In 1938 and while Segrè was visiting the Berkeley Radiation laboratory, Benito Mussolini's fascist government passed antisemitic laws barring Jews from university positions.

There, Segrè helped discover the element astatine and the isotope plutonium-239, which was later used to make the Fat Man nuclear bomb dropped on Nagasaki.

He found in April 1944 that Thin Man, the proposed plutonium gun-type nuclear weapon, would not work due to the presence of plutonium-240 impurities.

Segrè and Owen Chamberlain co-headed a research group at the Lawrence Radiation Laboratory that discovered the antiproton, for which the two shared the 1959 Nobel Prize in Physics.

Segrè was an active photographer who took many pictures documenting events and people in the history of modern science, which were donated to the American Institute of Physics after his death.

They attended the Como Conference in September 1927,[5] where Segrè heard lectures from notable physicists including Niels Bohr, Werner Heisenberg, Robert Millikan, Wolfgang Pauli, Max Planck and Ernest Rutherford.

[4] After a stint in the Italian Army from 1928 to 1929,[4] during which he was a commissioned as a second lieutenant in the antiaircraft artillery,[8] Segrè returned to the laboratory on Via Panisperna.

Segrè was awarded a Rockefeller Foundation fellowship and, on Fermi's advice, elected to use it to study under Otto Stern in Hamburg.

[17] In 1936 he paid a visit to Ernest O. Lawrence's Berkeley Radiation Laboratory, where he met Edwin McMillan, Donald Cooksey, Franz Kurie, Philip Abelson and Robert Oppenheimer.

Segrè enlisted Perrier's help to subject the strip to careful chemical and theoretical analysis, and they were able to prove that some of the radiation was being produced by a previously unknown element.

[19][20] In June 1938, Segrè paid a summer visit to California to study the short-lived isotopes of technetium, which did not survive being mailed to Italy.

While Segrè was en route, Benito Mussolini's fascist government passed racial laws barring Jews from university positions.

Segrè then asked whether he could do the chemistry and, with Kenneth Ross MacKenzie, successfully isolated the new element, which is today called astatine.

[33] He also worked with Seaborg, McMillan, Joseph W. Kennedy and Arthur C. Wahl to create plutonium-239 in Lawrence's 60-inch (150 cm) cyclotron in December 1940.

[34][35] 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.

[37] Segrè became the head of the laboratory's P-5 (Radioactivity) Group, which formed part of Robert Bacher's P (Experimental Physics) Division.

[40] Segrè's group set up its equipment in a disused Forest Service cabin in the Pajarito Canyon near Los Alamos in August 1943.

Segrè studied Berkeley's results and could find no error, while Kenneth Bainbridge likewise found no fault with New York's.

[44] 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.

[48] In August 1945, a few days before the surrender of Japan and the end of World War II, Segrè received an offer from Washington University in St. Louis of an associate professorship with a salary of US$5,000 (equivalent to $84,600 in 2023).

He also clashed with Lawrence over the latter's plan to create a rival nuclear-weapons laboratory to Los Alamos in Livermore, California, in order to develop the hydrogen bomb, a weapon that Segrè felt would be of dubious utility.

[51] Unhappy with his deteriorating relationships with his colleagues and with the poisonous political atmosphere at Berkeley caused by the loyalty oath controversy, Segrè accepted a job offer from the University of Illinois at Urbana–Champaign.

Kennedy, Seaborg, Wahl and Segrè were subsequently awarded the same amount for their discovery of plutonium, which came to $100,000 after being divided four ways, there being no legal fees this time.

He also edited the Annual Review of Nuclear and Particle Science from 1958 to 1977 and wrote an autobiography, A Mind Always in Motion (1993), which was published posthumously.