In 1942, Slotin was invited to participate in the Manhattan Project, and subsequently performed experiments with uranium and plutonium cores to determine their critical mass values. After World War II he continued his research at Los Alamos National Laboratory in New Mexico. On 21 May 1946, he accidentally triggered a fission reaction which released a burst of hard radiation. He was rushed to the hospital and died nine days later on 30 May. Slotin had become the victim of the second criticality accident in history following Harry Daghlian, who had been fatally exposed to radiation by the same plutonium " demon core " that killed Slotin.