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Nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...
was discovered in December 1938 by chemists
Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist who was a pioneer in the fields of radioactivity and radiochemistry. He is referred to as the father of nuclear chemistry and father of nuclear fission. Hahn and Lise Meitner ...
and
Fritz Strassmann Friedrich Wilhelm Strassmann (; 22 February 1902 – 22 April 1980) was a German chemist who, with Otto Hahn in December 1938, identified the element barium as a product of the bombardment of uranium with neutrons. Their observation was the ke ...
and physicists Lise Meitner and
Otto Robert Frisch Otto Robert Frisch FRS (1 October 1904 – 22 September 1979) was an Austrian-born British physicist who worked on nuclear physics. With Lise Meitner he advanced the first theoretical explanation of nuclear fission (coining the term) and firs ...
. Fission is a
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a transformatio ...
or
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
process in which the
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
of an
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...
splits into two or more smaller, lighter nuclei and often other particles. The fission process often produces
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
s and releases a very large amount of energy, even by the energetic standards of radioactive decay. Scientists already knew about
alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atom ...
and
beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
, but fission assumed great importance because the discovery that a
nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...
was possible led to the development of
nuclear power Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced b ...
and
nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...
s. Hahn was awarded the 1944
Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
for the discovery of nuclear fission. Hahn and Strassmann at the
Kaiser Wilhelm Institute for Chemistry The Kaiser Wilhelm Society for the Advancement of Science (German: ''Kaiser-Wilhelm-Gesellschaft zur Förderung der Wissenschaften'') was a German scientific institution established in the German Empire in 1911. Its functions were taken over by ...
in
Berlin Berlin ( , ) is the capital and largest city of Germany by both area and population. Its 3.7 million inhabitants make it the European Union's most populous city, according to population within city limits. One of Germany's sixteen constitue ...
bombarded
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
with slow
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
s and discovered that
barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...
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 Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic country located on ...
, who a few months earlier had fled
Nazi Germany Nazi Germany (lit. "National Socialist State"), ' (lit. "Nazi State") for short; also ' (lit. "National Socialist Germany") (officially known as the German Reich from 1933 until 1943, and the Greater German Reich from 1943 to 1945) was ...
. Meitner and her nephew Frisch theorised, and then proved, that the uranium nucleus had been split and published their findings in ''
Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...
''. Meitner calculated that the energy released by each disintegration was approximately 200
megaelectronvolt In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. ...
s, 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 Sir James Chadwick, (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932. In 1941, he wrote the final draft of the MAUD Report, which inspi ...
in 1932 created a new means of
nuclear transmutation Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed. A transmutatio ...
.
Enrico Fermi Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...
and his colleagues in
Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus (legendary) , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg , map_caption ...
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 Ausenium ( atomic symbol Ao) was the name assigned to the element with atomic number 93, now known as neptunium. It was named after a Greek name of Italy, Ausonia. The same team assigned the name hesperium to element 94, after Hesperia, a poeti ...
and
hesperium Hesperium (or esperium; atomic symbol Es) was the name assigned to the element with atomic number 94, now known as plutonium. It was named in Italian ''Esperio'' after a Greek name of Italy, Hesperia, "the land of the West". The same team assigned ...
. Fermi won the 1938
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
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 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 discover ...
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 Aristid von Grosse (January 1905 – July 21, 1985) was a German nuclear chemist. During his work with Otto Hahn, he got access to waste material from radium production, and with this starting material he was able in 1927 to isolate protact ...
suggested that what Fermi's group had found was an isotope of
protactinium Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds ...
. This spurred Hahn and Meitner, the discoverers of the most stable
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
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 Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
. Their work overturned long-held beliefs in physics and paved the way for the discovery of the real elements 93 (
neptunium Neptunium is a chemical element with the Symbol (chemistry), symbol Np and atomic number 93. A radioactivity, radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after ...
) and 94 (
plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
), for the discovery of fission in other elements, and for the determination of the role of the
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...
isotope in that of uranium.
Niels Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. B ...
and John Wheeler reworked the
liquid drop model In nuclear physics, the semi-empirical mass formula (SEMF) (sometimes also called the Weizsäcker formula, Bethe–Weizsäcker formula, or Bethe–Weizsäcker mass formula to distinguish it from the Bethe–Weizsäcker process) is used to approxim ...
to explain the mechanism of fission.


Background


Radioactivity

In the last years of the 19th century, scientists frequently experimented with the
cathode-ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms (oscilloscope), pictur ...
, which by then had become a standard piece of laboratory equipment. A common practice was to aim the
cathode ray Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to ele ...
s at various substances and to see what happened.
Wilhelm Röntgen Wilhelm Conrad Röntgen (; ; 27 March 184510 February 1923) was a German mechanical engineer and physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range known as X-rays or Röntgen rays, an achiev ...
had a screen coated with
barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...
platinocyanide Platinocyanide, also known as tetracyanoplatinate (IUPAC), cyanoplatinate, or platinocyanate, is a polyatomic ion with the molecular formula t(CN)4sup>2−. The name also applies to compounds containing this ion, which are salts of the hypothetical ...
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-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s. The following year
Henri Becquerel Antoine Henri Becquerel (; 15 December 1852 – 25 August 1908) was a French engineer, physicist, Nobel laureate, and the first person to discover evidence of radioactivity. For work in this field he, along with Marie Skłodowska-Curie and P ...
was experimenting with fluorescent
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
salts, and wondered if they too might produce X-rays. 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. Unlike 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.
Marie Curie Marie Salomea Skłodowska–Curie ( , , ; born Maria Salomea Skłodowska, ; 7 November 1867 – 4 July 1934) was a Polish and naturalized-French physicist and chemist who conducted pioneering research on radioactivity. She was the first ...
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 Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...
. She gave the phenomenon the name "radioactivity". Along with
Pierre Curie Pierre Curie ( , ; 15 May 1859 – 19 April 1906) was a French physicist, a pioneer in crystallography, magnetism, piezoelectricity, and radioactivity. In 1903, he received the Nobel Prize in Physics with his wife, Marie Curie, and Henri Becqu ...
and Gustave Bémont, she began investigating
pitchblende Uraninite, formerly pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2 but because of oxidation typically contains variable proportions of U3O8. Radioactive decay of the uranium causes t ...
, 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 Bismuth is a chemical element with the Symbol (chemistry), symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental ...
, 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 Polonium is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character ...
". 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 Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rather t ...
". 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 Radium chloride (RaCl2) is a salt of radium and chlorine, and the first radium compound isolated in a pure state. Marie Curie and André-Louis Debierne used it in their original separation of radium from barium. The first preparation of radium me ...
, and they never did manage to isolate polonium. In 1898,
Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand physicist who came to be known as the father of nuclear physics. ''Encyclopædia Britannica'' considers him to be the greatest ...
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 Paul Ulrich Villard (28 September 1860 – 13 January 1934) was a French chemist and physicist. He discovered gamma rays in 1900 while studying the radiation emanating from radium. Early research Villard was born in Saint-Germain-au-Mon ...
discovered a third type of Becquerel radiation which, following Rutherford's scheme, were called "
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
s", and Curie noted that radium also produced a radioactive gas. Identifying the gas chemically proved frustrating; Rutherford and
Frederick Soddy Frederick Soddy FRS (2 September 1877 – 22 September 1956) was an English radiochemist who explained, with Ernest Rutherford, that radioactivity is due to the transmutation of elements, now known to involve nuclear reactions. He also prov ...
found it to be inert, much like
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
. It later came to be known as
radon Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through ...
. Rutherford identified beta rays as cathode rays (electrons), and hypothesised—and in 1909 with
Thomas Royds Thomas Royds (April 11, 1884 – May 1, 1955) was a British solar physicist who worked with Ernest Rutherford on the identification of alpha radiation as the nucleus of the helium atom, and who was Director of the Kodaikanal Solar Observatory, I ...
proved—that alpha particles were
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
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 Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
. In 1903, Soddy and Margaret Todd applied the term "
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
" to atoms that were chemically and spectroscopically identical but had different radioactive half-lives. Rutherford proposed a model of the
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...
in which a very small, dense and positively charged
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
of
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s was surrounded by orbiting, negatively charged electrons (the
Rutherford model The Rutherford model was devised by the New Zealand-born physicist Ernest Rutherford to describe an atom. Rutherford directed the Geiger–Marsden experiment in 1909, which suggested, upon Rutherford's 1911 analysis, that J. J. Thomson's plum ...
).
Niels Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. B ...
improved upon this in 1913 by reconciling it with the
quantum In physics, a quantum (plural quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantizati ...
behaviour of electrons (the
Bohr model In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar Syste ...
).


Protactinium

Soddy and
Kasimir Fajans Kazimierz Fajans (Kasimir Fajans in many American publications; 27 May 1887 – 18 May 1975) was a Polish American physical chemist of Polish-Jewish origin, a pioneer in the science of radioactivity and the discoverer of chemical element protact ...
independently observed in 1913 that alpha decay caused atoms to shift down two places in the
periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...
, 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 Actinium is a chemical element with the symbol Ac and atomic number 89. It was first isolated by Friedrich Oskar Giesel in 1902, who gave it the name ''emanium''; the element got its name by being wrongly identified with a substance And ...
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 Dmitri Ivanovich Mendeleev (sometimes transliterated as Mendeleyev or Mendeleef) ( ; russian: links=no, Дмитрий Иванович Менделеев, tr. , ; 8 February Old_Style_and_New_Style_dates">O.S._27_January.html" ;"title="O ...
) as "ekatantalium", would be an alpha emitter with chemical properties similar to tantalium (now known as
tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that is ...
). It was not long before Fajans and
Oswald Helmuth Göhring Oswald Helmuth Göhring, also known as Otto Göhring, (1889 - 1915) was a German chemist who, with his teacher Kasimir Fajans, co-discovered the chemical element protactinium in 1913. Discovery of protactinium Protactinium was first identified ...
discovered it as a decay product of a beta-emitting product of thorium. Based on the
radioactive displacement law of Fajans and Soddy The law of radioactive displacements, also known as Fajans's and Soddy's law, in radiochemistry and nuclear physics, is a rule governing the Nuclear transmutation, transmutation of elements during radioactive decay. It is named after Frederick Sod ...
, 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 Dahlem ( or ) is a locality of the Steglitz-Zehlendorf borough in southwestern Berlin. Until Berlin's 2001 administrative reform it was a part of the former borough of Zehlendorf. It is located between the mansion settlements of Grunewald and L ...
took up the challenge of finding the missing isotope.
Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist who was a pioneer in the fields of radioactivity and radiochemistry. He is referred to as the father of nuclear chemistry and father of nuclear fission. Hahn and Lise Meitner ...
had graduated from the
University of Marburg The Philipps University of Marburg (german: Philipps-Universität Marburg) was founded in 1527 by Philip I, Landgrave of Hesse, which makes it one of Germany's oldest universities and the oldest still operating Protestant university in the wor ...
as an organic chemist, but had been a post-doctoral researcher at
University College London , mottoeng = Let all come who by merit deserve the most reward , established = , type = Public research university , endowment = £143 million (2020) , budget = ...
under Sir
William Ramsay Sir William Ramsay (; 2 October 1852 – 23 July 1916) was a Scottish chemist who discovered the noble gases and received the Nobel Prize in Chemistry in 1904 "in recognition of his services in the discovery of the inert gaseous elements ...
, and under Rutherford at
McGill University McGill University (french: link=no, Université McGill) is an English-language public research university located in Montreal, Quebec, Canada. Founded in 1821 by royal charter granted by King George IV,Frost, Stanley Brice. ''McGill Universit ...
, where he had studied radioactive isotopes. In 1906, he returned to Germany, where he became an assistant to
Emil Fischer Hermann Emil Louis Fischer (; 9 October 1852 – 15 July 1919) was a German chemist and 1902 recipient of the Nobel Prize in Chemistry. He discovered the Fischer esterification. He also developed the Fischer projection, a symbolic way of dra ...
at the
University of Berlin Humboldt-Universität zu Berlin (german: Humboldt-Universität zu Berlin, abbreviated HU Berlin) is a German public research university in the central borough of Mitte in Berlin. It was established by Frederick William III on the initiative o ...
. 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 The University of Vienna (german: Universität Wien) is a public research university located in Vienna, Austria. It was founded by Duke Rudolph IV in 1365 and is the oldest university in the German-speaking world. With its long and rich histor ...
in 1906, and had then moved to Berlin to study physics under
Max Planck Max Karl Ernst Ludwig Planck (, ; 23 April 1858 – 4 October 1947) was a German theoretical physicist whose discovery of energy quanta won him the Nobel Prize in Physics in 1918. Planck made many substantial contributions to theoretical p ...
at the Friedrich-Wilhelms-Universität. Meitner found Hahn, who was her own age, less intimidating than older, more distinguished colleagues. 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. 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 World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
in 1914. Hahn was called up into the German Army, and Meitner became a volunteer
radiographer Radiographers, also known as radiologic technologists, diagnostic radiographers and medical radiation technologists are healthcare professionals who specialize in the imaging of human anatomy for the diagnosis and treatment of pathology. Radi ...
in Austrian Army hospitals. 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 travelling between the western and eastern fronts, Berlin and Leverkusen between the summer of 1914 and late 1916. Most of the students, laboratory assistants and technicians had been called up, so Hahn, who was stationed in Berlin between January and September of 1917, 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 . 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 Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds ...
, 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 Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The decay ...
decayed into protactinium. It remained unsolved until
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...
was discovered in 1929. 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. In 1949, the International Union of Pure and Applied Chemistry (
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
) named the new element definitively protactinium, and confirmed Hahn and Meitner as discoverers.


Transmutation

Patrick Blackett Patrick Maynard Stuart Blackett, Baron Blackett (18 November 1897 – 13 July 1974) was a British experimental physicist known for his work on cloud chambers, cosmic rays, and paleomagnetism, winning the Nobel Prize for Physics in 1948. ...
was able to accomplish
nuclear transmutation Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed. A transmutatio ...
of nitrogen into oxygen in 1925, using alpha particles directed at nitrogen. In modern notation for the atomic nuclei, the reaction was: : + → + p This was the first observation of a
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a transformatio ...
, 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 Ernest Thomas Sinton Walton (6 October 1903 – 25 June 1995) was an Irish physicist and Nobel laureate. He is best known for his work with John Cockcroft to construct one of the earliest types of particle accelerator, the Cockcroft–Walton g ...
and
John Cockcroft Sir John Douglas Cockcroft, (27 May 1897 – 18 September 1967) was a British physicist who shared with Ernest Walton the Nobel Prize in Physics in 1951 for splitting the atomic nucleus, and was instrumental in the development of nuclea ...
, who used artificially accelerated protons against
lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...
, to break this nucleus into two alpha particles. The feat was popularly known as "splitting the atom", but was not
nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...
; as it was not the result of initiating an internal
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
process. Just a few weeks before Cockcroft and Walton's feat, another scientist at the
Cavendish Laboratory The Cavendish Laboratory is the Department of Physics at the University of Cambridge, and is part of the School of Physical Sciences. The laboratory was opened in 1874 on the New Museums Site as a laboratory for experimental physics and is named ...
,
James Chadwick Sir James Chadwick, (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932. In 1941, he wrote the final draft of the MAUD Report, which inspi ...
, discovered the
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
, using an ingenious device made with
sealing wax Sealing wax is a wax material of a seal which, after melting, hardens quickly (to paper, parchment, ribbons and wire, and other material) forming a bond that is difficult to separate without noticeable tampering. Wax is used to verify something su ...
, through the reaction of
beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form mi ...
with alpha particles: : + → + n Irène Curie and
Frédéric Joliot Frédéric and Frédérick are the French versions of the common male given name Frederick. They may refer to: In artistry: * Frédéric Back, Canadian award-winning animator * Frédéric Bartholdi, French sculptor * Frédéric Bazille, Impress ...
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: : + → + n which then decays to a stable isotope of
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
: → + 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 Positron emission, beta plus decay, or β+ decay is a subtype of radioactive decay called beta decay, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino (). Positron ...
, 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 would be able to penetrate the nucleus more easily than protons or alpha particles.
Enrico Fermi Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...
and his colleagues in
Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus (legendary) , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg , map_caption ...
Edoardo Amaldi Edoardo Amaldi (5 September 1908 – 5 December 1989) was an Italian physicist. He coined the term "neutrino" in conversations with Enrico Fermi distinguishing it from the heavier "neutron". He has been described as "one of the leading nuclear ...
,
Oscar D'Agostino Oscar D'Agostino (29 August 1901 – 16 March 1975) was an Italian chemist and one of the so-called ''Via Panisperna boys'', the group of young scientists led by Enrico Fermi: all of them were physicists, except for D'Agostino, who was a chemist. ...
,
Franco Rasetti Franco Dino Rasetti (August 10, 1901 – December 5, 2001) was an Italian (later naturalized American) physicist, paleontologist and botanist. Together with Enrico Fermi, he discovered key processes leading to nuclear fission. Rasetti refused ...
and
Emilio Segrè Emilio Gino Segrè (1 February 1905 – 22 April 1989) was an Italian-American physicist and Nobel laureate, who discovered the elements technetium and astatine, and the antiproton, a subatomic antiparticle, for which he was awarded the Nobel ...
—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 counter A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental ph ...
s and a
cloud chamber A cloud chamber, also known as a Wilson cloud chamber, is a particle detector used for visualizing the passage of ionizing radiation. A cloud chamber consists of a sealed environment containing a supersaturated vapour of water or alcohol. ...
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 havoc 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. Working in assembly-line fashion, they started by irradiating water, and then progressed up the periodic table through lithium, beryllium,
boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
and
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
, without inducing any radioactivity. When they got to
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
and then
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
, they had their first successes. Induced radioactivity was ultimately found through the neutron bombardment of 22 different elements. 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. When a new copy of ''La Ricerca Scientifica'' arrived at the Niels Bohr's Institute for Theoretical Physics at the
University of Copenhagen The University of Copenhagen ( da, Københavns Universitet, KU) is a prestigious public university, public research university in Copenhagen, Copenhagen, Denmark. Founded in 1479, the University of Copenhagen is the second-oldest university in ...
, 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 The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous sil ...
, but at Bohr's institute
George de Hevesy George Charles de Hevesy (born György Bischitz; hu, Hevesy György Károly; german: Georg Karl von Hevesy; 1 August 1885 – 5 July 1966) was a Hungarian radiochemist and Nobel Prize in Chemistry laureate, recognized in 1943 for his key role ...
had a complete set of their oxides that had been given to him by Auergesellschaft, so de Hevesy and
Hilde Levi Hilde Levi (9 May 1909 – 26 July 2003) was a German-Danish physicist. She was a pioneer of the use of radioactive isotopes in biology and medicine, notably the techniques of radiocarbon dating and autoradiography. In later life she bec ...
carried out the process with them. 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. What they observed was a complex mixture of half-lives. Following the displacement law, they checked for the presence of
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
, bismuth, radium, actinium, thorium and protactinium (skipping the elements whose chemical properties were unknown), and (correctly) found no indication of any of them. Fermi noted three types of reactions were caused by neutron irradiation: emission of an alpha particle (n, α);
proton emission Proton emission (also known as proton radioactivity) is a rare type of radioactive decay in which a proton is ejected from a nucleus. Proton emission can occur from high-lying excited states in a nucleus following a beta decay, in which case t ...
(n, p); and gamma emission (n, γ). Invariably, the new isotopes decayed by beta emission, which caused elements to move up the periodic table. Based on the periodic table of the time, Fermi believed that element 93 was ekarhenium—the element below rhenium—with characteristics similar to
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
and
rhenium Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...
. Such an element was found, and Fermi tentatively concluded that his experiments had created new elements with 93 and 94 protons, which he dubbed
ausonium Ausenium (atomic symbol Ao) was the name assigned to the element with atomic number 93, now known as neptunium. It was named after a Greek name of Italy, Ausonia (disambiguation), Ausonia. The same team assigned the name hesperium to element 94, ...
and
hesperium Hesperium (or esperium; atomic symbol Es) was the name assigned to the element with atomic number 94, now known as plutonium. It was named in Italian ''Esperio'' after a Greek name of Italy, Hesperia, "the land of the West". The same team assigned ...
. The results were published in ''
Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...
'' in June 1934. However, in this paper Fermi cautioned that "a careful search for such heavy 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." In retrospect, what they had detected was indeed an unknown rhenium-like element,
technetium Technetium is a chemical element with the symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. All available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous ...
, which lies between manganese and rhenium on the periodic table.
Leo Szilard Leo Szilard (; hu, Szilárd Leó, pronounced ; born Leó Spitz; February 11, 1898 – May 30, 1964) was a Hungarian-German-American physicist and inventor. He conceived the nuclear chain reaction in 1933, patented the idea of a nuclear ...
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 heavy 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. 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 Coulomb barrier, named after Coulomb's law, which is in turn named after physicist Charles-Augustin de Coulomb, is the energy barrier due to electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo a ...
; 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 Paraffin wax (or petroleum wax) is a soft colorless solid derived from petroleum, coal, or oil shale that consists of a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms. It is solid at room temperature and begins to m ...
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. 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. The current model of the nucleus in 1934 was the
liquid drop model In nuclear physics, the semi-empirical mass formula (SEMF) (sometimes also called the Weizsäcker formula, Bethe–Weizsäcker formula, or Bethe–Weizsäcker mass formula to distinguish it from the Bethe–Weizsäcker process) is used to approxim ...
first proposed by
George Gamow George Gamow (March 4, 1904 – August 19, 1968), born Georgiy Antonovich Gamov ( uk, Георгій Антонович Гамов, russian: Георгий Антонович Гамов), was a Russian-born Soviet and American polymath, theoreti ...
in 1930. His simple and elegant model was refined and developed by
Carl Friedrich von Weizsäcker Carl Friedrich Freiherr von Weizsäcker (; 28 June 1912 – 28 April 2007) was a German physicist and philosopher. He was the longest-living member of the team which performed nuclear research in Germany during the Second World War, under ...
and, after the discovery of the neutron, by
Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent series ...
in 1935 and Niels Bohr in 1936, it agreed closely with observations. In the model, the
nucleon In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons were ...
s were held together in the smallest possible volume (a sphere) by the
strong nuclear force The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the n ...
, 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, 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.


Discovery


Objections

Fermi won the 1938
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
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 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 discover ...
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. 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. Noddack and her husband,
Walter Noddack Walter Noddack (17 August 1893 – 7 December 1960) was a German chemist. He, Ida Tacke (who later married Noddack), and Otto Berg reported the discovery of element 43 and element 75 in 1925. Rhenium They named element 75 rhenium (Latin ''Rhe ...
, were renowned chemists who had been nominated for the
Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
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 Carlo Perrier (born July 7, 1886 in Turin , † May 22, 1948 in Genoa ) was an Italian mineralogist and chemist who did extensive research on the element technetium. With the discovery of technetium in 1937, he and Emilio Segrè Emilio Gino S ...
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, but Meitner was not afraid to tell Hahn ''Hähnchen, von Physik verstehst Du Nichts'' ("Hahn dear, of physics you understand nothing"). 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 Aristid von Grosse (January 1905 – July 21, 1985) was a German nuclear chemist. During his work with Otto Hahn, he got access to waste material from radium production, and with this starting material he was able in 1927 to isolate protact ...
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. Initially, Hahn was not interested, but von Grosse's mention of protactinium changed his mind. "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, having been the discoverers of protactinium." Hahn and Meitner were joined by
Fritz Strassmann Friedrich Wilhelm Strassmann (; 22 February 1902 – 22 April 1980) was a German chemist who, with Otto Hahn in December 1938, identified the element barium as a product of the bombardment of uranium with neutrons. Their observation was the ke ...
. Strassmann had received his doctorate in analytical chemistry from the
Technical University of Hannover Gottfried Wilhelm Leibniz University Hannover (german: Gottfried Wilhelm Leibniz Universität), also known as the University of Hannover, is a public research university located in Hanover, Germany. Founded on 2 May 1831 as Higher Vocational Sc ...
in 1929, 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 The Nazi Party, officially the National Socialist German Workers' Party (german: Nationalsozialistische Deutsche Arbeiterpartei or NSDAP), was a far-right politics, far-right political party in Germany active between 1920 and 1945 that crea ...
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 The German Chemical Society (German: ', GDCh) is a learned society and professional association founded in 1949 to represent the interests of German chemists in local, national and international contexts. GDCh "brings together people working in che ...
when it became part of the Nazi
German Labour Front The German Labour Front (german: Deutsche Arbeitsfront, ; DAF) was the labour organisation under the Nazi Party which replaced the various independent trade unions in Germany during Adolf Hitler's rise to power. History As early as March 1933, ...
. As a result, he could neither work in the chemical industry nor receive his
habilitation Habilitation is the highest university degree, or the procedure by which it is achieved, in many European countries. The candidate fulfills a university's set criteria of excellence in research, teaching and further education, usually including a ...
, 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. The 1933
Law for the Restoration of the Professional Civil Service The Law for the Restoration of the Professional Hitler Service (german: Gesetz zur Wiederherstellung des Berufsbeamtentums, shortened to ''Berufsbeamtengesetz''), also known as Civil Service Law, Civil Service Restoration Act, and Law to Re-es ...
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. 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.
Carl Bosch Carl Bosch (; 27 August 1874 – 26 April 1940) was a German chemist and engineer and Nobel Laureate in Chemistry. He was a pioneer in the field of high-pressure industrial chemistry and founder of IG Farben, at one point the world's largest ...
, the director of
IG Farben Interessengemeinschaft Farbenindustrie AG (), commonly known as IG Farben (German for 'IG Dyestuffs'), was a German chemical and pharmaceutical conglomerate (company), conglomerate. Formed in 1925 from a merger of six chemical companies—BASF, ...
, a major sponsor of the Kaiser Wilhelm Institute for Chemistry, assured Meitner that her position there was safe, and she agreed to stay. Meitner, Hahn and Strassmann drew closer together personally as their anti-Nazi politics increasingly alienated them from the rest of the organisation, but it gave them more time for research, as administration was devolved to Hahn's and Meitner's assistants.


Research

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 Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
perrhenate The perrhenate ion is the anion with the formula , or a compound containing this ion. The perrhenate anion is tetrahedral, being similar in size and shape to perchlorate and the valence isoelectronicity, isoelectronic permanganate. The perrhenate a ...
, platinum chloride and
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...
. What remained was then acidified with
hydrogen sulphide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
, 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. Moreover, the chemical reactions involved ruled out all elements from
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
and above on the periodic table. 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. 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. 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. 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.") Meitner was increasingly uncertain. They had now constructed three (n, γ) reactions: # + n → (10 seconds) → (2.2 minutes) → (59 minutes) → (66 hours) → (2.5 hours) → (?) # + n → (40 seconds) → (16 minutes) → (5.7 hours) → (?) # + n → (23 minutes) → Meitner 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 In nuclear physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. The neutron cross section σ can be defined as the area in cm2 for which the number of ...
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 isomer A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state, higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited ...
ism, 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. 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 Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the gene ...
() that overwhelmed weaker neutron-induced activity. 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 In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...
; when they had more, an (n, γ) reaction that formed was favoured. In Paris, Irene Curie and Pavel Savitch had also set out to replicate Fermi's findings. In collaboration with
Hans von Halban Hans Heinrich von Halban (24 January 1908 – 28 November 1964) was a French physicist, of Austrian-Jewish descent. Family He was descended on his father's side from Polish Jews, who left Kraków for Vienna in the 1850s. His grandfather, Heinr ...
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 Savitch detected a radioactive substance with a 3.5-hour half-life. 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. 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 Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lantha ...
(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 Yttrium is a chemical element with the symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost always found in com ...
, which is chemically similar.) By using Geiger counters and skipping the chemical precipitation, Curie and Savitch detected the 3.5-hour half-life in irradiated uranium. With the ''
Anschluss The (, or , ), also known as the (, en, Annexation of Austria), was the annexation of the Federal State of Austria into the German Reich on 13 March 1938. The idea of an (a united Austria and Germany that would form a " Greater Germany ...
'', Germany's unification with Austria on 12 March 1938, Meitner lost her Austrian citizenship.
James Franck James Franck (; 26 August 1882 – 21 May 1964) was a German physicist who won the 1925 Nobel Prize for Physics with Gustav Hertz "for their discovery of the laws governing the impact of an electron upon an atom". He completed his doctorate in ...
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. On 13 July 1938, Meitner departed for the Netherlands with Dutch physicist
Dirk Coster Dirk Coster (October 5, 1889 – February 12, 1950), was a Dutch physicist. He was a Professor of Physics and Meteorology at the University of Groningen. Coster was born in Amsterdam. On February 26, 1919, he married Lina Maria Wijsman, who ...
. Before she left, Otto Hahn gave 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 Adriaan Daniël Fokker (; 17 August 1887 – 24 September 1972) was a Dutch physicist. He worked in the fields of special relativity and statistical mechanics. He was the inventor of the Fokker organ, a 31-tone equal-tempered (31-TET) organ. ...
, she flew to Copenhagen, where she was greeted by Frisch, and stayed with Niels and
Margrethe Bohr Margrethe Nørlund Bohr (7 March 1890–21 December 1984) was the Danish wife of and collaborator, editor and transcriber for physicist Niels Bohr who received the Nobel Prize. She also influenced her son, Nobel Prize winner Aage Bohr. Biograph ...
at their holiday house in
Tisvilde Tisvilde is a small town with a population of 1,444 (1 January 2022) Stockholm Stockholm () is the Capital city, capital and List of urban areas in Sweden by population, largest city of Sweden as well as the List of urban areas in the Nordic countries, largest urban area in Scandinavia. Approximately 980,000 people liv ...
, where she was met by
Eva von Bahr Eva Elisabet von Bahr (born 17 December 1968) is a Swedish make-up artist and hair stylist. She is best known for her works in '' Arn: The Knight Templar'' (2007), ''The Girl with the Dragon Tattoo'' (2011), '' The 100-Year-Old Man Who Climbed O ...
.


Interpretation

The Paris group published their results in September 1938. 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: : + n → α + → α + Meitner found this very hard to believe. In November, Hahn travelled 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. Assisted by Clara Lieber and Irmgard Bohne, they 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 Barium bromide is the chemical compound with the formula BaBr2. It is ionic in nature. Structure and properties BaBr2 crystallizes in the lead chloride (cotunnite) motif, giving white orthorhombic crystals that are deliquescent. In aqueous ...
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. On 19 December, Hahn wrote to Meitner, informing her that the radium isotopes behaved chemically like barium. Anxious to finish up before the Christmas break, Hahn and Strassmann submitted their findings to ''Naturwissenschaften'' on 22 December without waiting for Meitner to reply. Hahn understood that a "burst" of the atomic nuclei had occurred, but he was unsure about that interpretation. Hahn concluded the article in "Naturwissenschaften" with: "As chemists... we should substitute the symbols Ba, La, Ce for Ra, Ac, Th. As 'nuclear chemists' fairly close to physics we cannot yet bring ourselves to take this step which contradicts all previous experience in physics." Frisch normally celebrated Christmas with Meitner in Berlin, but in 1938 she accepted an invitation from Eva von Bahr to spend it with her family at
Kungälv Kungälv () (old no, Konghelle) is a city and the seat of Kungälv Municipality in Västra Götaland County, Sweden. It had 22,768 inhabitants in 2010. In 2021, the main Kungälv - Ytterby - Kareby conurbation had a combined population approachin ...
, and Meitner asked Frisch to join her there. 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. 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. 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 Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
, although Hahn mistakenly believed that the
atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&nbs ...
es had to add up to 239 rather than the
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
s adding up to 92, and thought it was masurium (technetium), and so did not check for it: : + n → + + some n 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. Frisch conducted the experiment on 13 January 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. In their second publication on nuclear fission in February 1939, Hahn and Strassmann used the term ''Uranspaltung'' (uranium fission) for the first time, and predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a
nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...
.


Reception


Bohr brings the news to the United States

Before departing for the United States on 7 January 1939 with his son Erik to attend the Fifth Washington Conference on Theoretical Physics, Bohr promised Frisch that he would not mention fission until the papers appeared in print, but during the Atlantic crossing on the , Bohr discussed the mechanism of fission with
Leon Rosenfeld Leon, Léon (French) or León (Spanish) may refer to: Places Europe * León, Spain, capital city of the Province of León * Province of León, Spain * Kingdom of León, an independent state in the Iberian Peninsula from 910 to 1230 and again f ...
, and failed to inform him that the information was confidential. On arrival in
New York City New York, often called New York City or NYC, is the List of United States cities by population, most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the L ...
on 16 January, they were met by Fermi and his wife Laura Capon, and by John Wheeler, who had been a fellow at Bohr's institute in 1934–1935. As it happened, there was a meeting of
Princeton University Princeton University is a private university, private research university in Princeton, New Jersey. Founded in 1746 in Elizabeth, New Jersey, Elizabeth as the College of New Jersey, Princeton is the List of Colonial Colleges, fourth-oldest ins ...
's Physics Journal Club that evening, and when Wheeler asked Rosenfeld if he had any news to report, Rosenfeld told them. An embarrassed Bohr fired off a note to ''Nature'' defending Meitner and Frisch's claim to the priority of the discovery. Hahn was annoyed that while Bohr mentioned his and Strassmann's work in the note, he cited only Meitner and Frisch. News spread quickly of the new discovery, which was correctly seen as an entirely novel physical effect with great scientific—and potentially practical—possibilities.
Isidor Isaac Rabi Isidor Isaac Rabi (; born Israel Isaac Rabi, July 29, 1898 – January 11, 1988) was an American physicist who won the Nobel Prize in Physics in 1944 for his discovery of nuclear magnetic resonance, which is used in magnetic resonance ima ...
and
Willis Lamb Willis Eugene Lamb Jr. (; July 12, 1913 – May 15, 2008) was an American physicist who won the Nobel Prize in Physics in 1955 "for his discoveries concerning the fine structure of the hydrogen spectrum." The Nobel Committee that year awarded hal ...
, two
Columbia University Columbia University (also known as Columbia, and officially as Columbia University in the City of New York) is a private research university in New York City. Established in 1754 as King's College on the grounds of Trinity Church in Manhatt ...
physicists working at Princeton, heard the news and carried it back to Columbia. Rabi said he told Fermi; Fermi gave credit to Lamb. For Fermi, the news came as a profound embarrassment, as the
transuranic elements The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements. ...
that he had partly been awarded the Nobel Prize for discovering had not been transuranic elements at all, but
fission products Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
. 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 Herbert Lawrence Anderson (May 24, 1914 – July 16, 1988) was an American nuclear physicist who was Professor of Physics at the University of Chicago. He contributed to the Manhattan Project. He was also a member of the team which made the firs ...
. Bohr grabbed him by the shoulder and said: "Young man, let me explain to you about something new and exciting in physics."


Further research

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 Pupin Physics Laboratories , also known as Pupin Hall, is home to the physics and astronomy departments of Columbia University in New York City. The building is located on the south side of 120th Street, just east of Broadway. In 1965, Pupin was ...
. The experiment involved placing uranium oxide inside of an
ionization chamber The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...
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. ) , image_skyline = , image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
, under the joint auspices of
The George Washington University , mottoeng = "God is Our Trust" , established = , type = Private federally chartered research university , academic_affiliations = , endowment = $2.8 billion (2022) , preside ...
and the
Carnegie Institution of Washington The Carnegie Institution of Washington (the organization's legal name), known also for public purposes as the Carnegie Institution for Science (CIS), is an organization in the United States established to fund and perform scientific research. Th ...
. 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 ''Physical Review'' is a peer-reviewed scientific journal established in 1893 by Edward Nichols. It publishes original research as well as scientific and literature reviews on all aspects of physics. It is published by the American Physical S ...
'' on 1 September 1939, the day
Germany invaded Poland The invasion of Poland (1 September – 6 October 1939) was a joint attack on the Republic of Poland by Nazi Germany and the Soviet Union which marked the beginning of World War II. The German invasion began on 1 September 1939, one week afte ...
, starting
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
in Europe. As the experimental physicists studied fission, they uncovered more puzzling results.
George Placzek George Placzek (; September 26, 1905 – October 9, 1955) was a Moravian physicist. Biography Placzek was born into a wealthy Jewish family in Brünn, Moravia (now Brno, Czech Republic), the grandson of Chief Rabbi Baruch Placzek.PDF He studied ...
(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 Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it ...
isotope. 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 The Radiation Laboratory, commonly called the Rad Lab, was a microwave and radar research laboratory located at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. It was first created in October 1940 and operated until 31 ...
in
Berkeley, California Berkeley ( ) is a city on the eastern shore of San Francisco Bay in northern Alameda County, California, United States. It is named after the 18th-century Irish bishop and philosopher George Berkeley. It borders the cities of Oakland and Emer ...
, Emilio Segrè and
Edwin McMillan Edwin Mattison McMillan (September 18, 1907 – September 7, 1991) was an American physicist credited with being the first-ever to produce a transuranium element, neptunium. For this, he shared the 1951 Nobel Prize in Chemistry with Glenn Seabor ...
used the
cyclotron A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Janu ...
to create the isotope. They then detected a beta activity with a 2-day half-life, but it had
rare-earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
chemical characteristics, and element 93 was supposed to have chemistry akin to rhenium. It was therefore overlooked as just another fission product. Another year passed before McMillan and
Philip Abelson Philip, also Phillip, is a male given name, derived from the Greek (''Philippos'', lit. "horse-loving" or "fond of horses"), from a compound of (''philos'', "dear", "loved", "loving") and (''hippos'', "horse"). Prominent Philips who popularize ...
determined that the 2-day half-life element was that of the elusive element 93, which they named "
neptunium Neptunium is a chemical element with the Symbol (chemistry), symbol Np and atomic number 93. A radioactivity, radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after ...
". They paved the way for the discovery by
Glenn Seaborg Glenn Theodore Seaborg (; April 19, 1912February 25, 1999) was an American chemist whose involvement in the synthesis, discovery and investigation of ten transuranium elements earned him a share of the 1951 Nobel Prize in Chemistry. His work i ...
, Emilio Segrè and
Joseph W. Kennedy Joseph William Kennedy (May 30, 1916 – May 5, 1957) was an American chemist who was a co-discoverer of plutonium, along with Glenn T. Seaborg, Edwin McMillan and Arthur Wahl. During World War II he was head of the CM (Chemistry and Metallurgy) ...
of element 94, which they named "
plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
" in 1941. Another avenue of research, spearheaded by Meitner, was to determine if other elements could fission after being irradiated with neutrons. It was soon determined that thorium and protactinium could. Measurements were also made of the amount of energy released. Hans von Halban,
Frédéric Joliot-Curie Jean Frédéric Joliot-Curie (; ; 19 March 1900 – 14 August 1958) was a French physicist and husband of Irène Joliot-Curie, with whom he was jointly awarded the Nobel Prize in Chemistry in 1935 for their discovery of Induced radioactivity. T ...
and
Lew Kowarski Lew Kowarski (10 February 1907, Saint Petersburg – 30 July 1979, Geneva) was a naturalized French physicist. He was a lesser-known but important contributor to nuclear science. Early life Lew Kowarski was born in Saint Petersburg to Nicholas K ...
demonstrated that uranium bombarded by neutrons emitted more neutrons than it absorbed, suggesting the possibility of a
nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...
. Fermi and Anderson did so too a few weeks later. It was apparent to many scientists that, in theory at least, an extremely powerful energy source could be created, although most still considered an
atomic bomb A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions (thermonuclear bomb), producing a nuclear explosion. Both bomb ...
an impossibility.


Nobel Prize

Both Hahn and Meitner had been nominated for the chemistry and the physics Nobel Prizes many times even before the discovery of nuclear fission for their work on radioactive isotopes and protactinium. Several more nominations followed for the discovery of fission between 1940 and 1943. Nobel Prize nominations were vetted by committees of five, one for each award. Although both Hahn and Meitner received nominations for physics, radioactivity and radioactive elements had traditionally been seen as the domain of chemistry, and so the
Nobel Committee for Chemistry The Nobel Committee for Chemistry is the Nobel Committee responsible for proposing laureates for the Nobel Prize in Chemistry. The committee received reports from
Theodor Svedberg Theodor Svedberg (30 August 1884 – 25 February 1971) was a Swedish chemist and Nobel laureate for his research on colloids and proteins using the ultracentrifuge. Svedberg was active at Uppsala University from the mid 1900s to late 1940s. Wh ...
in 1941 and in 1942. These chemists were impressed by Hahn's work, but felt that the experimental work of Meitner and Frisch was not extraordinary. They did not understand why the physics community regarded their work as seminal. As for Strassmann, although his name was on the papers, there was a long-standing policy of conferring awards on the most senior scientist in a collaboration. In 1944 the Nobel Committee for Chemistry voted to recommend that Hahn alone be given the
Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
for 1944. However, Germans had been forbidden to accept Nobel Prizes after the
Nobel Peace Prize The Nobel Peace Prize is one of the five Nobel Prizes established by the will of Swedish industrialist, inventor and armaments (military weapons and equipment) manufacturer Alfred Nobel, along with the prizes in Nobel Prize in Chemistry, Chemi ...
had been awarded to
Carl von Ossietzky Carl von Ossietzky (; 3 October 1889 – 4 May 1938) was a German journalist and pacifist. He was the recipient of the 1935 Nobel Peace Prize for his work in exposing the clandestine German re-armament. As editor-in-chief of the magazine ''Die ...
in 1936. The committee's recommendation was rejected by the
Royal Swedish Academy of Sciences The Royal Swedish Academy of Sciences ( sv, Kungliga Vetenskapsakademien) is one of the Swedish Royal Academies, royal academies of Sweden. Founded on 2 June 1739, it is an independent, non-governmental scientific organization that takes special ...
, which decided to defer the award for one year. The war was over when the academy reconsidered the award in September 1945. The Nobel Committee for Chemistry had now become more cautious, as it was apparent that much research had been undertaken by the
Manhattan Project The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
in the United States in secret, and it suggested deferring the 1944 Nobel Prize in Chemistry for another year. The academy was swayed by
Göran Liljestrand Göran Liljestrand (16 April 1886 – 16 January 1968), Swedish pharmacologist, known for the discovery of the Euler-Liljestrand mechanism. Liljestrand was born in Gothenburg but finished school at the Norra Real school in Stockholm, before m ...
, who argued that it was important for the academy to assert its independence from the
Allies of World War II The Allies, formally referred to as the United Nations from 1942, were an international military coalition formed during the Second World War (1939–1945) to oppose the Axis powers, led by Nazi Germany, Imperial Japan, and Fascist Italy. ...
, and award the Nobel Prize in Chemistry to a German, as it had done after World War I when it had awarded it to
Fritz Haber Fritz Haber (; 9 December 186829 January 1934) was a German chemist who received the Nobel Prize in Chemistry in 1918 for his invention of the Haber–Bosch process, a method used in industry to synthesize ammonia from nitrogen gas and hydrogen ...
. Hahn therefore became the sole recipient of the 1944 Nobel Prize in Chemistry "for his discovery of the fission of heavy nuclei". Meitner wrote in a letter to her friend Birgit Broomé-Aminoff on 20 November 1945: In 1946, the
Nobel Committee for Physics The Nobel Committee for Physics is the Nobel Committee responsible for proposing laureates for the Nobel Prize for Physics.
considered nominations for Meitner and Frisch received from
Max von Laue Max Theodor Felix von Laue (; 9 October 1879 – 24 April 1960) was a German physicist who received the Nobel Prize in Physics in 1914 for his discovery of the diffraction of X-rays by crystals. In addition to his scientific endeavors with cont ...
, Niels Bohr, Oskar Klein, Egil Hylleraas and James Franck. Reports were written for the committee by Erik Hulthén, who held the chair of experimental physics at
Stockholm University Stockholm University ( sv, Stockholms universitet) is a public research university in Stockholm, Sweden, founded as a college in 1878, with university status since 1960. With over 33,000 students at four different faculties: law, humanities, so ...
, in 1945 and 1946. Hulthén argued that theoretical physics should be considered award-worthy only if it inspired great experiments. The role of Meitner and Frisch in being the first to understand and explain fission was not understood. There may also have been personal factors: the chairman of the committee,
Manne Siegbahn Karl Manne Georg Siegbahn FRS(For) HFRSE (3 December 1886 – 26 September 1978) was a Swedish physicist who was awarded the Nobel Prize in Physics in 1924 "for his discoveries and research in the field of X-ray spectroscopy". Biography Siegbahn ...
, disliked Meitner, and had a professional rivalry with Klein. Meitner and Frisch would continue to be nominated regularly for many years, but would never be awarded a Nobel Prize.


In history and memory

At the end of the war in Europe, Hahn was taken into custody and incarcerated at
Farm Hall A farm (also called an agricultural holding) is an area of land that is devoted primarily to agricultural processes with the primary objective of producing food and other crops; it is the basic facility in food production. The name is used fo ...
with nine other senior scientists, all of whom except Max von Laue had been involved with the
German nuclear weapons program The Uranverein ( en, "Uranium Club") or Uranprojekt ( en, "Uranium Project") was the name given to the project in Germany to research nuclear technology, including nuclear weapons and nuclear reactors, during World War II. It went through seve ...
, and all except Hahn and
Paul Harteck Paul Karl Maria Harteck (20 July 190222 January 1985) was an Austrian physical chemist. In 1945 under Operation Epsilon in "the big sweep" throughout Germany, Harteck was arrested by the allied British and American Armed Forces for suspicion of ...
were physicists. It was here that they heard the news of the
atomic bombings of Hiroshima and Nagasaki The United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August 1945, respectively. The two bombings killed between 129,000 and 226,000 people, most of whom were civilians, and remain the onl ...
. Unwilling to accept that they were years behind the Americans, and unaware that their conversations were being recorded, they concocted a story that they had never wanted their nuclear weapons program to succeed in the first place on moral grounds. Hahn was still there when his Nobel Prize was announced in November 1945. The Farm Hall scientists would spend the rest of their lives attempting to rehabilitate the image of German science that had been tarnished by the Nazi period. Inconvenient details like the thousands of female slave labourers from
Sachsenhausen concentration camp Sachsenhausen () or Sachsenhausen-Oranienburg was a German Nazi concentration camp in Oranienburg, Germany, used from 1936 until April 1945, shortly before the defeat of Nazi Germany in May later that year. It mainly held political prisoners ...
who mined uranium ore for their experiments were swept under the rug. For Hahn, this necessarily involved asserting his claim of the discovery of fission for himself, for chemistry, and for Germany. He used his Nobel Prize acceptance speech to assert this narrative. Hahn's message resonated strongly in Germany, where he was revered as the proverbial
good German __NOTOC__ ''Good Germans'' is an ironic term — usually placed between single quotes such as 'Good Germans' — referring to German citizens during and after World War II who claimed not to have supported the Nazi regime, but remained silent and ...
, a decent man who had been a staunch opponent of the Nazi regime, but had remained in Germany where he had pursued pure science. As president of the
Max Planck Society The Max Planck Society for the Advancement of Science (german: Max-Planck-Gesellschaft zur Förderung der Wissenschaften e. V.; abbreviated MPG) is a formally independent non-governmental and non-profit association of German research institutes. ...
from 1946 to 1960, he projected an image of German science as undiminished in brilliance and untainted by Nazism to an audience that wanted to believe it. After the Second World War, Hahn came out strongly against the use of nuclear energy for military purposes. He saw the application of his scientific discoveries to such ends as a misuse, or even a crime. Lawrence Badash wrote: "His wartime recognition of the perversion of science for the construction of weapons and his postwar activity in planning the direction of his country's scientific endeavours now inclined him increasingly toward being a spokesman for social responsibility." In contrast, in the immediate aftermath of the war Meitner and Frisch were hailed as the discoverers of fission in English-speaking countries. Japan was seen as a
puppet state A puppet state, puppet régime, puppet government or dummy government, is a State (polity), state that is ''de jure'' independent but ''de facto'' completely dependent upon an outside Power (international relations), power and subject to its o ...
of Germany and the destruction of Hiroshima and Nagasaki as poetic justice for the persecution of the Jewish people. In January 1946, Meitner toured the United States, where she gave lectures and received
honorary degree An honorary degree is an academic degree for which a university (or other degree-awarding institution) has waived all of the usual requirements. It is also known by the Latin phrases ''honoris causa'' ("for the sake of the honour") or ''ad hono ...
s. She attended a cocktail party for
Lieutenant General Lieutenant general (Lt Gen, LTG and similar) is a three-star military rank (NATO code OF-8) used in many countries. The rank traces its origins to the Middle Ages, where the title of lieutenant general was held by the second-in-command on the ...
Leslie Groves Lieutenant General Leslie Richard Groves Jr. (17 August 1896 – 13 July 1970) was a United States Army Corps of Engineers officer who oversaw the construction of the Pentagon and directed the Manhattan Project, a top secret research project ...
, the director of the
Manhattan Project The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
(who gave her sole credit for the discovery of fission in his 1962 memoirs), and was named Woman of the Year by the
Women's National Press Club The National Press Club is a professional organization and social community in Washington, D.C. for journalists and communications professionals. It hosts public and private gatherings with invited speakers from public life. The club also offers e ...
. At the reception for this award, she sat next to the
President of the United States The president of the United States (POTUS) is the head of state and head of government of the United States of America. The president directs the executive branch of the federal government and is the commander-in-chief of the United Stat ...
,
Harry S. Truman Harry S. Truman (May 8, 1884December 26, 1972) was the 33rd president of the United States, serving from 1945 to 1953. A leader of the Democratic Party, he previously served as the 34th vice president from January to April 1945 under Franklin ...
. But Meitner did not enjoy public speaking, especially in English, nor did she relish the role of a celebrity, and she declined the offer of a visiting professorship at
Wellesley College Wellesley College is a private women's liberal arts college in Wellesley, Massachusetts, United States. Founded in 1870 by Henry and Pauline Durant as a female seminary, it is a member of the original Seven Sisters Colleges, an unofficial g ...
. In 1966, the
United States Atomic Energy Commission The United States Atomic Energy Commission (AEC) was an agency of the United States government established after World War II by U.S. Congress to foster and control the peacetime development of atomic science and technology. President H ...
jointly awarded the
Enrico Fermi Prize The Enrico Fermi Prize, first awarded in 2001, is given by the Italian Physical Society (Società Italiana di Fisica). It is a yearly award of €30,000 honoring one or more Members of the Society who have "particularly honoured physics with thei ...
to Hahn, Strassmann and Meitner for their discovery of fission. The ceremony was held in the
Hofburg The Hofburg is the former principal imperial palace of the Habsburg dynasty. Located in the centre of Vienna, it was built in the 13th century and expanded several times afterwards. It also served as the imperial winter residence, as Schönbrunn ...
palace in Vienna. It was the first time that the Enrico Fermi Prize had been awarded to non-Americans, and the first time it was presented to a woman. Meitner's diploma bore the words: "For pioneering research in the naturally occurring radioactivities and extensive experimental studies leading to the discovery of fission". Hahn's diploma was slightly different: "For pioneering research in the naturally occurring radioactivities and extensive experimental studies culminating in the discovery of fission." Hahn and Strassmann were present, but Meitner was too ill to attend, so Frisch accepted the award on her behalf. During combined celebrations in Germany of the 100th birthdays of Einstein, Hahn, Meitner and von Laue in 1978, Hahn's narrative of the discovery of fission began to crumble. Hahn and Meitner had died in 1968, but Strassmann was still alive, and he asserted the importance of his analytical chemistry and Meitner's physics in the discovery, and their role as more than just assistants. A detailed biography of Strassmann appeared in 1981, a year after his death, and a prize-winning one of Meitner for young adults in 1986. Scientists questioned the focus on chemistry, historians challenged the accepted narrative of the Nazi period, and
feminists Feminism is a range of socio-political movements and ideologies that aim to define and establish the political, economic, personal, and social equality of the sexes. Feminism incorporates the position that society prioritizes the male poi ...
saw Meitner as yet another example of the
Matilda effect The Matilda effect is a bias against acknowledging the achievements of women scientists whose work is attributed to their male colleagues. This phenomenon was first described by suffragist and abolitionist Matilda Joslyn Gage (1826–98) in her ...
, where a woman had been airbrushed from the pages of history. By 1990, Meitner had been restored to the narrative, although her role remained contested, particularly in Germany. Weizsäcker, a colleague of Hahn and Meitner during their time in Berlin, and a fellow inmate with Hahn in Farm Hall, strongly supported Hahn's role in the discovery of nuclear fission. He told an audience that had gathered for the ceremonial inclusion of a bust of Meitner in the ''Ehrensaal'' (Hall of Fame) at the ''
Deutsches Museum The Deutsches Museum (''German Museum'', officially (English: ''German Museum of Masterpieces of Science and Technology'')) in Munich, Germany, is the world's largest museum of science and technology, with about 28,000 exhibited objects from ...
'' in Munich on 4 July 1991 that neither Meitner nor physics had contributed to the discovery of fission, which, he declared, was "a discovery of Hahn's and not of Lise Meitner's."


Notes


References

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Further reading

* {{DEFAULTSORT:Discovery of nuclear fission 1938 in science Fission, discovery of Nuclear fission Nuclear chemistry Radioactivity