Thediscovery of the neutron and its properties was central to the significant developments inatomic physics in the first half of the 20th century. Early in the century,Ernest Rutherford developed a crudemodel of the atom,[1]: 188 [2] based on thegold foil experiment ofHans Geiger andErnest Marsden. In this model, atoms had theirmass andpositive electric charge concentrated in a very smallnucleus.[3] By 1920,isotopes ofchemical elements had been discovered, theatomic masses had been determined to be approximatelyinteger multiples of the mass of thehydrogen atom,[4] and theatomic number had been identified as the charge on the nucleus.[5]: §1.1.2 Throughout the 1920s, the nucleus was viewed as composed of combinations ofprotons andelectrons, the twoelementary particles known at the time, but that model presented several experimental and theoretical contradictions.[1]: 298
The essential nature of the atomic nucleus was established with the discovery of theneutron byJames Chadwick in 1932[6] and the determination that it was a new elementary particle, distinct from the proton.[7][8]: 55
The uncharged neutron was immediately utilized as a new means to probe nuclear structure, leading to such discoveries as the creation of new radioactive elements by neutron irradiation (1934) and thefission ofuranium atoms by neutrons (1938).[9] The discovery of fission led to the creation of bothnuclear power andnuclear weapons by the end of World War II. Both the proton and the neutron were presumed to be elementary particles until the 1960s, when they were determined to be composite particles built fromquarks.[10]
At the start of the 20th century, the debate as to the existence of atoms had not yet been resolved. Philosophers such asErnst Mach andWilhelm Ostwald denied the existence of atoms, viewing them as a convenient mathematical construct, while scientists such asArnold Sommerfeld andLudwig Boltzmann saw that physical theories required the existence of atoms.[9]: 13–14
Radioactivity was discovered in 1896 by theFrench scientistHenri Becquerel, while working withphosphorescent materials. In 1898,Ernest Rutherford atCavendish Laboratory distinguished two types of radioactivity,alpha rays andbeta rays, which differed in their ability to penetrate, or travel into, ordinary objects or gases. Two years later,Paul Villard discoveredgamma rays, which possessed even more penetrating power.[1]: 8–9 These radiations were soon identified with known particles: beta rays were shown to be electrons byWalter Kaufmann in 1902, alpha rays were shown to be helium ions by Rutherford andThomas Royds in 1907, and gamma rays were shown to be electromagnetic radiation, that is, a form oflight, by Rutherford andEdward Andrade in 1914.[1]: 61–62, 87 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 utilized in scattering experiments to probe the interior of atoms.[11]: 112–115
At theUniversity of Manchester between 1908 and 1913, Rutherford directedHans Geiger andErnest Marsden in a series of experiments to determine what occurs whenalpha particles scatter from metal foil. Now called theRutherford 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 fewscattered 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, it was apparent to Rutherford by 1911 that the atom consisted of a small, massive nucleus with positive charge surrounded by a much larger cloud of negatively chargedelectrons. 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.[12]: 188 [2]
While the Rutherford model was largely ignored at the time,[12] whenNiels Bohr joined Rutherford's group, he developed theBohr model for electrons orbiting the nucleus in 1913.[13] The Bohr model eventually led to an atomic model based onquantum mechanics by the mid-1920s.
Concurrent with the work of Rutherford, Geiger, and Marsden, theradiochemistFrederick Soddy at theUniversity of Glasgow was studying chemistry-related problems on radioactive materials. Soddy had worked with Rutherford on radioactivity atMcGill University.[14] By 1910, about 40 different radioactive elements, referred to asradioelements, had been identified between uranium and lead, although the periodic table only allowed for 11 elements. Soddy andKazimierz Fajans 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. Additionally, those radioelements that reside in the same places in the periodic system are chemically identical. Soddy called these chemically identical elementsisotopes.[15]: 3–5 [16] For his study of radioactivity and the discovery of isotopes, Soddy was awarded the 1921 Nobel Prize in Chemistry.[17]
.jpg)
Building from work byJ. J. Thomson on the deflection of positively charged atoms by electric and magnetic fields,Francis Aston built the firstmass spectrograph at the Cavendish Laboratory in 1919. He was then able to separate the two isotopes ofneon,20
Ne and22
Ne. Aston discovered thewhole number rule: that the masses of all the particles have whole number relationships tooxygen-16,[18] which he took to have a mass of exactly 16.[4] Today, the whole-number rule is expressed in multiples of adalton (Da), which is defined relative tocarbon-12.[19]. Significantly, 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.
Since Einstein'smass-energy equivalence had been known since 1905, Aston and others quickly realized that the mass discrepancy is due to the binding energy of atoms. When the contents of a number of hydrogen atoms are bound into a single atom, the single atom's energy must be less than the sum of the energies of the separate hydrogen atoms, and, therefore, the single atom's mass is less than the sum of the hydrogen atom masses.[4] 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.[20] Noting Aston's recent discovery of nuclear binding energy, in 1920Arthur Eddington suggested that stars may obtain their energy by fusing hydrogen (protons) into helium and that the heavier elements may form in stars.[21]
Rutherford and others had noted the disparity between the atomic mass number of an atom, and the approximate charge required on the nucleus for the Rutherford model to work. The charge of the atomic nucleus in elementary charges was typically about half its atomic mass number.[22]: 82 Antonius van den Broek hypothesized that the required charge, denoted byZ, was not half of the atomic weight for elements, but instead was exactly equal to the element's ordinal position in theperiodic table.[1]: 228 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.[a][23]: 180
At theUniversity of Manchester in 1913Henry Moseley discussed the newBohr model of the atom with the visiting Bohr.[22] The model accounted for the electromagnetic emission spectrum from the hydrogen atom, and Moseley and Bohr speculated 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.[24]: 346 In 1913–1914 Moseley tested the question experimentally by usingX-ray diffraction techniques. He found that the most intenseshort-wavelength line in the X-ray spectrum of a particular element, known as theK-alpha line, was related to the element's position in the periodic table, that is, its atomic number,Z. Indeed, Moseley introduced this nomenclature.[5]: §1.1.2 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.[25][5]: 5 [23]: 181
Within a year, it was noted that the equation for the relation, now calledMoseley's law, could be explained in terms of the 1913 Bohr model with reasonable extra assumptions about atomic structure in other elements.[26]: 87 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 theperiodic system in order of atomic number, rather than atomic weight.[27]: 127 The result tied together the organization of the periodic table, the Bohr model for the atom,[28]: 56 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.[29]
Further research in atomic physics was interrupted by the onset ofWorld War I. Moseley was killed in 1915 at theBattle of Gallipoli,[30][23]: 182 while Rutherford's studentJames Chadwick was interned in Germany for the duration of the war, 1914–1918.[31] In Berlin,Lise Meitner's andOtto Hahn's research work on determining the radioactive decay chains of radium and uranium by precise chemical separation was interrupted.[9]: §4 Meitner spent much of the war working as aradiologist and medicalX-ray technician near the Austrian front, while Hahn, achemist, worked on research inpoison gas warfare.[9]: 61–62, 68
In 1920, Rutherford gave aBakerian lecture 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.[32][8]: 23 [5]: 5 By 1920, the existence of electrons within the atomic nucleus was widely assumed. It was also assumed the nucleus consisted of hydrogen nuclei in number equal to the atomic mass number. But, since each hydrogen nucleus had charge +1 e, the nucleus required a smaller number of "internal electrons" each of charge −1 e 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.[1]: 230–231 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 ...";[32]: 376–377 namely, it was known thatbeta radiation was electrons emitted from the nucleus.[8]: 21 [5]: 5–6
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,[33] 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.[8]: 25 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.[32]: 396 The former is the nucleus ofdeuterium, discovered in 1931 byHarold Urey.[34] 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 at the time.[32]: 396
Around 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 theneutron.[35][36][37][5]: 6 About that same time the wordproton was adopted for the hydrogen nucleus.[38] Neutron was apparently constructed from theLatin root forneutral and theGreek ending-on (by imitation ofelectron andproton).[39][40] References to the wordneutron in connection with the atom can be found in the literature as early as 1899, however.[1]: 398 [35]
Rutherford and Chadwick immediately began an experimental program at theCavendish Laboratory inCambridge to search for the neutron.[8]: 27 [1]: 398 The experiments continued throughout the 1920s without success.[6]
Rutherford's conjecture and the hypothetical "neutron" were not widely accepted. In his 1931 monograph on theConstitution of Atomic Nuclei and Radioactivity,George Gamow, then at the Institute for Theoretical Physics in Copenhagen, did not mention the neutron.[41] At the time of their 1932 measurements in Paris that would lead to the discovery of the neutron,Irène Joliot-Curie andFrédéric Joliot were unaware of the conjecture.[42]
Throughout the 1920s, physicists assumed that the atomic nucleus was composed of protons and "nuclear electrons".[8]: 29–32 [43] 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 +7elementary charge units and a mass of 14 atomic mass units. This nucleus would also be orbited by another 7 electrons, termed "external electrons" by Rutherford,[32]: 375 to complete the14N atom. However problems with the hypothesis soon became apparent.
Ralph Kronig pointed out in 1926 that the observedhyperfine structure 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 theZeeman effect, but no such effects were observed.[44]: 199 It seemed that the magnetic moment of the electron vanished when it was within the nucleus.[1]: 299
While on a visit toUtrecht University in 1928, Kronig learned of a surprising aspect of the rotational spectrum of N2+. The precision measurement made byLeonard Ornstein, the director of Utrecht's Physical Laboratory, showed that the spin of a 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. Due to this phenomenon, Kronig suggested the possibility that "protons and electrons do not retain their identity to the extent they do outside the nucleus".[1]: 299–301 [45]: 117
Observations of therotational energy levels of diatomic molecules usingRaman spectroscopy byFranco Rasetti 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 physicistsWalter Heitler andGerhard Herzberg showed that the hydrogen nuclei obey Fermi statistics and the nitrogen nuclei obey Bose statistics. However, a then unpublished result ofEugene Wigner 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."[45]: 117–118
TheKlein paradox,[46] discovered byOskar Klein in 1928, presented further quantum mechanical objections to the notion of an electron confined within a nucleus. Derived from theDirac equation, this paradox suggested that an electron approaching a high potential barrier has a high probability of passing through the barrier by apair creation process.[41] Apparently, an electron could not be confined within a nucleus by any potential well. The meaning of this paradox was subject to widespread debate at the time.[44]: 199–200
By about 1930, it was generally recognized that it was difficult to reconcile the proton–electron model for nuclei with theHeisenberg uncertainty relation of quantum mechanics.[44]: 199 [1]: 299 This relation,Δx⋅Δp ≥1⁄2ħ, implies that an electron confined to a region the size of an atomic nucleus typically has a kinetic energy of about 40 MeV,[1]: 299 [b] which is larger than the observed energy of beta particles emitted from the nucleus.[1] Such energy is also much larger than the binding energy of nucleons,[47]: 89 which Aston and others had shown to be less than 9 MeV per nucleon.[48]: 511
In 1927,Charles Ellis andW. 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. Furthermore, the continuous energy distribution seemed to indicate that energy was not conserved by this "nuclear electrons" process. In 1929, Bohr proposed to modify the law of energy conservation to account for the continuous energy distribution, a proposal that 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 of these considerations did not "prove" an electron could not exist in the nucleus, they were confusing and challenging forphysicists to interpret. Many theories were invented to explain how the above arguments could be wrong.[49]: 4–5 In his 1931 monograph, Gamow summarized all of these contradictions, marking the statements regarding electrons in the nucleus with warning symbols.[43]: 23
In 1930,Walther Bothe and his collaborator Herbert Becker inGiessen, Germany found that if the energeticalpha particles emitted frompolonium fell on certain light elements, specificallyberyllium (9
4Be),boron (11
5B), orlithium (7
3Li), an unusually penetrating radiation was produced.[50] Beryllium produced the most intense radiation. Polonium is highly radioactive, producing energetic alpha radiation, and was commonly used for scattering experiments at the time.[41]: 99–110 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 begamma radiation. The radiation was more penetrating than any gamma rays known, and the details of experimental results were difficult to interpret.[51][52][41]
Two years later,Irène Joliot-Curie andFrédéric Joliot in Paris showed that if this unknown radiation fell onparaffin wax, or any otherhydrogen-containing compound, it ejected protons of very high energy (5 MeV).[53] This observation was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but that interpretation (Compton scattering) 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.[5]: §1.3.1 In Rome, the young physicistEttore Majorana declared that the manner in which the new radiation interacted with protons required a neutral particle as heavy as a proton, but declined to publish his result despite the encouragement ofEnrico Fermi.[54]
On hearing of the Paris results, Rutherford andJames Chadwick at the Cavendish Laboratory also did not believe the gamma ray hypothesis since it failed toconserve energy.[55] Assisted byNorman Feather,[56] Chadwick quickly performed a series of experiments showing that the gamma ray hypothesis was untenable. The previous year, Chadwick, J.E.R. Constable, andE.C. Pollard had already conducted experiments on disintegrating light elements using alpha radiation from polonium.[57] 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:9Be +4He (α) →12C +1n. 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.[58] Measurements of the recoil energy showed that the mass of the radiation particles must be similar to the mass of the proton: the new radiation could not consist of gamma rays. Uncharged particles with similar mass to theproton matched the properties Rutherford described in 1920 and which had later been called neutrons.[59][6][60][61] Chadwick won theNobel Prize in Physics in 1935 for this discovery.[62]
The year 1932 was later referred to as the "annus mirabilis" for nuclear physics in the Cavendish Laboratory,[58] with discoveries of the neutron, artificial nuclear disintegration by theCockcroft–Walton particle accelerator, and thepositron.
Given the problems of theproton–electron model,[43][63] 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[64][65][66][61] andDmitri Ivanenko[67] had proposed proton–neutron models for the nucleus.[68] 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",[69][according to whom?] 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 nuclearisospin quantum numbers.
The proton–neutron model explained the confusion with dinitrogen. When14N was proposed to consist of 3 pairs each of protons and neutrons, with an additional unpaired neutron and proton each contributing a spin of1⁄2 ħ in the same direction for a total spin of 1 ħ, the model became viable.[70][71][72] Soon, neutrons were used to naturally explain spin differences within 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 at the time could account for how electrons or positrons could emanate from the nucleus.[73][74] In 1934,Enrico Fermi published his paper describing theprocess of beta decay, in which the neutron decays to a proton bycreating an electron and a (as yet undiscovered)neutrino.[75] The paper employed the analogy thatphotons, or electromagnetic radiation, were similarly created and destroyed in atomic processes. Ivanenko had suggested a similar analogy in 1932.[70][76] Fermi's theory requires the neutron to be a spin-1⁄2 particle. His theory preserved the principle of conservation of energy, which had been put 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.[75] While this paper has still influenced physics today, the ideas within it were so new that, when it was first submitted to the journalNature in 1933, it was rejected as being too speculative.[69]
The question of whether the neutron was a composite particle of a proton and an electron persisted for a few years after its discovery.[77][78] In 1932Harrie Massey explored a model for a composite neutron to account for its great penetrating power through matter and its electrical neutrality,[79] 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 7thSolvay Conference held in October 1933, attended by Heisenberg,Niels Bohr,Lise Meitner,Ernest Lawrence, Fermi, Chadwick, and others.[69][80] As posed by Chadwick in hisBakerian Lecture 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 (1.0078 Da), then the neutron could be a proton-electron composite because of the mass defect from thenuclear binding energy. If greater than the combined masses, then the neutron was elementary like the proton.[60] The question was challenging to answer because the electron's mass is only 0.05% of the proton's mass, 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 is1.00866 Da. In Chadwick's 1932 paper reporting on the discovery, he estimated the mass of the neutron to be between1.005 Da and1.008 Da.[55] By bombarding boron with alpha particles, Frédéric and Irène Joliot-Curie obtained a high value of1.012 Da, while Ernest Lawrence's team at the University of California measured the small value1.0006 Da using their newcyclotron.[81]
In 1935 Chadwick and his doctoral studentMaurice Goldhaber resolved the issue by reporting the first accurate measurement of the mass of the neutron. They used the 2.6 MeV gamma rays ofThallium-208 (208Tl) (then known asthorium C") tophotodisintegrate the deuteron.[82]
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, or0.0023 Da). The neutron's mass could then be determined by the simple mass balance
| md | + | b.e. | = | mp | + | mn |
wheremd,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 Da and 1.0081 Da, respectively. They found that the neutron's mass was slightly greater than the mass of the proton1.0084 Da or1.0090 Da, depending on the precise value used for the deuteron mass.[7] The mass of the neutron was too large to be a proton–electron composite, and the neutron was therefore identified as an elementary particle.[55] Chadwick and Goldhaber predicted that a free neutron would be able to decay into a proton, electron, and neutrino (beta decay).
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 byOtto Stern in 1933 inHamburg that the proton had an anomalously large magnetic moment.[83][84] By 1934 groups led by Stern, now inPittsburgh, andI. I. Rabi inNew York had independently deduced that the magnetic moment of the neutron was negative and unexpectedly large by measuring the magnetic moments of the proton anddeuteron.[78][85][86][87][88] Values for the magnetic moment of the neutron were also determined byRobert Bacher[89] (1933) atAnn Arbor andI.Y. Tamm andS.A. Altshuler[78][90] (1934) in theSoviet Union 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 developednuclear magnetic resonance techniques.[88] 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.[78]
The discovery of the neutron immediately gave scientists a new tool for examining 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 byNorman Feather, Chadwick's colleague and protege, in scattering experiments with nitrogen.[91] Feather was able to demonstrate that neutrons interacting with nitrogen nuclei scattered to protons or induced nitrogen to disintegrate to formboron with the emission of an alpha particle. Feather was therefore the first to demonstrate that neutrons produce nuclear disintegrations.
InRome, 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.[92] Thecross section 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 ofnuclear reactions brought about by slow neutrons".[93][94] Later, Fermi recounted toChandrasekhar that he was originally planning to put a piece of lead there, but an inexplicable, intuitive feeling made him put a paraffin in the spot instead.[95][96]
InBerlin, the collaboration ofLise Meitner andOtto Hahn, together with their assistantFritz Strassmann, 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 astransuranic.[97] 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 escapeantisemitic persecution inNazi Germany after theAnschluss, 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–mesothoriumfractionation") produced puzzling results: what they had understood to be three isotopes of radium were instead consistently behaving asbarium.[9] Radium (atomic number 88) and barium (atomic number 56) are in the samechemical group. By January 1939 Hahn had concluded that what they had thought were transuranic nuclides were instead much lighter nuclides, such as barium,lanthanum,cerium and lightplatinoids. Meitner and her nephewOtto Frisch immediately and correctly interpreted these observations as resulting fromnuclear fission, a term coined by Frisch.[98]
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.[9] In their second publication on nuclear fission, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process.[99]Frédéric Joliot and his team proved this phenomenon to be achain reaction in March 1939. In 1945, Hahn received the 1944Nobel Prize in Chemistry "for his discovery of the fission of heavy atomic nuclei".[100][101]
The discovery of nuclear fission at the end of 1938 marked a shift in the centers of nuclear research fromEurope to the United States. Large numbers of scientists were migrating to the United States to escape the troubles andantisemitism in Europe and the loomingwar[102]: 407–410 (SeeJewish scientists and the Manhattan Project). The new centers of nuclear research were the universities in the United States, particularlyColumbia University in New York and theUniversity of Chicago where Enrico Fermi had relocated,[103][104] and a secret research facility atLos Alamos,New Mexico, established in 1942, the new home of theManhattan Project.[105] This wartime project was focused on the construction ofnuclear weapons, utilizing the enormous energy released by the fission of uranium orplutonium through neutron-based chain reactions.
The discoveries of the neutron and positron in 1932 were the start of the discoveries of many new particles.Muons were discovered in 1936,pions andkaons were discovered in 1947, andlambda particles were discovered in 1950. Throughout the 1950s and 1960s, a large number of particles calledhadrons were discovered. A classification scheme for organizing all these particles, proposed independently byMurray Gell-Mann[106] andGeorge Zweig[107][108] in 1964, became known as thequark model. 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 calledpartons orquarks. The quark model received experimental verification beginning in the late 1960s and provided an explanation for the neutron's anomalous magnetic moment.[109][10]
{{citation}}: CS1 maint: publisher location (link)During the 1920s physicists came to accept the view that matter is built of only two kinds of elementary particles, electrons and protons.
In 1930 Bothe, in collaboration with H. Becker, bombarded beryllium of mass 9 (and also boron and lithium) with alpha rays derived from polonium, and obtained a new form of radiation ...
heisenberg proton neutron model.
neutron.
One day, as I came to the laboratory, it occurred to me that I should examine the effect of placing a piece of lead before the incident neutrons. Instead of my usual custom, I took great pains to have the piece of lead precisely machined. I was clearly dissatisfied with something; I tried every excuse to postpone putting the piece of lead in its place. when finally, with some reluctance, I was going to put it in place, I said to myself: 'No, I do not want this piece of lead here; what I want is a piece of paraffin'. It was just like that with no advance warning, no conscious prior reasoning. I immediately took some odd piece of paraffin and placed it where the piece of lead was to have been.
| Classical physics | |
|---|---|
| Modern physics |
|
| Recent developments |
|
| On specific discoveries | |
| By periods | |
| By groups | |
| Scientific disputes | |
