The demon core was a sphere of plutonium–gallium alloy that was involved in two fatal radiation accidents when scientists tested it as a fissile core of an early atomic bomb. It was manufactured in 1945 by the Manhattan Project, the U.S. nuclear weapon development effort during World War II. It was a subcritical mass that weighed and was in diameter. The core was prepared for shipment to the Pacific Theater as part of the third nuclear weapon intended to be dropped on Japan, but when Japan surrendered, the core was retained for testing and potential later use in the case of another conflict.
The two criticality accidents occurred at the Los Alamos Laboratory in New Mexico on August 21, 1945, and May 21, 1946. In both cases, an experiment was intended to demonstrate how close the core was to criticality, using a neutron-reflective tamper (layer of dense material surrounding the fissile material). In both accidents, the core was accidentally put into a critical configuration. Physicists Harry Daghlian (in the first accident) and Louis Slotin (in the second accident) both suffered acute radiation syndrome and died shortly afterward. Others present in the laboratory were also exposed to radiation during the accidents. The core was melted down during the summer of 1946, and the material was recycled for use in other cores.
Manufacturing and early history The demon core was a solid softball-sized sphere measuring in diameter; it was similar to the core used in the bombing of Nagasaki. It consisted of three parts made of plutonium–gallium: two hemispheres and an anti-jet ring, designed to keep neutron flux from "jetting" out of the joined surface between the hemispheres during implosion. The core of the device used in the Trinity Test at the Alamogordo Bombing and Gunnery Range in July did not have such a ring.
The refined plutonium was shipped from the Hanford Site in Washington to the Los Alamos Laboratory; an inventory document dated August 30 shows Los Alamos had expended "HS-1, 2, 3, 4; R-1" (the components of the Trinity and Nagasaki bombs) and had in its possession "HS-5, 6; R-2", finished and in the hands of quality control. Material for "HS-7, R-3" was in the Los Alamos metallurgy section and would also be ready by September 5 (it is not certain whether this date allowed for the unmentioned "HS-8s fabrication to complete the fourth core). The metallurgists used a plutonium–gallium alloy, which stabilized the delta () phase allotrope of plutonium so it could be hot pressed into the desired spherical shape. As plutonium was found to corrode readily, the sphere was then coated with nickel.
On August 10, Major General Leslie R. Groves Jr., wrote to General of the Army George C. Marshall, the Chief of Staff of the United States Army, to inform him that:
Marshall added an annotation, "It is not to be released on Japan without express authority from the President", on President Harry S. Truman's orders. On August 13, the third bomb was scheduled. It was anticipated that it would be ready by August 16 to be dropped on August 19. This was pre-empted by Japan's surrender on August 15, 1945, while preparations were still being made for it to be couriered to Kirtland Field. The third core remained at Los Alamos. Raemer Schreiber being interviewed by Richard Rhodes
First incident The core, once assembled, was designed to be at "−5 cents". In this state, there is only a small safety margin against extraneous factors that might increase reactivity, causing the core to become supercritical, and then prompt critical, a brief state of rapid energy increase. These factors are not common in the environment; they are only likely to occur under conditions such as the compression of the solid metallic core (which would eventually be the method used to explode the bomb), the addition of more nuclear material, or provision of an external reflector which would reflect outbound neutrons back into the core. The experiments conducted at Los Alamos leading to the two fatal accidents were designed to see how much neutron reflection was required to approach supercriticality, and included arranging such reflectors in a way that would guarantee that the core was indeed close to the critical point.
On August 21, 1945, the plutonium core produced a burst of neutron radiation that resulted in physicist Harry Daghlian's death. Daghlian made a mistake while performing neutron reflector experiments on the core. He was working alone; a security guard, Private Robert J. Hemmerly, was seated at a desk away. The core was placed within a stack of neutron-reflective tungsten carbide bricks, and the addition of each brick made the assembly closer to criticality. While attempting to stack another brick around the assembly, Daghlian accidentally dropped it onto the core and thereby caused the core to go well into supercriticality, a self-sustaining critical chain reaction. He quickly moved the brick off the assembly, but he received a fatal dose of radiation. He died 25 days later from acute radiation poisoning.
Name Age at accident Profession Dose Patient numbers in this document have been identified as: 1 – Daghlian, 2 – Hemmerly, 3 – Slotin, 4 – Graves, 5 – Kline, 6 – Young, 7 – Cleary, 8 – Cieleski, 9 – Schreiber, 10 – Perlman Aftermath Haroutune "Harry" Krikor Daghlian Jr. 24 Physicist neutron gamma Died 25 days after the accident of acute radiation syndrome, hematopoietic focus Private Robert J. Hemmerly 29 Special Engineer Detachment guard neutron gamma Died in 1978 (33 years after accident) of acute myelogenous leukemia at age 62
Second incident
Background On May 21, 1946, physicist Louis Slotin and seven other 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. Slotin, who was leaving Los Alamos, was showing the technique to Alvin C. Graves, who would use it in a final test before the Operation Crossroads nuclear tests scheduled a month later at Bikini Atoll. It required the operator to place two half-spherical shells of beryllium (a neutron reflector) around the core to be tested and manually lower the top reflector over the core using a thumb hole at the polar point. As the reflectors were manually moved closer and farther away from each other, neutron detectors indicated the core's neutron multiplication rate. The experimenter needed to maintain a slight separation between the reflector halves to allow enough neutrons to escape from the core in order to stay below criticality. The standard protocol was to use shims between the halves, as allowing them to close completely could result in the instantaneous formation of a critical mass and a lethal power excursion.
Using his own unapproved protocol, Slotin did not place any shims. The top half of the reflector was resting directly on the bottom half at one point, while 180 degrees from this point a gap was maintained by the blade of a flat-tipped screwdriver in Slotin's hand. The size of the gap between the reflectors was changed by twisting the screwdriver. Slotin became the local expert, performing the test on almost a dozen occasions, often in his trademark blue jeans and cowboy boots in front of a roomful of observers. Enrico Fermi reportedly told Slotin and others they would be "dead within a year" if they continued performing the test in that manner. Scientists referred to this flirtation with a nuclear chain reaction as "tickling the dragon's tail", based on a remark by physicist Richard Feynman.
On the day of the accident, Slotin's screwdriver slipped outward a fraction of an inch while he was lowering the top reflector, allowing the reflector to fall into place around the core. Instantly, there was a flash of light; the core had become supercritical, releasing an intense burst of neutron radiation. Slotin quickly twisted his wrist, flipping the top shell to the floor. There was an estimated half-second between when the sphere closed to when Slotin removed the top reflector. The reaction in the room was one of immediate, somber realization. Slotin received a lethal dose of 1,000 rad (10 Gy) neutron and 114 rad (1.14 Gy) gamma radiation in less than a second. While the position of his body over the apparatus shielded the others from much of the neutron radiation, his own exposure was irreversible. Slotin reportedly turned to his colleagues in the hushed room and famously uttered the remark, "Well, that does it." As an expert in criticality, Slotin knew he had received a lethal dose. He immediately began drawing a sketch of the room’s layout to assist medical teams in calculating the specific dosages received by each survivor, even as his own health began its rapid decline. He died nine days later from acute radiation poisoning.
Graves, the next nearest person to the core, was watching over Slotin's shoulder and was thus partially shielded by him. He received a high but non-lethal radiation dose, and was hospitalized for several weeks with severe radiation poisoning. He died 19 years later, at age 55, of heart failure. While Graves's heart failure may have been caused by exposure to radiation, it may have been caused by a hereditary condition, as his father also died of heart failure.
The second accident was reported by the Associated Press on May 26, 1946: "Four men injured through accidental exposure to radiation in the government's atomic laboratory here [Los Alamos] have been discharged from the hospital and 'immediate condition' of four others is satisfactory, the Army reported today. Dr. Norris E. Bradbury, project director, said the men were injured last Tuesday in what he described as an experiment with fissionable material."Associated Press, "Several at Atomic Bomb Laboratory Injured", The San Bernardino Daily Sun, San Bernardino, California, Monday 27 May 1946, Volume 52, page 1.
Medical studies Later research was performed concerning the health of the men. An early report was published in 1951. A later report was compiled for the U.S. government and submitted in 1979. A summary of its findings:
Name Origin Age at accident Profession Dose Aftermath Louis Alexander Slotin Winnipeg, Manitoba, Canada 35 Physicist neutron gamma Died 9 days after the accident of acute radiation syndrome, gastrointestinal focus. Alvin C. Graves Austin, Texas 36 Physicist neutron gamma Died in 1965 (19 years after the accident) of myocardial infarction, with aggravating "compensated myxedema and cataracts", while skiing. Samuel Allan Kline Chicago, Illinois 26 Physics student, later patent attorney Died in 2001 (55 years after the accident) at age 81; refused to participate with studies and was prevented from obtaining his own medical records from the incident. Marion Edward Cieslicki Mt. Lebanon, Pennsylvania 23 Physicist neutron gamma Died of acute myelocytic leukemia in 1965 (19 years after the accident). Dwight Smith Young Chicago, Illinois 54 Photographer neutron gamma Died of aplastic anemia and bacterial endocarditis in 1975 (29 years after the accident) at age 83. Raemer Edgar Schreiber McMinnville, Oregon 36 Physicist neutron gamma Died of natural causes in 1998 (52 years after the accident), at age 88. Theodore Perlman New Orleans, Louisiana 23 Engineer neutron gamma "Alive and in good health and spirits" as of 1978; most likely died in June 1988 (42 years after the accident), in Livermore, California. Private Patrick Joseph Cleary New York City, New York 21 Security guard neutron gamma Sergeant 1st Class Cleary was killed in action on September 3, 1950 (4 years after the accident), while serving with the 8th Cavalry Regiment, US Army in the Korean War.
Two machinists, Paul Long and another, unidentified, in another part of the building, away, were not treated.