The nuclear reaction theorised by Meitner and Frisch with the following nuclear chain reaction theorized by Hahn and Strassmann[1]
Nuclear fission was discovered in December 1938 by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Fission is a nuclear reaction or radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei and often other particles. The fission process often produces gamma rays and releases a very large amount of energy, even by the energetic standards of radioactive decay. Scientists already knew about alpha decay and beta decay, but fission assumed great importance because the discovery that a nuclear chain reaction was possible led to the development of nuclear power and nuclear weapons. Hahn was awarded the 1944 Nobel Prize in Chemistry for the discovery of nuclear fission.
Hahn and Strassmann at the Kaiser Wilhelm Institute for Chemistry in Berlin bombarded uranium with slow neutrons and discovered that barium had been produced. Hahn suggested a bursting of the nucleus, but he was unsure of what the physical basis for the results were. They reported their findings by mail to Meitner in Sweden, who a few months earlier had fled Nazi Germany. Meitner and her nephew Frisch theorised, and then proved, that the uranium nucleus had been split and published their findings in Nature. Meitner calculated that the energy released by each disintegration was approximately 200 megaelectronvolts, and Frisch observed this. By analogy with the division of biological cells, he named the process "fission".
The discovery came after forty years of investigation into the nature and properties of radioactivity and radioactive substances. The discovery of the neutron by James Chadwick in 1932 created a new means of nuclear transmutation. Enrico Fermi and his colleagues in Rome studied the results of bombarding uranium with neutrons, and Fermi concluded that his experiments had created new elements with 93 and 94 protons, which his group dubbed ausenium and hesperium. Fermi won the 1938 Nobel Prize in Physics for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons".[2] However, not everyone was convinced by Fermi's analysis of his results. Ida Noddack suggested that instead of creating a new, heier element 93, it was conceivable that the nucleus had broken up into large fragments, and Aristid von Grosse suggested that what Fermi's group had found was an isotope of protactinium.
This spurred Hahn and Meitner, the discoverers of the most stable isotope of protactinium, to conduct a four-year-long investigation into the process with their colleague Strassmann. After much hard work and many discoveries, they determined that what they were observing was fission, and that the new elements that Fermi had found were fission products. Their work overturned long-held beliefs in physics and ped the way for the discovery of the real elements 93 (neptunium) and 94 (plutonium), for the discovery of fission in other elements, and for the determination of the role of the uranium-235 isotope in that of uranium. Niels Bohr and John Wheeler reworked the liquid drop model to explain the mechanism of fission.
Background[edit] Radioactivity[edit]In the last years of the 19th century, scientists frequently experimented with the cathode-ray tube, which by then had become a standard piece of laboratory equipment. A common practice was to aim the cathode rays at various substances and to see what happened. Wilhelm Röntgen had a screen coated with barium platinocyanide that would fluoresce when exposed to cathode rays. On 8 November 1895, he noticed that even though his cathode-ray tube was not pointed at his screen, which was covered in black cardboard, the screen still fluoresced. He soon became convinced that he had discovered a new type of rays, which are today called X-rays. The following year Henri Becquerel was experimenting with fluorescent uranium salts, and wondered if they too might produce X-rays.[3] On 1 March 1896 he discovered that they did indeed produce rays, but of a different kind, and even when the uranium salt was kept in a dark drawer, it still made an intense image on an X-ray plate, indicating that the rays came from within, and did not require an external energy source.[4]
The periodic table c. 1930Unlike Röntgen's discovery, which was the object of widespread curiosity from scientists and lay people alike for the ability of X-rays to make visible the bones within the human body, Becquerel's discovery made little impact at the time, and Becquerel himself soon moved on to other research.[5] Marie Curie tested samples of as many elements and minerals as she could find for signs of Becquerel rays, and in April 1898 also found them in thorium. She ge the phenomenon the name "radioactivity".[6] Along with Pierre Curie and Guste Bémont, she began investigating pitchblende, a uranium-bearing ore, which was found to be more radioactive than the uranium it contained. This indicated the existence of additional radioactive elements. One was chemically akin to bismuth, but strongly radioactive, and in July 1898 they published a paper in which they concluded that it was a new element, which they named "polonium". The other was chemically like barium, and in a December 1898 paper they announced the discovery of a second hitherto unknown element, which they called "radium". Convincing the scientific community was another matter. Separating radium from the barium in the ore proved very difficult. It took three years for them to produce a tenth of a gram of radium chloride, and they never did manage to isolate polonium.[7]
In 1898, Ernest Rutherford noted that thorium ge off a radioactive gas. In examining the radiation, he classified Becquerel radiation into two types, which he called α (alpha) and β (beta) radiation.[8] Subsequently, Paul Villard discovered a third type of Becquerel radiation which, following Rutherford's scheme, were called "gamma rays", and Curie noted that radium also produced a radioactive gas. Identifying the gas chemically proved frustrating; Rutherford and Frederick Soddy found it to be inert, much like argon. It later came to be known as radon. Rutherford identified beta rays as cathode rays (electrons), and hypothesised—and in 1909 with Thomas Royds proved—that alpha particles were helium nuclei.[9][10] Observing the radioactive disintegration of elements, Rutherford and Soddy classified the radioactive products according to their characteristic rates of decay, introducing the concept of a half-life.[9][11] In 1903, Soddy and Margaret Todd applied the term "isotope" to atoms that were chemically and spectroscopically identical but had different radioactive half-lives.[12][13] Rutherford proposed a model of the atom in which a very small, dense and positively charged nucleus of protons was surrounded by orbiting, negatively charged electrons (the Rutherford model).[14] Niels Bohr improved upon this in 1913 by reconciling it with the quantum behiour of electrons (the Bohr model).[15][16][17]
Protactinium[edit] The decay chain of actinium. Alpha decay shifts two elements down; beta decay shifts one element up.Soddy and Kasimir Fajans independently observed in 1913 that alpha decay caused atoms to shift down two places in the periodic table, while the loss of two beta particles restored it to its original position. In the resulting reorganisation of the periodic table, radium was placed in group II, actinium in group III, thorium in group IV and uranium in group VI. This left a gap between thorium and uranium. Soddy predicted that this unknown element, which he referred to (after Dmitri Mendeleev) as "ekatantalium", would be an alpha emitter with chemical properties similar to tantalium (now known as tantalum).[18][19][20] It was not long before Fajans and Oswald Helmuth Göhring discovered it as a decay product of a beta-emitting product of thorium. Based on the radioactive displacement law of Fajans and Soddy, this was an isotope of the missing element, which they named "brevium" after its short half-life. However, it was a beta emitter, and therefore could not be the mother isotope of actinium. This had to be another isotope.[18]
Two scientists at the Kaiser Wilhelm Institute (KWI) in Berlin-Dahlem took up the challenge of finding the missing isotope. Otto Hahn had graduated from the University of Marburg as an organic chemist, but had been a post-doctoral researcher at University College London under Sir William Ramsay, and under Rutherford at McGill University, where he had studied radioactive isotopes. In 1906, he returned to Germany, where he became an assistant to Emil Fischer at the University of Berlin. At McGill he had become accustomed to working closely with a physicist, so he teamed up with Lise Meitner, who had received her doctorate from the University of Vienna in 1906, and had then moved to Berlin to study physics under Max Planck at the Friedrich-Wilhelms-Universität. Meitner found Hahn, who was about the same age as her, less intimidating than older, more distinguished colleagues.[21] Hahn and Meitner moved to the recently established Kaiser Wilhelm Institute for Chemistry in 1913, and by 1920 had become the heads of their own laboratories there, with their own students, research programs and equipment.[21] The new laboratories offered new opportunities, as the old ones had become too contaminated with radioactive substances to investigate feebly radioactive substances. They developed a new technique for separating the tantalum group from pitchblende, which they hoped would speed the isolation of the new isotope.[18]
Otto Hahn and Lise Meitner in 1912The work was interrupted by the outbreak of the First World War in 1914. Hahn was called up into the German Army, and Meitner became a volunteer radiographer in Austrian Army hospitals.[22] She returned to the Kaiser Wilhelm Institute in October 1916. Hahn joined the new gas command unit at Imperial Headquarters in Berlin in December 1916 after trelling between the western and eastern fronts, Berlin and Leverkusen between the summer of 1914 and late 1916.[23]
Most of the students, laboratory assistants and technicians had been called up, so Hahn, who was stationed in Berlin between January and September 1917,[24] and Meitner had to do everything themselves. By December 1917 she was able to isolate the substance, and after further work were able to prove that it was indeed the missing isotope. Meitner submitted her and Hahn's findings for publication in March 1918 to the scientific paper Physikalischen Zeitschrift under the title Die Muttersubstanz des Actiniums; ein neues radioaktives Element von langer Lebensdauer.[18][25]
Although Fajans and Göhring had been the first to discover the element, custom required that an element was represented by its longest-lived and most abundant isotope, and brevium did not seem appropriate. Fajans agreed to Meitner and Hahn naming the element protactinium, and assigning it the chemical symbol Pa. In June 1918, Soddy and John Cranston announced that they had extracted a sample of the isotope, but unlike Hahn and Meitner were unable to describe its characteristics. They acknowledged Hahn's and Meitner's priority, and agreed to the name. The connection to uranium remained a mystery, as neither of the known isotopes of uranium decayed into protactinium. It remained unsolved until uranium-235 was discovered in 1929.[18][26]
For their discovery Hahn and Meitner were repeatedly nominated for the Nobel Prize in Chemistry in the 1920s by several scientists, among them Max Planck, Heinrich Goldschmidt, and Fajans himself.[27][28] In 1949, the International Union of Pure and Applied Chemistry (IUPAC) named the new element definitively protactinium, and confirmed Hahn and Meitner as discoverers.[29]
Transmutation[edit] Irène Curie and Frédéric Joliot in their Paris laboratory in 1935Patrick Blackett was able to accomplish nuclear transmutation of nitrogen into oxygen in 1925, using alpha particles directed at nitrogen. In modern notation for the atomic nuclei, the reaction was:
147N + 42He → 178O + pThis was the first observation of a nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus.[30] A fully artificial nuclear reaction and nuclear transmutation was achieved in April 1932 by Ernest Walton and John Cockcroft, who used artificially accelerated protons against lithium, to break this nucleus into two alpha particles. The feat was popularly known as "splitting the atom", but was not nuclear fission;[31][32] as it was not the result of initiating an internal radioactive decay process.[33] Just a few weeks before Cockcroft and Walton's feat, another scientist at the Cendish Laboratory, James Chadwick, discovered the neutron, using an ingenious device made with sealing wax, through the reaction of beryllium with alpha particles:[34][35]
94Be + 42He → 126C + nIrène Curie and Frédéric Joliot irradiated aluminium foil with alpha particles and found that this results in a short-lived radioactive isotope of phosphorus with a half-life of around three minutes:
2713Al + 42He → 3015P + nwhich then decays to a stable isotope of silicon:
3015P → 3014Si + e+They noted that radioactivity continued after the neutron emissions ceased. Not only had they discovered a new form of radioactive decay in the form of positron emission, they had transmuted an element into a hitherto unknown radioactive isotope of another, thereby inducing radioactivity where there had been none before. Radiochemistry was now no longer confined to certain hey elements, but extended to the entire periodic table.[36][37][38]
Chadwick noted that being electrically neutral, neutrons would be able to penetrate the nucleus more easily than protons or alpha particles.[39] Enrico Fermi and his colleagues in Rome—Edoardo Amaldi, Oscar D'Agostino, Franco Rasetti and Emilio Segrè—picked up on this idea.[40] Rasetti visited Meitner's laboratory in 1931, and again in 1932 after Chadwick's discovery of the neutron. Meitner showed him how to prepare a polonium-beryllium neutron source. On returning to Rome, Rasetti built Geiger counters and a cloud chamber modelled after Meitner's. Fermi initially intended to use polonium as a source of alpha particles, as Chadwick and Curie had done. Radon was a stronger source of alpha particles than polonium, but it also emitted beta and gamma rays, which played hoc with the detection equipment in the laboratory. But Rasetti went on his Easter vacation without preparing the polonium-beryllium source, and Fermi realised that since he was interested in the products of the reaction, he could irradiate his sample in one laboratory and test it in another down the hall. The neutron source was easy to prepare by mixing with powdered beryllium in a sealed capsule. Moreover, radon was easily obtained; Giulio Cesare Trabacchi had more than a gram of radium and was happy to supply Fermi with radon. With a half-life of only 3.82 days it would only go to waste otherwise, and the radium continually produced more.[40][41]
Enrico Fermi and his research group (the Via Panisperna boys), c. 1934. Left to right: Oscar D'Agostino, Emilio Segrè, Edoardo Amaldi, Franco Rasetti and FermiWorking in assembly-line fashion, they started by irradiating water, and then progressed up the periodic table through lithium, beryllium, boron and carbon, without inducing any radioactivity. When they got to aluminium and then fluorine, they had their first successes. Induced radioactivity was ultimately found through the neutron bombardment of 22 different elements.[42][43] Meitner was one of the select group of physicists to whom Fermi mailed advance copies of his papers, and she was able to report that she had verified his findings with respect to aluminium, silicon, phosphorus, copper and zinc.[41] When a new copy of La Ricerca Scientifica arrived at the Niels Bohr's Institute for Theoretical Physics at the University of Copenhagen, her nephew, Otto Frisch, as the only physicist there who could read Italian, found himself in demand from colleagues wanting a translation. The Rome group had no samples of the rare earth metals, but at Bohr's institute George de Hevesy had a complete set of their oxides that had been given to him by Auergesellschaft, so de Hevesy and Hilde Levi carried out the process with them.[44]
When the Rome group reached uranium, they had a problem: the radioactivity of natural uranium was almost as great as that of their neutron source.[45] What they observed was a complex mixture of half-lives. Following the displacement law, they checked for the presence of lead, bismuth, radium, actinium, thorium and protactinium (skipping the elements whose chemical properties were unknown), and (correctly) found no indication of any of them.[45] Fermi noted three types of reactions were caused by neutron irradiation: emission of an alpha particle (n, α); proton emission (n, p); and gamma emission (n, γ). Invariably, the new isotopes decayed by beta emission, which caused elements to move up the periodic table.[46]
Based on the periodic table of the time, Fermi believed that element 93 was ekarhenium—the element below rhenium—with characteristics similar to manganese and rhenium. Such an element was found, and Fermi tentatively concluded that his experiments had created new elements with 93 and 94 protons,[47] which he dubbed ausenium and hesperium.[48][49] The results were published in Nature in June 1934.[47] However, in this paper Fermi cautioned that "a careful search for such hey particles has not yet been carried out, as they require for their observation that the active product should be in the form of a very thin layer. It seems therefore at present premature to form any definite hypothesis on the chain of disintegrations involved."[47] In retrospect, what they had detected was indeed an unknown rhenium-like element, technetium, which lies between manganese and rhenium on the periodic table.[45]
Leo Szilard and Thomas A. Chalmers reported that neutrons generated by gamma rays acting on beryllium were captured by iodine, a reaction that Fermi had also noted. When Meitner repeated their experiment, she found that neutrons from the gamma-beryllium sources were captured by hey elements like iodine, silver and gold, but not by lighter ones like sodium, aluminium and silicon. She concluded that slow neutrons were more likely to be captured than fast ones, a finding she reported to Naturwissenschaften in October 1934.[50][51] Everyone had been thinking that energetic neutrons were required, as was the case with alpha particles and protons, but that was required to overcome the Coulomb barrier; the neutrally charged neutrons were more likely to be captured by the nucleus if they spent more time in its vicinity. A few days later, Fermi considered a curiosity that his group had noted: uranium seemed to react differently in different parts of the laboratory; neutron irradiation conducted on a wooden table induced more radioactivity than on a marble table in the same room. Fermi thought about this and tried placing a piece of paraffin wax between the neutron source and the uranium. This resulted in a dramatic increase in activity. He reasoned that the neutrons had been slowed by collisions with hydrogen atoms in the paraffin and wood.[52] The departure of D'Agostino meant that the Rome group no longer had a chemist, and the subsequent loss of Rasetti and Segrè reduced the group to just Fermi and Amaldi, who abandoned the research into transmutation to concentrate on exploring the physics of slow neutrons.[45]
The current model of the nucleus in 1934 was the liquid drop model first proposed by George Gamow in 1930.[53] His simple and elegant model was refined and developed by Carl Friedrich von Weizsäcker and, after the discovery of the neutron, by Werner Heisenberg in 1935 and Niels Bohr in 1936. It agreed closely with observations. In the model, the nucleons were held together in the smallest possible volume (a sphere) by the strong nuclear force, which was capable of overcoming the longer ranged Coulomb electrical repulsion between the protons. The model remained in use for certain applications into the 21st century, when it attracted the attention of mathematicians interested in its properties,[54][55][56] but in its 1934 form it confirmed what physicists thought they already knew: that nuclei were static, and that the odds of a collision chipping off more than an alpha particle were practically zero.[57]
Discovery[edit] Objections[edit]Fermi won the 1938 Nobel Prize in Physics for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons".[2] However, not everyone was convinced by Fermi's analysis of his results. Ida Noddack suggested in September 1934 that instead of creating a new, heier element 93, that:
One could assume equally well that when neutrons are used to produce nuclear disintegrations, some distinctly new nuclear reactions take place which he not been observed previously with proton or alpha-particle bombardment of atomic nuclei. In the past one has found that transmutations of nuclei only take place with the emission of electrons, protons, or helium nuclei, so that the hey elements change their mass only a small amount to produce near neighbouring elements. When hey nuclei are bombarded by neutrons, it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbours of the irradiated element.[58]
Noddack's article was read by Fermi's team in Rome, Curie and Joliot in Paris, and Meitner and Hahn in Berlin.[45] However, the quoted objection comes some distance down, and is but one of several gaps she noted in Fermi's claim.[59] Bohr's liquid drop model had not yet been formulated, so there was no theoretical way to calculate whether it was physically possible for the uranium atoms to break into large pieces.[60] Noddack and her husband, Walter Noddack, were renowned chemists who had been nominated for the Nobel Prize in Chemistry for the discovery of rhenium, although at the time they were also embroiled in a controversy over the discovery of element 43, which they called "masurium". The discovery of technetium by Emilio Segrè and Carlo Perrier put an end to their claim, but did not occur until 1937. It is unlikely that Meitner or Curie had any prejudice against Noddack because of her sex,[61] but Meitner was not afraid to tell Hahn Hähnchen, von Physik verstehst Du Nichts ("Hahn dear, of physics you understand nothing").[62] The same attitude carried over to Noddack, who did not propose an alternative nuclear model, nor conduct experiments to support her claim. Although Noddack was a renowned analytical chemist, she lacked the background in physics to appreciate the enormity of what she was proposing.[59]
Former Kaiser Wilhelm Institute for Chemistry building in Berlin. After the Second World War it became part of the Free University of Berlin. It was renamed the Otto Hahn Building in 1956, and the Hahn-Meitner Building in 2010.[63][64]Noddack was not the only critic of Fermi's claim. Aristid von Grosse suggested that what Fermi had found was an isotope of protactinium.[65][66] Meitner was eager to investigate Fermi's results, but she recognised that a highly skilled chemist was required, and she wanted the best one she knew: Hahn, although they had not collaborated for many years. Initially, Hahn was not interested, but von Grosse's mention of protactinium changed his mind.[67] "The only question", Hahn later wrote, "seemed to be whether Fermi had found isotopes of transuranian elements, or isotopes of the next-lower element, protactinium. At that time Lise Meitner and I decided to repeat Fermi's experiments in order to find out whether the 13-minute isotope was a protactinium isotope or not. It was a logical decision, hing been the discoverers of protactinium."[68]
Hahn and Meitner were joined by Fritz Strassmann. Strassmann had received his doctorate in analytical chemistry from the Technical University of Hannover in 1929,[69] and had come to the Kaiser Wilhelm Institute for Chemistry to study under Hahn, believing that this would improve his employment prospects. He enjoyed the work and the people so much that he stayed on after his stipend expired in 1932. After the Nazi Party came to power in Germany in 1933, he declined a lucrative offer of employment because it required political training and Nazi Party membership, and he resigned from the Society of German Chemists when it became part of the Nazi German Labour Front. As a result, he could neither work in the chemical industry nor receive his habilitation, which was required to become an independent researcher in Germany. Meitner persuaded Hahn to hire Strassmann using money from the director's special circumstances fund. In 1935, Strassmann became an assistant on half pay. Soon he would be credited as a collaborator on the papers they produced.[70]
The 1933 Law for the Restoration of the Professional Civil Service removed Jewish people from the civil service, which included academia. Meitner never tried to conceal her Jewish descent, but initially was exempt from its impact on multiple grounds: she had been employed before 1914, had served in the military during the World War, was an Austrian rather than a German citizen, and the Kaiser Wilhelm Institute was a government-industry partnership.[71] However, she was dismissed from her adjunct professorship at the University of Berlin on the grounds that her World War I service was not at the front, and she had not completed her habilitation until 1922.[72] Carl Bosch, the director of IG Farben, a major sponsor of the Kaiser Wilhelm Institute for Chemistry, assured Meitner that her position there was safe, and she agreed to stay.[71] Meitner, Hahn and Strassmann drew closer together personally as their anti-Nazi politics increasingly alienated them from the rest of the organisation, but it ge them more time for research, as administration was devolved to Hahn's and Meitner's assistants.[70]
Research[edit] The nuclear fission display at the Deutsches Museum in Munich. The table and instruments are originals,[73][74] but would not he been together in the same room. Pressure from historians, scientists and feminists caused the museum to alter the display in 1988 to acknowledge Lise Meitner, Otto Frisch and Fritz Strassmann.[75]The Berlin group started by irradiating uranium salt with neutrons from a radon-beryllium source similar to the one that Fermi had used. Powdered beryllium was mixed with radon in a sealed capsule. This provided a much stronger neutron source than polonium-beryllium mixtures.[76] They dissolved the uranium salt and added potassium perrhenate, platinum chloride and sodium hydroxide. What remained was then acidified with hydrogen sulphide, resulting in platinum sulphide and rhenium sulphide precipitation. Fermi had noted four radioactive isotopes with the longest-lived hing 13- and 90-minute half-lives, and these were detected in the precipitate.[77]
The Berlin group then tested for protactinium by adding protactinium-234 to the solution. When this was precipitated, it was found to be separated from the 13- and 90-minute half-life isotopes, demonstrating that von Grosse was incorrect, and they were not isotopes of protactinium. Moreover, the chemical reactions involved ruled out all elements from mercury and above on the periodic table.[77] They were able to precipitate the 90-minute activity with osmium sulphide and the 13-minute one with rhenium sulphide, which ruled out them being isotopes of the same element. All this provided strong evidence that they were indeed transuranium elements, with chemical properties similar to osmium and rhenium.[76][78]
Fermi had also reported that fast and slow neutrons had produced different activities. This indicated that more than one reaction was taking place. When the Berlin group could not replicate the Rome group's findings, they commenced their own research into the effects of fast and slow neutrons. To minimise radioactive contamination if there were an accident, different phases were carried out in different rooms, all in Meitner's section on the ground floor of the Kaiser Wilhelm Institute. Neutron irradiation was carried out in one laboratory, chemical separation in another, and measurements were conducted in a third. The equipment they used was simple and mostly hand made.[79]
By March 1936, they had identified ten different half-lives, with varying degrees of certainty. To account for them, Meitner had to hypothesise a new (n, 2n) class of reaction and the alpha decay of uranium, neither of which had ever been reported before, and for which physical evidence was lacking. So while Hahn and Strassmann refined their chemical procedures, Meitner devised new experiments to shine more light on the reaction processes. In May 1937, they issued parallel reports, one in Zeitschrift für Physik with Meitner as the principal author, and one in Chemische Berichte with Hahn as the principal author.[79][80][81] Hahn concluded his by stating emphatically: Vor allem steht ihre chemische Verschiedenheit von allen bisher bekannten Elementen außerhalb jeder Diskussion ("Above all, their chemical distinction from all previously known elements needs no further discussion."[81])
Meitner was increasingly uncertain. They had now constructed three (n, γ) reactions:
23892U + n → 23992U (10 seconds) → 23993ekaRe (2.2 minutes) → 23994ekaOs (59 minutes) → 23995ekaIr (66 hours) → 23996ekaPt (2.5 hours) → 23997ekAu (?) 23892U + n → 23992U (40 seconds) → 23993ekaRe (16 minutes) → 23994ekaOs (5.7 hours) → 23995ekaIr (?) 23892U + n → 23992U (23 minutes) → 23993ekaReMeitner was certain that these had to be (n, γ) reactions, as slow neutrons lacked the energy to chip off protons or alpha particles. She considered the possibility that the reactions were from different isotopes of uranium; three were known: uranium-238, uranium-235 and uranium-234. However, when she calculated the neutron cross section it was too large to be anything other than the most abundant isotope, uranium-238. She concluded that it must be a case of nuclear isomerism, which had been discovered in protactinium by Hahn in 1922. Nuclear isomerism had been given a physical explanation by von Weizsäcker, who had been Meitner's assistant in 1936, but had since taken a position at the Kaiser Wilhelm Institute for Physics. Different nuclear isomers of protactinium had different half-lives, and this could be the case for uranium too, but if so it was somehow being inherited by the daughter and granddaughter products, which seemed to be stretching the argument to breaking point. Then there was the third reaction, an (n, γ) one, which occurred only with slow neutrons.[82] Meitner therefore ended her report on a very different note to Hahn, reporting that: "The process must be neutron capture by uranium-238, which leads to three isomeric nuclei of uranium-239. This result is very difficult to reconcile with current concepts of the nucleus."[80][83]
Exhibition to mark the 75th anniversary of the discovery of nuclear fission, at the Vienna International Centre in 2013. Images of Meitner and Strassmann are prominently displayed.After this, the Berlin group moved on to working with thorium, as Strassmann put it, "to recover from, the horror of the work with uranium".[84] However, thorium was not easier to work with than uranium. For a start, it had a decay product, radiothorium (22890Th) that overwhelmed weaker neutron-induced activity. But Hahn and Meitner had a sample from which they had regularly removed its mother isotope, mesothorium (22888Ra), over a period of several years, allowing the radiothorium to decay away. Even then, it was still more difficult to work with because its induced decay products from neutron irradiation were isotopes of the same elements produced by thorium's own radioactive decay. What they found was three different decay series, all alpha emitters—a form of decay not found in any other hey element, and for which Meitner once again had to postulate multiple isomers. They did find an interesting result: under bombardment with 2.5 MeV fast neutrons, these (n, α) decay series occurred simultaneously; for slow neutrons, an (n, γ) reaction that formed 23390Th was foured.[85]
In Paris, Irene Curie and Pel Sitch had also set out to replicate Fermi's findings. In collaboration with Hans von Halban and Peter Preiswerk, they irradiated thorium and produced the isotope with a 22-minute half-life that Fermi had noted. In all, Curie's group detected eight different half-lives in their irradiated thorium. Curie and Sitch detected a radioactive substance with a 3.5-hour half-life.[45][39][86] The Paris group proposed that it might be an isotope of thorium. Meitner asked Strassmann, who was now doing most of the chemistry work, to check. He detected no sign of thorium. Meitner wrote to Curie with their results, and suggested a quiet retraction.[87] Nonetheless, Curie persisted. They investigated the chemistry, and found that the 3.5-hour activity was coming from something that seemed to be chemically similar to lanthanum (which in fact it was), which they attempted unsuccessfully to isolate with a fractional crystallization process. (It is possible that their precipitate was contaminated with yttrium, which is chemically similar.) By using Geiger counters and skipping the chemical precipitation, Curie and Sitch detected the 3.5-hour half-life in irradiated uranium.[88]
With the Anschluss, Germany's unification with Austria on 12 March 1938, Meitner lost her Austrian citizenship.[89] James Franck offered to sponsor her immigration to the United States, and Bohr offered a temporary place at his institute, but when she went to the Danish embassy for a visa, she was told that Denmark no longer recognised her Austrian passport as valid.[90] On 13 July 1938, Meitner departed for the Netherlands with Dutch physicist Dirk Coster. Before she left, Otto Hahn ge her a diamond ring he had inherited from his mother to sell if necessary. She reached safety, but with only her summer clothes. Meitner later said that she left Germany forever with 10 marks in her purse. With the help of Coster and Adriaan Fokker, she flew to Copenhagen, where she was greeted by Frisch, and stayed with Niels and Margrethe Bohr at their holiday house in Tisvilde. On 1 August she took the train to Stockholm, where she was met by Eva von Bahr.[91]
Interpretation[edit]The Paris group published their results in September 1938.[88] Hahn dismissed the isotope with the 3.5-hour half-life as contamination, but after looking at the details of the Paris group's experiments and the decay curves, Strassmann was worried. He decided to repeat the experiment, using his more efficient method of separating radium. This time, they found what they thought was radium, which Hahn suggested resulted from two alpha decays:
23892U + n → α + 23590Th → α + 23188RaMeitner found this very hard to believe.[92][93]
The mechanism of fission. A neutron caused the nucleus to wobble, elongate, and split.In November, Hahn trelled to Copenhagen, where he met with Bohr and Meitner. They told him that they were very unhappy about the proposed radium isomers. On Meitner's instructions, Hahn and Strassmann began to redo the experiments, even as Fermi was collecting his Nobel Prize in Stockholm.[94] Assisted by Clara Lieber and Irmgard Bohne, Hahn and Strassmann isolated the three radium isotopes (verified by their half-lives) and used fractional crystallisation to separate them from the barium carrier by adding barium bromide crystals in four steps. Since radium precipitates preferentially in a solution of barium bromide, at each step the fraction drawn off would contain less radium than the one before. However, they found no difference between each of the fractions. In case their process was faulty in some way, they verified it with known isotopes of radium; the process was fine. Hahn and Strassmann found a fourth radium isotope. Their half-lives were formulated as such by Hahn and Strassmann:
R a I ? →