A plutonium sphere killed two Manhattan Project scientists in separate accidents — scientists called it 'tickling the dragon's tail.' The dragon bit back twice.
The demon core was a 6.2-kilogram sphere of plutonium-gallium alloy built in 1945 for use in a potential third atomic bomb. After Japan surrendered, it wasn't needed — so Los Alamos researchers kept using it for dangerous criticality experiments.
On August 21, 1945, physicist Harry Daghlian accidentally dropped a tungsten carbide brick onto the core, causing it to go supercritical and emit a lethal burst of radiation. He died 25 days later, the first American to be killed by a criticality accident.
Less than a year later, on May 21, 1946, physicist Louis Slotin was demonstrating the same experiment to colleagues when his screwdriver slipped. The core went critical again. Slotin instinctively yanked the assembly apart with his bare hands, saving the seven others in the room — but absorbed a fatal dose himself and died nine days later.
The dangerous technique Slotin was using — holding two neutron-reflective beryllium hemispheres apart with just a screwdriver — had been nicknamed 'tickling the dragon's tail' by physicist Richard Feynman, who considered it recklessly unsafe.
After the second death, all such manual criticality experiments were immediately banned at Los Alamos. Remote-controlled machinery was mandated for any future work. The demon core itself was melted down in the summer of 1946 and its plutonium recycled into the U.S. nuclear arsenal.