbahn’s new institute of physics, where she found herself at odds with Siegbahn and without the resources necessary to continue her work. Although she had produced the theory of fission, she had no way to conduct experiments to verify it (2, p 247). These factors are reflected in early decisions made by the Nobel Prize selection committees.15 Hahn and Meitner were nominated for the physics prize early in the 1940s, but the committee ruled that the discovery of fission belonged to chemistry. In special reports written by members of the chemistry committee, “Hahn’s work was considered important, while Meitner’s and Frisch’s experimental work was not extraordinary, and if there was a significant theoretical contribution, then Bohr16 should be given credit” (21, pp 26– 32). Nor was there any consideration of the political situation that forced Meitner’s separation from her team, or of the effect of Nazi anti-Semitic policies on the published record, conditions that contributed to this superficial portrait of the discovery. The chemistry committee recommended the 1944 Nobel Prize in Chemistry be awarded to Hahn alone for “his” discovery of fission. Although the Nobel Academy did not immediately accept this recommendation, by the end of 1945 Hahn had received his Nobel Prize (21, pp 26–32). In 1945 and 1946, Meitner and Frisch were nominated for the Nobel Prize in Physics for their contributions to the discovery of fission.17 The person chosen to evaluate these nominations was Erick Hulthén, a former student of Siegbahn’s. In both years, he recommended against awarding Meitner and Frisch the prize. Although the same misinformed perceptions that guided the chemistry committee appeared in his report, the internal politics of the physics committee also influenced his decision. Manne Siegbahn, Meitner’s ungracious host in Stockholm, was both an influential physicist and a member of the physics committee (21, p 28). Furthermore, the physics committee favored experimentalists, thus creating a prejudice against the theoretical work of Meitner and Frisch. Hulthén addresses this in his report, discounting the theoretical explanation because it had no impact on the experiment. Another factor that possibly influenced the committee was Sweden’s traditional orientation toward Germany as the leading force in science. However, Meitner and Frisch’s theory of fission served as the basis of successful work in the United States and Britain, not Germany. More obvious, however, is the way in which Hulthén reviewed the facts associated with the discovery. He based his report almost wholly on the original articles written by Hahn and Strassmann and by Meitner and Frisch, making no reference to other publications that provided evidence for the importance of Meitner and Frisch’s contributions. He dismissed as insignificant the experiments Frisch conducted to verify the theoretical explanation and saw nothing extraordinary in the theory that Meitner and Frisch had developed. Furthermore, he believed they had left important questions unanswered, questions that he felt Bohr answered later. He therefore concluded that any additional credit for the discovery of fission

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should be awarded to Bohr. On the basis of Hulthén’s reports, the Nobel committees in 1945 and 1946 denied Meitner and Frisch the Nobel Prize (21, pp 29–30). Conclusion The discovery of fission represented the culmination of the long and illustrious scientific careers of Lise Meitner and Otto Hahn. Unfortunately, events surrounding the discovery did not allow both scientists to receive equal recognition. However, with the passage of time and extensive historical research, equality is being restored. It is interesting to note the appearance of meitnerium and the disappearance of hahnium as names for transuranic elements.18 Notes 1. Radioactive recoil describes the recoil of an atom after the emission of a particle, thereby conserving momentum. It is analogous to the recoil in a gun after it is fired. By studying recoil, one can better understand the energies and masses involved in the radioactive decay process (3, p 58). 2. Meitner had special permission to attend Planck’s lectures and her experimental studies were an exception. When Meitner first began work with Hahn, she was confined to the basement, away from male students. Later, when these restrictions were lifted, many male students protested (2, pp 23, 29). 3. Meitner went unpaid until Planck, now a personal friend, made her his assistant. Her promotion to partner in Hahn’s lab at the KWI was most likely the result of the efforts of Emil Fischer, head of the institute and a friend of Meitner’s as well (2, pp 45–47). 4. Meitner also served on the Russian front from August 1915 to October 1916 as an X-ray nurse-technician in the Austrian army. 5. According to the Random House Unabridged Dictionary, a (nuclear) isomerism is the relation of two or more nuclides that have the same atomic number and mass number but different energy levels and half-lives (3, pp 95–103). An isomeric state is a longlived excited state of a nucleus. 6. The Joliot-Curies were successful in penetrating the aluminum nucleus with the charged α particles because aluminum has a relatively low atomic number. The momentum of a fast α particle could overcome the repulsive electrical force between it and the nucleus. However, this method would not work on heavier elements. The neutron, on the other hand, had no electrical charges to overcome (6, pp 202–204). 7. Radiochemists seeking to identify an unknown nuclide separate it from other reaction products and reactants using chemical procedures. It is then identified using radiation characteristics such as energy or half-life, atomic mass, or atomic number. The amount of radioactive material produced in a nuclear reaction is usually small. For example, the mass of 39Sr required to produce 108 disintegrations per second is only 1 × 10᎑7 g. Thus it is often impossible to weigh the nuclide isolated in a radiochemical separation. Traditionally, physicists set up the nuclear reaction and concentrate on measurements, whereas chemists focus on separation and purification procedures. 8. Although the results from Berlin did appear to contradict the current theory of nuclear isomers, isomerism was still a relatively new and largely unexplained concept.


Chemistry for Everyone 9. EkaAu was identified and named neptunium by McMillan and Abelson in 1941. 10. Irene Curie in Paris, Phillip Abelson at the University of California Berkeley, and a group at the University of Michigan all obtained similar results from irradiated uranium (10, p 101). 11. This conception of the nucleus certainly limited the vision of scientists involved. It also distorted experiments. To screen out various emissions, scientists placed a thin metal foil in front of their detectors, thus blocking the large fission products (10, p 102). 12. In this model nuclear particles did not function individually but rather acted together as a “fluid” that was bound by strong nuclear forces similar to the surface tension on a drop of water. 13. They first considered the uranium nuclei might be like a charged drop of liquid whose charge might be large enough to overcome the effect of the surface tension. Thus the nucleus resembled a wobbly, unstable drop ready to split at the slightest provocation, such as the impact of a single neutron. Once separated, the two smaller drops would acquire a high speed requiring a total energy of about 200 MeV. The source of this energy was determined using Einstein’s famous equation, E = ∆m c 2, where ∆m is the mass difference between the reactants and products of a nuclear reaction. 14. After World War II ended, Hahn overlooked the atrocities committed by the Nazis and focused on his own sufferings in Germany (2, p 263). 15. The recent release of the Nobel records for 1945 and 1946 has been the impetus for a number of papers on Meitner, fission, and the Nobel Prize. See Crawford, N.; Sime, R.; Walker, M. Nature 1996, 382, 393–395. 16. Bohr elaborated on the theory proposed by Meitner and Frisch. In particular, he sorted out the cross-sections of different uranium isotopes (6, pp 284–285). Bohr and John Wheeler published a paper, “The Mechanism of Nuclear Fission”, in the September 1939 issue of Physical Review in which they concluded that 235U was probably the isotope responsible for slow neutron fission. 17. Bohr was among the distinguished physicists who nominated Meitner and Frisch in 1946 (21, p 28). 18. In 1997, element atomic number 109, discovered in 1982, was officially named meitnerium. Element 105, discovered in 1967, was unofficially known as hahnium until 1997, when it was offi-

cially named dubnium after the Joint Nuclear Institute at Dubna, Russia, one of the laboratories where it was first observed.

Literature Cited 1. Sime, R. L. J. Chem. Educ. 1989, 66, 373–376. 2. Sime, R. L. Lise Meitner: A Life in Physics; University of California Press: Berkeley, 1996. 3. Hahn, O. Otto Hahn: A Scientific Autobiography; Charles Scribner’s Sons: New York, 1966. 4. Rife, P. Lise Meitner and the Dawn of the Atomic Age; Birkhauser: Boston, 1998. 5. Krafft, F. Internal and External Conditions for the Discovery of Fission by the Berlin Team. In Otto Hahn and the Rise of Nuclear Physics; Shea, W., Ed.; D. Reidel: Boston, 1983. 6. Rhodes, R. The Making of the Atomic Bomb; Simon and Schuster: New York, 1988. 7. Fermi, E. Nature 1934, 133, 898–899. 8. Graetzer, H. G.; Anderson, D. L. The Discovery of Nuclear Fission: A Documentary History; Van Nostrand Reinhold: New York, 1971; Chapter 2. 9. Meitner, L.; Hahn, O.; Strassmann, F. Z. Phys. 1937, 106, 249–270. 10. Weart, S. The Discovery of Fission and a Nuclear Physics Paradigm. In Otto Hahn and the Rise of Nuclear Physics; Shea, W., Ed.; D. Reidel: Boston, 1983. 11. Habashi, F. Bull. Hist. Chem. 1989, 3, 1516. 12. Noddack, I. Z. Angew. Chem. 1934, 47, 653. 13. Curie, I.; Savitch, P. J. Phys. Radium 1938, 9 (7), 355. 14. Hahn, O.; Strassmann, F. Naturwissenschaften 1938, 26, 755. 15. Hahn, O.; Strassmann, F. Naturwissenschaften 1939, 27, 11. 16. Meitner, L.; Frisch, O. Nature 1939, 143, 239. 17. Frisch, O. R. Nature 1939, 143, 276. 18. McMillian, E. Phys. Rev. 1939, 55, 510. 19. The Atomic Bomb: Its First Explosion Opens a New Era; Life, Aug 20, 1945, p 89. 20. Operation Epsilon: The Farm Hall Transcripts; University of California Press: Berkeley, 1993. 21. Crawford, N.; Sime, R. L.; Walker, M. Phys. Today 1997, September, 26–32.


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