The demon core was a spherical 6.2-kilogram (14 lb) subcritical mass of plutonium 89 millimetres (3.5 in) in diameter, manufactured during World War II by the United States nuclear weapon development effort, the Manhattan Project, as a fissile core for an early atomic bomb. The core was prepared for shipment as part of the third nuclear weapon to be used in Japan, but when Japan surrendered, the core was retained at Los Alamos for testing and potential later use. It was involved in two criticality accidents at the Los Alamos Laboratory on August 21, 1945, and May 21, 1946, each resulting in a fatality. Both experiments were designed to demonstrate how close the core was to criticality with a tamper, but in each case, the core was accidentally placed into a supercritical configuration. Physicists Harry Daghlian and Louis Slotin suffered Acute Radiation Syndrome (ARS) and died soon after, while others present in the lab were also exposed.



Haroutune Krikor Daghlian Jr. (May 4, 1921 – September 15, 1945) was an American physicist with the Manhattan Project, which designed and produced the atomic bombs that were used in World War II. He accidentally irradiated himself on August 21, 1945, during a critical mass experiment at the remote Omega Site of the Los Alamos Laboratory in New Mexico, and died 25 days later from the resultant radiation poisoning.

Daghlian was irradiated as a result of a criticality accident that occurred when he accidentally dropped a tungsten carbide brick onto a 6.2 kg plutonium–gallium alloy bomb core. This core, subsequently nicknamed the "demon core", was later involved in the death of another physicist, Louis Slotin.