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The discovery of 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 behav ...
and its properties was central to the extraordinary developments in
atomic physics Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. Atomic physics typically refers to the study of atomic structure and the interaction between atoms. It is primarily concerned w ...
in the first half of the 20th century. Early in the century,
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 ...
developed a crude
model A model is an informative representation of an object, person or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a measure. Models c ...
of the atom, based on the gold foil experiment of
Hans Geiger Johannes Wilhelm "Hans" Geiger (; ; 30 September 1882 – 24 September 1945) was a German physicist. He is best known as the co-inventor of the detector component of the Geiger counter and for the Geiger–Marsden experiment which discover ...
and
Ernest Marsden Sir Ernest Marsden (19 February 1889 – 15 December 1970) was an English-New Zealand physicist. He is recognised internationally for his contributions to science while working under Ernest Rutherford, which led to the discovery of new theories ...
. In this model, atoms had their
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementa ...
and positive electric charge concentrated in a very small
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 Nucle ...
. By 1920 chemical
isotopes 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 numbe ...
had been discovered, 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 been determined to be (approximately) integer multiples of the mass of the
hydrogen atom A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen consti ...
, and 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 ...
had been identified as the charge on the nucleus.Byrne, J. ''Neutrons, Nuclei, and Matter'', Dover Publications, Mineola, New York, 2011, Throughout the 1920s, the nucleus was viewed as composed of combinations of
protons 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 ...
and
electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kno ...
, the two elementary particles known at the time, but that model presented several experimental and theoretical contradictions. The essential nature of the atomic nucleus was established with the discovery of the neutron by
James Chadwick 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 insp ...
in 1932 and the determination that it was a new elementary particle, distinct from the proton. The uncharged neutron was immediately exploited as a new means to probe nuclear structure, leading to such discoveries as the creation of new radioactive elements by neutron irradiation (1934) and the fission of
uranium 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 weakly ...
atoms by neutrons (1938). The discovery of fission led to the creation of both
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 ...
and
nuclear weapons 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 ...
by the end of World War II. Both the proton and the neutron were presumed to be elementary particles until the 1960s, when they were determined to be composite particles built from
quarks A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...
.


Discovery of radioactivity

At the start of the 20th century, the vigorous debate as to the existence of atoms had not yet been resolved. Philosophers such as
Ernst Mach Ernst Waldfried Josef Wenzel Mach ( , ; 18 February 1838 – 19 February 1916) was a Moravian-born Austrian physicist and philosopher, who contributed to the physics of shock waves. The ratio of one's speed to that of sound is named the Mach ...
and
Wilhelm Ostwald Friedrich Wilhelm Ostwald (; 4 April 1932) was a Baltic German chemist and philosopher. Ostwald is credited with being one of the founders of the field of physical chemistry, with Jacobus Henricus van 't Hoff, Walther Nernst, and Svante Arrhen ...
denied that atoms were real, viewing them as a convenient mathematical construct, while scientists such as
Arnold Sommerfeld Arnold Johannes Wilhelm Sommerfeld, (; 5 December 1868 – 26 April 1951) was a German theoretical physicist who pioneered developments in atomic and quantum physics, and also educated and mentored many students for the new era of theoretica ...
and
Ludwig Boltzmann Ludwig Eduard Boltzmann (; 20 February 1844 – 5 September 1906) was an Austrian physicist and philosopher. His greatest achievements were the development of statistical mechanics, and the statistical explanation of the second law of thermodyn ...
saw that physical theories required the existence of atoms. Radioactivity was discovered in 1896 by the French scientist
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 Pi ...
, while working with
phosphorescent Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluor ...
materials. 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 ...
at
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 ...
distinguished two types of radioactivity,
alpha ray Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...
s and
beta ray A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β� ...
s, which differed in their ability to penetrate, or travel into, ordinary objects or gases. Two years later,
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-Mont ...
discovered
gamma Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter r ...
rays, which possessed even more penetrating power. These radiations were soon identified with known particles: beta rays were shown to be electrons by Walter Kaufmann in 1902; alpha rays were shown to be helium ions by Rutherford and
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 ...
in 1907; and gamma rays were shown to be electromagnetic radiation, that is, a form of
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahe ...
, by Rutherford and
Edward Andrade Edward Neville da Costa Andrade FRS (27 December 1887 – 6 June 1971) was an English physicist, writer, and poet. He told ''The Literary Digest'' his name was pronounced "as written, i.e., like ''air raid'', with ''and'' substituted for ''air' ...
in 1914. These radiations had also been identified as emanating from atoms, hence they provided clues to processes occurring within atoms. Conversely, the radiations were also recognized as tools that could be exploited in scattering experiments to probe the interior of atoms.


The gold foil experiment and the discovery of the atomic nucleus

At the
University of Manchester , mottoeng = Knowledge, Wisdom, Humanity , established = 2004 – University of Manchester Predecessor institutions: 1956 – UMIST (as university college; university 1994) 1904 – Victoria University of Manchester 1880 – Victoria Univer ...
between 1908 and 1913, Rutherford directed
Hans Geiger Johannes Wilhelm "Hans" Geiger (; ; 30 September 1882 – 24 September 1945) was a German physicist. He is best known as the co-inventor of the detector component of the Geiger counter and for the Geiger–Marsden experiment which discover ...
and
Ernest Marsden Sir Ernest Marsden (19 February 1889 – 15 December 1970) was an English-New Zealand physicist. He is recognised internationally for his contributions to science while working under Ernest Rutherford, which led to the discovery of new theories ...
in a series of experiments to determine what happens when
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...
s scatter from metal foil. Now called the Rutherford gold foil experiment, or the Geiger–Marsden experiment, these measurements made the extraordinary discovery that although most alpha particles passing through a thin gold foil experienced little deflection, a few scattered to a high angle. The scattering indicated that some of the alpha particles ricocheted back from a small, but dense, component inside the atoms. Based on these measurements, by 1911 it was apparent to Rutherford that the atom consisted of a small, massive nucleus with positive charge surrounded by a much larger cloud of negatively charged
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kno ...
s. The concentrated atomic mass was required to provide the observed deflection of the alpha particles, and Rutherford developed a mathematical model that accounted for the scattering. The Rutherford model was very influential, motivating 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 ...
for electrons orbiting the nucleus in 1913 and eventually leading to
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, qu ...
by the mid-1920s.


Discovery of isotopes

Concurrent with the work of Rutherford, Geiger, and Marsden, the radiochemist
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 prove ...
at the University of Glasgow was studying chemistry related problems on radioactive materials. Soddy had worked with Rutherford on radioactivity 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 University ...
. By 1910, about 40 different radioactive elements, referred to as ''radioelements'', had been identified between uranium and lead, although the periodic table only allowed for 11 elements. Soddy and
Kazimierz 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 found in 1913 that an element undergoing alpha decay will produce an element two places to the left in the periodic system and an element undergoing beta decay will produce an element one place to the right in the periodic system. Also, those radioelements that reside in the same places in the periodic system are chemically identical. Soddy called these chemically identical elements
isotopes 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 numbe ...
. For his study of radioactivity and the discovery of isotopes, Soddy was awarded the 1921 Nobel Prize in Chemistry. Building from work by
J. J. Thomson Sir Joseph John Thomson (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered. In 1897, Thomson showed that ...
on the deflection of positively charged atoms by electric and magnetic fields,
Francis Aston Francis William Aston FRS (1 September 1877 – 20 November 1945) was a British chemist and physicist who won the 1922 Nobel Prize in Chemistry for his discovery, by means of his mass spectrograph, of isotopes in many non-radioactive elements a ...
built the first
mass spectrograph Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...
at the Cavendish Laboratory in 1919. His aim, which he easily achieved, was to separate the two isotopes of
neon Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton ...
, and . Aston discovered that the masses of all the particles are whole numbers (the
whole number rule In chemistry, the whole number rule states that the masses of the isotopes are whole number multiples of the mass of the hydrogen atom. The rule is a modified version of Prout's hypothesis proposed in 1815, to the effect that atomic weights are m ...
): that is, the masses of all the isotopes are whole number multiples of the mass of the
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxi ...
atom. In these measurements, Aston arbitrarily computed his masses relative to
oxygen-16 Oxygen-16 (16O) is a stable isotope of oxygen, having 8 neutrons and 8 protons in its nucleus. It has a mass of . Oxygen-16 is the most abundant isotope of oxygen and accounts for 99.762% of oxygen's natural abundance. The relative and absolut ...
, which he took to have a mass of exactly 16. (Today the
atomic mass unit The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at r ...
(amu) is relative to
carbon-12 Carbon-12 (12C) is the most abundant of the two stable isotopes of carbon (carbon-13 being the other), amounting to 98.93% of element carbon on Earth; its abundance is due to the triple-alpha process by which it is created in stars. Carbon-12 i ...
.) Ironically, the one exception to this rule was hydrogen itself, which had a mass value of 1.008. The excess mass was small, but well outside the limits of experimental uncertainty. Aston and others quickly realized that the discrepancy is due to the binding energy of atoms, that is, the mass of a number of hydrogen atoms bound into a single atom must be less than the sum of the masses of the separate hydrogen atoms. Aston's work on isotopes won him the 1922 Nobel Prize in Chemistry for the discovery of isotopes in a large number of non-radioactive elements, and for his enunciation of the whole number rule. Noting Aston's recent discovery of nuclear binding energy, in 1920
Arthur Eddington Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a philosopher of science and a populariser of science. The Eddington limit, the natural limit to the lumi ...
suggested that stars may obtain their energy by fusing hydrogen (protons) into helium and that the heavier elements may form in stars.


Atomic number and Moseley's law

Rutherford and others had noted the disparity between the mass of an atom, computed in atomic mass units, and the approximate charge required on the nucleus for the Rutherford model to work. The required charge of the atomic nucleus was usually about half its atomic mass.
Antonius van den Broek Antonius Johannes van den Broek (4 May 1870, Zoetermeer – 25 October 1926, Bilthoven) was a Dutch amateur physicist notable for being the first who realized that the number of an element in the periodic table (now called atomic number) correspond ...
boldly hypothesized that the required charge, denoted by ''Z'', was not half of the atomic weight for elements, but instead was exactly equal to the element's ordinal position 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 ...
. At that time, the positions of the elements in the periodic table were not known to have any physical significance. If the elements were ordered based on increasing atomic mass, however, periodicity in chemical properties was exhibited. Exceptions to this periodicity were apparent, however, such as cobalt and nickel. At the
University of Manchester , mottoeng = Knowledge, Wisdom, Humanity , established = 2004 – University of Manchester Predecessor institutions: 1956 – UMIST (as university college; university 1994) 1904 – Victoria University of Manchester 1880 – Victoria Univer ...
in 1913
Henry Moseley Henry Gwyn Jeffreys Moseley (; 23 November 1887 – 10 August 1915) was an English physicist, whose contribution to the science of physics was the justification from physical laws of the previous empirical and chemical concept of the atomic nu ...
discussed the new
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 ...
of the atom with the visiting Bohr. The model accounted for the electromagnetic emission spectrum from the hydrogen atom, and Moseley and Bohr wondered if the electromagnetic emission spectra of heavier elements such as cobalt and nickel would follow their ordering by weight, or by their position in the periodic table. In 1913–1914 Moseley tested the question experimentally by using
X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angle ...
techniques. He found that the most intense short-wavelength line in the X-ray spectrum of a particular element, known as the
K-alpha Characteristic X-rays are emitted when outer-shell electrons fill a vacancy in the inner shell of an atom, releasing X-rays in a pattern that is "characteristic" to each element. Characteristic X-rays were discovered by Charles Glover Barkla in 1909 ...
line, was related to the element's position in the periodic table, that is, its atomic number, ''Z''. Indeed, Moseley introduced this nomenclature. Moseley found that the frequencies of the radiation were related in a simple way to the atomic number of the elements for a large number of elements. Within a year it was noted that the equation for the relation, now called
Moseley's law Moseley's law is an empirical law concerning the characteristic x-rays emitted by atoms. The law had been discovered and published by the English physicist Henry Moseley in 1913-1914. Until Moseley's work, "atomic number" was merely an element's ...
, could be explained in terms of the 1913 Bohr model, with reasonable extra assumptions about atomic structure in other elements. Moseley's result, by Bohr's later account, not only established atomic number as a measurable experimental quantity, but gave it a physical meaning as the positive charge on the atomic nucleus. The elements could be ordered in the
periodic system 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 o ...
in order of atomic number, rather than atomic weight. The result tied together the organization of the periodic table, the Bohr model for the atom, and Rutherford's model for alpha scattering from nuclei. It was cited by Rutherford, Bohr, and others as a critical advance in understanding the nature of the atomic nucleus. Further research in atomic physics was interrupted by the outbreak of World War I. Moseley was killed in 1915 at the Battle of Gallipoli, while Rutherford's student
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 insp ...
was interned in Germany for the duration of the war, 1914–1918. In Berlin,
Lise Meitner Elise Meitner ( , ; 7 November 1878 – 27 October 1968) was an Austrian-Swedish physicist who was one of those responsible for the discovery of the element protactinium and nuclear fission. While working at the Kaiser Wilhelm Institute on rad ...
's and
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 ...
's research work on determining the radioactive decay chains of radium and uranium by precise chemical separation was interrupted. Meitner spent much of the war working as a
radiologist Radiology ( ) is the medical discipline that uses medical imaging to diagnose diseases and guide their treatment, within the bodies of humans and other animals. It began with radiography (which is why its name has a root referring to radiatio ...
and medical
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  ...
technician near the Austrian front, while Hahn, a
chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe th ...
, worked on research in poison gas warfare.


Rutherford atom

In 1920 Rutherford gave a
Bakerian lecture The Bakerian Medal is one of the premier medals of the Royal Society that recognizes exceptional and outstanding science. It comes with a medal award and a prize lecture. The medalist is required to give a lecture on any topic related to physical ...
at the Royal Society entitled the "Nuclear Constitution of Atoms", a summary of recent experiments on atomic nuclei and conclusions as to the structure of atomic nuclei. By 1920, the existence of electrons within the atomic nucleus was widely assumed. It was assumed the nucleus consisted of hydrogen nuclei in number equal to the atomic mass. But since each hydrogen nucleus had charge +1, the nucleus required a smaller number of "internal electrons" each of charge −1 to give the nucleus its correct total charge. The mass of protons is about 1800 times greater than that of electrons, so the mass of the electrons is incidental in this computation. Such a model was consistent with the scattering of alpha particles from heavy nuclei, as well as the charge and mass of the many isotopes that had been identified. There were other motivations for the proton–electron model. As noted by Rutherford at the time, "We have strong reason for believing that the nuclei of atoms contain electrons as well as positively charged bodies...", namely, it was known that
beta radiation A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β� ...
was electrons emitted from the nucleus. In that lecture, Rutherford conjectured the existence of new particles. The alpha particle was known to be very stable, and it was assumed to retain its identity within the nucleus. The alpha particle was presumed to consist of four protons and two closely bound electrons to give it +2 charge and mass 4. In a 1919 paper, Rutherford had reported the apparent discovery of a new doubly charged particle of mass 3, denoted the X++, interpreted to consist of three protons and a closely bound electron. This result suggested to Rutherford the likely existence of two new particles: one of two protons with a closely bound electron, and another of one proton and a closely bound electron. The X++ particle was later determined to have mass 4 and to be just a low-energy alpha particle. Nevertheless, Rutherford had conjectured the existence of the deuteron, a +1 charge particle of mass 2, and the neutron, a neutral particle of mass 1. The former is the nucleus of
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one n ...
, discovered in 1931 by
Harold Urey Harold Clayton Urey ( ; April 29, 1893 – January 5, 1981) was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934 for the discovery of deuterium. He played a significant role in the d ...
. The mass of the hypothetical neutral particle would be little different from that of the proton. Rutherford determined that such a zero-charge particle would be difficult to detect by available techniques. About the time of Rutherford's lecture, other publications appeared with similar suggestions of a proton–electron composite in the nucleus, and in 1921 William Harkins, an American chemist, named the uncharged particle the ''neutron''. About that same time the word ''proton'' was adopted for the hydrogen nucleus. Neutron was apparently constructed from the
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
root for ''neutral'' and the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
ending ''-on'' (by imitation of ''
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kno ...
'' and ''
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 ...
''). References to the word ''neutron'' in connection with the atom can be found in the literature as early as 1899, however. Rutherford and Chadwick immediately began an experimental program 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 ...
in
Cambridge Cambridge ( ) is a university city and the county town in Cambridgeshire, England. It is located on the River Cam approximately north of London. As of the 2021 United Kingdom census, the population of Cambridge was 145,700. Cambridge becam ...
to search for the neutron. The experiments continued throughout the 1920s without success. Rutherford's conjecture and the hypothetical "neutron" were not widely accepted. In his 1931 monograph on the ''Constitution of Atomic Nuclei and Radioactivity'',
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, theore ...
, then at the Institute for Theoretical Physics in Copenhagen, did not mention the neutron.Gamow G. (1931) ''Constitution of Atomic Nuclei and Radiation'', Oxford: Clarendon Press At the time of their 1932 measurements in Paris that would lead to the discovery of the neutron,
Irène Joliot-Curie Irène Joliot-Curie (; ; 12 September 1897 – 17 March 1956) was a French chemist, physicist and politician, the elder daughter of Pierre and Marie Curie, and the wife of Frédéric Joliot-Curie. Jointly with her husband, Joliot-Curie was award ...
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 ...
were unaware of the conjecture.


Problems of the nuclear electrons hypothesis

Throughout the 1920s, physicists assumed that the atomic nucleus was composed of protons and "nuclear electrons". Under this hypothesis, the nitrogen-14 (14N) nucleus would be composed of 14 protons and 7 electrons, so that it would have a net charge of +7
elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
units and a mass of 14 atomic mass units. This nucleus would also be orbited by another 7 electrons, termed "external electrons" by Rutherford, to complete the 14N atom. However problems with the hypothesis soon became apparent.
Ralph Kronig Ralph Kronig (10 March 1904 – 16 November 1995) was a German physicist. He is noted for the discovery of particle spin and for his theory of X-ray absorption spectroscopy. His theories include the Kronig–Penney model, the Coster–Kronig ...
pointed out in 1926 that the observed
hyperfine structure In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the nucl ...
of atomic spectra was inconsistent with the proton–electron hypothesis. This structure is caused by the influence of the nucleus on the dynamics of orbiting electrons. The magnetic moments of supposed "nuclear electrons" should produce hyperfine spectral line splittings similar to the
Zeeman effect The Zeeman effect (; ) is the effect of splitting of a spectral line into several components in the presence of a static magnetic field. It is named after the Dutch physicist Pieter Zeeman, who discovered it in 1896 and received a Nobel prize ...
, but no such effects were observed. It seemed that the magnetic moment of the electron vanished when it was within the nucleus. While on a visit to
Utrecht University Utrecht University (UU; nl, Universiteit Utrecht, formerly ''Rijksuniversiteit Utrecht'') is a public research university in Utrecht, Netherlands. Established , it is one of the oldest universities in the Netherlands. In 2018, it had an enrollm ...
in 1928, Kronig learned of a surprising aspect of the rotational spectrum of N2+. The precision measurement made by
Leonard Ornstein Leonard Salomon Ornstein (November 12, 1880 in Nijmegen, the Netherlands – May 20, 1941 in Utrecht, the Netherlands) was a Dutch physicist. Biography Ornstein studied theoretical physics with Hendrik Antoon Lorentz at University of Lei ...
, the director of Utrecht's Physical Laboratory, showed that the spin of nitrogen nucleus must be equal to one. However, if the nitrogen-14 (14N) nucleus was composed of 14 protons and 7 electrons, an odd number of spin-1/2 particles, then the resultant nuclear spin should be half-integer. Kronig therefore suggested that perhaps "protons and electrons do not retain their identity to the extent they do outside the nucleus". Observations of the rotational energy levels of diatomic molecules using
Raman spectroscopy Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman s ...
by
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 ...
in 1929 were inconsistent with the statistics expected from the proton–electron hypothesis. Rasetti obtained band spectra for H2 and N2 molecules. While the lines for both diatomic molecules showed alternation in intensity between light and dark, the pattern of alternation for H2 is opposite to that of the N2. After carefully analyzing these experimental results, German physicists
Walter Heitler Walter Heinrich Heitler (; 2 January 1904 – 15 November 1981) was a German physicist who made contributions to quantum electrodynamics and quantum field theory. He brought chemistry under quantum mechanics through his theory of valence bondi ...
and
Gerhard Herzberg Gerhard Heinrich Friedrich Otto Julius Herzberg, (; December 25, 1904 – March 3, 1999) was a German-Canadian pioneering physicist and physical chemist, who won the Nobel Prize for Chemistry in 1971, "for his contributions to the knowledge ...
showed that the hydrogen nuclei obeys Fermi statistics and the nitrogen nuclei obeys Bose statistics. However, a then unpublished result of
Eugene Wigner Eugene Paul "E. P." Wigner ( hu, Wigner Jenő Pál, ; November 17, 1902 – January 1, 1995) was a Hungarian-American theoretical physicist who also contributed to mathematical physics. He received the Nobel Prize in Physics in 1963 "for his con ...
showed that a composite system with an odd number of spin-1/2 particles must obey Fermi statistics; a system with an even number of spin-1/2 particle obeys Bose statistics. If the nitrogen nucleus had 21 particles, it should obey Fermi statistics, contrary to fact. Thus, Heitler and Herzberg concluded: "the electron in the nucleus ... loses its ability to determine the statistics of the nucleus." The Klein paradox, discovered by
Oskar Klein Oskar Benjamin Klein (; 15 September 1894 – 5 February 1977) was a Swedish theoretical physicist. Biography Klein was born in Danderyd outside Stockholm, son of the chief rabbi of Stockholm, Gottlieb Klein from Humenné in Kingdom of Hunga ...
in 1928, presented further quantum mechanical objections to the notion of an electron confined within a nucleus. Derived from the
Dirac equation In particle physics, the Dirac equation is a relativistic wave equation derived by British physicist Paul Dirac in 1928. In its free form, or including electromagnetic interactions, it describes all spin- massive particles, called "Dirac p ...
, this clear and precise paradox suggested that an electron approaching a high potential barrier has a high probability of passing through the barrier by a pair creation process. Apparently, an electron could not be confined within a nucleus by any potential well. The meaning of this paradox was intensely debated at the time. By about 1930 it was generally recognized that it was difficult to reconcile the proton–electron model for nuclei with the Heisenberg uncertainty relation of quantum mechanics. This relation, , implies that an electron confined to a region the size of an atomic nucleus typically has a kinetic energy of about 40 MeV, which is larger than the observed energy of beta particles emitted from the nucleus. Such energy is also much larger than the binding energy of nucleons, which Aston and others had shown to be less than 9 MeV per nucleon. In 1927, Charles Ellis and W. Wooster at the Cavendish Laboratory measured the energies of β-decay electrons. They found that the distribution of energies from any particular radioactive nuclei was broad and continuous, a result that contrasted notably with the distinct energy values observed in alpha and gamma decay. Further, the continuous energy distribution seemed to indicate that energy was not conserved by this "nuclear electrons" process. Indeed, in 1929 Bohr proposed to modify the law of energy conservation to account for the continuous energy distribution. The proposal earned the support of Werner Heisenberg. Such considerations were apparently reasonable, inasmuch as the laws of quantum mechanics had so recently overturned the laws of classical mechanics. While all these considerations did not "prove" an electron could not exist in the nucleus, they were confusing and challenging for
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate caus ...
s to interpret. Many theories were invented to explain how the above arguments could be wrong. In his 1931 monograph, Gamow summarized all these contradictions, marking the statements regarding electrons in the nucleus with warning symbols.


Discovery of the neutron

In 1930,
Walther Bothe Walther Wilhelm Georg Bothe (; 8 January 1891 – 8 February 1957) was a German nuclear physicist, who shared the Nobel Prize in Physics in 1954 with Max Born. In 1913, he joined the newly created Laboratory for Radioactivity at the Reich Physi ...
and Herbert Becker in
Giessen Giessen, spelled Gießen in German (), is a town in the German state (''Bundesland'') of Hesse, capital of both the district of Giessen and the administrative region of Giessen. The population is approximately 90,000, with roughly 37,000 univer ...
, Germany found that if the energetic
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...
s emitted from
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 ...
fell on certain light elements, specifically
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 ...
(),
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 ...
(), or
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 e ...
(), an unusually penetrating radiation was produced. Beryllium produced the most intense radiation. Polonium is highly radioactive, producing energetic alpha radiation, and it was commonly used for scattering experiments at the time. Alpha radiation can be influenced by an electric field, because it is composed of charged particles. The observed penetrating radiation was not influenced by an electric field, however, so it was thought to be
gamma radiation 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 sh ...
. The radiation was more penetrating than any gamma rays known, and the details of experimental results were difficult to interpret. Two years later
Irène Joliot-Curie Irène Joliot-Curie (; ; 12 September 1897 – 17 March 1956) was a French chemist, physicist and politician, the elder daughter of Pierre and Marie Curie, and the wife of Frédéric Joliot-Curie. Jointly with her husband, Joliot-Curie was award ...
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 ...
in Paris showed that if this unknown radiation fell on paraffin wax, or any other
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxi ...
-containing compound, it ejected protons of very high energy (5 MeV). This observation was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but that interpretation (
Compton scattering Compton scattering, discovered by Arthur Holly Compton, is the scattering of a high frequency photon after an interaction with a charged particle, usually an electron. If it results in a decrease in energy (increase in wavelength) of the photon ...
) had a logical problem. From energy and momentum considerations, a gamma ray would have to have impossibly high energy (50 MeV) to scatter a massive proton. In Rome, the young physicist
Ettore Majorana Ettore Majorana (,, uploaded 19 April 2013, retrieved 14 December 2019 ; born on 5 August 1906 – possibly dying after 1959) was an Italian theoretical physicist who worked on neutrino masses. On 25 March 1938, he disappeared under mysteri ...
declared that the manner in which the new radiation interacted with protons required a new neutral particle. On hearing of the Paris results, neither Rutherford nor
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 insp ...
at the Cavendish Laboratory believed the gamma ray hypothesis. Assisted by
Norman Feather Norman Feather FRS FRSE PRSE (16 November 1904 – 14 August 1978), was an English nuclear physicist. Feather and Egon Bretscher were working at the Cavendish Laboratory, Cambridge in 1940, when they proposed that the 239 isotope of element ...
, Chadwick quickly performed a series of experiments showing that the gamma ray hypothesis was untenable. The previous year, Chadwick, J.E.R. Constable, and E.C. Pollard had already conducted experiments on disintegrating light elements using alpha radiation from polonium. They had also developed more accurate and efficient methods for detecting, counting, and recording the ejected protons. Chadwick repeated the creation of the radiation using beryllium to absorb the alpha particles: Following the Paris experiment, he aimed the radiation at paraffin wax, a hydrocarbon high in hydrogen content, hence offering a target dense with protons. As in the Paris experiment, the radiation energetically scattered some of the protons. Chadwick measured the range of these protons, and also measured how the new radiation impacted the atoms of various gases. He found that the new radiation consisted of not gamma rays, but uncharged particles with about the same mass as the
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 ...
. These particles were neutrons. Chadwick won the
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 " ...
in 1935 for this discovery. The year 1932 was later referred to as the "
annus mirabilis ''Annus mirabilis'' (pl. ''anni mirabiles'') is a Latin phrase that means "marvelous year", "wonderful year", "miraculous year", or "amazing year". This term has been used to refer to several years during which events of major importance are rem ...
" for nuclear physics in the Cavendish Laboratory, with discoveries of the neutron, artificial nuclear disintegration by the Cockcroft–Walton particle accelerator, and the
positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
.


Proton–neutron model of the nucleus

Given the problems of the ''proton–electron model'',Friedlander, G.; Kennedy, J.W.; Miller, J.M. (1964) ''Nuclear and Radiochemistry'' (2nd edition), Wiley, pp. 22–23 and 38–39 it was quickly accepted that the atomic nucleus is composed of protons and neutrons, although the precise nature of the neutron was initially unclear. Within months after the discovery of the neutron,
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 Über quantentheoretische Umdeutung kinematis ...
and
Dmitri Ivanenko Dmitri Dmitrievich Ivanenko (russian: Дми́трий Дми́триевич Иване́нко; July 29, 1904 – December 30, 1994) was a Ukrainian theoretical physicist who made great contributions to the physical science of the twentieth cen ...
had proposed proton–neutron models for the nucleus. Heisenberg's landmark papers approached the description of protons and neutrons in the nucleus through quantum mechanics. While Heisenberg's theory for protons and neutrons in the nucleus was a "major step toward understanding the nucleus as a quantum mechanical system," he still assumed the presence of nuclear electrons. In particular, Heisenberg assumed the neutron was a proton–electron composite, for which there is no quantum mechanical explanation. Heisenberg had no explanation for how lightweight electrons could be bound within the nucleus. Heisenberg introduced the first theory of nuclear exchange forces that bind the nucleons. He considered protons and neutrons to be different quantum states of the same particle, i.e., nucleons distinguished by the value of their nuclear
isospin In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions o ...
quantum numbers. The proton–neutron model explained the puzzle of dinitrogen. When 14N was proposed to consist of 3 pairs each of protons and neutrons, with an additional unpaired neutron and proton each contributing a spin of  ħ in the same direction for a total spin of 1 ħ, the model became viable. Soon, neutrons were used to naturally explain spin differences in many different nuclides in the same way. If the proton–neutron model for the nucleus resolved many issues, it highlighted the problem of explaining the origins of beta radiation. No existing theory could account for how electrons, or positrons, could emanate from the nucleus. In 1934,
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 t ...
published his classic paper describing the process of beta decay, in which the neutron decays to a proton by ''creating'' an electron and a (as yet undiscovered)
neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is ...
. The paper employed the analogy that
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always m ...
s, or electromagnetic radiation, were similarly created and destroyed in atomic processes. Ivanenko had suggested a similar analogy in 1932. Fermi's theory requires the neutron to be a spin- particle. The theory preserved the principle of conservation of energy, which had been thrown into question by the continuous energy distribution of beta particles. The basic theory for beta decay proposed by Fermi was the first to show how particles could be created and destroyed. It established a general, basic theory for the interaction of particles by weak or strong forces. While this influential paper has stood the test of time, the ideas within it were so new that when it was first submitted to the journal
Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the 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. Although humans are p ...
in 1933 it was rejected as being too speculative.


The nature of the neutron

The question of whether the neutron was a composite particle of a proton and an electron persisted for a few years after its discovery. In 1932
Harrie Massey Sir Harrie Stewart Wilson Massey (16 May 1908 – 27 November 1983) was an Australian mathematical physicist who worked primarily in the fields of atomic and atmospheric physics. A graduate of the University of Melbourne and Cambridge Unive ...
explored a model for a composite neutron to account for its great penetrating power through matter and its electrical neutrality, for example. The issue was a legacy of the prevailing view from the 1920s that the only elementary particles were the proton and electron. The nature of the neutron was a primary topic of discussion at the 7th
Solvay Conference The Solvay Conferences (french: Conseils Solvay) have been devoted to outstanding preeminent open problems in both physics and chemistry. They began with the historic invitation-only 1911 Solvay Conference on Physics, considered a turning point ...
held in October 1933, attended by Heisenberg,
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 ...
,
Lise Meitner Elise Meitner ( , ; 7 November 1878 – 27 October 1968) was an Austrian-Swedish physicist who was one of those responsible for the discovery of the element protactinium and nuclear fission. While working at the Kaiser Wilhelm Institute on rad ...
,
Ernest Lawrence Ernest Orlando Lawrence (August 8, 1901 – August 27, 1958) was an American nuclear physicist 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 fo ...
, Fermi, Chadwick, and others. As posed by Chadwick in his
Bakerian Lecture The Bakerian Medal is one of the premier medals of the Royal Society that recognizes exceptional and outstanding science. It comes with a medal award and a prize lecture. The medalist is required to give a lecture on any topic related to physical ...
in 1933, the primary question was the mass of the neutron relative to the proton. If the neutron's mass was less than the combined masses of a proton and an electron (), then the neutron could be a proton-electron composite because of the mass defect from the
nuclear binding energy Nuclear binding energy in experimental physics is the minimum energy that is required to disassemble the nucleus of an atom into its constituent protons and neutrons, known collectively as nucleons. The binding energy for stable nuclei is always ...
. If greater than the combined masses, then the neutron was elementary like the proton. The question was challenging to answer because the electron's mass is only 0.05% of the proton's, hence exceptionally precise measurements were required. The difficulty of making the measurement is illustrated by the wide-ranging values for the mass of the neutron obtained from 1932 to 1934. The accepted value today is . In Chadwick's 1932 paper reporting on the discovery, he estimated the mass of the neutron to be between and . By bombarding boron with alpha particles, Frédéric and Irène Joliot-Curie obtained a high value of , while Ernest Lawrence's team at the University of California measured the small value using their new
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: Jan ...
. In 1935 Chadwick and his doctoral student Maurice Goldhaber resolved the issue by reporting the first accurate measurement of the mass of the neutron. They used the 2.6 MeV gamma rays of
Thallium Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes and ...
-208 (208Tl) (then known as thorium C") to photodisintegrate the deuteron : In this reaction, the resulting proton and neutron have about equal kinetic energy, since their masses are about equal. The kinetic energy of the resulting proton could be measured (0.24 MeV), and therefore the deuteron's binding energy could be determined (2.6 MeV − 2(0.24 MeV) = 2.1 MeV, or ). The neutron's mass could then be determined by the simple mass balance : where md,p,n refer to the deuteron, proton, or neutron mass, and "b.e." is the binding energy. The masses of the deuteron and proton were known; Chadwick and Goldhaber used values 2.0142 u and 1.0081 u, respectively. They found that the neutron's mass was slightly greater than the mass of the proton or , depending on the precise value used for the deuteron mass. The mass of the neutron was too large to be a proton–electron composite, and the neutron was therefore identified as an elementary particle. Chadwick and Goldhaber predicted that a free neutron would be able to decay into a proton, electron, and neutrino (
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 e ...
).


Neutron physics in the 1930s

Soon after the discovery of the neutron, indirect evidence suggested the neutron had an unexpected non-zero value for its magnetic moment. Attempts to measure the neutron's magnetic moment originated with the discovery by
Otto Stern :''Otto Stern was also the pen name of German women's rights activist Louise Otto-Peters (1819–1895)''. Otto Stern (; 17 February 1888 – 17 August 1969) was a German-American physicist and Nobel laureate in physics. He was the second most ...
in 1933 in
Hamburg (male), (female) en, Hamburger(s), Hamburgian(s) , timezone1 = Central (CET) , utc_offset1 = +1 , timezone1_DST = Central (CEST) , utc_offset1_DST = +2 , postal ...
that the proton had an anomalously large magnetic moment. By 1934 groups led by Stern, now in
Pittsburgh Pittsburgh ( ) is a city in the Commonwealth of Pennsylvania, United States, and the county seat of Allegheny County. It is the most populous city in both Allegheny County and Western Pennsylvania, the second-most populous city in Pennsylv ...
, and I. I. Rabi in
New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * ...
had independently deduced that the magnetic moment of the neutron was negative and unexpectedly large by measuring the magnetic moments of the proton and
deuteron Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one n ...
. Values for the magnetic moment of the neutron were also determined by Robert Bacher (1933) at
Ann Arbor Anne, alternatively spelled Ann, is a form of the Latin female given name Anna. This in turn is a representation of the Hebrew Hannah, which means 'favour' or 'grace'. Related names include Annie. Anne is sometimes used as a male name in th ...
and I.Y. Tamm and S.A. Altshuler (1934) in the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...
from studies of the hyperfine structure of atomic spectra. By the late 1930s accurate values for the magnetic moment of the neutron had been deduced by the Rabi group using measurements employing newly developed
nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
techniques. The large value for the proton's magnetic moment and the inferred negative value for the neutron's magnetic moment were unexpected and raised many questions. The discovery of the neutron immediately gave scientists a new tool for probing the properties of atomic nuclei. Alpha particles had been used over the previous decades in scattering experiments, but such particles, which are helium nuclei, have +2 charge. This charge makes it difficult for alpha particles to overcome the Coulomb repulsive force and interact directly with the nuclei of atoms. Since neutrons have no electric charge, they do not have to overcome this force to interact with nuclei. Almost coincident with their discovery, neutrons were used by
Norman Feather Norman Feather FRS FRSE PRSE (16 November 1904 – 14 August 1978), was an English nuclear physicist. Feather and Egon Bretscher were working at the Cavendish Laboratory, Cambridge in 1940, when they proposed that the 239 isotope of element ...
, Chadwick's colleague and protege, in scattering experiments with nitrogen. Feather was able to show that neutrons interacting with nitrogen nuclei scattered to protons or induced nitrogen to disintegrate to form
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 ...
with the emission of an alpha particle. Feather was therefore the first to show that neutrons produce nuclear disintegrations. In
Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus (Romulus and Remus, legendary) , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg ...
, Enrico Fermi and his team bombarded heavier elements with neutrons and found the products to be radioactive. By 1934 they had used neutrons to induce radioactivity in 22 different elements, many of these elements of high atomic number. Noticing that other experiments with neutrons at his laboratory seemed to work better on a wooden table than a marble table, Fermi suspected that the protons of the wood were slowing the neutrons and so increasing the chance for the neutron to interact with nuclei. Fermi therefore passed neutrons through paraffin wax to slow them and found that the radioactivity of some bombarded elements increased by a factor of tens to hundreds. The
cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...
for interaction with nuclei is much larger for slow neutrons than for fast neutrons. In 1938 Fermi received the 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 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 transformation o ...
s brought about by slow neutrons"''. In Berlin, the collaboration of
Lise Meitner Elise Meitner ( , ; 7 November 1878 – 27 October 1968) was an Austrian-Swedish physicist who was one of those responsible for the discovery of the element protactinium and nuclear fission. While working at the Kaiser Wilhelm Institute on rad ...
and
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 ...
, together with their assistant
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 k ...
, furthered the research begun by Fermi and his team when they bombarded uranium with neutrons. Between 1934 and 1938, Hahn, Meitner, and Strassmann found a great number of radioactive transmutation products from these experiments, all of which they regarded as
transuranic 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. ...
. Transuranic nuclides are those that have an atomic number greater than uranium (92), formed by neutron absorption; such nuclides are not naturally occurring. In July 1938, Meitner was forced to escape
antisemitic Antisemitism (also spelled anti-semitism or anti-Semitism) is hostility to, prejudice towards, or discrimination against Jews. A person who holds such positions is called an antisemite. Antisemitism is considered to be a form of racism. Antis ...
persecution in
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 ...
after the
Anschluss The (, or , ), also known as the (, en, Annexation of Austria), was the annexation of the Federal State of Austria into the Nazi Germany, German Reich on 13 March 1938. The idea of an (a united Austria and Germany that would form a "Ger ...
, and she was able to secure a new position in Sweden. The decisive experiment on 16–17 December 1938 (using a chemical process called "radium–barium–mesothorium
fractionation Fractionation is a separation process in which a certain quantity of a mixture (of gases, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in which the ...
") produced puzzling results: what they had understood to be three isotopes of radium were instead consistently behaving as
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. The ...
. Radium (atomic number 88) and barium (atomic number 56) are in the same
chemical group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the res ...
. By January 1939 Hahn had concluded that what they had thought were transuranic nuclides were instead much lighter nuclides, such as barium,
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 lant ...
,
cerium Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 o ...
and light platinoids. Meitner and her nephew
Otto 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 first ...
immediately and correctly interpreted these observations as resulting from
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 radioa ...
, a term coined by Frisch. Hahn and his collaborators had detected the splitting of uranium nuclei, made unstable by neutron absorption, into lighter elements. Meitner and Frisch also showed that the fission of each uranium atom would release about 200 MeV of energy. The discovery of fission electrified the global community of atomic physicists and the public. In their second publication on nuclear fission, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process.
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 ...
and his team proved this phenomenon to be a
chain reaction A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events. Chain reactions are one way that sys ...
in March 1939. In 1945 Hahn received 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 his discovery of the fission of heavy atomic nuclei."''


After 1939

The discovery of nuclear fission at the end of 1938 marked a shift in the centers of nuclear research from Europe to the United States. Large numbers of scientists were migrating to the United States to escape the troubles and
antisemitism Antisemitism (also spelled anti-semitism or anti-Semitism) is hostility to, prejudice towards, or discrimination against Jews. A person who holds such positions is called an antisemite. Antisemitism is considered to be a form of racism. Antis ...
in Europe and the looming
war War is an intense armed conflict between states, governments, societies, or paramilitary groups such as mercenaries, insurgents, and militias. It is generally characterized by extreme violence, destruction, and mortality, using regular o ...
(See Jewish scientists and the Manhattan Project). The new centers of nuclear research were the universities in the United States, particularly Columbia University in New York and the
University of Chicago The University of Chicago (UChicago, Chicago, U of C, or UChi) is a private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park neighborhood. The University of Chicago is consistently ranked among the be ...
where Enrico Fermi had relocated, and a secret research facility at Los Alamos,
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...
, established in 1942, the new home 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 ...
. This wartime project was focussed on the construction of
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 bomb ...
s, exploiting the enormous energy released by the fission of uranium or
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 exhibits ...
through neutron-based chain reactions. The discoveries of the neutron and positron in 1932 were the start of the discoveries of many new particles.
Muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wit ...
s were discovered in 1936.
Pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gen ...
s and
kaon KAON (Karlsruhe ontology) is an ontology infrastructure developed by the University of Karlsruhe and the Research Center for Information Technologies in Karlsruhe. Its first incarnation was developed in 2002 and supported an enhanced version of ...
s were discovered in 1947, while lambda particles were discovered in 1950. Throughout the 1950s and 1960s, a large number of particles called
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the e ...
s were discovered. A classification scheme for organizing all these particles, proposed independently by
Murray Gell-Mann Murray Gell-Mann (; September 15, 1929 – May 24, 2019) was an American physicist who received the 1969 Nobel Prize in Physics for his work on the theory of elementary particles. He was the Robert Andrews Millikan Professor of Theoretical ...
and
George Zweig George Zweig (; born May 30, 1937) is a Russian-American physicist. He was trained as a particle physicist under Richard Feynman. He introduced, independently of Murray Gell-Mann, the quark model (although he named it "aces"). He later turned his ...
in 1964, became known as the
quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the ...
. By this model, particles such as the proton and neutron were not elementary, but composed of various configurations of a small number of other truly elementary particles called partons or
quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...
s. The quark model received experimental verification beginning in the late 1960s and finally provided an explanation for the neutron's anomalous magnetic moment.


Videos


Ernest Rutherford summarizes the state of nuclear physics in 1935.
(7 min., Nobelprize.org)
Hans Bethe discusses Chadwick and Goldhaber's work on deuteron disintegration.
(2 min., Web of Stories)


Explanatory notes


References


Further reading


Annotated bibliography for neutrons from the Alsos Digital Library for Nuclear Issues
*
Abraham Pais Abraham Pais (; May 19, 1918 – July 28, 2000) was a Dutch- American physicist and science historian. Pais earned his Ph.D. from University of Utrecht just prior to a Nazi ban on Jewish participation in Dutch universities during World War II ...
, ''Inward Bound'', Oxford: Oxford University Press, 1986. . *
Herwig Schopper Herwig Franz Schopper (born 28 February 1924) is a Czech-born experimental physicist and was the director general of CERN from 1981 to 1988. Biography Schopper was born in Lanškroun, Bohemia, to a family of Austrian descent. He obtained his d ...
, ''Weak interactions and nuclear beta decay'', Publisher, North-Holland Pub. Co., 1966. * Ruth Lewin Sime, ''Lise Meitner: A Life in Physics'', Berkeley, University of California Press, 1996. . * Roger H. Stuewer, "The Nuclear Electron Hypothesis". In ''Otto Hahn and the Rise of Nuclear Physics'', William R. Shea, ed. Dordrecht, Holland: D. Riedel Publishing Company. pp. 19–67, 1983. . * Sin-Itiro Tomonaga, ''The Story of Spin'', The University of Chicago Press, 1997. {{ISBN, 9780226807942 Neutron