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. It was involved in two criticality accidents, on August 21, 1945, and May 21, 1946, each of which killed a person.

The core was intended for use in a possible third nuclear weapon to be dropped on Japan, but when Japan's surrender made this unnecessary, it was repurposed for testing. It was designed with a small safety margin to ensure a successful explosion of the bomb. The device briefly went supercritical when it was accidentally placed in supercritical configurations during two separate experiments intended to guarantee the core was close to the critical point. The incidents happened at the Los Alamos Laboratory, resulting in the acute radiation poisoning and subsequent deaths of scientists Harry Daghlian and Louis Slotin.

Louis Alexander Slotin (1 December 1910 – 30 May 1946) was a Canadian physicist and chemist who took part in the Manhattan Project. He was born and raised in the North End of Winnipeg, Manitoba. After earning both his Bachelor of Science and Master of Science degrees from the University of Manitoba, Slotin attended King's College London, where he obtained his doctorate in physical chemistry in 1936. Afterwards, he joined the University of Chicago as a research associate to help design a cyclotron. In 1942, he was invited to participate in the Manhattan Project.

As part of the Manhattan Project, Slotin performed experiments with uranium and plutonium cores to determine their critical mass values. After World War II, Slotin continued his research at Los Alamos National Laboratory. On 21 May 1946, Slotin accidentally began a fission reaction, which released a burst of hard radiation. Slotin was rushed to the hospital, and died nine days later on 30 May, the victim of the second criticality accident in history, following the death of Harry Daghlian, who had been exposed to radiation by the same "demon core" that killed Slotin.

Slotin was hailed as a hero by the United States government for reacting quickly enough to prevent the deaths of his colleagues. Some physicists argue that this was a preventable accident. The accident and its aftermath have been dramatized in several fictional and non-fiction accounts.