This is extraordinary. Extraordinarily stupid, really. In the early days of nuclear research, scientists didn’t really take precautions when working with masses that could easily release life-threatening amounts of radiation. The demon core was a 6.2 kg mass of plutonium that was involved in not one but two lethal accidents at the Los Alamos lab in 1945 and 1946. The mass was used in experiments to determine its critical mass without actually having it go critical and start a dangerous nuclear fission reaction. On August 21, 1945, a researcher named Harry Daghlian was performing an experiment that involved manually stacking bricks of tungsten carbide around the core. If he slipped up at the wrong time, the plutonium would go critical and release a lethal dose of neutron radiation, and that’s exactly what happened.
In a nuclear fission reaction, atomic cores split, releasing radiation. Some of the neutrons that zoom out of the splitting core may hit neighboring atomic nuclei, splitting them up, creating a chain reaction; when, on average, one neutron is released for each one that disappears without splitting another core, the reaction is self-sustaining and the mass is said to be critical. If more than one neutron is freed per neutron “lost”, the mass is supercritical, and the amount of radiation is increasing. Tungsten carbide reflects neutrons, which means some of the neutrons in a subcritical fission process that would have not split up other nuclei may be reflected back and get a second chance to start a chain reaction. This means that reflectors can make a subcritical mass go critical. Daghlian noticed a neutron counter indicated that stacking another brick would result in critical mass; as he withdrew his hand, the tungsten brick slipped onto the core, which went critical, beaming enough radiation into Daghlian that he died from the damages a month later.
One would think this incident would hint to the researchers that maybe they ought to take some more security precautions when working with such a dangerous core. Not so. In the experiment simulated in the photo above, two half-spheres of beryllium, another neutron reflector, are moved around the core. One half-sphere sits below the core, another is lowered onto the core from above. The only thing separating the half-spheres was a simple screwdriver. If the two half-spheres were to connect, the mass would go critical. The physicist Louis Slotin had done the experiment, “tickling the dragon’s tail”, several times before. On May 21, 1946, he was demonstrating it to seven other scientists. As he manually lowered the upper half-sphere, his screwdriver slipped an inch, the half-sphere fell into place, the plutonium went critical and the researchers observed a blue flash of light as a lethal dose of radiation shot into Slotin. Slotin was quick-witted enough to immediately throw the beryllium half-sphere off the core, saving the lives of the seven other men in the room, but it was too late for him. He died nine days later from radiation poisoning.
The demon core was finally destroyed in an atomic test at Bikini Atoll on July 1, 1946.
dailymeh posted this on February 1, 2011
