On this day in history, August 21, 1945... Physicist Harry Daghlian was fatally irradiated in an accident during an experiment with the Demon Core at Los Alamos National Laboratory. The demon core was a 6.2-kilogram subcritical mass of plutonium. In 1946, Physicist Louis Slotin was the second scientist to die from radiation poisoning involving this same mass of plutonium. After these incidents the spherical plutonium core was referred to as the "demon core."