In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay. Nuclear fission of heavy elements was discovered on December 17, 1938 by German Otto Hahn and his assistant Fritz Strassmann, and explained theoretically in January 1939 by Lise Meitner and her nephew Otto Robert Frisch. Frisch named the process by analogy with biological fission of living cells.
Introduction
Nuclear fission
Nuclear fission | Introduction
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Nuclear fission is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei.
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In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay. Nuclear fission of heavy elements was discovered on December 17, 1938 by German Otto Hahn and his assistant Fritz Strassmann, and explained theoretically in January 1939 by Lise Meitner and her nephew Otto Robert Frisch. Frisch named the process by analogy with biological fission of living cells.
Nuclear fission | Introduction
Nuclear fission was discovered in December 1938 by physicists Lise Meitner and Otto Robert Frisch and chemists Otto Hahn and Fritz Strassmann. Fission is a nuclear reaction or radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei. 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.
Discovery of nuclear fission | Introduction
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 ausonium 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".
Discovery of nuclear fission | Introduction
Hahn and Strassmann at the Kaiser Wilhelm Institute for Chemistry in Berlin bombarded uranium with slow neutrons, and discovered that barium had been produced. 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". Hahn was awarded the 1944 Nobel Prize in Chemistry for the discovery.
Discovery of nuclear fission | Introduction
Frisch conducted the experiment on 13 February 1939, and found the pulses caused by the reaction just as they had predicted. He decided he needed a name for the newly discovered nuclear process. He spoke to William A. Arnold, an American biologist working with de Hevesy and asked him what biologists called the process by which living cells divided into two cells. Arnold told him that biologists called it fission. Frisch then applied that name to the nuclear process in his paper. Frisch mailed both the jointly-authored note on fission and his paper on the recoil experiment to "Nature" on 16 January 1939; the former appeared in print on 11 February and the latter on 18 February.
Discovery of nuclear fission | Eureka!
The discovery of nuclear fission occurred in 1938 in the buildings of Kaiser Wilhelm Society for Chemistry, today part of the Free University of Berlin, following over four decades of work on the science of radioactivity and the elaboration of new nuclear physics that described the components of atoms. In 1911, Ernest 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). Niels Bohr improved upon this in 1913 by reconciling the quantum behavior of electrons (the Bohr model). Work by Henri Becquerel, Marie Curie, Pierre Curie, and Rutherford further elaborated that the nucleus, though tightly bound, could undergo different forms of radioactive decay, and thereby transmute into other elements.
Nuclear fission | Discovery of nuclear fission
However, not everyone was convinced by Fermi's analysis of his results. Ida Noddack suggested that instead of creating a new, heavier 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.
Discovery of nuclear fission | Introduction
It is important to note that efforts to understand fission processes are still an ongoing and have been a very difficult problem to solve since fission was first discovered by Otto Hahn and Fritz Strassmann in 1938. While nuclear physicists understand many aspects of the fission process, there is currently no encompassing theoretical framework that gives a satisfactory account of the basic observations.
Fission barrier | Introduction
As research on nuclear fission progressed in early 1939, Edwin McMillan at the Berkeley Radiation Laboratory of the University of California, Berkeley decided to run an experiment bombarding uranium using the powerful 60-inch (1.52 m) cyclotron that had recently been built at the university. The purpose was to separate the various fission products produced by the bombardment by exploiting the enormous force that the fragments gain from their mutual electrical repulsion after fissioning. Although he did not discover anything of note from this, McMillan did observe two new beta decay half-lives in the uranium trioxide target itself, which meant that whatever was producing the radioactivity had not violently repelled each other like normal fission products.
Neptunium | Discovery
Niels Bohr improved upon this in 1913 by reconciling it with the quantum behaviour of electrons (the Bohr model).
Discovery of nuclear fission | Radioactivity
Just a few weeks before Cockcroft and Walton's feat, another scientist at the Cavendish Laboratory, James Chadwick, discovered the neutron, using an ingenious device made with sealing wax, through the reaction of beryllium with alpha particles: Irène Curie and Frédéric Joliot irradiated aluminium foil with alpha particles, they found that this results in a short-lived radioactive isotope of phosphorus with a half-life of around three minutes: which then decays to a stable isotope of silicon 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.
Discovery of nuclear fission | Transmutation
The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932. Two months later, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. In December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slow neutrons, and discovered that barium had been produced. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Frisch, proved that the uranium nucleus had been split. They published their finding in "Nature" in 1939. This phenomenon was a new type of nuclear disintegration, and was more powerful than any seen before.
MAUD Committee | The discovery of fission
The feat was popularly known as "splitting the atom", and would win them the 1951 Nobel Prize in Physics for ""Transmutation of atomic nuclei by artificially accelerated atomic particles"", although it was not the nuclear fission reaction later discovered in heavy elements. After English physicist James Chadwick discovered the neutron in 1932, Enrico Fermi and his colleagues in Rome studied the results of bombarding uranium with neutrons in 1934. Fermi concluded that his experiments had created new elements with 93 and 94 protons, which the group dubbed ausonium and hesperium. However, not all were convinced by Fermi's analysis of his results, though he would win 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".
Nuclear fission | Discovery of nuclear fission
Frisch conducted the experiment on 13 February, and found the pulses caused by the reaction just as they had predicted. He decided he needed a name for the newly discovered nuclear process. He spoke to William A. Arnold, an American biologist working with de Hevesy, and asked him what biologists called the process by which living cells divided into two cells. Arnold told him that biologists called it fission. Frisch then applied that name to the nuclear process in his paper. Frisch mailed both papers to "Nature" on 16 January; the jointly-authored note appeared in print on 11 February and Frisch's paper on recoil on 18 February.
Lise Meitner | Nuclear fission
Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found Herbert L. Anderson. Bohr grabbed him by the shoulder and said: “Young man, let me explain to you about something new and exciting in physics.” It was clear to a number of scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University team conducted the first nuclear fission experiment in the United States, which was done in the basement of Pupin Hall.
Nuclear fission | Discovery of nuclear fission
It was therefore overlooked as just another fission product. Another year passed before McMillan and Philip Abelson determined that the 2-day half-life element was that of the elusive element 93, which they named "neptunium". They paved the way for the discovery by Glenn Seaborg, Emilio Segrè and Joseph W. Kennedy of element 94, which they named "plutonium" in 1941.
Discovery of nuclear fission | Further research
After James Chadwick discovered the neutron in 1932, Irène Curie and Frédéric Joliot irradiated aluminium foil with alpha particles, they found that this results in a short-lived radioactive isotope of phosphorus. They noted that positron emission continued after the neutron emissions ceased. Not only had they discovered a new form of radioactive decay, 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 heavy elements, but extended to the entire periodic table. Chadwick noted that being electrically neutral, neutrons could penetrate the atomic nucleus more easily than protons or alpha particles.
Otto Hahn | Discovery of nuclear fission
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 another case of the nuclear isomerism that Hahn had discovered in protactinium. She 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."
Otto Hahn | Discovery of nuclear fission
For more details of the original discovery of nuclear fission see the work of Otto Hahn.
Nuclear chemistry | Study of nuclear reactions
The German chemist Ida Noddack notably suggested in print in 1934 that instead of creating a new, heavier element 93, that "it is conceivable that the nucleus breaks up into several large fragments." However, Noddack's conclusion was not pursued at the time. After the Fermi publication, Otto Hahn, Lise Meitner, and Fritz Strassmann began performing similar experiments in Berlin. Meitner, an Austrian Jew, lost her Austrian citizenship with the "Anschluss", the union of Austria with Germany in March 1938, but she fled in July 1938 to Sweden and started a correspondence by mail with Hahn in Berlin. By coincidence, her nephew Otto Robert Frisch, also a refugee, was also in Sweden when Meitner received a letter from Hahn dated 19 December describing his chemical proof that some of the product of the bombardment of uranium with neutrons was barium.
Nuclear fission | Discovery of nuclear fission
Hahn suggested a "bursting" of the nucleus, but he was unsure of what the physical basis for the results were. Barium had an atomic mass 40% less than uranium, and no previously known methods of radioactive decay could account for such a large difference in the mass of the nucleus. Frisch was skeptical, but Meitner trusted Hahn's ability as a chemist. Marie Curie had been separating barium from radium for many years, and the techniques were well-known. Meitner and Frisch then correctly interpreted Hahn's results to mean that the nucleus of uranium had split roughly in half. Frisch suggested the process be named "nuclear fission", by analogy to the process of living cell division into two cells, which was then called binary fission.
Nuclear fission | Discovery of nuclear fission
The experiment involved placing uranium oxide inside of an ionization chamber and irradiating it with neutrons, and measuring the energy thus released. The results confirmed that fission was occurring and hinted strongly that it was the isotope uranium 235 in particular that was fissioning. The next day, the Fifth Washington Conference on Theoretical Physics began in Washington, D.C. under the joint auspices of the George Washington University and the Carnegie Institution of Washington. There, the news on nuclear fission was spread even further, which fostered many more experimental demonstrations.
Nuclear fission | Discovery of nuclear fission
The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932, and in April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. In December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slow neutrons, and discovered that barium had been produced. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Frisch, determined that the uranium nucleus had been split, a conclusion they published in "Nature" in 1939. By analogy with the division of biological cells, they named the process "fission".
Nuclear weapons of the United Kingdom | Tube Alloys
News of the discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation by Lise Meitner and Otto Frisch, was brought to the United States by Niels Bohr. Based on his liquid drop model of the nucleus, he theorized that it was the uranium-235 isotope and not the more abundant uranium-238 that was primarily responsible for fission with thermal neutrons. To verify this Alfred O. C. Nier at the University of Minnesota used a mass spectrometer to create a microscopic amount of enriched uranium-235 in April 1940. John R. Dunning, Aristid von Grosse and Eugene T.
Calutron | Origins
During the Christmas holiday in 1938 he visited his aunt Lise Meitner in Kungälv. While there she received the news that Otto Hahn and Fritz Strassmann in Berlin had discovered that the collision of a neutron with a uranium nucleus produced the element barium as one of its byproducts. Hahn, in a letter to Meitner, called this new reaction a "bursting" of the uranium nucleus. Frisch and Meitner hypothesized that the uranium nucleus had split in two, explained the process, and estimated the energy released, and Frisch coined the term fission to describe it. Political restraints of the Nazi era forced the team of Hahn and that of Frisch and Meitner (both of whom were Jewish) to publish separately.
Otto Robert Frisch | Nuclear fission
A team led by Enrico Fermi in 1934 observed that bombarding uranium with neutrons produces the emission of beta rays (electrons or positrons from the elements produced; see beta particle). The fission products were at first mistaken for new elements with atomic numbers 93 and 94, which the Dean of the Faculty of Rome, Orso Mario Corbino, christened "ausonium" and "hesperium", respectively. The experiments leading to the discovery of uranium's ability to fission (break apart) into lighter elements and release binding energy were conducted by Otto Hahn and Fritz Strassmann in Hahn's laboratory in Berlin. Lise Meitner and her nephew, the physicist Otto Robert Frisch, published the physical explanation in February 1939 and named the process "nuclear fission".
Uranium | Fission research
Patrick 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: This 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. 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; as it was not the result of initiating an internal radioactive decay process.
Discovery of nuclear fission | Transmutation
Hahn therefore became the sole recipient of the 1944 Nobel Prize in Chemistry "for his discovery of the fission of heavy nuclei".
Discovery of nuclear fission | Nobel Prize
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. 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.
Discovery of nuclear fission | Protactinium
A fossil natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions have occurred. This can be examined by analysis of isotope ratios. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda. The phenomenon was discovered in 1972 in Oklo, Gabon by French physicist Francis Perrin under conditions very similar to what was predicted. Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions are thought to have taken place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging probably less than 100 kW of thermal power during that time.
Natural nuclear fission reactor | Introduction
The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932. In April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. Enrico Fermi and his team in Rome conducted experiments involving the bombardment of elements by slow neutrons, which produced heavier elements and isotopes. Then, in December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slowed neutrons, and discovered that barium had been produced, and therefore that the uranium atomic nucleus had been split. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Robert Frisch, developed a theoretical justification which they published in "Nature" in 1939.
Tube Alloys | Discovery of fission
Frisch also experimentally verified the fission reaction by means of a cloud chamber, confirming the energy release. Therefore, Noddack's original hypothesis was finally accepted.
Ida Noddack | Nuclear fission
This was based on Meitner's 1937 measurements of the neutron capture cross-sections. This would be experimentally verified in February 1940, after Alfred Nier was able to produce sufficient pure uranium-235 for John R. Dunning, Aristid von Grosse and Eugene T. Booth to test. Other scientists resumed the search for the elusive element 93, which seemed to be straightforward, as they now knew it resulted from the 23-minute half-life. At the Radiation Laboratory in Berkeley, California, Emilio Segrè and Edwin McMillan used the cyclotron to create the isotope. They then detected a beta activity with a 2-day half-life, but it had rare-earth element chemical characteristics, and element 93 was supposed to have chemistry akin to rhenium.
Discovery of nuclear fission | Further research
The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932, and in April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. In December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slow neutrons, and discovered that barium had been produced, and therefore that the uranium nucleus had been split. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Robert Frisch, developed a theoretical explanation of the process. By analogy with the division of biological cells, they named the process "fission".
British hydrogen bomb programme | Tube Alloys
In December 1938, German chemist Otto Hahn and his assistant Fritz Strassmann sent a manuscript to the German science journal "Naturwissenschaften" ("Natural Sciences") reporting that they had detected and identified the element barium after bombarding uranium with neutrons. Their article was published on 6 January 1939. On 19 December 1938, eighteen days before the publication, Otto Hahn communicated these results and his conclusion of a "bursting" of the uranium nucleus in a letter to his colleague and friend Lise Meitner, who had fled Germany in July to the Netherlands and then to Sweden. Meitner and her nephew Otto Robert Frisch confirmed Hahn's conclusion of a "bursting" and correctly interpreted the results as "nuclear fission" – a term coined by Frisch.
German nuclear weapons program | Discovery of nuclear fission
The first information about the atomic nucleus was obtained at the beginning of the 20th century by studying radioactivity. For a long period of time only three kinds of nuclear decay modes (alpha, beta, and gamma) were known. They illustrate three of the fundamental interactions in nature: strong, weak, and electromagnetic. Spontaneous fission became better studied soon after its discovery in 1940 by Konstantin Petrzhak and Georgy Flyorov because of both the military and the peaceful applications induced fission. This was discovered circa 1939 by Otto Hahn, Lise Meitner, and Fritz Strassmann. There are many other kinds of radioactivity, e.g.
Cluster decay | History
Following the discovery of nuclear fission in uranium by Otto Hahn and Fritz Strassmann in 1939, McMillan began experimenting with uranium. He bombarded it with neutrons produced in the Radiation Laboratory's cyclotron through bombarding beryllium with deuterons. In addition to the nuclear fission products reported by Hahn and Strassmann, they detected two unusual radioactive isotopes, one with a half-life of about 2.3 days, and the other with one of around 23 minutes. McMillan identified the short-lived isotope as uranium-239, which had been reported by Hahn and Strassmann. McMillan suspected that the other was an isotope of a new, undiscovered element, with an atomic number of 93.
Edwin McMillan | Discovery of neptunium
Further work by Enrico Fermi in the 1930s focused on using slow neutrons to increase the effectiveness of induced radioactivity. Experiments bombarding uranium with neutrons led Fermi to believe he had created a new transuranic element, which was dubbed hesperium. In 1938, German chemists Otto Hahn and Fritz Strassmann, along with Austrian physicist Lise Meitner and Meitner's nephew, Otto Robert Frisch, conducted experiments with the products of neutron-bombarded uranium, as a means of further investigating Fermi's claims. They determined that the relatively tiny neutron split the nucleus of the massive uranium atoms into two roughly equal pieces, contradicting Fermi. This was an extremely surprising result: all other forms of nuclear decay involved only small changes to the mass of the nucleus, whereas this process—dubbed "fission" as a reference to biology—involved a complete rupture of the nucleus.
Nuclear power | Origins
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.
Discovery of nuclear fission | Research
(For example, by alpha decay: the emission of an alpha particle—two protons and two neutrons bound together into a particle identical to a helium nucleus.) Some work in nuclear transmutation had been done. In 1917, Rutherford was able to accomplish transmutation of nitrogen into oxygen, using alpha particles directed at nitrogen N + α → O + p. This 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. Eventually, in 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleagues Ernest Walton and John Cockcroft, who used artificially accelerated protons against lithium-7, to split this nucleus into two alpha particles.
Nuclear fission | Discovery of nuclear fission
All the nuclear reactions that had been documented involved chipping protons or alpha particles from the nucleus. Breaking it up seemed far more difficult. However the liquid drop model that Gamow had postulated suggested the possibility that an atomic nucleus could become elongated and overcome the surface tension that held it together. According to Frisch: Meitner and Frisch had correctly interpreted Hahn's results to mean that the nucleus of uranium had split roughly in half. The first two reactions that the Berlin group had observed were light elements created by the breakup of uranium nuclei; the third, the 23-minute one, was a decay into the real element 93.
Discovery of nuclear fission | Eureka!
Just as the term nuclear "chain reaction" would later be borrowed from chemistry, so the term "fission" was borrowed from biology. News spread quickly of the new discovery, which was correctly seen as an entirely novel physical effect with great scientific—and potentially practical—possibilities. Meitner's and Frisch's interpretation of the discovery of Hahn and Strassmann crossed the Atlantic Ocean with Niels Bohr, who was to lecture at Princeton University. I.I. Rabi and Willis Lamb, two Columbia University physicists working at Princeton, heard the news and carried it back to Columbia. Rabi said he told Enrico Fermi; Fermi gave credit to Lamb.
Nuclear fission | Discovery of nuclear fission
From there, the news on nuclear fission spread even further, which fostered many more experimental demonstrations. Bohr and Wheeler overhauled the liquid drop model to explain the mechanism of nuclear fission, with conspicuous success. Their paper appeared in "Physical Review" on 1 September 1939, the day Germany invaded Poland, starting World War II in Europe. As the experimental physicists studied fission, they uncovered more puzzling results. George Placzek (who had measured the slow neutron absorption of gold in 1934 using Bohr's Nobel Prize medal) asked Bohr why uranium fissioned with both very fast and very slow neutrons. Walking to a meeting with Wheeler, Bohr had an insight that the fission at low energies was due to the uranium-235 isotope, while at high energies it was mainly due to the far more abundant uranium-238 isotope.
Discovery of nuclear fission | Further research
Then there was the third reaction, an (n, γ) one, which occurred only with slow neutrons. 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." 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". However, thorium was not easier to work with than uranium. For a start, it had a decay product, radiothorium () that overwhelmed weaker neutron-induced activity.
Discovery of nuclear fission | Research
During the Christmas holiday in 1938, Frisch visited his aunt Lise Meitner in Kungälv in Sweden, where she had relocated after Germany's annexation of Austria. While there she received the news that her former colleagues Otto Hahn and Fritz Strassmann in Berlin had discovered that the collision of a neutron with a uranium nucleus produced barium as one of its byproducts. Frisch and Meitner hypothesized that the uranium nucleus had split in two. They estimated the energy released at around 200 MeV, and Frisch appropriated the term fission from biology to describe it. Hahn's paper described the experiment and the finding of the barium byproduct.
Frisch–Peierls memorandum | Nuclear fission
Further investigations into this uranium deposit discovered uranium ore with a concentration as low as 0.44%. Subsequent examination of isotopes of fission products such as neodymium and ruthenium also showed anomalies, as described in more detail below. This loss in is exactly what happens in a nuclear reactor. A possible explanation, therefore, was that the uranium ore had operated as a natural fission reactor. Other observations led to the same conclusion, and on September 25, 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.
Natural nuclear fission reactor | History
Hahn's paper described the experiment and the finding of the barium byproduct. Meitner's and Frisch's paper explained the physics behind the phenomenon. Frisch went back to Copenhagen, where he was quickly able to isolate the pieces produced by fission reactions. As Frisch himself later recalled, a fundamental idea of the direct experimental proof of the nuclear fission was suggested to him by George Placzek. Many feel that Meitner and Frisch deserved Nobel Prize recognition for their contributions to understanding fission.
Otto Robert Frisch | Nuclear fission
It was clear to many scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University group conducted the first nuclear fission experiment in the United States, which was done in the basement of Pupin Hall. The experiment involved placing uranium oxide inside of an ionization chamber and irradiating it with neutrons, and measuring the energy thus released. The next day, the Fifth Washington Conference on Theoretical Physics began in Washington, D.C., under the joint auspices of The George Washington University and the Carnegie Institution of Washington.
Discovery of nuclear fission | Further research
With the discovery of the neutron, scientists could at last calculate what fraction of binding energy each nucleus had, by comparing the nuclear mass with that of the protons and neutrons which composed it. Differences between nuclear masses were calculated in this way. When nuclear reactions were measured, these were found to agree with Einstein's calculation of the equivalence of mass and energy to within 1% as of 1934.
Nuclear physics | James Chadwick discovers the neutron
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". However, not everyone was convinced by Fermi's analysis of his results. Ida Noddack suggested in September 1934 that instead of creating a new, heavier element 93, that: Noddack's article was read by Fermi's team in Rome, Curie and Joliot in Paris, and Meitner and Hahn in Berlin. However, the quoted objection comes some distance down, and is but one of several gaps she noted in Fermi's claim.
Discovery of nuclear fission | Objections
The discovery of fission led to the creation of both nuclear power and weapons by the end of World War II. Both the proton and the neutron were presumed to be elementary particles until the 1960s, when they were determined to be composite particles built from quarks.
Discovery of the neutron | Introduction
Meitner's and Frisch's paper explained the physics behind the phenomenon. Frisch went back to Copenhagen, where he was able to isolate the fragments produced by fission reactions. Frisch later recalled that: The news of the discovery of fission was brought to America by Bohr in January 1939. Bohr and John A. Wheeler set to work applying the liquid drop model developed by Bohr and Fritz Kalckar to explain the mechanism of nuclear fission. George Placzek, who was skeptical about the whole idea of fission, challenged Bohr to explain why uranium seemed to fission with both very fast and very slow neutrons.
Frisch–Peierls memorandum | Nuclear fission
The discovery of nuclear fission in uranium by German chemists Otto Hahn and Fritz Strassmann in December 1938, and its theoretical explanation (and naming) by Lise Meitner and Otto Frisch soon after, opened up the possibility that neutrons produced by fission could create a controlled nuclear chain reaction. At Columbia University, Enrico Fermi and Leo Szilard began exploring how this might be achieved. In August 1939, Szilard drafted a confidential letter to the President of the United States, Franklin D. Roosevelt, warning of the possibility of a German nuclear weapon project, and convinced his old friend and collaborator Albert Einstein to co-sign it.
Metallurgical Laboratory | Origins
But Hahn and Meitner had a sample from which they had regularly removed its mother isotope, mesothorium (), 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 heavy element, and for which Meitner once again had to postulate multiple isomers. They did find an interesting result: these (n, α) decay series occurred simultaneously when the energy of the incident neutrons was less than 2.5 MeV; when they had more, an (n, γ) reaction that formed was favoured.
Discovery of nuclear fission | Research
He filed a patent for his idea of a simple nuclear reactor the following year. The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation (and naming) by their collaborators Lise Meitner and Otto Frisch, opened up the possibility of creating a nuclear chain reaction with uranium or indium, but initial experiments were unsuccessful. In order for a chain reaction to occur, fissioning uranium atoms had to emit additional neutrons to keep the reaction going. At Columbia University in New York, Italian physicist, Enrico Fermi, with Americans John Dunning, Herbert L.
Chicago Pile-1 | Origins
Meitner received the letter from Hahn describing his chemical proof that some of the product of the bombardment of uranium with neutrons was barium. Barium had an atomic mass 40% less than uranium, and no previously known methods of radioactive decay could account for such a large difference in the mass of the nucleus. Nonetheless, she had immediately written back to Hahn to say: "At the moment the assumption of such a thoroughgoing breakup seems very difficult to me, but in nuclear physics we have experienced so many surprises, that one cannot unconditionally say: 'It is impossible.'" Meitner felt that Hahn was too careful a chemist to make an elementary blunder, but found the results difficult to explain.
Discovery of nuclear fission | Eureka!
The discovery of nuclear fission at the end of 1938 marked a shift in the centers of nuclear research from Europe to the United States. Large numbers of scientists were migrating to the United States to escape the troubles and antisemitism in Europe and the looming war (See Jewish scientists and the Manhattan Project). The new centers of nuclear research were the universities in the United States, particularly Columbia University in New York and the University of Chicago where Enrico Fermi had relocated, and a secret research facility at Los Alamos, New Mexico, established in 1942, the new home of the Manhattan project.
Discovery of the neutron | After 1939
Frisch and Meitner calculated that the energy released by each disintegration was approximately . By analogy with the division of biological cells, they named the process "fission". Niels Bohr and John A. Wheeler applied the liquid drop model developed by Bohr and Fritz Kalckar to explain the mechanism of nuclear fission. Bohr had an epiphany that the fission at low energies was principally due to the uranium-235 isotope, while at high energies it was mainly due to the more abundant uranium-238 isotope. The former makes up just 0.7% of natural uranium, while the latter accounts for 99.3%. Frédéric Joliot-Curie and his Paris colleagues Hans von Halban and Lew Kowarski raised the possibility of a nuclear chain reaction in a paper published in "Nature" in April 1939.
MAUD Committee | The discovery of fission
Enrico Fermi and his colleagues in Rome picked up on this idea, and began irradiating elements with neutrons. The radioactive displacement law of Fajans and Soddy said that beta decay causes isotopes to move one element up on the periodic table, and alpha decay causes them to move two down. When Fermi's group bombarded uranium atoms with neutrons, they found a complex mix of half lives. Fermi therefore concluded that the new elements with atomic numbers greater than 92 (known as transuranium elements) had been created. Meitner and Hahn had not collaborated for many years, but Meitner was eager to investigate Fermi's results.
Otto Hahn | Discovery of nuclear fission
Noddack's idea of nuclear fission was not confirmed until much later. Experiments along a similar line to Fermi's, by Irène Joliot-Curie, Frédéric Joliot-Curie and Pavle Savić in 1938 raised what they called "interpretational difficulties" when the supposed transuranics exhibited the properties of rare earths rather than those of adjacent elements. Ultimately on December 17, 1938, Otto Hahn and Fritz Strassmann provided chemical proof that the previously presumed transuranic elements were isotopes of barium, and Hahn wrote these exciting results to his exiled colleague Lise Meitner, explaining the process as a 'bursting' of the uranium nucleus into lighter elements. Meitner and Otto Frisch utilized Fritz Kalckar and Niels Bohr's liquid drop hypothesis (first proposed by George Gamow in 1935) to provide a first theoretical model and mathematical proof of what Frisch coined nuclear fission.
Ida Noddack | Nuclear fission
Nuclear fission, the creation of a nuclear chain reaction in uranium, was discovered in 1939 following experiments by Otto Hahn and Fritz Strassman, and the interpretation of their results by physicists such as Lise Meitner and Otto Frisch. Shortly thereafter, word of the discovery spread throughout the international physics community. In order for the fission process to chain react, the neutrons created by uranium fission must be slowed down by interacting with a neutron moderator (an element with a low atomic weight, that will "bounce", when hit by a neutron) before they will be captured by other uranium atoms. By late 1939, it became well known that the two most promising moderators were heavy water and graphite.
Nuclear graphite | History
By 1908, the process of alpha decay was known to consist of the ejection of helium nuclei from the decaying atom; however, as with cluster decay, alpha decay is not typically categorized as a process of fission. The first nuclear fission process discovered was fission induced by neutrons. Because cosmic rays produce some neutrons, it was difficult to distinguish between induced and spontaneous events. Cosmic rays can be reliably shielded by a thick layer of rock or water. Spontaneous fission was identified in 1940 by Soviet physicists Georgy Flyorov and Konstantin Petrzhak by their observations of uranium in the Moscow Metro Dinamo station, underground.
Spontaneous fission | History
The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, followed by its theoretical explanation (and naming) by Lise Meitner and Otto Frisch, opened up the possibility of a controlled nuclear chain reaction with uranium. At Columbia University, Enrico Fermi and Leo Szilard began exploring how this might be done. Szilard drafted a confidential letter to the President of the United States, Franklin D. Roosevelt, explaining the possibility of atomic bombs, and warning of the danger of a German nuclear weapon project. He convinced his old friend and collaborator Albert Einstein to co-sign it, lending his fame to the proposal.
X-10 Graphite Reactor | Origins
However in the history of nuclear weapons, the development of fission weapons was never strongly hindered by basic design questions except in the very first nuclear weapons programs.
Nth Country Experiment | Introduction
Numerous scientists, including Leó Szilárd, who was one of the first, recognized that if fission reactions released additional neutrons, a self-sustaining nuclear chain reaction could result. Once this was experimentally confirmed and announced by Frédéric Joliot-Curie in 1939, scientists in many countries (including the United States, the United Kingdom, France, Germany, and the Soviet Union) petitioned their governments for support of nuclear fission research, just on the cusp of World War II, for the development of a nuclear weapon.
Nuclear power | Origins
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. Noddack was not the only critic of Fermi's claim. Aristid von Grosse suggested that what Fermi had found was an isotope of protactinium. 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.
Discovery of nuclear fission | Objections
The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932. In April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. Then, in December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slowed neutrons, and discovered that barium had been produced. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Frisch, explained that the uranium nucleus had been split. By analogy with the division of biological cells, they named the process "fission". The discovery of fission raised the possibility that an extremely powerful atomic bomb could be created.
Quebec Agreement | Tube Alloys
She gave the phenomenon the name "radioactivity". Along with Pierre Curie and Gustave 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.
Discovery of nuclear fission | Radioactivity
Frisch confirmed this experimentally on 13 January 1939.
German nuclear weapons program | Discovery of nuclear fission
Since these elements comprise a large percentage of fission products, Segrè and McMillan decided that the half-life must have been simply another fission product, titling the paper "An Unsuccessful Search for Transuranium Elements". However, as more information about fission became available, the possibility that the fragments of nuclear fission could still have been present in the target became more remote. McMillan and several scientists, including Philip H. Abelson, attempted again to determine what was producing the unknown half-life. In early 1940, McMillan realized that his 1939 experiment with Segrè had failed to test the chemical reactions of the radioactive source with sufficient rigor.
Neptunium | Discovery
Meitner, and her nephew Otto Robert Frisch, Discovery of fission}}correctly interpreted these results as being nuclear fission. Frisch confirmed this experimentally on 13 January 1939. Droste and Siegfried Flügge, an assistant to Hahn, independently also predicted a large energy release from nuclear fission.
Gottfried von Droste | Career
Enrico Fermi and his colleagues studied the results of bombarding uranium with neutrons in 1934. That year, Ida Noddack first mentioned the concept of nuclear fission. In December 1938, four years after the Fermi publication, Lise Meitner and Otto Robert Frisch correctly interpreted Otto Hahn and Fritz Strassmann's radiochemical experimental results as evidence of nuclear fission. News of the discovery spread quickly among physicists and it was realized that if chain reactions could be controlled, fission might be a new source of great power. What was needed was a substance which could moderate the energy of the secondary neutrons emitted by fission, so they could be captured by other fissile nuclei.
Norwegian heavy water sabotage | Background
Noddack correctly criticized Enrico Fermi's chemical proofs in his 1934 neutron bombardment experiments, from which he postulated that transuranic elements might have been produced. This theory was widely accepted for a few years. However, Noddack's paper "On Element 93" suggested a number of possibilities, but centered on Fermi's failure to chemically eliminate "all" lighter than uranium elements in his proofs, rather than only down to lead. The paper is considered historically significant today not simply because she correctly pointed out the flaw in Fermi's chemical proof but because she suggested the possibility that "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 neighbors of the irradiated element." In doing so she presaged what would become known a few years later as nuclear fission.
Ida Noddack | Nuclear fission
They acknowledged 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.
Discovery of nuclear fission | Protactinium
The neutron was discovered in 1932 by British physicist James Chadwick. The concept of a nuclear chain reaction brought about by nuclear reactions mediated by neutrons was first realized shortly thereafter, by Hungarian scientist Leó Szilárd, in 1933. He filed a patent for his idea of a simple reactor the following year while working at the Admiralty in London. However, Szilárd's idea did not incorporate the idea of nuclear fission as a neutron source, since that process was not yet discovered. Szilárd's ideas for nuclear reactors using neutron-mediated nuclear chain reactions in light elements proved unworkable. Inspiration for a new type of reactor using uranium came from the discovery by Lise Meitner, Fritz Strassmann and Otto Hahn in 1938 that bombardment of uranium with neutrons (provided by an alpha-on-beryllium fusion reaction, a "neutron howitzer") produced a barium residue, which they reasoned was created by the fissioning of the uranium nuclei.
Nuclear reactor | Early reactors
Ernest Rutherford commented in the article that inefficiencies in the process precluded use of it for power generation. However, the neutron had been discovered in 1932, shortly before, as the product of a nuclear reaction. Szilárd, who had been trained as an engineer and physicist, put the two nuclear experimental results together in his mind and realized that if a nuclear reaction produced neutrons, which then caused further similar nuclear reactions, the process might be a self-perpetuating nuclear chain-reaction, spontaneously producing new isotopes and power without the need for protons or an accelerator. Szilárd, however, did not propose fission as the mechanism for his chain reaction, since the fission reaction was not yet discovered, or even suspected.
Nuclear chain reaction | History
The Berlin group started by irradiating uranium salt with neutrons from a radon-beryllium source similar to the one that Fermi had used. They dissolved it 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 having 13- and 90-minute half-lives, and these were detected in the precipitate. 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.
Discovery of nuclear fission | Research
The chemists then turned to techniques for removing light element impurities from the plutonium; but on 14 July 1944, Oppenheimer informed Kennedy that this would no longer be required. The notion of spontaneous fission had been raised by Niels Bohr and John Archibald Wheeler in their 1939 treatment of the mechanism of nuclear fission. The first attempt to discover spontaneous fission in uranium was made by Willard Libby, but he failed to detect it. It had been observed in Britain by Frisch and Titterton, and independently in the Soviet Union by Georgy Flyorov and Konstantin Petrzhak in 1940; the latter are generally credited with the discovery.
Project Y | Plutonium
On returning to Copenhagen, Frisch informed Bohr, who slapped his forehead and exclaimed "What idiots we have been!" Bohr promised not to say anything until they had a paper ready for publication. To speed the process, they decided to submit a one-page note to "Nature". At this point, the only evidence that they had was the barium. Logically, if barium was formed, the other element must be krypton, although Hahn mistakenly believed that the atomic masses had to add up to 239 rather than the atomic numbers adding up to 92, and thought it was masurium (technetium), and so did not check for it: Over a series of long-distance phone calls, Meitner and Frisch came up with a simple experiment to bolster their claim: to measure the recoil of the fission fragments, using a Geiger counter with the threshold set above that of the alpha particles.
Discovery of nuclear fission | Eureka!
Throughout the 1920s, the nucleus was viewed as composed of combinations of protons and electrons, the two elementary particles known at the time, but that model presented several experimental and theoretical contradictions. The essential nature of the atomic nucleus was established with the discovery of the neutron by James Chadwick in 1932 and the determination that it was a new elementary particle, distinct from the proton. The uncharged neutron was immediately exploited as a new means to probe nuclear structure, leading to such discoveries as the creation of new radioactive elements by neutron irradiation (1934) and the fission of uranium atoms by neutrons (1938).
Discovery of the neutron | Introduction
In 1935, Strassmann became an assistant on half pay. Soon he would be credited as a collaborator on the papers they produced.
Discovery of nuclear fission | Objections
He added a footnote to this effect to his Nobel Prize acceptance speech. Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found Herbert L. Anderson. Bohr grabbed him by the shoulder and said: "Young man, let me explain to you about something new and exciting in physics."
Discovery of nuclear fission | Bohr brings the news to the United States
Ida Noddack (25 February 1896 – 24 September 1978), "née" Tacke, was a German chemist and physicist. In 1934 she was the first to mention the idea later named nuclear fission. With her husband - Walter Noddack - and Otto Berg she discovered element 75, rhenium. She was nominated three times for the Nobel Prize in Chemistry.
Ida Noddack | Introduction
Photofission is a process in which a nucleus, after absorbing a gamma ray, undergoes nuclear fission and splits into two or more fragments. The reaction was discovered in 1940 by a small team of engineers and scientists operating the Westinghouse Atom Smasher at the company's Research Laboratories in Forest Hills, Pennsylvania. They used a 5 MeV proton beam to bombard fluorine and generate high-energy photons, which then irradiated samples of uranium and thorium. Gamma radiation of modest energies, in the low tens of MeV, can induce fission in traditionally fissile elements such as the actinides thorium, uranium, plutonium, and neptunium.
Photofission | Introduction
Instead, Szilárd proposed using mixtures of lighter known isotopes which produced neutrons in copious amounts. He filed a patent for his idea of a simple nuclear reactor the following year. In 1936, Szilárd attempted to create a chain reaction using beryllium and indium, but was unsuccessful. Nuclear fission was discovered by Otto Hahn and Fritz Strassmann in December 1938 and explained theoretically in January 1939 by Lise Meitner and her nephew Otto Robert Frisch. A few months later, Frédéric Joliot-Curie, H. Von Halban and L. Kowarski in Paris searched for, and discovered, neutron multiplication in uranium, proving that a nuclear chain reaction by this mechanism was indeed possible.
Nuclear chain reaction | History
Nuclear fission is the reverse process to fusion. For nuclei heavier than nickel-62 the binding energy per nucleon decreases with the mass number. It is therefore possible for energy to be released if a heavy nucleus breaks apart into two lighter ones. The process of alpha decay is in essence a special type of spontaneous nuclear fission. It is a highly asymmetrical fission because the four particles which make up the alpha particle are especially tightly bound to each other, making production of this nucleus in fission particularly likely. From certain of the heaviest nuclei whose fission produces free neutrons, and which also easily absorb neutrons to initiate fission, a self-igniting type of neutron-initiated fission can be obtained, in a chain reaction.
Nuclear physics | Nuclear fission
In 1938, the German chemist Otto Hahn, a student of Rutherford, directed neutrons onto uranium atoms expecting to get transuranium elements. Instead, his chemical experiments showed barium as a product. A year later, Lise Meitner and her nephew Otto Frisch verified that Hahn's result were the first experimental "nuclear fission". In 1944, Hahn received the Nobel Prize in Chemistry. Despite Hahn's efforts, the contributions of Meitner and Frisch were not recognized. In the 1950s, the development of improved particle accelerators and particle detectors allowed scientists to study the impacts of atoms moving at high energies. Neutrons and protons were found to be hadrons, or composites of smaller particles called quarks.
Atom | Fission, high-energy physics and condensed matter
By then, Frederick Soddy's researches of radioactive elements, and experiments of J. J. Thomson and F.W. Aston conclusively demonstrated existence of isotopes, whose nuclei have different masses in spite of identical atomic numbers. It prompted Rutherford to conjecture that all nuclei other than hydrogen contain chargeless particles, which he named the neutron. Evidences that atomic nuclei consist of some smaller particles (now called "nucleons") grew; it became obvious that, while protons repulse each other electrostatically, nucleons attract each other by some new force (nuclear force). It culminated in proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn), and nuclear fusion by Hans Bethe in that same year.
History of subatomic physics | Inside the atom
Otto Hahn and Fritz Strassmann reported the discovery of uranium fission in the January 6, 1939 issue of "Die Naturwissenschaften", and Lise Meitner identified it as nuclear fission in the February 11, 1939 issue of "Nature". This generated intense interest among physicists. Danish physicist Niels Bohr brought the news to the United States, and the U.S. opened the Fifth Washington Conference on Theoretical Physics with Enrico Fermi on January 26, 1939. The results were quickly corroborated by experimental physicists, most notably Fermi and John R. Dunning at Columbia University. Hungarian physicist Leó Szilárd was living in the United States at the time and realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction which could yield vast amounts of energy for electric power generation or atomic bombs.
Einstein–Szilárd letter | Origin
Radiochemistry was now no longer confined to certain heavy elements, but extended to the entire periodic table. Chadwick noted that being electrically neutral, neutrons would be able to penetrate the nucleus more easily than protons or alpha particles. Enrico Fermi and his colleagues in Rome—Edoardo Amaldi, Oscar D'Agostino, Franco Rasetti and Emilio Segrè—picked up on this idea. 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.
Discovery of nuclear fission | Transmutation
Frisch confirmed this experimentally on January 13, 1939. In 1944, Hahn received the Nobel Prize for Chemistry for the discovery of nuclear fission. Some historians have documented the history of the discovery of nuclear fission and believe Meitner should have been awarded the Nobel Prize with Hahn.
Herbert L. Anderson | Education
The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932. In April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. Enrico Fermi and his team in Rome conducted experiments involving the bombardment of elements by slow neutrons, which produced heavier elements and isotopes. Then, in December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slowed neutrons, and discovered that barium had been produced, and therefore that the uranium nucleus had been split. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Frisch, developed a theoretical justification for the process, which they published in "Nature" in 1939.
High Explosive Research | Tube Alloys
They had assumed that almost every collision between a neutron of any energy and a uranium-235 nucleus would produce a fission. The value they used for the fission cross section was that published by French researchers in 1939, but data published by the Americans in the 15 March and 15 April 1940 issues of the American journal "Physical Review" indicated that it was much smaller.
MAUD Committee | University of Liverpool
A mass of fissile material large enough (and in a suitable configuration) to induce a self-sustaining chain reaction is called a critical mass. When a neutron is captured by a suitable nucleus, fission may occur immediately, or the nucleus may persist in an unstable state for a short time. If there are enough immediate decays to carry on the chain reaction, the mass is said to be prompt critical, and the energy release will grow rapidly and uncontrollably, usually leading to an explosion. When discovered on the eve of World War II, this insight led multiple countries to begin programs investigating the possibility of constructing an atomic bomb — a weapon which utilized fission reactions to generate far more energy than could be created with chemical explosives.
Nuclear technology | Nuclear fission
The discovery of the neutron by James Chadwick in 1932, followed by the discovery of nuclear fission by chemists Otto Hahn and Fritz Strassmann in 1938, and its explanation (and naming) by physicists Lise Meitner and Otto Frisch soon after, opened up the possibility of a controlled nuclear chain reaction using uranium. At the time, few scientists in the United States thought that an atomic bomb was practical, but the possibility that a German atomic bomb project would develop atomic weapons concerned refugee scientists from Nazi Germany and other fascist countries, leading to the drafting of the Einstein–Szilard letter to warn President Franklin D.
Project Y | Nuclear fission and atomic bombs
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. The 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. She returned to the Kaiser Wilhelm Institute in October 1916, when not only Hahn, but most of the students, laboratory assistants and technicians had been called up.
Discovery of nuclear fission | Protactinium
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. In 1898, Ernest Rutherford noted that thorium gave off a radioactive gas. In examining the radiation, he classified Becquerel radiation into two types, which he called α (alpha) and β (beta) radiation. 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.
Discovery of nuclear fission | Radioactivity
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. 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. In 1903, Soddy and Margaret Todd applied the term "isotope" to atoms that were chemically and spectroscopically indistinct but had different radioactive half-lives. 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).
Discovery of nuclear fission | Radioactivity
It was apparent to many scientists that, in theory at least, an extremely powerful explosive could be created, although most still considered an atomic bomb an impossibility. The term was already familiar to the British public through the writings of H. G. Wells, in his 1913 novel "The World Set Free".
MAUD Committee | The discovery of fission
Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found Anderson. Bohr grabbed him by the shoulder and said: "Young man, let me explain to you about something new and exciting in physics." It was clear to a number of scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, Glasoe was a member of the experimental team at Columbia University which conducted the first nuclear fission experiment in the United States, which was done in the basement of Pupin Hall; the other members of the team were Herbert L.
G. N. Glasoe | Career