Demon Core

The experiments conducted at Los Alamos leading to the two fatal accidents were designed to guarantee that the core was indeed close to the critical point by arranging such reflectors and seeing how much neutron reflection was required to approach supercriticality. [6]

On August 21, 1945, the plutonium core produced a burst of neutron radiation that led to physicist Harry Daghlian’s death. [9]

On May 21, 1946,[11] physicist Louis Slotin and seven other Los Alamos personnel were in a Los Alamos laboratory conducting another experiment to verify the closeness of the core to criticality by the positioning of neutron reflectors.

Source: en.wikipedia.org