On Aug. 21, 1945, physicist Harry Daghlian accidentally dropped a brick of tungsten carbide into a plutonium bomb core at the Los Alamos National Laboratory. The mass went critical, and Daghlian died of radiation sickness.

Exactly nine months later, physicist Louis Slotin was conducting an experiment on the same mass of plutonium when his screwdriver slipped and the mass again went critical. He too died of radiation sickness.

The mass became known as “the demon core.”