Lise Meitner and the Discovery of Nuclear Fission Author(S): Ruth Lewin Sime Source: Scientific American , Vol

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Lise Meitner and the Discovery of Nuclear Fission Author(s): Ruth Lewin Sime Source: Scientific American , Vol. 278, No. 1 (JANUARY 1998), pp. 80-85 Published by: Scientific American, a division of Nature America, Inc. Stable URL: https://www.jstor.org/stable/10.2307/26057626

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This content downloaded from 129.2.19.102 on Mon, 18 Nov 2019 16:13:00 UTC All use subject to https://about.jstor.org/terms Lise Meitner and the Discovery of Nuclear Fission

by Ruth Lewin Sime

One of the discoverers of ﬁssion in 1938, Meitner was at the time overlooked by the Nobel judges. Racial persecution, fear and opportunism combined to obscure her contributions

This content downloaded from 129.2.19.102 on Mon, 18 Nov 2019 16:13:00 UTC All use subject to https://about.jstor.org/terms hen scientists first recog- Questions from nuclear physics initiated believed that history would be on her nized, in late 1938, that a the work. Data and assumptions from side. Fifty years later, it is. Wneutron could split an atom’s both chemistry and physics guided and core, the discovery came as a complete misguided their progress. And private Investigating Uranium surprise. Indeed, no physical theory had letters reveal that Meitner made essen- predicted nuclear fission, and its discov- tial contributions until the very end. orn and educated in Vienna, Lise erers had not the slightest foreknowl- By any normal standards of scientific BMeitner moved to Berlin in 1907 at edge of its eventual use in atomic bombs attribution, the Nobel committees should the age of 28. There she teamed up with and power plants. That much of the have recognized her influence. But in Otto Hahn, a chemist just her age, to story is undisputed. Germany the conditions were anything study radioactivity, the process by which The question of who deserved credit but normal. The country’s anti-Jewish one nucleus is transformed into another for the breakthrough, however, has long policies forced Meitner to emigrate, sep- by the emission of alpha or beta parti- been debated. Physicist Lise Meitner and arated her from her laboratory and pro- cles. Their collaboration was capped by two chemists, Otto Hahn and Fritz hibited her from being a co-author with their discovery in 1918 of protactini- Strassmann, conducted a four-year-long Hahn and Strassmann in reporting the um, a particularly heavy radioactive ele- investigation that resulted in the discov- fission result. Because of political op- ment. As their careers progressed, they ery of fission in their laboratory in Berlin. pression and fear, Hahn distanced him- remained equals scientifically and pro- Meitner fled Nazi Germany in 1938 to self and fission from Meitner and phys- fessionally: both were professors at the escape the persecution of Jews, and soon ics soon after the discovery took place. Kaiser Wilhelm Institute for Chemistry, after, Hahn and Strassmann reported the In time, the Nobel awards sealed these and each maintained an independent discovery. Meitner and her nephew, Otto injustices into scientific history. Recently section in the institute—his for radio- R. Frisch, published the correct theoreti- released documents show that the No- chemistry, hers for physics. cal interpretation of fission a few weeks bel committees did not grasp the extent During the 1920s, Hahn continued later. But the 1944 Nobel Prize in Chem- to which the result relied on both phys- developing radiochemical techniques, istry was awarded to Hahn alone. ics and chemistry, and they did not rec- whereas Meitner entered the new field That Strassmann did not get the No- ognize that Hahn had distanced himself of nuclear physics. Hahn later described bel with Hahn is probably because he from Meitner not on scientific grounds this period as a time when her work, was the junior investigator on the team, but because of political oppression, fear more than his, brought international and Nobel committees tend to favor se- and opportunism. recognition to the institute. Her promi- nior scientists. But Meitner and Hahn Other factors also served to margin- nence, and her Austrian citizenship, held equal professional standing. Why alize Meitner, including her outsider sta- shielded Meitner when Hitler came to was she excluded? Hahn offered what tus as a refugee in Sweden, a postwar power in 1933; unlike most others of became the standard account, which was unwillingness in Germany to confront Jewish origin, she was not dismissed uncritically accepted for many years. Nazi crimes, and a general perception— from her position. And although many According to him, the discovery had re- held much more strongly then than it is of her students and assistants were Nazi lied solely on chemical experiments that now—that women scientists were un- enthusiasts, Meitner found the physics were done after Meitner left Berlin. She important, subordinate or wrong. Pub- too exciting to leave. She was particu- and physics, he maintained, had noth- licly, Meitner said little at the time. Pri- larly intrigued by the experiments of En- ing to do with his success, except per- vately, she described Hahn’s behavior rico Fermi and his co-workers in Rome, haps to delay it. as “simply suppressing the past,” a past who began using neutrons to bombard Strassmann, who was very much in in which they had been the closest of elements throughout the periodic table. Hahn’s shadow, disagreed. He insisted colleagues and friends. She must have Fermi observed that when a neutron that Meitner had been their intellectual leader and that she remained one of them through her correspondence with Hahn, even after she left Berlin. The available documents support Strassmann’s view. Scientific publications show that the investigation that led to the discovery of fission was intensely interdisciplinary.

O COURTESY OF ARCHIV ZUR GESCHICHTE DER MAX-PLANCK-GESELLSCHAFT, BERLIN OF ARCHIV ZUR GESCHICHTE DER MAX-PLANCK-GESELLSCHAFT, O COURTESY LISE MEITNER (shown at left in about

HOT 1930, at the age of 50) was regarded as , P

AF one of the leading nuclear physicists of her day. Although she smoked and worked with radioactivity all her adult life, she lived to the age of 90. Otto Hahn and Meitner (right), photographed in their laboratory at the University of Berlin around 1910, were colleagues and good AIT © ATELIER LOTTE MEITNER-GR LOTTE AIT © ATELIER friends from 1907 until Meitner was BERLIN TESY OF ARCHIV ZUR GESCHICHTE DER MAX-PLANCK-GESELLSCHAFT, TR COUR POR forced to ﬂee from Germany in 1938.

This content downloaded from 129.2.19.102 on Mon, 18 Nov 2019 16:13:00 UTC All use subject to https://about.jstor.org/terms MEITNER’S PHYSICAL APPARATUS was used by the Berlin team from 1934 to 1938 for work that resulted in the discovery of nuclear fission. Beginning in the 1950s, it was displayed in the Deutsches Museum for some 30 years as the “Worktable of Otto Hahn,” with only a passing reference to Fritz Strass- mann and no mention of Meitner. COURTESY OF DEUTSCHES MUSEUM, MUNICH OF DEUTSCHES COURTESY

though reluctant at ﬁrst, agreed to help, and Fritz Strassmann, an analytical chemist from the institute, also joined the collaboration. The three were polit- ically compatible: Meitner was “non- Aryan,” Hahn was anti-Nazi, and Strassmann had refused to join the Na- tional Socialist–associated German Chemical Society, making him unem- ployable outside the institute. By the end of 1934, the team reported that the beta emitters Fermi observed reaction occurred, the targeted nucleus Fermi irradiated the heaviest known ele- could not be attributed to any other did not change dramatically: the incom- ment, uranium, with neutrons, he ob- known element and that they behaved ing neutron would most often cause the served several new beta emitters, none in a manner expected for transuranics: target nucleus to emit a proton or an al- with the chemical properties of uranium they could be separated out of the reac- ) pha particle, nothing more. Heavy ele- or the elements near it. Thus, he cau- tion mixture along with transition met- right ments, he found, favored neutron cap- tiously suggested that he had synthesized als, such as platinum and rhenium sul- ture. That is, a heavy nucleus would new elements beyond uranium. All over ﬁdes. Thus, like Fermi, the Berlin scien-

ALIFORNIA ( gain an extra neutron; if radioactive, the the world, scientists were fascinated. tists tentatively suggested that these

Y OF C heavier nucleus would invariably decay Meitner had been verifying Fermi’s activities were new elements beyond by emitting beta rays, which transformed results up to this point. The work per- uranium. As it turned out, the interpre-

, UNIVERSIT it into the next higher element. When fectly suited her interests and expertise, tation was incorrect: it rested on two

ORY and she was then in her assumptions—one from physics and one prime: one of the ﬁrst women from chemistry—that would prove false PERIODIC TABLE of the 1920s and 1930s (below to enter the upper ranks of only several years later. left) led researchers to expect that the elements fol- German science, she was a From physics, it had until then been lowing uranium would be transition elements. After leading nuclear physicist of observed that only small changes could the discovery of several transuranic elements in the 1940s, Glenn T. Seaborg recognized that the ac- her day. To study these new take place during nuclear reactions, tinides form a second rare-earth series homologous “transuranics” in detail, how- leaving an event such as ﬁssion unimag- to the lanthanides (1995 periodic table, below right). ever, Meitner needed an out- inable. And from chemistry it appeared Element 109 was named meitnerium in 1994. standing radiochemist. Hahn, that transuranic elements would be ); COURTESY OF LAWRENCE BERKELEY NATIONAL LABORAT BERKELEY NATIONAL OF LAWRENCE ); COURTESY left dam, 1969 ( er vier, Amst lse vier, onsen, E pr y J. W. Van S Van y J. W. b dic System of Chemical Elements, Elements, of Chemical dic System io er he P T OM FR

This content downloaded from 129.2.19.102 on Mon, 18 Nov 2019 16:13:00 UTC All use subject to https://about.jstor.org/terms transition elements. It was a simple mis- take: the chemistry of thorium and uranium is quite similar to that of transi- Discovering Fission tion elements, so chemists in the 1930s The Berlin group found that a large number of beta emitters (radioactive nuclei also expected that the elements beyond that emit electrons) were formed when neutrons hit uranium nuclei. The research- uranium would be transitionlike, resem- ers proposed two chains, which they believed consisted of elements beyond ura- bling rhenium, osmium, iridium and nium, each with its own rate of beta decay: platinum. Process 1 Untangling Decay Chains ELECTRON

he two false assumptions reinforced 92U + 92U 93 94 95 96 97 each other, misleading the investi- 10 2.2 59 66 2.5 T NEUTRON SECONDS MINUTES MINUTES HOURS HOURS gation for several years. Later Hahn blamed physicists and their mistaken Process 2 faith in small nuclear changes for ob- U structing the discovery. If anything, how- 92 + 92U 93 94 95 ever, the scientific publications indicate 40 16 5.7 that the chemists were complacent and SECONDS MINUTES HOURS the physicists were more skeptical. Physics did not predict fission, to be sure, In addition, they identified a simpler reaction: but it detected discrepancies that chemistry could not. Process 3 The Berlin scientists had tried to sep- U arate the presumed transuranics, which 92 + 92U 93 23 had extremely weak activities, from ura- MINUTES nium and its decay products, which had much stronger, natural radioactivity. After irradiating a uranium sample with Meitner regarded process three as the most understandable, and later it was neutrons, they would dissolve the sam- shown to be correct. But she was puzzled by processes one and two because the ple and then separate from the solution decay chains were so long and paralleled each other. Ultimately, when Hahn and just those activities with the chemistry Strassmann identified one of the reaction products as barium, Meitner and Frisch of transition metals, generally by using realized that the uranium nucleus had split into nuclei of barium and krypton, transition-metal compounds as carriers. which began a series of beta emissions: The precipitate itself was a mixture of several beta emitters, which the Berlin 56Ba 57La 58Ce 59Pr team painstakingly began to disentangle. U + Over two years, they identified two 92 parallel beta-decay chains, which they 36Kr 37Rb 38Sr 39Y

referred to as processes one and two [see JARED SCHNEIDMAN DESIGN box at right]. The sequence of these de- These nuclei and other fission fragments account for the decay chains of pro- cays corresponded to the properties ex- cesses one and two. Meitner and Frisch proposed the name “nuclear fission,” pub- pected for the elements lished the first theoretical explanation of the process and predicted the enormous following uranium: they energy released. —R.L.S. resembled the transition elements rhenium, osmium and so on. The fit between the sequences and theories thin. Meitner struggled to inte- Meitner regarded it as odd that three the predicted chemistry grate the results from chemistry, radio- different neutron-capture processes all seemed too good not to chemistry and her own physical mea- originated from the same uranium 238 be true. Publishing in surements into a cogent model of the isotope. She suspected that something Chemische Berichte in nuclear processes involved. She estab- was very wrong with processes one and 1936 and 1937, with lished that thermal—exceedingly slow— two. From theoretical considerations, Hahn as the senior au- neutrons enhanced the yield of process- she could not understand how the cap- thor, the elated group re- es one and two, evidence that these ture of a single neutron could produce peatedly referred to these events involved neutron capture. But such great instability that it would take transuranics as “unques- fast neutrons generated the same results. four or five beta emissions to relieve it. tionable,” there being “no Thus, she concluded that both processes And it was even harder to understand doubt” about their exis- originated with the most abundant ura- that the two long beta-decay chains tence and “no need for nium isotope, uranium 238. She also paralleled each other for several steps. further discussion.” identified a third process—involving the Theory offered no explanation. In a All the while, the data capture of moderately slow neutrons— 1937 report to Zeitschrift für Physik, were stretching physical for which there was no long beta chain. Meitner concluded that the results were

This content downloaded from 129.2.19.102 on Mon, 18 Nov 2019 16:13:00 UTC All use subject to https://about.jstor.org/terms followed a barium carrier, identified it as an isotope of radium. Meitner and Hahn corresponded con- stantly, and mail between Stockholm and Berlin was delivered overnight. She could scarcely believe the radium result. To form radium, the uranium nucleus would have to emit two alpha particles. Meitner was convinced that it was en- ergetically impossible for a thermal neutron to knock out even one alpha particle—and certainly not two. In Novem- ber 1938 Meitner visited Niels Bohr’s Institute for Theoretical Physics in Co- penhagen, and Hahn met her there on November 13. Outside the city their meeting was kept secret to avoid politi- TESY OF ARCHIV ZUR GESCHICHTE DER MAX-PLANCK-GESELLSCHAFT, BERLIN TESY OF ARCHIV ZUR GESCHICHTE DER MAX-PLANCK-GESELLSCHAFT, cal difficulties for Hahn, and he never COUR mentioned it later in his memoirs. But IN THE 1920s Meitner, as professor and head of her own section for physics at the we know from Hahn’s own pocket di- Kaiser Wilhelm Institute for Chemistry, became prominent in nuclear physics. In this ary that they met, and we know that photograph, taken in 1920 when Niels Bohr first visited Berlin, are some of her closest Meitner objected strenuously to the ra- colleagues and friends; nearly half would win Nobel Prizes. Front row: Otto Stern (No- bel, 1943), James Franck (1925), Bohr (1922). Second from right: Gustav Hertz dium result. That was the message Hahn (1925). To Meitner’s right and back: Hahn (1944) and George de Hevesy (1943). brought back to Strassmann in Berlin. According to Strassmann, Hahn told him that Meitner “urgently pleaded” “difficult to reconcile with current con- such decay chains because uranium can that they verify the radium one more cepts of nuclear structure.” split in many ways.) Meitner regarded time. “Fortunately, her opinion and Once fission was recognized, research- process three as the most normal, and judgment carried so much weight with ers understood that processes one and later this was shown to be correct: the us that we immediately began the nec- two were fission processes: the uranium uranium 239 isotope formed in this essary control experiments,” Strassmann splits into fragments that are highly neutron-capture reaction decays by beta remembered. With these experiments, radioactive and form a long sequence emission to element 93. In 1940 it was they intended to verify the presence of of beta decays. (There can be many identified by Edwin McMillan and radium by partially separating it from Philip Abelson and later named neptu- its barium carrier. But no separation oc- nium. Had the Berlin scientists been curred, and they were forced to con- able to detect neptunium, they would clude that their “radium” was in fact have found that it is a rare-earth ele- an isotope of barium, an element much ment, and they would have realized lighter than uranium. that the activities in processes one and In December 1938, just before Christ- two are not transuranics. But they did mas, Hahn told Meitner about the bari- not detect it; their neutron sources were um. It was a “frightful result,” he wrote. too weak. “We know uranium cannot really break up into barium!” He hoped she could Identifying Barium propose “some fantastic explanation.” Meitner answered by return mail. Al-

COLLECTION he most serious error the Berlin team though she found it difﬁcult to think of Tmade was that the investigators sep- a “thorough-going breakup,” she as-

SICS TODAY arated out and studied only those activ- sured him that “one cannot uncondi- PHY ities with transition-metal chemistry, ig- tionally say: it is impossible.” Her letter noring all others. In 1938 in Paris, Irène must have been the best Christmas Curie and Pavel Savitch used a different present he ever received. She had vehe- technique to examine the entire mix- mently objected to the radium result, but ture of uranium products and found a she was ready to consider the barium new, strong activity whose chemistry result as expanding, rather than contra- they could not ascertain. Like the pre- dicting, existing theory. AIP EMILIO SEGRÈ VISUAL ARCHIVES, AIP EMILIO SEGRÈ VISUAL sumed transuranics, its yield was en- Later, Hahn was known to say that if OTTO R. FRISCH and Meitner were the hanced by thermal neutrons. By the time Meitner had still been in Berlin, she first to explain, in 1939, the fission pro- the Berlin team looked into it in Octo- cess. In England in 1940 he and fellow might have talked him out of the bari- émigré Rudolf Peierls analyzed the poten- ber 1938, however, Meitner had been um result and might have “forbidden” tial of nuclear fission for use in weapons forced to flee Germany for Stockholm. him from making the discovery. But and helped to launch the Allied atomic Hahn and Strassmann analyzed the Meitner’s letter, which Hahn always had bomb project. Curie activity alone and, finding that it in his possession, demonstrates that the

This content downloaded from 129.2.19.102 on Mon, 18 Nov 2019 16:13:00 UTC All use subject to https://about.jstor.org/terms opposite is true. And at the time, Hahn as physicists had only interpreted it. clearly found her letter reassuring, be- In the weeks following the discovery, cause only after he received it did he add Hahn exploited that artificial division. a paragraph to the galley proofs of his He knew Meitner’s forced emigration barium publication, suggesting that the was unjust. He knew she had fully par- uranium nucleus had split in two. Meit- ticipated in the discovery. But he could ner was bitterly disappointed that she not say so. He was afraid for himself could not share in this “beautiful dis- and for his position and terribly afraid covery,” as she called it, but they all that others would find out that he and knew that it was impossible to include a Strassmann had continued to collabo- “non-Aryan” in the publication. rate with Meitner after she left Berlin. He decided that the discovery of fission Revising Nuclear Theory consisted of just those chemical experiments that he and Strassmann had or Christmas, Meitner visited a done in December. In February 1939 he Ffriend in western Sweden, and her wrote to Meitner, “We absolutely never nephew, Otto Frisch, a physicist at touched on physics, but instead we did Bohr’s institute, joined her. When Meit- chemical separations over and over ner and Frisch came together, so, too, again.” He described fission as a “gift did the various strands of nuclear theo- from heaven,” a miracle that would pro- ry. Both were accustomed to thinking tect him and his institute. of the nucleus as a liquid drop, but now As it turned out, it may not have been they visualized it as a wobbly, oscillat- necessary for Hahn to divorce himself TESY OF IRMGARD STRASSMANN OUR ing drop that was ready to split in two. from Meitner and physics to make the C Frisch realized that the surface tension “miracle” come true. That spring the FRITZ STRASSMANN worked with of a nucleus as large as uranium might German military took an active interest Meitner and Hahn on the investigations be vanishingly small. Meitner did the in the potential uses of the new discov- that led to the discovery of nuclear fission. His knowledge of analytical mass defect calculation in her head and ery, and by the summer of 1939 Hahn chemistry was crucial to the identification estimated the lost mass that was con- and his institute were secure. Later he of barium. A courageous anti-Nazi, he verted to enormous energy when the recalled that “fission saved that whole helped to save the life of a Jewish friend nucleus split. Everything fell into place: situation.” during the war. the theoretical interpretation itself was After the atomic bomb, fission was a beautiful discovery—and it was recog- more sensational than ever, and Hahn nized as such. The physics community was a very famous man. In postwar Ger- have been awarded a Nobel Prize had immediately adopted the term “fission” many, he was a major public figure for she emigrated anywhere else. In 1943 that Meitner and Frisch proposed, and a generation, lionized as a Nobel laure- she was invited to Los Alamos to work Bohr used their work as a starting point ate and a decent German who never on the atomic bomb, but she refused. for a more extensive theory. gave in to the Nazis, a scientist who did For a brief period after the war ended, Hahn and Strassmann’s barium find- not build a bomb. His treatment of she was a celebrity in the U.S. and ing appeared in Naturwissenschaften in Meitner, however, was anything but de- Britain, miscast as the Jewish refugee January 1939; Meitner and Frisch pub- cent. Not once in his numerous articles, who escaped the Nazis with the secret lished their interpretation in Nature a interviews, memoirs or autobiographies of the bomb. But Meitner was a private few weeks later. On the surface, the dis- did he mention her initiative for the person who detested publicity. She nev- covery of fission was now completely uranium project, her leadership of their er wrote an autobiography or autho- divided—chemistry from physics, exper- team in Berlin or their collaboration af- rized a biography. She left Stockholm iment from theory, Germans from refu- ter she left. He died in Göttingen in for Cambridge, England, in 1960 and gees. To those who did not understand 1968 at the age of 89. died there in 1968, a few days before her the science or who did not care to un- In Sweden during the war, Meitner’s 90th birthday. Sadly, she died some 30 derstand the politics, it might appear that professional status was poor. Her friends years before she received proper recog- chemists had discovered fission, where- believed that she almost surely would nition for her work. SA

The Author Further Reading

RUTH LEWIN SIME was born in New York City in 1939. She Looking Back. Lise Meitner in Bulletin of the Atomic Scientists, received a bachelor’s degree in mathematics from Barnard College Vol. 20, pages 1–7; November 1964. in 1960 and obtained a doctorate in chemistry from Harvard What Little I Remember. Otto R. Frisch. Cambridge University University in 1964. Since 1968, she has taught chemistry at Sacra- Press, 1979. mento City College. Her interest in Lise Meitner began some 25 Im Schatten der Sensation: Leben und Wirken von Fritz years ago, when she taught a class on women in science and dis- Strassmann. Fritz Krafft. Verlag Chemie, Weinheim, 1981. covered that little scholarly attention had been paid to Meitner’s A Nobel Tale of Postwar Injustice. Elisabeth Crawford, Ruth life and work. Her biography Lise Meitner: A Life in Physics was Lewin Sime and Mark Walker in Physics Today, Vol. 50, No. 9, published in 1996 by the University of California Press. pages 26–32; September 1997.

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