In 1945, physicist Harry Daghlian was working on a 6.2 kg (14 lb) spherical mass of plutonium at the Los Alamos laboratory. He was stacking bricks of tungsten carbide around the plutonium core when he noticed a nearby neutron counter signaling that the addition of the final brick would make the assembly supercritical. Daghlian immediately withdrew his hand ... and the brick slipped onto the center of the plutonium core and the assembly went critical. Daghlian was able to dissemble the bricks (the core didn't explode), but he died from radiation poisoning 28 days later.
Nine months later, physicist Louis Slotin, an expert in triggering devices, and seven other scientists gathered in the laboratory to perform a dangerous experiment he called "tickling the dragon's tail." The experiment involved creating the beginning steps of a nuclear fission reactor by placing two half-spheres of beryllium around the plutonium core. The trick was to keep the beryllium from touching the plutonium core, which Slotin had done many times before.
But on that day, Slotin decided to use a screwdriver instead of shims, and his hand slipped and the beryllium hemisphere touched the plutonium core, which instantly went critical. Slotin realized his mistake, and used his hand to lift the beryllium just a fraction of a second later ... but that was enough to give him a lethal dose of radiation. The other scientists saw a "blue glow" of air ionization and felt a "heat wave" - they were saved from immediate death (though later 3 of them died from side effects of radiation years later). Slotin, on the other hand, died 9 days later.
Both of Daghlian and Slotin's accidents were on Tuesday the 21st, both used the same plutonium core, and both died in the same room at the same hospital. The plutonium core was later named the "Demon Core" and was put to use in the Able test of the Operation Crossroads nuclear weapon test at the Bikini Atoll in the summer of 1946.
The Able test of Operation Crossroads, July 1, 1946.
Photo: Office of History & Heritage Resources