aa r X i v : . [ phy s i c s . h i s t - ph ] J u l July 15, 2020
James Chadwick: ahead of his time
Gerhard Ecker
University of Vienna, Faculty of PhysicsBoltzmanngasse 5, A-1090 Wien, Austria
Abstract
James Chadwick is known for his discovery of the neutron. Many of his earlier findingsand ideas in the context of weak and strong nuclear forces are much less known. Thisbiographical sketch attempts to highlight the achievements of a scientist who pavedthe way for contemporary subatomic physics.
Early years
James Chadwick was born on Oct. 20, 1891 in Bollington, Cheshire in the northwest ofEngland, as the eldest son of John Joseph Chadwick and his wife Anne Mary. His fatherwas a cotton spinner while his mother worked as a domestic servant. In 1895 the parentsleft Bollington to seek a better life in Manchester. James was left behind in the care ofhis grandparents, a parallel with his famous predecessor Isaac Newton who also grew upwith his grandmother. It might be an interesting topic for sociologists of science to findout whether there is a correlation between children educated by their grandmothers andfuture scientific geniuses.James attended Bollington Cross School. He was very attached to his grandmother,much less to his parents. Nevertheless, he joined his parents in Manchester around 1902but found it difficult to adjust to the new environment. The family felt they could not affordto send James to Manchester Grammar School although he had been offered a scholarship.Instead, he attended the less prestigious Central Grammar School where the teaching wasactually very good, as Chadwick later emphasised.Chadwick’s modest background did not prevent him from receiving an excellent generaleducation. Especially his mathematics teacher encouraged him and finally persuaded himto enter a competition for a scholarship at Manchester University, which he won at the ageof sixteen [1].
In May 1907, Ernest Rutherford moved from McGill University in Canada to ManchesterUniversity. He inherited from his predecessor a modern and well-equipped department withone major deficiency: there was no radium. The problem was solved by Stefan Meyer fromthe Radium Institute in Vienna with a generous gift of some 300 milligrams of radium chlo-ride. The Manchester School of radioactivity under Rutherford’s leadership soon producedresults that would revolutionise science.In the fall of 1908, Chadwick came to the university for a preliminary interview. Al-though he intended to take up mathematics, he ended up being interviewed by a lecturerfrom the physics department. Chadwick was too shy to admit his mistake and thus startedhis life as a physicist by accident. He was not too impressed by his first-year courses butthings changed substantially when in his second year he attended lectures on electricityand magnetism delivered by Rutherford, “the first stimulating lectures I had ever had inphysics” [2].At the end of the second year, there was no more formal education in physics. Instead,the few remaining honours students in physics were assigned specific research projects byRutherford. As Chadwick later remarked [2]: “I had half an education in physics. Therewere whole aspects of physics I knew little about.”1t that time, there were no generally accepted units for radioactivity. At an interna-tional congress in Brussels in September 1910 it was agreed that the amount of radioactivityreleased by a gram of radium would serve as the standard unit, later to be known as acurie. The third-year research project assigned to Chadwick was a highly topical one. Hewas instructed to investigate a method initially devised by Rutherford to compare differentradium sources. When Chadwick became aware of a small problem in Rutherford’s originalsetup he did not dare to mention it to the master, risking rather to disappoint Rutherfordwho noticed the problem after the first measurements. The approach was applied to a com-parison of the two standard radium sources available at the time, one initially providedby Marie Curie and the other by Stefan Meyer. The comparison was complety convincing,leading at one hand to the acceptance of an agreed world standard and, on the other hand,to Chadwick’s first published paper together with Rutherford [3].In the summer of 1911, Chadwick graduated with first class honours although the finalexam was by no means straightforward. The written part presented no problems. On theother hand, Chadwick found out only shortly before the exam that he also had to undergoa practical examination, with J.J. Thomson as external examiner. Chadwick claims thathe was terrified by Thomson’s charisma and could hardly say a word [2]. In any case,Rutherford was convinced of Chadwick’s talents and accepted him as a graduate student.As a demonstrator he received his first salary, modest enough but sufficient for regularlunches after three years as an impoverished undergraduate. The growing impact of theManchester School attracted many visitors, among them Niels Bohr who came in March1912 as a postdoctoral fellow and made friends with Chadwick. The most prominent resultof Bohr’s stay was his atomic model, which dominated the physicists’ view of the atom tillthe emergence of quantum mechanics and beyond.The next project of Chadwick was a thorough investigation of γ -ray absorption in gases.Until that time all information on γ -ray absorption coefficients had been indirect. Usingsimple but ingenious techniques, Chadwick was able to measure these coefficients withuncertainties of only a few percent. This research definitely established Chadwick as animaginative experimenter. Moreover, in addition to obtaining precise results he was ableto draw concise and far-reaching conclusions in the ensuing publications. In the presentcase, his first paper as sole author [4], he pointed out that the high concentration of ionsfound in the upper atmosphere could not be wholly due to radiation from the radioactivematerial in the earth and, therefore, must be the result of radiation from outer space. Inthe same year 1912 the Austrian physicist Viktor Hess established the existence of cosmicrays after ionization measurements on a series of seven balloon flights [5].Chadwick received his M.Sc. in the summer of 1912. In the following year, the universitynominated him for a prestigious 1851 Exhibition Science Research Scholarship. Rutherfordwanted Chadwick to spend at least one year of this scholarship in his group in Manchesterbut the terms of the scholarship foresaw that the recipient would have to work at aninstitute other than the nominating university. Despite Rutherford’s interventions, thescholarship commissioners remained adamant: take it or leave it. It was finally agreed thatChadwick would spend the first year of tenure either in Berlin with Rutherford’s former2ollaborator Hans Geiger or at the University of Vienna. No one, least the scholarshipcommissioners, could foresee that Chadwick was about to embark on one of his life’s greatadventures [1]. In the fall of 1913, Chadwick arrived in Berlin to spend the first year of his stipend inGeiger’s laboratory at the Physikalisch-Technische Reichsanstalt in Charlottenburg, a sub-urb of Berlin. Geiger had returned from Manchester to Germany in 1912 after performingseminal experiments with his student Ernest Marsden that established Rutherford’s atomicmodel. He gave Chadwick a warm welcome and introduced him to other colleagues such asOtto Hahn and Lise Meitner from the Kaiser Wilhelm Institute for Chemistry. After quicklyobtaining a working knowledge of German and getting used to German bureaucracy [2],Chadwick soon enjoyed the friendly atmosphere and Geiger’s helpful guidance. His chosenarea of work was β radiation. He could not anticipate that this topic would remain a crucialand much debated subject in the development of subatomic physics for at least anothertwenty years. Nor could he anticipate that his contribution, although completely correct,would remain contested for almost fifteen years. During the first decade of the 20 th century it was established that α particles are heliumnuclei and β particles are electrons. It was also found that in a given α decay the emitted α particles all had the same energy corresponding to the mass difference between initialand final nuclei. It was natural to assume the same behaviour for the emitted electrons in β decay. But by 1913, researchers from both the Manchester School and Hahn’s laboratoryhad discarded the hypothesis that the β spectrum was monochromatic. Instead, the exper-iments seemed to show that β spectra consisted of several discrete lines. To that date, theweak point of all experiments was the detection of the emitted electrons on photographicplates. When Chadwick entered the game he had the advantage of employing instead ofa photographic plate a particle counter that had just been developed by Geiger and thathas been carrying his name ever since. After initial doubts about his results he convincedhimself that the line spectrum was actually an artefact of the photographic detection. Inthe publication [6], a hallmark of modern physics that is unfortunately difficult to access(the original publication was probably translated into German by Geiger [1]), he made itabsolutely clear that the accepted picture of Rutherford, Hahn and others was incorrectand that the β spectrum was actually continuous. While Rutherford immediately acceptedChadwick’s results, many others like Meitner remained sceptical.After the war, Rutherford suggested to Charles Ellis, a young member of the Manchestergroup (see also Subsec. 3.3), to reexamine the issue of the β spectrum. Ellis not onlyconfirmed Chadwick’s findings of 1914 but he also explained the occurrence of discrete lines3uperimposed on the continuous spectrum as being due to internal conversion involvingelectrons in the atomic shell [7]. Lise Meitner was not convinced and insisted [8] that theprimary electrons in β decay must all have the same energy because of energy conservation.Although her reasoning was theoretically correct she could not explain the continuousspectrum found by Chadwick and Ellis blaming it on problems with their experimentalsetups. Since Chadwick’s original result was called into question by Meitner he teamed upwith Ellis for a measurement of the intensity distributions of electrons in the β decays of P b (radium B) and Bi (radium C) by an ionisation method. They summarised theirresults as follows [9]: “Firstly, the continuous spectrum has a real existence which is notdependent on the experimental arrangement and any explanation of it as due to secondarycauses is untenable . . . ” Due to her theoretical “prejudice”, Meitner was still not convinced.It took another five years of hard work by Ellis and collaborators before the matter wasfinally settled. In 1927, Ellis and Wooster undertook an absolute measurement [10] of theheat produced by the total absorption of the electrons emitted in the β decay of Bi .They demonstrated that the average energy released per individual β decay was equal tothe mean energy of the continuous spectrum and that secondary processes, as called for byMeitner, did not exist. In a follow-up experiment, Meitner and her colleague Orthmann notonly confirmed [11] the results of Ellis and Wooster but they arrived at an even strongerconclusion. Ellis and Wooster had speculated that some continuous γ spectra could saveenergy conservation because γ rays could not be observed in their calorimeter. But Meitnerand Orthmann showed employing special counters that such a continuous γ -ray spectrumdid not exist.The experimental situation was now settled but the theoretical dilemma became evenworse. As the results appeared to contradict the sacrosanct conservation of energy, Bohrspeculated at the end of the 1920s that in the microcosm energy conservation might onlyhold on average, but that an individual decay could violate the energy balance. But therewas also a problem with the conservation of angular momentum if the electron with itsspin 1/2 were the only decay product in addition to the final nucleus. Moreover, there werealso problems with quantum statistics. According to the general picture of the nucleusat the time, the nucleus of the nitrogen isotope N should contain fourteen protons andseven electrons. Because of the odd number of particles with spin 1/2 the nitrogen nucleuswould have half-integer spin and should satisfy Fermi-Dirac statistics. But experimentactually showed that the N nucleus had integer spin and was therefore a boson. Finally,it was difficult to reconcile with quantum mechanics and in particular with the uncertaintyrelation that particles as light as electrons could be confined in such a small volume as anatomic nucleus.In December 1930, Pauli broke the Gordian knot in his famous letter to the “radioactiveladies and gentlemen” who gathered for a meeting in T¨ubingen. He proposed as a solution ofthe various problems that the electron in β decay is accompanied by an additional particlethat would have to be electrically neutral and have spin 1/2. Pauli named the postulatedparticle neutron but soon the name neutrino (the small neutron) proposed by Enrico Fermi Actually, Bohr upheld his view at least until 1932. β decay [12]. Back in 1914, Chadwick started a project scattering β particles on a thin gold foil, inanalogy to the famous experiments of Geiger and Marsden with α particles. In the courseof that work he suggested to Geiger [2] “that perhaps electrons might be scattered from acrystal surface in much the same way as X rays. Geiger said there was nothing in it, it wasrather a silly suggestion to make.” Nine years later, a certain Louis de Broglie had the sameidea [13], inspiring in particular Erwin Schr¨odinger to set up his wave mechanics. In thiscase, the relevant experiments were indeed performed [14, 15], demonstrating wave-particleduality also for matter particles. Once again, Chadwick was ahead of his time but thefollowing events would have prevented him anyway from actively pursuing his idea. Chadwick’s career came to an abrupt end in August 1914 when the First World War brokeout. After the invasion of Belgium by the German army Great Britain declared war onGermany and the situation became precarious for a British citizen in Berlin. Although hisGerman colleagues in the laboratory were very helpful as Chadwick later acknowledgedgratefully he was arrested in November 1914 as an enemy alien. Together with hundredsof other British civilians, he was interned in a camp (Engl¨anderlager) in Ruhleben nearSpandau west of Berlin. Initial hopes that the war would be over by Christmas soon fadedand four years full of privation followed.This period is described in much detail in Brown’s biography [1]. Here, I confine myselfto the remarkable social activities in the camp, in particular in the form of an Arts andScience Union where Chadwick played a prominent role. For instance, he delivered regularlectures on electricity and magnetism and even on radioactivity that were well receivedby the participants. One of them was Charles Ellis who as a cadet of the Royal MilitaryAcademy had the bad luck of spending his summer holidays together with colleagues inGermany at the outbreak of the war. All of them were interned in Ruhleben. Wave-particle duality was already well established for photons at that time. β and γ rays (see Subsec. 3.1). While in thecamp, Chadwick managed to organise a small laboratory where they performed severalexperiments mainly in chemistry. He was even given for inspection some radioactive tooth-paste that became popular in Germany at that time. For getting both equipment andscientific literature the camp authorities were remarkably cooperative. Chadwick was evenallowed to leave the camp for visiting the prominent German scientists Heinrich Rubens,Walther Nernst and Emil Warburg who all offered help.The general living conditions were less agreeable. By 1917 the blockade of the BritishNavy on merchant ships had a devastating effect on food supplies to Germany in general andto the inmates of the Ruhleben camp in particular. Chadwick was severely undernourishedand had serious digestive problems that would accompany him for the rest of his life. Afterthe armistice in November 1918 all internees were released. After a long journey via theBaltic and the North Sea, Chadwick finally arrived at his parents’ home in Manchester. As early as 1815, the English chemist William Prout suspected on the basis of existingmeasurements of atomic masses that all atoms are built up of hydrogen atoms (Prout’shypothesis). Scattering α particles on light atoms, in particular on nitrogen, Rutherforddemonstrated that the hydrogen nucleus (denoted proton by him in 1920) does indeedoccur in all nuclei [16]. But Rutherford also recognised that additional, electrically neu-tral constituents must be contained in the nuclei in order to explain nuclear masses. Hecalled these constituents neutrons and he pictured them as bound states of protons andelectrons. Two reasons seemed to support such a picture. The mass of these neutrons wascomparable with the proton mass and the negatively charged electrons would compensatethe electrostatic repulsion between the protons at least to some extent.Scattering α particles on protons, Rutherford found deviations from the Coulomb law(electrostatic repulsion between α and p ) [17]: “. . . not inconsistent with the view that theforces between colliding atoms augment rapidly for values of d < . · − cm.” He associ-ated the deviations with the complex nature of α particles as bound states of four protonsand two electrons. With an improved experimental setup, Chadwick repeated the experi-ment, first as part of his doctoral thesis at Cambridge and then more thoroughly togetherwith a young colleague, the Swiss-born Canadian Etienne Bieler. Their results confirmedRutherford’s findings but their conclusions went beyond. Investigating the differential crosssection for various scattering angles, they observed agreement with expectations for slow α particles (low energies). On the other hand, the measured numbers of scattered protonsgreatly exceeded the expectations for fast α particles (high energies), in one case 100 timesas large, with only the Coulomb force between point charges. In the latter case also the6ngular distribution of the scattered protons was different. Their main conclusion [18] washailed by Abraham Pais [19] as marking the birth of the strong interactions: “. . . no systemof four H nuclei (i.e. protons) and two electrons united by inverse square law forces couldgive a field of force of such intensity over so large an extent . . . It is our task to find somefield of force which will reproduce these effects . . . The present experiments do not seem tothrow any light on the nature or the law of variation of the forces at the seat of an electriccharge, but merely show that the forces are of very great intensity.”During the following years, the Cambridge group extended their studies by scattering α particles on heavier atoms, more specifically on helium, magnesium and aluminium. Theseattempts were reviewed in the classic monograph of Rutherford, Chadwick and Ellis in1930 [20]. The results for α α scattering were similar to those from α p scattering [21].Extending the experiments to the heavier atoms magnesium and aluminium did not clarifythe situation. One reason was that the lever arm for distinguishing between the Coulombforce and any additional force is smaller for heavier atoms. From his results, Bieler [22]concluded that the additional force was attractive and seemed to vary with distance as r − although a dependence as r − could not be ruled out either. On the other hand,Debye and Hardmeier claimed that a force varying as r − would also describe the data byassuming that the incoming α particle strongly polarises the heavy nuclei [23]. However,as emphasised in Ref. [20], it did not seem possible to explain the collisions with hydrogenor helium nuclei in the same way.At the end of the 1920s, the situation was as unclear as at the beginning of the decade.Once again, Chadwick and his colleagues were some fifteen years ahead of their time. Foran understanding of the experimental results two fundamental theoretical developmentswere necessary, the quantum mechanical scattering theory and the Yukawa potential withthe pion mass setting the scale for the onset of the new force [24].From 1935 on, significant progress was made in the understanding of the strong inter-actions of nucleons and mesons. Mainly on the basis of nonrelativistic potential modelsinvolving mesons, it became possible to explain nuclear structure and nuclear reactions.However, these achievements were restricted to reactions where nucleons have small rela-tive velocities. The developments leading to the formulation of a relativistic quantum fieldtheory of the strong interactions (quantum chromodynamics) and its incorporation in theStandard Model of the fundamental interactions can for instance be found in Ref. [25]. Asked by Charles Weiner whether he had thought that his work done on the neutron mightbe of Nobel Prize calibre, Chadwick answered [2] : “The award of a Prize, it seems to me,to be not so much a question of luck but a question of being there at the right time.” In1932, Chadwick was indeed right on time. In consequence, he was ennobled by the NobelFoundation in 1935 and by the English King in 1945 where the second knighthood alsoacknowledged his role in the Manhattan Project.7hadwick’s discovery of the neutron thus differed from many of his earlier achievementswhere he happened to be ahead of his time. Since the neutron discovery is described inmuch detail in many books [1] and articles [26], I will confine myself here to a brief summaryof events for completeness.Rutherford had introduced the notion of a neutron as bound state of proton and elec-tron already in 1920 in order to understand nuclear masses. Especially after the advent ofquantum theory, the difficulties of this picture became more and more acute. As alreadymentioned in Subsec. 3.1, a serious discrepancy with experiment had to do with quantumstatistics. In Rutherford’s model the nucleus of the nitrogen isotope N contained four-teen protons and seven electrons. Because of the odd number of particles with spin 1/2the nitrogen nucleus would have half-integer spin and should satisfy Fermi-Dirac statistics,contradicting experimental evidence . In addition, while a bound state of proton and elec-tron was well understood in the form of the hydrogen atom, it was difficult to reconcilewith the uncertainty relation that particles as light as electrons could be confined in suchsmall volumes as atomic nuclei.Already in 1930, Walther Bothe and Herbert Becker [27] had scattered energetic α particles from a polonium source on several light elements such as Be . They observeda very penetrating radiation that was not deflected by an electric field and was there-fore interpreted as γ rays. Two years later, Ir`ene and Fr´ed´eric Joliot-Curie repeated theexperiment [28]. When the unknown radiation was directed at some hydrogen-containingmaterial such as paraffin wax, it released high-energy protons. Therefore, the process wasinterpreted as proton Compton scattering. The problem was that this would have required γ rays with unrealistically high energy (50 MeV). The Italian theorist Ettore Majoranacommented sarcastically [29]: “What fools, they have discovered the neutral proton andthey do not recognize it.”Neither Chadwick nor his mentor Rutherford believed the interpretation as protonCompton scattering. Chadwick immediately set to work and within three strenuous weeksnot only repeated the French experiment but also scattered the radiation on various atomsother than hydrogen. He found that the new radiation consisted not of γ rays, but ofuncharged particles with about the same mass as the proton. The last sentence in hisNature article [30] is crystal clear: “Up to the present, all the evidence is in favour of theneutron, while the quantum hypothesis (i.e. the γ -ray hypothesis) can only be upheld ifthe conservation of energy and momentum is relinquished at some point.”One may ask the question why Chadwick was more successful than the Joliot-Curies,both experienced scientists who would receive the Nobel Prize in chemistry in 1935 fortheir discovery of artificial radioactivity. A plausible answer is that Chadwick was notonly right on time but he was also in the right surroundings. While there was a generalconsensus that atomic nuclei consisted of protons and electrons, the idea of a neutron asbound state of proton and electron was rather specific to the Cavendish Laboratory. Thisis for instance spelled out by Joliot [31] when commenting on Chadwick’s discovery: “The The same problem existed for the Li nucleus. m n > m p + m e , (5.1)the neutron cannot be a bound state and it would be as elementary a particle as the proton.The precise measurement of the neutron mass turned out to be very demanding. Severalmeasurements of the neutron mass published by different groups were not conclusive andallowed for both possibilities. Initially, Chadwick favoured the bound-state nature of theneutron [32]: “It is, of course, possible to suppose that the neutron may be an elementaryparticle. This view has little to recommend it at present, except the possibility of explainingthe statistics of such nuclei as N .”In 1933 Maurice Goldhaber, a young refugee from Nazi-Germany, was offered a researchposition at the Cavendish by Rutherford. At some point, he suggested to Chadwick thatthe deuteron (usually called diplon at the time) might be a good candidate for a precisemeasurement of the neutron mass if it could be split by bombarding it with γ rays: γ + H → p + n . (5.2)Their experiment [33] produced a precise value for the neutron mass showing that theneutron cannot be a bound state. Electrons were banished from atomic nuclei, which consistof protons and neutrons only. In the fall of 1935, shortly before receiving the Nobel Prize, Chadwick moved to LiverpoolUniversity. He refurbished the old-fashioned laboratories and initiated the construction ofa cyclotron, making Liverpool one of the European centers of nuclear physics. As leadingexpert on neutron physics, he was chosen to write the final draft of the so-called MAUDReport, the basis of American-British collaboration in the Manhattan Project. His presenceat the Trinity nuclear test was characterised by a science journalist associated to theManhattan Project [34]: “Never before in history had any man lived to see his own discoverymaterialize itself with such telling effect on the destiny of man.” In 1948, Chadwick moved9ack to Cambridge to become Master of Gonville and Caius College, where he had beena research student in the early 1920s. He retired at the end of 1958 and moved to NorthWales with his wife. Ten years later they once more moved back to Cambridge to be neartheir daughters. James Chadwick died in his sleep on July 24, 1974.
Acknowledgements
I have made extensive use of the excellent biography of AndrewBrown [1] and of the interviews recorded by Charles Weiner [2]. For more detailed studiesof Chadwick’s biography, especially also of his personality, I very much recommend thesetwo references. For suggestions, corrections and other help, I thank Walter Grimus, HelmutNeufeld, Maria Probst, Michael Springer and Brigitte Strohmaier.
References [1] A. Brown,
The neutron and the bomb: a biography of Sir James Chadwick , OxfordUniv. Press, New York, 1997.[2] C. Weiner,
Sir James Chadwick, oral history , American Institute of Physics, CollegePark, Maryland, 1969.[3] E. Rutherford and J. Chadwick, Proc. Phys. Soc. 24 (1912) 141.[4] J. Chadwick, Proc. Phys. Soc. 24 (1912) 152.[5] V.F. Hess, Phys. Zeits. 13 (1912) 1084.[6] J. Chadwick, Verhandlungen der Deutschen Phys. Gesellschaft 16 (1914) 383.[7] C.D. Ellis, Proc. Royal Soc. A99 (1921) 261.[8] L. Meitner, Zeits. Physik 9 (1922) 131.[9] J. Chadwick and C.D. Ellis, Proc. Cambridge Phil. Soc. 21 (1922) 274.[10] C.D. Ellis and W.A. Wooster, Proc. Royal Soc. A117 (1927) 109.[11] L. Meitner and W. Orthmann, Zeits. Physik 60 (1930) 143.[12] E. Fermi, Zeits. Physik 88 (1934) 161.[13] L. de Broglie, Ann. de Physique 3 (1925) 22.[14] C. Davisson and L.H. Germer, Nature 119 (1927) 558.[15] G.P. Thomson and A. Reid, Nature 119 (1927) 890.1016] E. Rutherford, Phil. Mag., Ser. 6, Vol. 37 (1919) 581.[17] E. Rutherford, Phil. Mag., Ser. 6, Vol. 37 (1919) 537.[18] J. Chadwick and E.S. Bieler, Phil. Mag., Ser. 6, Vol. 42 (1921) 923.[19] A. Pais,
Inward bound: of matter and forces in the physical world , Oxford Univ. Press,New York, 1986.[20] E. Rutherford, J. Chadwick and C.D. Ellis,
Radiations from radioactive substances ,Cambridge Univ. Press, Cambridge, 1930.[21] E. Rutherford and J. Chadwick, Phil. Mag., Ser. 7, Vol. 4 (1927) 605.[22] E.S. Bieler, Proc. Royal Soc. A105 (1924) 434.[23] P. Debye and W. Hardmeier, Phys. Zeits. 27 (1926) 196.[24] H. Yukawa, Proc. Phys. Math. Soc. Japan 17 (1935) 48.[25] G. Ecker,
Particles, fields, quanta: from quantum mechanics to the Standard Model ofparticle physics , Springer Nature Switzerland AG, Cham, 2019.[26] https://en.wikipedia.org/wiki/Discovery of the neutron[27] W. Bothe and H. Becker, Zeits. Physik 66 (1930) 289.[28] I. Joliot-Curie and F. Joliot, Comptes Rendus 194 (1932) 273.[29] In E. Segr`e,
From x-rays to quarks , W.H. Freeman, New York, 1980.[30] J. Chadwick, Nature 129 (1932) 312.[31] In M. Goldsmith,
Fr´ed´eric Joliot-Curie , Lawrence and Wishart, London, 1976.[32] J. Chadwick, Proc. Royal Soc. A136 (1932) 692.[33] J. Chadwick and M. Goldhaber, Nature 134 (1934) 237.[34] W.L. Laurence,