In May 1946, Slotin, among others, was in a laboratory doing an experiment that involved creation of the beginning of the fission reaction by placing two half-spheres of beryllium (a neutron reflector) around a plutonium core. The experiment was nicknamed “tickling the dragon’s tail” after a remark by Richard Feynman that it was “tickling the tail of a sleeping dragon” due to its flirtations with nuclear chain reaction. Slotin grasped the upper beryllium hemisphere with his left hand through a thumb hole at the top while he maintained the separation of the half-spheres by a blade of a screwdriver with his right hand, having removed the shims normally used. Using a screwdriver was not a normal part of the experimental protocol.
Nine months previously on August 21, 1945, the same 6.2 kg plutonium core (later nicknamed the “demon core” because of these accidents) had produced a burst of ionizing radiation that caused lethal radiation poisoning to Harry Daghlian, an experimentor who had made a mistake while working alone doing neutron reflection experiments on it. This core, subject to experiments so shortly after the end of the war, had probably been the intended core for the 3rd nuclear weapon never used on Japan.
On May 21, the screwdriver slipped, the upper beryllium hemisphere fell and caused a “prompt critical” reaction, resulting in a burst of hard radiation. The “blue glow” of air ionization was observed and a “heat wave” was felt by the scientists in the room. Slotin instinctively jerked his left hand upward, lifting the upper beryllium hemisphere and dropping it to the floor. He exposed himself to a lethal dose (around 2100 rems, or 21 Sv) of neutron and gamma radiation, in history’s second criticality accident. In addition to the blue glow and heat, Slotin experienced a sour taste in his mouth and an intense burning sensation in his left hand. As soon as Slotin left the building, he vomited, a common reaction from exposure to extremely intense ionizing radiation. The official line was that Slotin, by quickly removing the upper hemisphere, was a hero for ending the critical reaction and protecting seven other observers in the room. The official release from the authorities while Slotin was dying in the hospital after the accident was: “Dr. Slotin’s quick reaction at the immediate risk of his own life prevented a more serious development of the experiment which would certainly have resulted in the death of the seven men working with him, as well as serious injury to others in the general vicinity.” The designation as a hero is moderated by criticisms (from, for example, Robert B. Brode) that the accident was avoidable and that Slotin was not using proper procedures, endangering the others in the lab along with himself