Editing James Chadwick (section)

== Second World War ==
=== Tube Alloys and the MAUD Report ===
During the Second World War, Chadwick carried out research as part of the [[Tube Alloys]] project to build an atom bomb, while his Manchester lab and environs were harassed by [[Luftwaffe]] bombing. When the [[Quebec Agreement]] merged his project with the American Manhattan Project, he became part of the British Mission, and worked at the [[Los Alamos Laboratory]] and in Washington, D.C. He surprised everyone by earning the almost-complete trust of project director [[Leslie R. Groves, Jr.]] For his efforts, Chadwick received a knighthood in the [[1945 New Year Honours|New Year Honours on 1 January 1945]]. In July 1945, he viewed the [[Trinity nuclear test]]. After this, he served as the British scientific advisor to the United Nations Atomic Energy Commission. Uncomfortable with the trend toward [[Big Science]], he returned to Cambridge where he became the [[Master (college)|Master]] of Gonville and Caius College in 1948. 

In Germany, [[Otto Hahn]] and [[Fritz Strassmann]] bombarded uranium with neutrons, and noted that [[barium]], a lighter element, was among the products produced. Hitherto, only the same or heavier elements had been produced by the process. In January 1939, Meitner and her nephew [[Otto Frisch]] astounded the physics community with a paper that [[discovery of nuclear fission|explained this result]].{{sfn|Brown|1997|p=170}} They theorised that uranium atoms bombarded with neutrons can break into two roughly equal fragments, a process they called [[nuclear fission|fission]]. They calculated that this would result in the release of about 200 [[Electronvolt|MeV]], implying an energy release orders of magnitude greater than chemical reactions,{{sfn|Meitner|Frisch|1939}} and Frisch confirmed their theory experimentally.{{sfn|Frisch|1939}} It was soon noted by Hahn that if neutrons were released during fission, then a chain reaction was possible.{{sfn|Hahn|Strassmann|1939}} French scientists, [[Pierre Joliot]], [[Hans von Halban]] and [[Lew Kowarski]], soon verified that more than one neutron was indeed emitted per fission.{{sfn|von Halban|Joliot|Kowarski|1939}} In a paper co-authored with the American physicist [[John Archibald Wheeler|John Wheeler]], Bohr theorised that fission was more likely to occur in the [[uranium-235]] [[isotope]], which made up only 0.7 per cent of natural uranium.{{sfn|Gowing|1964|pp=24–27}}{{sfn|Bohr|Wheeler|1939}}

[[File:William Penney, Otto Frisch, Rudolf Peierls and John Cockroft.jpg|thumb|upright=1.3|right|Key British physicists. Left to right: [[William Penney, Baron Penney|William Penney]], [[Otto Robert Frisch]], [[Rudolf Peierls]] and [[John Cockcroft]]. They are wearing the American [[Medal of Freedom (1945)|Medal of Freedom]].]]
Chadwick did not believe that there was any likelihood of another war with Germany in 1939, and took his family for a holiday on a remote lake in northern Sweden. The news of the outbreak of the [[Second World War]] therefore came as a shock. Determined not to spend another war in an internment camp, Chadwick made his way to [[Stockholm]] as fast as he could, but when he arrived there with his family, he found that all air traffic between Stockholm and London had been suspended. They made their way back to England on a [[tramp steamer]]. When he reached Liverpool, Chadwick found [[Joseph Rotblat]], a Polish post-doctoral fellow who had come to work with the cyclotron, was now destitute, as he was cut off from funds from Poland. Chadwick promptly hired Rotblat as a lecturer, despite his poor grasp of English.{{sfn|Brown|1997|pp=174–178}}

In October 1939, Chadwick received a letter from Sir [[Edward Victor Appleton|Edward Appleton]], the Secretary of the Department of Scientific and Industrial Research, asking for his opinion on the feasibility of an [[atom bomb]]. Chadwick responded cautiously. He did not dismiss the possibility, but carefully went over the many theoretical and practical difficulties involved. Chadwick decided to investigate the properties of [[uranium oxide]] further with Rotblat.{{sfn|Gowing|1964|pp=38–39}} In March 1940, Otto Frisch and [[Rudolf Peierls]] at the [[University of Birmingham]] re-examined the theoretical issues involved in a paper that became known as the [[Frisch–Peierls memorandum]]. Instead of looking at unenriched uranium oxide, they considered what would happen to a sphere of pure uranium-235, and found that not only could a chain reaction occur, but that it might require as little as {{convert|1|kg}} of uranium-235, and unleash the energy of tons of dynamite.{{sfn|Gowing|1964|pp=39–41}}

[[File:Liverpool Blitz D 5984.jpg|thumb|left|400px|Part of [[Liverpool]] devastated by the [[The Blitz|Blitz]]]]
A special subcommittee of the Committee for the Scientific Survey of Air Warfare (CSSAW), known as the [[MAUD Committee]], was created to investigate the matter further. It was chaired by Sir [[George Paget Thomson|George Thomson]] and its original membership included Chadwick, along with Mark Oliphant, John Cockcroft and [[Philip Moon]].{{sfn|Gowing|1964|p=45}} While other teams investigated [[uranium enrichment]] techniques, Chadwick's team at Liverpool concentrated on determining the [[nuclear cross section]] of uranium-235.{{sfn|Gowing|1964|p=63}} By April 1941, it had been experimentally confirmed that the [[critical mass]] of uranium-235 might be {{convert|8|kg}} or less.{{sfn|Brown|1997|p=206}} His research into such matters was complicated by all-but-incessant [[Luftwaffe]] bombings of the environs of his Liverpool lab; the windows were blown out so often that they were replaced by cardboard.{{sfn|Brown|1997|p=204}}

In July 1941, Chadwick was chosen to write the final draft of the MAUD Report, which, when presented by [[Vannevar Bush]] to [[President of the United States|President]] [[Franklin D. Roosevelt]] in October 1941, inspired the U.S. government to pour millions of dollars into the pursuit of an atom bomb.{{sfn|Bundy|1988|pp=48–49}} When [[George B. Pegram]] and [[Harold Urey]] visited Britain to see how the project,{{sfn|Gowing|1964|p=85}} now known as [[Tube Alloys]],{{sfn|Gowing|1964|p=109}} was going, Chadwick was able to tell them: "I wish I could tell you that the bomb is not going to work, but I am 90 per cent sure that it will."{{sfn|Gowing|1964|p=85}}

In a recent book about the Bomb project, [[Graham Farmelo]] wrote that "Chadwick did more than any other scientist to give Churchill the Bomb.&nbsp;... Chadwick was tested almost to the breaking point."{{sfn|Farmelo|2013|p=119}} So worried that he could not sleep, Chadwick resorted to sleeping pills, which he continued to take for most of his remaining years. Chadwick later said that he realised that "a nuclear bomb was not only possible—it was inevitable. Sooner or later these ideas could not be peculiar to us. Everybody would think about them before long, and some country would put them into action".{{sfn|Brown|1997|p=205}} Sir [[Hermann Bondi]] suggested that it was fortunate that Chadwick, not Rutherford, was the doyen of UK physics at the time, as the latter's prestige might otherwise have overpowered Chadwick's interest in "looking forward" to the Bomb's prospects.{{sfn|Bondi|1997}}

=== Manhattan Project ===
[[File:Quebec conference 1943.png|thumb|right|[[Mackenzie King]], [[Franklin D. Roosevelt]] and [[Winston Churchill]] at the [[Quebec Conference, 1943|first Quebec Conference]] in 1943]]
Owing to the danger from aerial bombardment, the Chadwicks sent their twins to Canada as part of a [[Evacuations of civilians in Britain during World War II|government evacuation scheme]].{{sfn|Brown|1997|pp=197–198}} Chadwick was reluctant to move Tube Alloys there, believing that the United Kingdom was a better location for the isotope separation plant.{{sfn|Brown|1997|pp=218–219}} The enormous scope of the effort became more apparent in 1942: even a pilot separation plant would cost over £1 million and strain Britain's resources, to say nothing of a full-scale plant, which was estimated to cost somewhere in the vicinity of £25 million. It would have to be built in America.{{sfn|Gowing|1964|pp=141–142}} At the same time that the British became convinced that a joint project was necessary, the progress of the American [[Manhattan Project]] was such that British cooperation seemed less essential, although the Americans were still eager to use Chadwick's talents.{{sfn|Gowing|1964|p=152}}

The matter of cooperation had to be taken up at the highest level. In September 1943, the [[Prime Minister of the United Kingdom|Prime Minister]], [[Winston Churchill]], and President Roosevelt negotiated the [[Quebec Agreement]], which reinstated cooperation between Britain, the United States and Canada. Chadwick, Oliphant, Peierls and [[Francis Simon|Simon]] were summoned to the United States by the director of Tube Alloys, Sir [[Wallace Akers]], to work with the Manhattan Project. The Quebec Agreement established a new [[Combined Policy Committee]] to direct the joint project. The Americans disliked Akers, so Chadwick was appointed technical advisor to the Combined Policy Committee, and the head of the British Mission.{{sfn|Gowing|1964|pp=166–171}}

Leaving Rotblat in charge in Liverpool, Chadwick began a tour of the Manhattan Project facilities in November 1943, except for the [[Hanford Site]] where plutonium was produced, which he was not allowed to see. He became the only man, apart from Groves and his second in command, to have access to all the American research and production facilities for the uranium bomb. Observing the work on the [[K-25]] [[gaseous diffusion]] facility at [[Oak Ridge, Tennessee]], Chadwick realised how wrong he had been about building the plant in wartime Britain. The enormous structure could never have been concealed from the Luftwaffe.{{sfn|Brown|1997|p=253}} In early 1944, he moved to [[Los Alamos, New Mexico]], with his wife and their twins, who now spoke with Canadian accents.{{sfn|Brown|1997|pp=250–261}} For security reasons, he was given the cover name of James Chaffee.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=95}}

[[File:Groves and Chadwick 830308.jpg|thumb|left|Chadwick (left) with [[Major General (United States)|Major General]] [[Leslie R. Groves, Jr.]], the director of the [[Manhattan Project]]]]
Chadwick accepted that the Americans did not need British help, but that it could still be useful in bringing the project to an early and successful conclusion. Working closely with the director of the Manhattan Project, [[Major General (United States)|Major General]] [[Leslie R. Groves, Jr.]], he attempted to do everything he could to support the effort.{{sfn|Brown|1997|pp=247–51}} He also endeavoured to place British scientists in as many parts of the project as possible to facilitate a post-war British nuclear weapons project to which Chadwick was committed. Requests from Groves via Chadwick for particular scientists tended to be met with an immediate rejection by the company, ministry or university currently employing them, only to be overcome by the overriding priority accorded to Tube Alloys.{{sfn|Gowing|1964|pp=241–244}} As a result, the British team was critical to the Project's success.{{sfn|Szasz|1992|p=xvi}}

Although he had more knowledge of the project than anyone else from Britain,{{sfn|Gowing|1964|p=329}} Chadwick had no access to the Hanford site. [[Charles Portal, 1st Viscount Portal of Hungerford|Lord Portal]] was offered a tour of Hanford in 1946. "This was the only plant to which Chadwick had been denied access in wartime, and now he asked Groves if he could accompany Portal. Groves replied that he could, but if he did then 'Portal will not see very much'."{{sfn|Brown|1997|p=317}} For his efforts, Chadwick received a [[knighthood]] in the [[1945 New Year Honours|New Year Honours on 1 January 1945]].<ref name="Kt">{{London Gazette|issue=36866 |supp=y|page=1|date=29 December 1944}} ''Knight Bachelor''</ref> He considered this to be a recognition of the work of the whole Tube Alloys project.{{sfn|Brown|1997|p=279}}

By early 1945, Chadwick was spending most of his time in Washington, D.C., and his family relocated from Los Alamos to a house on Washington's [[Dupont Circle]] in April 1945.{{sfn|Brown|1997|p=279}} He was present at the meeting of the Combined Policy Committee on 4 July when [[Field Marshal (United Kingdom)|Field Marshal]] Sir [[Henry Maitland Wilson]] gave Britain's agreement to use the atom bomb against Japan,{{sfn|Brown|1997|p=290}} and at the [[Trinity nuclear test]] on 16 July, when the first atom bomb was detonated.{{sfn|Brown|1997|p=292}} Inside its [[Pit (nuclear weapon)|pit]] was a polonium-beryllium [[modulated neutron initiator]], a development of the technique that Chadwick had used to discover the neutron over a decade before.{{sfn|Brown|1997|p=287}} [[William L. Laurence]], ''[[The New York Times]]'' reporter attached to the Manhattan Project, wrote that "never before in history had any man lived to see his own discovery materialize itself with such telling effect on the destiny of man."{{sfn|Laurence|1946|p=26}}