Chadwick and Meitner: Discoveries that changed the world

Professor Gerry Lander


Speaker: Professor Gerry Lander, Former Director of the Institut Laue-Langevin Grenoble, France

Prof. Lander retired at the end of 2005 as Director of the Institute for Transuranium Elements (an European Commission Laboratory) in Karlsruhe, Germany, and is now living in Grenoble. He has returned to doing science, at least part time!

His main interests are centred on the science of actinide (5f electrons) elements and compounds. He has used neutrons, both elastic and inelastic, and synchrotron x-rays.

He obtained his Ph.D at Cambridge University (with Jane Brown as supervisor!) many years ago and worked for 20 years at Argonne National Laboratory, where for the last 5 years (1981-1986) he was Director of the IPNS source. After that he went to ITU and spent 16 years there doing science before he again fell into administration. He has published over 400 papers, many from the ILL, where he first came on sabbatical in 1976. He has used, and published, from 16 instruments at the ILL, which if not a record might be close!

He was also the founding Editor of Neutron News, and did that job from 1989-2000.

The Institut Laue Langevin is the world’s leading facility in neutron science & technology. Its purpose is to provide the international scientific community with: the brightest possible beams of neutrons, state-of-the-art scientific instruments and the expertise of its scientists, engineers and technicians.

Talk Description

From the discovery of the neutron (1932) to the first demonstration of controlled fission (1942) was just ten years; a period that took physics from an occupation of a small number of eccentric gentlemen and (even fewer) ladies to something of concern to, and funding decisions of, Governments all over the world. The shadows of those tumultuous years are still with us, for better or worse.

This talk will recount those ten years through the lives of James Chadwick (1891-1974) and Lise Meitner (1878-1968), contemporaries who played pivotal roles in the events, even though, partly because of their retiring personalities, they are often over-shadowed by “larger” figures.

My notes from the lecture (if they don’t make sense then it is entirely my fault)

James Chadwick


Sir James Chadwick, CH, FRS (20 October 1891 – 24 July 1974) was a British 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 inspired the U.S. government to begin serious atomic bomb research efforts. He was the head of the British team that worked on the Manhattan Project during the Second World War. He was knighted in Britain in 1945 for his achievements in physics.


The photograph was taken in 1925 at Chadwick’s wedding which gave Rutherford “a real good laugh“. Peter Kapitza (left), in borrowed top hat, is James Chadwick‘s best man. Apparently Chadwick didn’t even smile for his wedding photographs


Lise Meitner (7 November 1878 – 27 October 1968) was an Austrian-Swedish physicist who worked on radioactivity and nuclear physics. Meitner, Otto Hahn and Otto Robert Frisch led the small group of scientists who first discovered nuclear fission of uranium when it absorbed an extra neutron; the results were published in early 1939.

She died just a few days before her 90th birthday

She is buried in Hampshire, UK, next to her brother with a simple gravestone reading:

“Lise Meitner: a physicist who never lost her humanity”



The above image (courtesy of the University of Cambridge Cavendish Laboratory) shows the staff and research students at the Cavendish Laboratory, Laboratory in 1923.Front row left to right: Chadwick, Stead, Aston, Thomson, Rutherford, Crowther, Trevelyan and Taylor. Second row left to right: Kapitza, Smyth, Alty, Crackson, Robinson, Curliss, Bieler, West and Merrier. Back row left to right: Blackett, Clay, Skinner, Griffith, Barton, Bates, Rogers and Emeleus. Nobel Laureates in the image are Thomson, Rutherford, Chadwick, Kapitza and Blackett.

Cambridge, at this time, was the centre of experimental work but the theoretical centre was in Berlin.

Ernest Rutherford, 1st Baron Rutherford of Nelson, OM, FRS, HFRSE (30 August 1871 – 19 October 1937), was a New Zealand-born British physicist who came to be known as the father of nuclear physics. Encyclopædia Britannica considers him to be the greatest experimentalist since Michael Faraday (1791–1867).

Rutherford holds a record of being involved in the most Nobel Prizes (ten including his own “Chemistry 1908 – investigations into the disintegration of the elements, and the chemistry of radioactive substances”. He was rather put out with the chemistry label as he believed only physics was important “everything else is simply stamp collecting”)


He was widely involved in the early studies into the structure of the atom.

J. J. Thomson started the ball rolling by “discovering” the electron and coming up with a new idea about the structure of the atom.


Sir Joseph John Thomson OM PRS (18 December 1856 – 30 August 1940) was an English physicist and Nobel Laureate in Physics, credited with the discovery and identification of the electron, the first subatomic particle to be discovered.

The plum pudding model is one of several historical scientific models of the atom. First proposed by J. J. Thomson in 1904 soon after the discovery of the electron, but before the discovery of the atomic nucleus, the model tried to explain two properties of atoms then known: that electrons are negatively-charged particles and that atoms have no net electric charge. The plum pudding model has electrons surrounded by a volume of positive charge, like negatively-charged “plums” embedded in a positively-charged “pudding”.


Rutherford wasn’t happy with this model. In 1911, although he could not prove that it was positive or negative, he theorized that atoms have their charge concentrated in a very small nucleus, and thereby pioneered the Rutherford model of the atom, through his discovery and interpretation of Rutherford scattering by the gold foil experiment of Hans Geiger and Ernest Marsden. He performed the first artificially induced nuclear reaction in 1917 in experiments where nitrogen nuclei were bombarded with alpha particles. As a result, he discovered the emission of a subatomic particle which, in 1919, he called the “hydrogen atom” but, in 1920, he more accurately named the proton.

The Geiger–Marsden experiments (also called the Rutherford gold foil experiment) were a landmark series of experiments by which scientists discovered that every atom contains a nucleus where all of its positive charge and most of its mass are concentrated. They deduced this by measuring how an alpha particle beam is scattered when it strikes a thin metal foil. The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.

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This apparatus was described in a 1913 paper by Geiger and Marsden. It was designed to accurately measure the scattering pattern of the alpha particles produced by the metal foil (F). The microscope (M) and screen (S) were affixed to a rotating cylinder and could be moved a full circle around the foil so that they could count scintillations from every angle.


These results allowed Rutherford to reject Thomson’s model of the atom, and instead proposed a model where the atom consisted of mostly empty space, with all of its positive charge concentrated in its centre in a very tiny volume, surrounded by a cloud of electrons.

When Geiger reported to Rutherford that he had spotted alpha particles being strongly deflected, Rutherford was astounded. In a lecture Rutherford delivered at Cambridge University, he said:

“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.” — Ernest Rutherford

Niels Bohr received an invitation from Rutherford to conduct post-doctoral work at Victoria University of Manchester. He adapted Rutherford’s nuclear structure to Max Planck’s quantum theory and so created his Bohr model of the atom.


The Bohr model of the hydrogen atom (Z = 1) or a hydrogen-like ion (Z > 1), where the negatively charged electron confined to an atomic shell encircles a small, positively charged atomic nucleus and where an electron jumps between orbits it is accompanied by an emitted or absorbed amount of electromagnetic energy (hn) (can also be written as hf where h is Planck’s constant and n or f is the frequency of the photon). The orbits in which the electron may travel are shown as grey circles; their radius increases as n2, where n is the principal quantum number. The 3 → 2 transition depicted here produces the first line of the Balmer series, and for hydrogen (Z = 1) it results in a photon of wavelength 656 nm (red light).

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.


The discovery of isotopes was further evidence that atoms were not simple.

Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.

The term isotope is formed from the Greek roots isos (ἴσος “equal”) and topos (τόπος “place”), meaning “the same place”; thus, the meaning behind the name is that different isotopes of a single element occupy the same position on the periodic table. It was coined by a Scottish doctor and writer Margaret Todd in 1913 in a suggestion to chemist Frederick Soddy.

The number of protons within the atom’s nucleus is called atomic number and is equal to the number of electrons in the neutral (non-ionized) atom. Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons. The number of nucleons (both protons and neutrons) in the nucleus is the atom’s mass number, and each isotope of a given element has a different mass number.

For example, carbon-12, carbon-13, and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13, and 14, respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that the neutron numbers of these isotopes are 6, 7, and 8 respectively.

Could a proton gain an electron to become neutral?

In 1921, while working with Niels Bohr (who postulated that electrons moved in specific orbits), Rutherford theorized about the existence of neutrons “neutral doublet”, (which he had christened in his 1920 Bakerian Lecture), which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keep the nuclei from flying apart from the repulsion between protons. The only alternative to neutrons was the existence of “nuclear electrons” which would counteract some of the proton charges in the nucleus, since by then it was known that nuclei had about twice the mass that could be accounted for if they were simply assembled from hydrogen nuclei (protons). But how these nuclear electrons could be trapped in the nucleus, was a mystery.

James Chadwick attended Victoria University of Manchester in 1908. He meant to study mathematics, but enrolled in physics by mistake. Like most students, he lived at home, walking the 6.4 km to the university and back each day. At the end of his first year, he was awarded a Heginbottom Scholarship to study physics. The physics department was headed by Rutherford, who assigned research projects to final-year students, and he instructed Chadwick to devise a means of comparing the amount of radioactive energy of two different sources. The idea was that they could be measured in terms of the activity of 1 gram of radium, a unit of measurement which would become known as the curie. Rutherford’s suggested approach was unworkable—something Chadwick knew but was afraid to tell Rutherford—so Chadwick pressed on, and eventually devised the required method. The results became Chadwick’s first paper, which, co-authored with Rutherford, was published in 1912. He graduated with first class honours in 1911, obtaining an MSc in 1912.

Rutherford suggested that Chadwick should go to Berlin in 1913 to further his studies. Unfortunately, he was still there when the first world war began and he was interned in the Ruhleben internment camp near Berlin. He was allowed to set up a laboratory in the stables and conduct scientific experiments using improvised materials such as radioactive toothpaste.

He was released after the Armistice with Germany came into effect in November 1918, and returned to his parents’ home in Manchester, where he wrote up his findings over the previous four years for the 1851 Exhibition commissioners.

Rutherford gave Chadwick a part-time teaching position at Manchester, allowing him to continue research.

In April 1919, Rutherford became director of the Cavendish Laboratory at the University of Cambridge, and Chadwick joined him there a few months later. Chadwick was awarded a Clerk-Maxwell studentship in 1920, and enrolled as a Doctor of Philosophy (PhD) student at Gonville and Caius College, Cambridge. The first half of his thesis was his work with atomic numbers. In the second, he looked at the forces inside the nucleus. His degree was awarded in June 1921. In November, he became a Fellow of Gonville and Caius College.

Chadwick’s studentship ended in 1923 and he became Rutherford’s assistant director of research. In this role, Chadwick helped Rutherford select PhD students.


The room which Rutherford and Chadwick used for their scattering experiments in the 1920s. The work was carried out in the dark to count scintillations.

In his research, Chadwick continued to probe the nucleus. At the time it was believed that the nucleus consisted of protons and electrons, so nitrogen’s nucleus, for example, with a mass number of 14, was assumed to contain 14 protons and 7 electrons. This gave it the right mass and charge, but the wrong spin. In quantum mechanics and particle physics, spin is an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei.

In Germany, Walther Bothe and his student Herbert Becker had used polonium to bombard beryllium with alpha particles, producing an unusual form of radiation. Chadwick had his Australian 1851 Exhibition scholar, Hugh Webster, duplicate their results. To Chadwick, this was evidence of something that he and Rutherford had been hypothesising for years: the neutron, a theoretical nuclear particle with no electric charge. Then in January 1932, Feather drew Chadwick’s attention to another surprising result. Frédéric and Irène Joliot-Curie had succeeded in knocking protons from paraffin wax using polonium and beryllium as a source for what they thought was 55MeV gamma radiation. Rutherford and Chadwick disagreed; protons were too heavy for that. But neutrons would need only a small amount of energy to achieve the same effect. In Rome, Ettore Majorana came to the same conclusion: the Joliot-Curies had discovered the neutron but did not know it.

The Curie’s incorrectly published the following in January 1932

9Be + 4Be –> 12C + 1nédéric_Joliot-Curieène_Joliot-Curie

Chadwick dropped all his other responsibilities to concentrate on proving the existence of the neutron, assisted by Feather and frequently working late at night. He devised a simple apparatus that consisted of a cylinder containing a polonium source and beryllium target. The resulting radiation could then be directed at a material such as paraffin wax; the displaced particles, which were protons, would go into a small ionisation chamber where they could be detected with an oscilloscope.

In February 1932, after only about two weeks of experimentation with neutrons, Chadwick sent a letter to Nature titled “Possible Existence of a Neutron”. It was only 200 words long. He communicated his findings in detail in an article sent to Proceedings of the Royal Society A titled “The Existence of a Neutron” in May. His discovery of the neutron was a milestone in understanding the nucleus. Reading Chadwick’s paper, Robert Bacher and Edward Condon realised that anomalies in the then-current theory, like the spin of nitrogen, would be resolved if the neutron has a spin of 1/2 and that a nitrogen nucleus consisted of seven protons and seven neutrons.


A schematic diagram of the experiment James Chadwick used to discover the neutron in 1932. At left, a polonium source was used to irradiate beryllium with alpha particles, which induced a type of uncharged radiation initially thought to be gamma rays. When this radiation struck paraffin wax, protons were ejected with ca. 5.5 MeV kinetic energy. The protons were observed using a small ionization chamber, called a counter, that detected a signal recorded on an oscillograph. Adapted from J. Chadwick, The Existence of a Neutron, Proc. Royal Society London, Series A, 136, 692-708, 1932. Adapted from Chadwick (1932).


Chadwick‘s neutron chamber in which a- particles from a polonium source, at the right-hand end, bombarded a beryllium source, at the left-hand end. From there, neutrons emerged to pass through a paraffin wax target, releasing enough protons to register on an oscilloscope. A vacuum pump was attached to the finger-like chimney.

In his 1933 Bakerian Lecture, Chadwick estimated that a neutron had a mass of about 1.0067 u. Since a proton and an electron had a combined mass of 1.0078 u, this implied the neutron as a proton–electron composite had a binding energy of about 2 MeV, which sounded reasonable, although it was hard to understand how a particle with so little binding energy could be stable.

The modern accepted value for the mass of the neutron is 1.00866 u. This mass was too large to be a proton–electron pair.

Chadwick found the ratio of neutron to proton to be 1.0090. The modern value is 1.0085.

Chadwick was a very complex, nervous man. He was never seen to smile and his wedding photographs implied he didn’t smile on his wedding day.

Nobody really knows where he got the polonium from (although Marie Curie was a friend). Another account was he obtained it from old radon.


The 1935 Nobel Laureates at the Nobel Prize Award Ceremony in the Golden Hall of the Stockholm City Hall, 10 December 1935. From left: James Chadwick, Irène Joliot-Curie and Frédéric Joliot. Chadwick was awarded the Nobel prize for physics and the Joliot-Curies were awarded the Nobel prize for physics.


Chadwick, on the right, receiving the 1935 Nobel Prize from King Gustav in Stockholm.


Chadwick‘s laboratory with its vibration-sensitive equipment was through the open door. Rutherford is talking to J.A. Ratcliffe.

In 1932 Cockcroft and Walton produced the first nuclear disintegration.

The Cockcroft–Walton (CW) generator, or multiplier, is an electric circuit that generates a high DC voltage from a low-voltage AC or pulsing DC input. It was named after the British and Irish physicists John Douglas Cockcroft and Ernest Thomas Sinton Walton, who in 1932 used this circuit design to power their particle accelerator, performing the first artificial nuclear disintegration in history.


Walton observing alpha-particles with the Cockcroft-Walton machine (1932)

Black and white photograph of the Cockcroft-Walton machine with Walton sitting in the observation ‘tent’ and Cockcroft in the background taken in 1932.


The Solvay Physics Conference, held in Brussels, Belgium, October 22-29, 1933. It was devoted to the atomic nucleus. Attendees included two future key Manhattan Project scientists (Fermi and Lawrence), the future head of the Nazi atomic bomb program (Heisenberg), and numerous leading pre-war physicists. A partial list of those attending: Niels Bohr (seated, third from left); James Chadwick (seated, farthest right); J. B. Cockroft (middle row, third from right); Marie Curie (seated, fifth from left); Enrico Fermi (middle row, fifth from left); Werner Heisenberg (middle row, fourth from left); Ernest O. Lawrence (back row, second from right); Lise Meitner (seated, second from right); Rudolf Peierls (middle row, second from right); Ernest Rutherford (seated, fifth from right); E. T. S. Walton (middle row, sixth from left). The photograph is courtesy Lawrence Berkeley National Laboratory. Only three of the people in the picture were women. Lisa Meitner met Chadwick at this time.

Only Bohr, Curie and Rutherford were known outside the physics world.

In March 1935, Chadwick received an offer of the Lyon Jones Chair of physics at the University of Liverpool and he set about acquiring a cyclotron.

Rutherford died in 1937 and Lawrence Bragg became the next head of the Cavendish Laboratory.

The budget for physics at this time was the same as the budget for opera.

The neutron was well suited to penetrating the “atom” and Enrico Fermi [1901–1954] in Rome rapidly became the expert. Fermi’s source of neutrons was radon gas (from a radium source) mixed with Be. This was much more intense than Po-Be sources then in use in other places, but must have been lethal!


The experimental physicists proceeded to bombard all the known elements in the Periodic Table. They also discovered that slow neutrons were better for possible “transmutations”.

When Fermi (and also the Joliot-Curie’s in Paris) got to uranium they assumed the transuranium elements were being formed. No less than 4 new b decay products were discovered in 1935 – quite surprising! Fermi even named them: ausonium and hesperium, this was a mistake.

By 1938 the situation in Italy had become difficult as Laura Fermi was part Jewish; Fermi receives Nobel 1938 in Stockholm and goes straight to Columbia University, NY.

Part 2 – The road to fission


The above picture shows Grete (or Emma) Planck, Meitner, and Elisabeth Schiemann in about 1913.

Meitner was born and brought up in Vienna in a loving family. Because she was female, she couldn’t attend university so she had to take science courses privately before graduating. Her inspiration was Ludwig Boltzmann.

In 1905 she became the second woman ever to receive a doctorate in physics from the city’s university.

She arrived in Berlin in 1907 at the Kaiser Wilhelm Institute and had a job in radiochemistry with Otto Hahn. She wasn’t allowed to enter the building through the front door and had to use the back entrance.

Emil Fischer, a director of the KWI for chemistry and vice-president of the Kaiser Wilhelm Society, didn’t like having women in the labs as he thought they would set fire to their hair.


Hahn, Meitner, and 0ne of the Planck twins, about 1910. The other twin was probably taking the photograph.


Meitner and Hahn, in their laboratory in Fischer‘s institute, about 1910.


The pictures above show a small instrument known as the simple beta spectrometer (figure 1). It was first used by Meitner, Hahn, and Otto von Baeyer in 1910. Meitner used the larger instrument (figure 2) for her studies of beta-gamma spectra in the 1920s.

In the small instrument, figure 1, sample A emits beta particles that travel upward through slit B and are recorded as a dark line on photographic plate C.

In a magnetic field perpendicular to the plane of the diagram, the electrons are deflected into a circular path, the deflection being greater for less energetic electrons. Meitner, Hahn, and von Baeyer observed discrete lines on the photographic plate, evidence of monoenergetic electron groups whose energy they determined from the position of the lines. In the larger instrument, figure 2, the orientation of the photographic plate C has been changed to improve the resolution of the electron lines.

Meitner moved to Dahlem in 1911 and got an unpaid “position” in 1912. She was now 34.

During the first world war Meitner volunteered for the ambulance service and helped with medical x-rays. She fell ill and returned to Berlin. She later helped bring scientific instruments to Ruhleben where Chadwick was held a prisoner.

Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It was first identified in 1913 by Kasimir Fajans and Oswald Helmuth Göhring and named brevium because of the short half-life of the specific isotope studied, i.e. protactinium-234. A more stable isotope of protactinium, 231Pa, was discovered in 1917/18 by Otto Hahn (the chemist) and Lise Meitner (the physicist), and they chose the name proto-actinium, but the IUPAC finally named it “protactinium” in 1949 and confirmed Hahn and Meitner as discoverers. The new name meant “(nuclear) precursor of actinium” and reflected that actinium is a product of radioactive decay of protactinium.


Attendees at the “bonzenfreie Kolloquium” (which translates to “colloquium without bigwigs”), a conference organized by Lise Meitner for Niels Bohr in Berlin, Germany, 1920.

Left to right: Otto Stern, Wilhelm Lenz, James Franck, Rudolph Ladenburg, Paul Knipping, Niels Bohr, Ernst Wagner, Otto von Baeyer, Otto Hahn, George de Hevesy, Lise Meitner, Wilhelm Westphal, Hans Geiger, Gustav Hertz, Peter Pringsheim.

Credit: Prof. Wilhelm Westfall, courtesy AIP Emilio Segrè Visual Archives

Hahn and Meitner’s work established the KWI as a major “player“ in the 1930‘s in studies of nuclear structure.


Meitner with Eva von BahrBergius, Kaiser Wilhelm Institute for Chemistry, in about 1920. The women met before World War I, when Eva was a student in Berlin; later, Eva would be Lise‘s closest friend in Sweden.


The image above shows the Kaiser Wilhelm Institute for Chemistry in about 1930; view from Thielallee. The smaller building at the left is the institute villa, where Meitner lived in an apartment during this period.

Meitner was very happy in Berlin. She loved music. Unfortunately, her life became more complicated on the 30th January 1933. Hitler became Chancellor of Germany.

Immediately the Jews in Germany experienced problems. Leitner, a non-practising Jew (she was a baptised protestant), was relatively well protected because she was Austrian.

Scientists like Einstein (who did not attend the Solvay conference as he went to Caltech in the US and never returned to Germany) and Schrodinger left; others like Planck & von Laue stayed and tried to protect younger colleagues.

Meitner had a chance to leave to Bohr’s Institute, but could not believe her world would change. In this respect she was completely apolitical.

In the years spanning 1934 to 1938, Enrico Fermi with his team in Rome, Otto Hahn and Lise Meitner in Berlin, Irene Curie and Paul Savitch in Paris discovered a large number of new radioisotopes. They all attempted to create heavier, ‘transuranic’, elements by bombarding uranium with neutrons (fission was not anticipated) but Meitner did not actually think this was right.

By 1936 the physicists in Berlin were working full time on the problem of neutrons and uranium. They made the reactions and then searched the precipitates. They did not analyse the filtrates because uranium was in the filtrates and the radioactivity from it was too high (they thought) to see other materials. This turned out to be a huge mistake!

Meitner’s experiment was spread over three rooms; an irradiation room; a chemistry laboratory; and a measuring room. In the irradiation room, a uranium sample was irradiated by a neutron source (a mixture of radium and beryllium), which was sealed in brass tubes and placed in a paraffin block, which slowed down the neutrons. Neutrons at the time were a relatively new discovery and as they are neutrally charged, they could interact with an atom’s nucleus with less interference from its electrons and protons. As the neutrons bombarded the uranium sample, nuclear fission occurred.

To measure radioactivity and the extremely small quantities of radioactive substances produced, the measuring room was equipped with home-made Geiger-Müller radioactivity counters to determine the decay of the extremely small quantities of radioactive substances produced. Unstable radioactive isotopes, like uranium used in the experiment, transmute over time into other elements in a process known as decay. Because different elements decay at different rates and release different types of radiation, plotting the decay of the uranium sample as a curve on a graph helped reveal the kind of atoms present, and was essential for determining what elements were produced from the nuclear fission.

Initially a major error was to think that all the decays were beta. This involves a neutron breaking up to form a proton, a beta particle and a neutrino. In fact, only one of the decays was a beta decay. Beta decay would enable lead to become bismuth, mercury to become thallium etc.

A chemical error was to think that D elements were the same as F elements.

The irradiated uranium sample was then brought to the chemistry laboratory where the subsequent radioactive elements from the nuclear fission were isolated using chemical methods.

The reactions seemed to be formed with both fast and slow neutrons and the chains were far too long given the very small energy of the neutrons.

In late 1937 the Curie’s reported a strong t½ = 3.5 hr activity in the filtrate and proposed it to be thorium (Th). Meitner showed this was impossible and the Paris group retracted, and then said it was La. In Berlin they were sceptical and called it “curiosium”!

But, even by early 1938, the Berlin group had still not looked at the filtrate.

On the 12th of March 1938 Germany annexed Austria and Meitner lost the protection of being an Austrian citizen. Initially she wasn’t worried and she went travelling during the Easter holidays.

Hahn becomes the Director of KWI and is questioned about Meitner. She now became very nervous.

By June 1938 she had her passport confiscated, but she was able to leave for Holland on the 15th of July. From there she travelled to Copenhagen and then to Stockholm.

Friedrich Wilhelm “Fritz” Strassmann (22 February 1902 – 22 April 1980) was a German chemist who, with Otto Hahn in early 1939, identified barium in the residue after bombarding uranium with neutrons, results which, when confirmed, demonstrated the previously unknown phenomenon of nuclear fission.


The above image shows Staussmann in 1936 at the age of 34.

Strassmann returned to the 3.5 hr intense b activity reported by the Curies. He then proposed the following chain:

238U + 1n → a + 235Th → 231Ra + a

In November 1938 Hahn met Meitner (now 60) in Copenhagen. She urged Hahn to check the chemistry of the filtrates. Back in Berlin he found it was not Ra (Z=88) but Ba, Z = 56. He wrote to Meitner on 19th of December 1938; sent a paper on the 24th of December and it was published on 6th January 1939.

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 first experimentally detected the fission by-products. Later, with his collaborator Rudolf Peierls he designed the first theoretical mechanism for the detonation of an atomic bomb in 1940.

The following image shows Otto Robert Frisch, aged 29, shortly before emigrating from Germany in 1933.


Meitner received the news and then moved to a friend’s house in Sweden, where she was joined by her nephew, Otto Frisch , then in Copenhagen. They took their famous walk in the woods and realised that the uranium nucleus was unstable and had broken in two via the ideas of the liquid-drop model of Bohr.

Liquid Drop Model of Nucleus

The liquid drop model was formulated by Niels Bohr as a theory as to how nuclear fission takes place. Nuclear fission is the splitting of a nucleus into several smaller parts. Bohr thought that this process would mimic that of the molecules of a liquid drop splitting apart.


He thought that the positive charges in the nucleus would repel from each other, thereby splitting the nucleus apart. Although inadequate to explain all nuclear phenomena, the concept behind the idea provided an excellent estimate to the properties of nuclei.

According to this model, the atomic nucleus behaves like the molecules in a drop of liquid. But in this nuclear scale, the fluid is made of nucleons (protons and neutrons), which are held together by the strong nuclear force. The liquid drop model of the nucleus takes into account the fact that the nuclear forces on the nucleons on the surface are different from those on nucleons in the interior of the nucleus. The interior nucleons are completely surrounded by other attracting nucleons. Here is the analogy with the forces that form a drop of liquid.

In the ground state the nucleus is spherical. If the sufficient kinetic or binding energy is added, this spherical nucleus may be distorted into a dumbbell shape and then may be split into two fragments. Since these fragments are a more stable configuration, the splitting of such heavy nuclei must be accompanied by energy release. This model does not explain all the properties of the atomic nucleus, but does explain the predicted nuclear binding energies.

Nuclear binding energy is the minimum energy that would be required to disassemble the nucleus of an atom into its component parts.

Meitner and Frisch calculated the charge effect and surface tension, and realised it was possible. They used the E = mc2 to show that ~ 0.2 of a proton has disappeared and 200 MeV/fission energy released. They predicted Kr (Z = 36) should also be there.

Frisch expected to see fission fragments, which he does. He told Bohr of his findings on the 2nd of January 1939. On the 6th January Bohr sailed to the USA where he told Rosenfeld but forgot to ask him not to tell anyone.

Arthur Hinton “Art” Rosenfeld (June 22, 1926 – January 27, 2017) was a UC Berkeley physicist and California energy commissioner, dubbed the “godfather of energy efficiency”, for developing new standards which helped improve energy efficiency in California and subsequently worldwide.

He entered graduate school at the University of Chicago, and studied particle physics under Enrico Fermi, a Nobel Prize-winning Italian physicist. He coauthored a book on nuclear physics with Fermi, who was noted for building the world’s first nuclear reactor.

The word was out in Princeton. Frisch submitted two papers to Nature on 16/1/39 and these were published on 11/2/39 (too late). “Fission” is borrowed from biology.

235U92 + 1n0144Ba56 + 89Kr36 + 31n0 + 200 MeV


Absorption of a neutron can cause uranium-235 to fission and release a huge amount of energy


The APO target room at the Carnegie Institution’s Department of Terrestrial Magnetism after the demonstration of fission there on Jan. 28, 1939.

L to r, Robert Meyer, Merle Tuve, Enrico Fermi, Richard Roberts, Leon Rosenfeld, Erik Bohr, Niels Bohr, Gregory Breit, John Fleming.

Courtesy of the Department of Terrestrial Magnetism, Carnegie Institution of Washington

January 1939

Neils Bohr brings the news of fission to the U.S. when he travels there for a conference.

Scientists quickly duplicate Frisch’s experiment and confirm nuclear fission.

The vertical pulses on the oscilloscope screen indicate the fission of U-235 nuclei. It was these pulses that Fermi and his team missed but that Frisch saw in his experiments to verify nuclear fission in January 1939.


Courtesy of F.W. Goro/Life

Part 3 – The road to the bomb

The physics of fission in early 1939

• By March 1939, Bohr & Wheeler had understood quantitatively the process, and identified 235U as being the key ingredient. Their paper “The mechanism of nuclear fission”, submitted 28th June 1939; published Phys. Rev. 56 426-450 (1939)

John Archibald Wheeler (July 9, 1911 – April 13, 2008) was an American theoretical physicist. He was largely responsible for reviving interest in general relativity in the United States after World War II. Wheeler also worked with Niels Bohr in explaining the basic principles behind nuclear fission.

• Leo Szilard was very active at the University of Chicago and realised that 239Pu94 should also be fissionable.

Questions that the physicists needed answers?

1) How much 235U is needed?

2) How could it be separated?

3) How many neutrons are produced; is it greater than 1?

4) Are the neutrons prompt or delayed? Prompt could not be controlled.

On the 1st of August 1939 Szilard & Teller convinced Einstein to write the famous letter to President Roosevelt.

Edward Teller (January 15, 1908 – September 9, 2003) was a Hungarian-American theoretical physicist who is known colloquially as “the father of the hydrogen bomb”, although he did not care for the title, and was only part of a team who developed the technology.

The Einstein–Szilárd letter was a letter written by Leó Szilárd and signed by Albert Einstein that was sent to the United States President Franklin D. Roosevelt on August 2, 1939. Written by Szilárd in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner, the letter warned that Germany might develop atomic bombs and suggested that the United States should start its own nuclear program. It prompted action by Roosevelt, which eventually resulted in the Manhattan Project developing the first atomic bombs.

On the 3rd of September 1939 War was declared in Europe because the Germans invaded Poland.

At this time a great deal of progress was made in nuclear physics within the UK. However, there was very little progress in the USA and Germany.

By early 1940, Chadwick (now at Liverpool with his cyclotron) and G. P. Thomson (Imperial College, London) were doing experiments on UO2. At ICL they were unable to sustain any chain reaction and were pessimistic.

Sir George Paget Thomson, FRS (3 May 1892 – 10 September 1975) was an English physicist and Nobel laureate in physics recognised for his discovery of the wave properties of the electron by electron diffraction. He was the son of J. J. Thomson.

Meanwhile in Birmingham Otto Frisch and Rudolf Peierls (both theorists) studied the matter in detail and proposed methods for enriching U (diffusion using UF6) and, assuming fast neutron fission, ~ 1 kg of pure 235U might be enough. (We know now the amount is ~ 6 kg). Chadwick & Thompson immediately co-opted Frisch & Peierls and arranged for them to continue working on the project. German ex-pats were allowed to work on this because it didn’t involve radon.

Sir Rudolf Ernst Peierls, CBE FRS (5 June 1907 – 19 September 1995) was a Jewish German-born British physicist who played a major role in the Manhattan Project and Tube Alloys, Britain’s nuclear programme. His obituary in Physics Today described him as “a major player in the drama of the eruption of nuclear physics into world affairs”.

Fast neutron fission cross section was first measured in Paris by Joliot et al, and was ~ 2.5 n. After June 1940 they took their D2O to UK and then later to Canada.

Nuclear research in Germany was a mess. It was not given high priority and there were many interdepartmental squabbles. In 1942 Albert Speer ended the project.

Albert Speer (March 19, 1905 – September 1, 1981) was the Minister of Armaments and War Production in Nazi Germany during most of World War II. A close ally of Adolf Hitler, he was convicted at the Nuremberg trials and sentenced to 20 years in prison.

The “Uranium Club” went on, under Werner Heisenberg’s direction and an effort to make a reactor was made at Haigerloch in the Black Forest with uranium cubes and D2O.

Werner Karl Heisenberg (5 December 1901 – 1 February 1976)[3] was a German theoretical physicist and one of the key pioneers of quantum mechanics.

Heavy water (deuterium oxide, 2H2O, D2O) is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (2H or D, also known as heavy hydrogen), rather than the common hydrogen-1 isotope (1H or H, also called protium) that makes up most of the hydrogen in normal water. The presence of deuterium gives the water different nuclear properties, and the increase of mass gives it slightly different physical and chemical properties when compared to normal water. It can be used to create ice and snow at higher temperatures since its melting point is 3.82 C.

Haigerloch is a town in the north-western part of the Swabian Alb in Germany.

The German nuclear weapons project (Uranium Society or Uranium Club) was a scientific effort led by Germany to develop and produce nuclear weapons during World War II. The first effort started in April 1939, just months after the discovery of nuclear fission in December 1938, but ended only months later shortly ahead of the German invasion of Poland, when many notable physicists were drafted into the Wehrmacht.

A second effort began under the administrative purview of the Wehrmacht’s Heereswaffenamt on 1 September 1939, the day of the invasion of Poland. The program eventually expanded into three main efforts: the Uranmaschine (nuclear reactor), uranium and heavy water production, and uranium isotope separation. Eventually it was assessed that nuclear fission would not contribute significantly to ending the war, and in January 1942, the Heereswaffenamt turned the program over to the Reich Research Council (Reichsforschungsrat) while continuing to fund the program. The program was split up among nine major institutes where the directors dominated the research and set their own objectives. Subsequently, the number of scientists working on applied nuclear fission began to diminish, with many applying their talents to more pressing war-time demands.


An array of uranium cubes hanging from the lid of the reactor.

Very little initially happened in the USA. The report of the MAUD committee went to Briggs (head of NBS and also the Uranium Committee) in July 1941 but he did nothing. The report stated that the critical mass is ~8 kg of 235U would be needed (Heisenberg thought 1000kg would be needed). Ernest Lawrence (Berkeley) produced the first measurable amounts of 235U and started to galvanise the US effort.

The MAUD Committee was a British scientific working group formed during the Second World War. It was established to perform the research required to determine if an atomic bomb was feasible. The name MAUD came from a strange line in a telegram from Danish physicist Niels Bohr referring to his housekeeper, Maud Ray.

The MAUD Committee was founded in response to the Frisch-Peierls memorandum, which was written in March 1940 by Rudolf Peierls and Otto Robert Frisch, two physicists who were refugees from Nazi Germany working at the University of Birmingham under the direction of Mark Oliphant. The memorandum argued that a small sphere of pure uranium-235 could have the explosive power of thousands of tons of TNT.

Lyman James Briggs (May 7, 1874 – March 25, 1963) was an American engineer, physicist and administrator. He was a director of the National Bureau of Standards during the Great Depression and chairman of the Uranium Committee before America entered the Second World War. The Lyman Briggs College at Michigan State University is named in his honour.

Ernest Orlando Lawrence (August 8, 1901 – August 27, 1958) was a pioneering American nuclear scientist and winner of the Nobel Prize in Physics in 1939 for his invention of the cyclotron. He is known for his work on uranium-isotope separation for the Manhattan Project, as well as for founding the Lawrence Berkeley National Laboratory and the Lawrence Livermore National Laboratory.

In November 1941 a delegation from the US visited the UK. They were greatly impressed. In early 1942, with the US now at war, a UK delegation went to the US, but Chadwick did not go.

The US now began to increase the pace of activity, spurred on by Lawrence, Fermi, Conant, Bush, and Compton.

James Bryant Conant (March 26, 1893 – February 11, 1978) was an American chemist, a transformative President of Harvard University, and the first U.S. Ambassador to West Germany.

Vannevar Bush (March 11, 1890 – June 28, 1974) was an American engineer, inventor and science administrator, who during World War II headed the U.S. Office of Scientific Research and Development (OSRD), through which almost all wartime military R&D was carried out, including important developments in radar and the initiation and early administration of the Manhattan Project.

Arthur Holly Compton (September 10, 1892 – March 15, 1962) was an American physicist who won the Nobel Prize in Physics in 1927 for his 1923 discovery of the Compton effect, which demonstrated the particle nature of electromagnetic radiation. It was a sensational discovery at the time: the wave nature of light had been well-demonstrated, but the idea that light had both wave and particle properties was not easily accepted. He is also known for his leadership of the Manhattan Project’s Metallurgical Laboratory at the University of Chicago, and served as Chancellor of Washington University in St. Louis from 1945 to 1953.

The UK, even Churchill, was at first reluctant to join, and this attitude later had negative consequences.

Sir Winston Leonard Spencer-Churchill (30 November 1874 – 24 January 1965) was a British politician, army officer, and writer. He was Prime Minister of the United Kingdom from 1940 to 1945, when he led Britain to victory in the Second World War, and again from 1951 to 1955. Churchill represented five constituencies during his career as a Member of Parliament (MP).

Chadwick finally went to Washington in Nov. 1943. He spent all of 1944 at Los Alamos and saw the Trinity test in July 1945.

Trinity was the code name of the first detonation of a nuclear device. It was conducted by the United States Army at 5:29 a.m. on July 16, 1945, as part of the Manhattan Project. The test was conducted in the Jornada del Muerto desert about 35 miles (56 km) southeast of Socorro, New Mexico, on what was then the USAAF Alamogordo Bombing and Gunnery Range, now part of White Sands Missile Range.

The image below shows James Chadwick (left), the head of the British Mission, conferring with Major General Leslie R. Groves, Jr. (right), the director of the Manhattan Project (since 1942)


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 that developed the atomic bomb during World War II.

Enrico Fermi – “The Italian Navigator has landed in the New World. The natives are very friendly.” A conversation between Compton & Conant. Key other participants were Leo Szilard & Eugene Wigner.

Eugene Paul “E. P.” Wigner (November 17, 1902 – January 1, 1995) was a Hungarian-American theoretical physicist and mathematician. He received the Nobel Prize in Physics in 1963 “for his contributions to the theory of the atomic nucleus and the elementary particles, particularly through the discovery and application of fundamental symmetry principles”.

Chicago Pile-1 (CP-1) was the world’s first artificial nuclear reactor. On 2 December 1942, the first human-made self-sustaining nuclear chain reaction was initiated in CP-1, during an experiment led by Enrico Fermi. The secret development of the reactor was the first major technical achievement for the Manhattan Project, the Allied effort to create atomic bombs during World War II. Although the project’s civilian and military leaders had misgivings about the possibility of a disastrous runaway reaction, they trusted Fermi’s safety calculations and decided they could carry out the experiment in a densely populated area. It was built by the Metallurgical Laboratory at the University of Chicago, under the west viewing stands of the original Stagg Field. Fermi described the apparatus as “a crude pile of black bricks and wooden timbers”.

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CP-1 at the University of Chicago. 2nd December 1942: 1553 hrs. 349,263 kg graphite; 36,507 kg UO2, and 5617 kg of U metal (from Ames Lab).

Compton was nervous of the experiment but Fermi thought it was safe

Part 4 – Aftermath

On the 6th August 1945, at 8:15am the United States dropped an atomic bomb on Hiroshima

Hiroshima is the capital of Hiroshima Prefecture in Japan. As of June 1, 2019, the city had an estimated population of 2.089 million.


The images below show Hiroshima just after the bomb

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Atomic Bomb Dome by Jan Letzel and modern Hiroshima

Germany did not escape, however they were lucky that the bomb wasn’t ready then

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At the end of World War II, many of the Kaiser Wilhelm Institutes were heavily damaged. The above right image shows Kaiser Wilhelm Institute for Chemistry and the villa (right) after air raids, February 1944 (Courtesy Archiv zur Geschichte der Max-Planck-Gesellschaft, Berlin).

On the 16th November 1945 the Nobel Committee announced that the 1944 Chemistry Prize would go to Otto Hahn alone. (Physics to I. Rabi). There were nominations for Meintner, Frisch, & Strassman, but was almost certainly “blackballed” by Siegbahn, who did not like her.

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 imaging. He was also one of the first scientists in the United States to work on the cavity magnetron, which is used in microwave radar and microwave ovens.

Kai Manne Börje Siegbahn (20 April 1918 – 20 July 2007) was a Swedish physicist.

Otto Hahn (a German “hero”) never really accepted the role that Meitner had played, nor did he ever acknowledge the difficulties she faced, and this caused a deep gulf between the two of them. However, Hahn shared the prize money with her.


Meitner and President Harry S. Truman, 9 February 1946, Washington, D.C. Meitner was honoured as “Woman of the Year“ by the National Women‘s Press Club. She dined with Chadwick on this visit, but they did not agree on the use of nuclear weapons. Meitner was horrified by the bomb. (AP Photo)

Meitner returned to Germany first in 1947 for the funeral of Max Planck. She returned a number of times, but was never completely comfortable there. Strassman offered her a position in Mainz, and she thought long about it. She enjoyed Vienna more.

In 1946 she joined the Swedish nuclear programme, where she was much happier. She retired in 1954.

Grand opening ceremony of Hahn-Meitner Institute for nuclear research in Wannsee, Berlin. 14th March 1959


Source: © American Institute of Physics/Science Photo Library. Hahn and Meitner were both recognised in different ways; Hahn with a Nobel prize and Meitner with element 109 named in her honour.

Element 109 Meitnerium (symbol Mt) has been named after Meitner

Max von Laue suggested to call the Institute the “Meitner Institute” (as there was already a Hahn Institute) but the fame of Otto Hahn was unavoidable from a historical perspective. Opening 14 March 1959 with Willy Brandt and at which Meitner was very happy to attend.

Willy Brandt (born Herbert Ernst Karl Frahm; 18 December 1913 – 8 October 1992) was a German politician and statesman who was leader of the Social Democratic Party of Germany (SPD) from 1964 to 1987 and served as Chancellor of the Federal Republic of Germany (West Germany) from 1969 to 1974. He was awarded the Nobel Peace Prize in 1971 for his efforts to strengthen cooperation in western Europe through the EEC and to achieve reconciliation between West Germany and the countries of Eastern Europe. He was the first Social Democrat chancellor since 1930.

Fleeing to Norway and then Sweden during the Nazi regime and working as a left-wing journalist, he took the name Willy Brandt as a pseudonym to avoid detection by Nazi agents, and then formally adopted the name in 1948.

In 1960 Meitner moved to Cambridge to be close to Otto Frisch and his family.


The 1966 Enrico Fermi Prize was presented to Lise Meitner (& Hahn & Strassman) in October 1966 in Cambridge by Glenn T. Seaborg, chairman of the United States Atomic Energy Commission. Otto Frisch is at Meitner‘s right. (Courtesy Max Perutz)


Hahn, Heisenberg, Meitner, and Born at the 12th Lindau conference of Nobel Laureates, 1962. © Lindau Nobel Laureate Meetings

Chadwick returned to Liverpool in 1946. In 1948 he was elected as Master of Gonville and Caius College in Cambridge (where he was a student & Fellow).,_Cambridge

He continued working there for 10 years, but did not much enjoy the squabbles with Fellows.

He retired to Wales for 10 years but then moved back to Cambridge in 1968. He died at 83 in July 1974.

Further credits

• Andrew Brown, The Neutron & the Bomb: A biography of Sir James Chadwick; Oxford U Press, 1997

• Ruth Lewin Sime: Lise Meitner: A life in Physics U of C Press, 1996

• Patricia Rife: Lise Meitner and the dawn of the nuclear age. Birkhäuser, 1999 (published first in German in 1992)

• Richard Rhodes: The making of the atomic bomb Simon Schuster 1986

• Per F Dahl, From Nuclear Transmutation to Nuclear Fission; 1932–1939, IoP, 2002

• Per F Dahl: Heavy Water and the wartime race for Nuclear Energy, IoP, 1999

• Hitler’s Uranium Club: The Farm Hall tapes edited by J. Bernstein, 1996

• Gregg Harken: Brotherhood of the Bomb, Holt & Co. NY 2002

• Special thanks to colleagues of Professor Lander who helped him so much with his presentation.

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