Editing James Chadwick (section)

== Researcher ==
=== Cambridge ===
Chadwick's Clerk-Maxwell studentship expired in 1923, and he was succeeded by the Russian physicist [[Pyotr Kapitza]]. The Chairman of the Advisory Council of the [[Department of Scientific and Industrial Research (United Kingdom)|Department of Scientific and Industrial Research]], Sir [[William Symington McCormick|William McCormick]] arranged for Chadwick to become Rutherford's assistant director of research. In this role, Chadwick helped Rutherford select PhD students. Over the next few years these would include [[John Cockcroft]], [[Norman Feather]] and [[Mark Oliphant]], who would become firm friends with Chadwick. As many students had no idea what they wanted to research, Rutherford and Chadwick would suggest topics. Chadwick edited all the papers produced by the laboratory.{{sfn|Brown|1997|pp=73–76}}

[[File:The Cavendish Laboratory - geograph.org.uk - 631839.jpg|thumb|upright=1.35|left|The original building of the  [[Cavendish Laboratory]] was the home of some of the great discoveries in physics. It was founded in 1874 by the [[William Cavendish, 7th Duke of Devonshire|Duke of Devonshire]] (whose family name was Cavendish), and its first professor was [[James Clerk Maxwell]]. The Laboratory has since moved to [[West Cambridge]].<ref>{{cite web |url=http://www.phy.cam.ac.uk/history |title=The History of the Cavendish |date=13 August 2013 |publisher=University of Cambridge |access-date=15 August 2014 }}</ref> ]]
In 1925, Chadwick met Aileen Stewart-Brown, the daughter of a Liverpool stockbroker. The two were married in August 1925,{{sfn|Brown|1997|pp=73–76}} with Kapitza as Best Man. The couple had twin daughters, Joanna and Judith, who were born in February 1927.{{sfn|Brown|1997|p=85}}

In his research, Chadwick continued to probe the nucleus. In 1925, the concept of [[Spin (physics)|spin]] had allowed physicists to explain the [[Zeeman effect]], but it also created unexplained anomalies. At the time it was believed that the nucleus consisted of [[proton]]s and electrons, so [[nitrogen]]'s nucleus, for example, with a [[mass number]] of 14, was assumed to contain 14 protons and 7 electrons. This gave it the right [[mass]] and charge, but the wrong spin.{{sfn|Brown|1997|pp=92–93}}

At a conference at Cambridge on beta particles and gamma rays in 1928, Chadwick met Geiger again. Geiger had brought with him a new model of his Geiger counter, which had been improved by his post-doctoral student [[Walther Müller]]. Chadwick had not used one since the war, and the new [[Geiger counter|Geiger–Müller counter]] was potentially a major improvement over the [[Scintillation (physics)|scintillation]] techniques then in use at Cambridge, which relied on the human eye for observation. The major drawback with it was that it detected [[Alpha decay|alpha]], [[Beta particle|beta]] and [[Gamma ray|gamma]] radiation, and radium, which the Cavendish laboratory normally used in its experiments, emitted all three, and was therefore unsuitable for what Chadwick had in mind. However, [[polonium]] is an alpha emitter, and [[Lise Meitner]] sent Chadwick about 2 millicuries (about {{val|0.5|u=ug}}) from [[Germany]].{{sfn|Brown|1997|pp=95–97}}{{sfn|Sublette|2006}}

In Germany, [[Walther Bothe]] and his student [[Herbert Becker (physicist)|Herbert Becker]] had used polonium to bombard [[beryllium]] with alpha particles, producing an unusual form of radiation. Chadwick had his Australian 1851 Exhibition scholar, Hugh Webster, duplicate their results. To Chadwick, this was evidence of something that he and Rutherford had been hypothesising for years: the [[neutron]], a theoretical nuclear particle with no electric charge.{{sfn|Brown|1997|pp=95–97}} Then in January 1932, Feather drew Chadwick's attention to another surprising result. [[Frédéric Joliot-Curie|Frédéric]] and [[Irène Joliot-Curie]] had succeeded in knocking protons from [[paraffin wax]] using polonium and beryllium as a source for what they thought was gamma radiation. Rutherford and Chadwick disagreed; protons were too heavy for that. But neutrons would need only a small amount of energy to achieve the same effect. In Rome, [[Ettore Majorana]] came to the same conclusion: the Joliot-Curies had discovered the neutron but did not know it.{{sfn|Brown|1997|pp=103–104}}

[[File:Sir Ernest Rutherfords laboratory, early 20th century. (9660575343).jpg|thumb|right|Sir Ernest Rutherford's laboratory]]

Chadwick dropped all his other responsibilities to concentrate on proving the existence of the neutron, assisted by Feather<ref name="sessI">{{Cite web | title=Oral History interview transcript with Norman Feather, Session I | date= 25 February 1971 | publisher=American Institute of Physics, Niels Bohr Library and Archives | author=<!--Staff writer(s); no by-line.--> | url=https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4559-1}}</ref> and frequently working late at night. He devised a simple apparatus that consisted of a cylinder containing a polonium source and beryllium target. The resulting radiation could then be directed at a material such as paraffin wax.  The displaced particles, which were protons, would go into a small ionisation chamber where they could be detected with an [[oscilloscope]].{{sfn|Brown|1997|pp=103–104}}
In February 1932, after only about two weeks of experimentation with neutrons,<ref name=APSNews /> Chadwick sent a letter to ''[[Nature (journal)|Nature]]'' titled "Possible Existence of a Neutron".{{sfn|Chadwick|1932a}} He communicated his findings in detail in an article sent to ''[[Proceedings of the Royal Society A]]'' titled "The Existence of a Neutron" in May.{{sfn|Chadwick|1932b}}{{sfn|Chadwick|1933}} His [[discovery of the neutron]] was a milestone in understanding the nucleus. Reading Chadwick's paper, [[Robert Bacher]] and [[Edward Condon]] realised that anomalies in the then-current theory, like the spin of nitrogen, would be resolved if the neutron has a [[spin-½|spin of 1/2]] and that a nitrogen nucleus consisted of seven protons and seven neutrons.{{sfn|Whaling|2009|pp=8–9}}{{sfn|Bacher|Condon|1932}}

The theoretical physicists [[Niels Bohr]] and [[Werner Heisenberg]] considered whether the neutron could be a fundamental [[nucleon|nuclear particle]] like the proton and electron, rather than a proton–electron pair.{{sfn|Heisenberg|1932a}}{{sfn|Heisenberg|1932b}}{{sfn|Heisenberg|1933}}{{sfn|Bromberg|1971}}  Heisenberg showed that the neutron was best described as a new nuclear particle,{{sfn|Heisenberg|1933}}{{sfn|Bromberg|1971}} but its exact nature remained unclear.  In his 1933 [[Royal Society Bakerian Medal|Bakerian Lecture]], Chadwick estimated that a neutron had a mass of about {{val|1.0067|ul=u}}. Since a proton and an electron had a combined mass of {{val|1.0078|u=u}}, this implied the neutron as a proton–electron composite had a binding energy of about {{val|2|ul=MeV}}, which sounded reasonable,{{sfn|Brown|1997|pp=115–116}} although it was hard to understand how a particle with so little binding energy could be stable.{{sfn|Bromberg|1971}}  Estimating such a small mass difference required challenging precise measurements, however, and several conflicting results were obtained in 1933–4.  By bombarding [[boron]] with alpha particles, Frédéric and Irène Joliot-Curie obtained a large value for the mass of a neutron, but [[Ernest Lawrence]]'s team at the [[University of California]] produced a small one.{{sfn|Heilbron|Seidel|1989|pp=153–157}}  Then [[Maurice Goldhaber]], a refugee from [[Nazi Germany]] and a graduate student at the Cavendish Laboratory, suggested to Chadwick that [[deuteron]]s could be [[Photodisintegration|photodisintegrated]] by the 2.6 MeV gamma rays of <sup>208</sup>Tl (then known as [[Decay chain#Thorium series|thorium C"]]):
:{| border="0"
|- style="height:2em;"
|{{nuclide|link=yes|deuterium|2}}&nbsp;||+&nbsp;||{{Subatomic particle|link=yes|gamma}}&nbsp;||→&nbsp;||{{nuclide|link=yes|hydrogen|1}}&nbsp;||+&nbsp;||{{Subatomic particle|link=yes|neutron}}
|}
An accurate value for the mass of the neutron could be determined from this process. Chadwick and Goldhaber tried this and found that it worked.{{sfn|Goldhaber|1934}}{{sfn|Chadwick|Goldhaber|1934}}{{sfn|Chadwick|Goldhaber|1935}}  They measured the kinetic energy of the proton produced as 1.05 MeV, leaving the mass of the neutron as the unknown in the equation. Chadwick and Goldhaber calculated that it was either 1.0084 or 1.0090 atomic units, depending on the values used for the masses of the proton and deuteron.{{sfn|Brown|1997|pp=122–125}}{{sfn|Chadwick|Goldhaber|1935}}  (The modern accepted value for the mass of the neutron is {{val|1.00866|ul=u}}.) The mass of the neutron was too large to be a proton–electron pair.{{sfn|Brown|1997|pp=122–125}}

For his discovery of the neutron, Chadwick was awarded the [[Hughes Medal]] by the [[Royal Society]] in 1932, the [[Nobel Prize in Physics]] in 1935, the [[Copley Medal]] in 1950 and the [[Franklin Medal]] in 1951.<ref name="Nobel">{{cite web |url=http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-bio.html |title=James Chadwick – Biography |publisher=[[The Nobel Foundation]] |access-date=21 April 2013}}</ref> His discovery of the neutron made it possible to produce elements heavier than [[uranium]] in the laboratory by the capture of slow neutrons followed by [[beta decay]]. Unlike the positively charged [[alpha particle]]s, which are repelled by the electrical forces present in the nuclei of other atoms, neutrons do not need to overcome any [[Coulomb barrier]], and can therefore penetrate and enter the nuclei of even the heaviest elements such as uranium. This inspired [[Enrico Fermi]] to investigate the nuclear reactions brought about by collisions of nuclei with slow neutrons, work for which Fermi would receive the Nobel Prize in 1938.{{sfn|Brown|1997|pp=125}}

[[Wolfgang Pauli]] proposed another kind of particle on 4 December 1930 to explain the continuous spectrum of beta radiation that Chadwick had reported in 1914. Since not all of the energy of beta radiation could be accounted for, the law of [[conservation of energy]] appeared to be violated, but Pauli argued that this could be redressed if another, undiscovered, particle was involved.{{sfn|Brown|1997|pp=119–120}} Pauli also called this particle a neutron, but it was clearly not the same particle as Chadwick's neutron. Fermi renamed it the [[neutrino]], Italian for "little neutron".{{sfn|Close|2012|pp=15–18}} In 1934, Fermi proposed his [[Fermi's interaction|theory of beta decay]] which explained that the electrons emitted from the nucleus were created by the decay of a neutron into a proton, an electron, and a neutrino.{{sfn|Fermi|1968}}{{sfn|Close|2012|pp=22–25}}  The neutrino could account for the missing energy, but a particle with little mass and no electric charge was difficult to observe. [[Rudolf Peierls]] and [[Hans Bethe]] calculated that neutrinos could easily pass through the Earth, so the chances of detecting them were slim.{{sfn|Close|2012|pp=26–28}}<ref>{{cite journal |title=The Neutrino |journal=[[Nature (journal)|Nature]] |issn=0028-0836 |volume=133 |issue=3362 |pages=532 |date=7 April 1934 |doi=10.1038/133532a0 |bibcode = 1934Natur.133..532B |last1=Bethe |first1=H |last2=Peierls |first2=R |s2cid=4001646 |doi-access=free }}</ref> [[Frederick Reines]] and [[Clyde Cowan]] would [[neutrino experiment|confirm the neutrino]] on 14 June 1956 by placing a detector within a large antineutrino flux from a nearby nuclear reactor.{{sfn|Close|2012|pp=37–41}}

=== Liverpool ===
With the onset of the [[Great Depression in the United Kingdom]], the government became more parsimonious with funding for science. At the same time, Lawrence's recent invention, the [[cyclotron]], promised to revolutionise experimental nuclear physics, and Chadwick felt that the Cavendish laboratory would fall behind unless it also acquired one. He therefore chafed under Rutherford, who clung to the belief that good nuclear physics could still be done without large, expensive equipment, and turned down the request for a cyclotron.{{sfn|Brown|1997|pp=129–132}}

[[File:Victoria Clock Tower, Liverpool University - geograph.org.uk - 374422.jpg|thumb|left|upright|"[[Red brick university|Red brick]]" [[Victoria Building, University of Liverpool|Victoria Building]] at the [[University of Liverpool]] ]]
Chadwick was himself a critic of [[Big Science]] in general, and Lawrence in particular, whose approach he considered careless and focused on technology at the expense of science. When Lawrence postulated the existence of a new and hitherto unknown particle that he claimed was a possible source of limitless energy at the [[Solvay Conference]] in 1933, Chadwick responded that the results were more likely attributable to contamination of the equipment.{{sfn|Herken|2002|p=10}} While Lawrence rechecked his results at Berkeley only to find that Chadwick was correct, Rutherford and Oliphant conducted an investigation at the Cavendish that found that deuterium [[Nuclear fusion|fuses]] to form [[helium-3]], thereby causing the effect that Lawrence had observed. This was another major discovery, but the Oliphant-Rutherford [[particle accelerator]] was an expensive state-of-the-art piece of equipment.{{sfn|Heilbron|Seidel|1989|pp=165–167}}{{sfn|Oliphant|Rutherford|1933}}{{sfn|Oliphant|Kinsey|Rutherford|1933}}{{sfn|Oliphant|Harteck|Rutherford|1934}}

In March 1935, Chadwick received an offer of the Lyon Jones Chair of physics at the [[University of Liverpool]], in his wife's home town, to succeed [[Lionel Wilberforce]]. The laboratory was so antiquated that it still ran on [[direct current]] electricity, but Chadwick seized the opportunity, assuming the chair on 1 October 1935. The university's prestige was soon bolstered by Chadwick's Nobel Prize, which was announced in November 1935.{{sfn|Brown|1997|pp=134–139}} His medal was sold at auction in 2014 for $329,000.<ref>{{cite news |newspaper=Yahoo News |title=Sold! Nobel Prize for Neutron Discovery Auctioned for $329,000 |first=Megan |last=Gannon |date=4 June 2014 |url=https://news.yahoo.com/sold-nobel-prize-neutron-discovery-auctioned-329-000-161620108.html |access-date=16 September 2014 }}</ref>

Chadwick set about acquiring a cyclotron for Liverpool. He started by spending £700 to refurbish the antiquated laboratories at Liverpool, so some components could be made in-house.{{sfn|Brown|1997|p=142}} He was able to persuade the university to provide £2,000 and obtained a grant for another £2,000 from the Royal Society.{{sfn|Brown|1997|pp=149–151}} To build his cyclotron, Chadwick brought in two young experts, Bernard Kinsey and Harold Walke, who had worked with Lawrence at the University of California. A local cable manufacturer donated the copper conductor for the coils. The cyclotron's 50-ton magnet was manufactured in [[Trafford Park]] by [[Metropolitan-Vickers]], which also made the vacuum chamber.{{sfn|Holt|1994}} The cyclotron was completely installed and running in July 1939. The total cost of £5,184 was more than Chadwick had received from the university and the Royal Society, so Chadwick paid the rest from his {{SEK|159,917|link=yes}} (£8,243) Nobel Prize money.{{sfn|Brown|1997|pp=173–174}}

At Liverpool the Medicine and Science faculties worked together closely. Chadwick was automatically a committee member of both faculties, and in 1938 he was appointed to a commission headed by [[Edward Stanley, 17th Earl of Derby|Lord Derby]] to investigate the arrangements for cancer treatment in Liverpool. Chadwick anticipated that neutrons and radioactive isotopes produced with the 37-inch cyclotron could be used to study biochemical processes, and might become a weapon in the fight against cancer.{{sfn|King|1997}}{{sfn|Brown|1997|p=150}}