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