1938 achievement in physics
Nuclear fission was discovered in December 1938 by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Fission is a nuclear reaction or radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei and often other particles. The fission process often produces gamma rays and releases a very large amount of energy, even by the energetic standards of radioactive decay. Scientists already knew about alpha decay and beta decay, but fission assumed great importance because the discovery that a nuclear chain reaction was possible led to the development of nuclear power and nuclear weapons. Hahn was awarded the 1944 Nobel Prize in Chemistry for the discovery of nuclear fission.
Hahn and Strassmann at the Kaiser Wilhelm Institute for Chemistry in Berlin bombarded uranium with slow neutrons and discovered that barium had been produced. Hahn suggested a bursting of the nucleus, but he was unsure of what the physical basis for the results were. They reported their findings by mail to Meitner in Sweden, who a few months earlier had fled Nazi Germany. Meitner and her nephew Frisch theorised, and then proved, that the uranium nucleus had been split and published their findings in "Nature". Meitner calculated that the energy released by each disintegration was approximately 200 megaelectronvolts, and Frisch observed this. By analogy with the division of biological cells, he named the process "fission".
The discovery came after forty years of investigation into the nature and properties of radioactivity and radioactive substances. The discovery of the neutron by James Chadwick in 1932 created a new means of nuclear transmutation. Enrico Fermi and his colleagues in Rome studied the results of bombarding uranium with neutrons, and Fermi concluded that his experiments had created new elements with 93 and 94 protons, which his group dubbed "ausenium" and "hesperium". Fermi won the 1938 Nobel Prize in Physics for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". However, not everyone was convinced by Fermi's analysis of his results. Ida Noddack suggested that instead of creating a new, heavier element 93, it was conceivable that the nucleus had broken up into large fragments, and Aristid von Grosse suggested that what Fermi's group had found was an isotope of protactinium.
This spurred Hahn and Meitner, the discoverers of the most stable isotope of protactinium, to conduct a four-year-long investigation into the process with their colleague Strassmann. After much hard work and many discoveries, they determined that what they were observing was fission, and that the new elements that Fermi had found were fission products. Their work overturned long-held beliefs in physics and paved the way for the discovery of the real elements 93 (neptunium) and 94 (plutonium), for the discovery of fission in other elements, and for the determination of the role of the uranium-235 isotope in that of uranium. Niels Bohr and John Wheeler reworked the liquid drop model to explain the mechanism of fission.
Background.
Radioactivity.
In the last years of the 19th century, scientists frequently experimented with the cathode-ray tube, which by then had become a standard piece of laboratory equipment. A common practice was to aim the cathode rays at various substances and to see what happened. Wilhelm Röntgen had a screen coated with barium platinocyanide that would fluoresce when exposed to cathode rays. On 8 November 1895, he noticed that even though his cathode-ray tube was not pointed at his screen, which was covered in black cardboard, the screen still fluoresced. He soon became convinced that he had discovered a new type of rays, which are today called X-rays. The following year Henri Becquerel was experimenting with fluorescent uranium salts, and wondered if they too might produce X-rays. 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 tested samples of as many elements and minerals as she could find for signs of Becquerel rays, and in April 1898 also found them in thorium. She gave the phenomenon the name "radioactivity". Along with Pierre Curie and Gustave Bémont, she began investigating pitchblende, a uranium-bearing ore, which was found to be more radioactive than the uranium it contained. This indicated the existence of additional radioactive elements. One was chemically akin to bismuth, but strongly radioactive, and in July 1898 they published a paper in which they concluded that it was a new element, which they named "polonium". The other was chemically like barium, and in a December 1898 paper they announced the discovery of a second hitherto unknown element, which they called "radium". Convincing the scientific community was another matter. Separating radium from the barium in the ore proved very difficult. It took three years for them to produce a tenth of a gram of radium chloride, and they never did manage to isolate polonium.
In 1898, Ernest Rutherford noted that thorium gave off a radioactive gas. In examining the radiation, he classified Becquerel radiation into two types, which he called α (alpha) and β (beta) radiation. Subsequently, Paul Villard discovered a third type of Becquerel radiation which, following Rutherford's scheme, were called "gamma rays", and Curie noted that radium also produced a radioactive gas. Identifying the gas chemically proved frustrating; Rutherford and Frederick Soddy found it to be inert, much like argon. It later came to be known as radon. Rutherford identified beta rays as cathode rays (electrons), and hypothesised—and in 1909 with Thomas Royds proved—that alpha particles were helium nuclei. Observing the radioactive disintegration of elements, Rutherford and Soddy classified the radioactive products according to their characteristic rates of decay, introducing the concept of a half-life. In 1903, Soddy and Margaret Todd applied the term "isotope" to atoms that were chemically and spectroscopically identical but had different radioactive half-lives. Rutherford proposed a model of the atom in which a very small, dense and positively charged nucleus of protons was surrounded by orbiting, negatively charged electrons (the Rutherford model). Niels Bohr improved upon this in 1913 by reconciling it with the quantum behaviour of electrons (the Bohr model).
Protactinium.
Soddy and Kasimir Fajans independently observed in 1913 that alpha decay caused atoms to shift down two places in the periodic table, while the loss of two beta particles restored it to its original position. In the resulting reorganisation of the periodic table, radium was placed in group II, actinium in group III, thorium in group IV and uranium in group VI. This left a gap between thorium and uranium. Soddy predicted that this unknown element, which he referred to (after Dmitri Mendeleev) as "ekatantalium", would be an alpha emitter with chemical properties similar to tantalium (now known as tantalum). It was not long before Fajans and Oswald Helmuth Göhring discovered it as a decay product of a beta-emitting product of thorium. Based on the radioactive displacement law of Fajans and Soddy, this was an isotope of the missing element, which they named "brevium" after its short half-life. However, it was a beta emitter, and therefore could not be the mother isotope of actinium. This had to be another isotope.
Two scientists at the Kaiser Wilhelm Institute (KWI) in Berlin-Dahlem took up the challenge of finding the missing isotope. Otto Hahn had graduated from the University of Marburg as an organic chemist, but had been a post-doctoral researcher at University College London under Sir William Ramsay, and under Rutherford at McGill University, where he had studied radioactive isotopes. In 1906, he returned to Germany, where he became an assistant to Emil Fischer at the University of Berlin. At McGill he had become accustomed to working closely with a physicist, so he teamed up with Lise Meitner, who had received her doctorate from the University of Vienna in 1906, and had then moved to Berlin to study physics under Max Planck at the Friedrich-Wilhelms-Universität. Meitner found Hahn, who was 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 in 1914. Hahn was called up into the German Army, and Meitner became a volunteer radiographer in Austrian Army hospitals. She returned to the Kaiser Wilhelm Institute in October 1916. 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 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, and assigning it the chemical symbol Pa. In June 1918, Soddy and John Cranston announced that they had extracted a sample of the isotope, but unlike Hahn and Meitner were unable to describe its characteristics. They acknowledged Hahn's and Meitner's priority, and agreed to the name. The connection to uranium remained a mystery, as neither of the known isotopes of uranium decayed into protactinium. It remained unsolved until uranium-235 was discovered in 1929.
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) named the new element definitively protactinium, and confirmed Hahn and Meitner as discoverers.
Transmutation.
Patrick Blackett was able to accomplish nuclear transmutation of nitrogen into oxygen in 1925, using alpha particles directed at nitrogen. In modern notation for the atomic nuclei, the reaction was:
147N + 42He → 178O + p
This was the first observation of a nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. A fully artificial nuclear reaction and nuclear transmutation was achieved in April 1932 by Ernest Walton and John Cockcroft, who used artificially accelerated protons against lithium, to break this nucleus into two alpha particles. The feat was popularly known as "splitting the atom", but was not nuclear fission; as it was not the result of initiating an internal radioactive decay process.
Just a few weeks before Cockcroft and Walton's feat, another scientist at the Cavendish Laboratory, James Chadwick, discovered the neutron, using an ingenious device made with sealing wax, through the reaction of beryllium with alpha particles:
94Be + 42He → 126C + n
Irène Curie and Frédéric Joliot irradiated aluminium foil with alpha particles and found that this results in a short-lived radioactive isotope of phosphorus with a half-life of around three minutes:
2713Al + 42He → 3015P + n
which then decays to a stable isotope of silicon
3015P → 3014Si + e+
They noted that radioactivity continued after the neutron emissions ceased. Not only had they discovered a new form of radioactive decay in the form of positron emission, they had transmuted