what part of the atom did chadwick discover?
Neutron
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 Chadwick dropped all his other responsibilities to concentrate on proving the existence of the neutron , 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 . 

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 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 . He was present at the meeting of the Combined Policy Committee on 4 July when Field Marshal Sir Henry Maitland Wilson gave Britain 's agreement to use the atomic bomb against Japan , and at the Trinity nuclear test on 16 July , when the first atomic bomb was detonated . Inside its 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 . 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 . " 

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 Chadwick was still in Germany at the start of the First World War , and was interned in the Ruhleben internment camp near Berlin , where he was allowed to set up a laboratory in the stables and conduct scientific experiments using improvised materials such as radioactive toothpaste . With the help of Charles Drummond Ellis , he worked on the ionisation of phosphorus , and the photochemical reaction of carbon monoxide and chlorine . He was released after the Armistice with Germany came into effect in November 1918 , and returned to his parents ' home in Manchester , where he wrote up his findings over the previous four years for the 1851 Exhibition commissioners . 

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 In his research , Chadwick continued to probe the nucleus . In 1925 , the concept of 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 protons 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 . 

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 After the war , Chadwick followed Rutherford to the Cavendish Laboratory at the University of Cambridge , where Chadwick earned his Doctor of Philosophy degree under Rutherford 's supervision from Gonville and Caius College , Cambridge , in June 1921 . He was Rutherford 's assistant director of research at the Cavendish Laboratory for over a decade at a time when it was one of the world 's foremost centres for the study of physics , attracting students like John Cockcroft , Norman Feather , and Mark Oliphant . Chadwick followed his discovery of the neutron by measuring its mass . He anticipated that neutrons would become a major weapon in the fight against cancer . Chadwick left the Cavendish Laboratory in 1935 to become a professor of physics at the University of Liverpool , where he overhauled an antiquated laboratory and , by installing a cyclotron , made it an important centre for the study of nuclear physics . 
