text: The Japanese attack on Pearl Harbor in December 1941 and the subsequent United States declaration of war upon Italy rendered Segrè an enemy alien and cut him off from communication with his parents. Physicists began leaving the Radiation Laboratory to do war work, and Raymond T. Birge asked him to teach classes to the remaining students. This provided a useful supplement to Segrè's income, and he established important friendships and professional associations with some of these students, who included Owen Chamberlain and Clyde Wiegand.
  In late 1942, Oppenheimer asked Segrè to join the Manhattan Project at its Los Alamos Laboratory. Segrè became the head of the laboratorys P-5 (Radioactivity) Group, which formed part of Robert Bachers P (Experimental Physics) Division. For security reasons, he was given the cover name of Earl Seaman. He moved to Los Alamos with his family in June 1943.
  Segrès group set up its equipment in a disused Forest Service cabin in the Pajarito Canyon near Los Alamos in August 1943. His groups task was to measure and catalog the radioactivity of various fission products. An important line of research was determining the degree of isotope enrichment achieved with various samples of enriched uranium. Initially, the tests using mass spectrometry, used by Columbia University, and neutron assay, used by Berkeley, gave different results. Segrè studied Berkeleys results and could find no error, while Kenneth Bainbridge likewise found no fault with New Yorks. However, analysis of another sample showed close agreement. Higher rates of spontaneous fission were observed at Los Alamos, which Segrè's group concluded were due to cosmic rays, which were more prevalent at Los Alamos due to its high altitude.
  The group measured the activity of thorium, uranium-234, uranium-235 and uranium-238, but only had access to microgram quantities of plutonium-239.  The first sample plutonium produced in the nuclear reactor at Oak Ridge was received in April 1944. Within days the group observed five times the rate of spontaneous fission as with the cyclotron-produced plutonium.  This was not news that the leaders of the project wanted to hear. It meant that Thin Man, the proposed plutonium gun-type nuclear weapon, would not work and implied that the projects investment in plutonium production facilities at the Hanford Site was wasted. Segrès group carefully checked their results and concluded that the increased activity was due to the plutonium-240 isotope.
  In June 1944, Segrè was summoned into Oppenheimer's office and informed that while his father was safe, his mother had been rounded up by the Nazis in October 1943. Segrè never saw either of his parents again. His father died in Rome in October 1944. In late 1944, Segrè and Elfriede became naturalized citizens of the United States. His group, now designated R-4, was given responsibility for measuring the gamma radiation from the Trinity nuclear test in July 1945. The blast damaged or destroyed most of the experiments, but enough data was recovered to measure the gamma rays.