Demon core The demon core was a , subcritical mass of plutonium which went briefly critical in two separate accidents at the Los Alamos laboratory in 1945 and 1946 . Each incident resulted in the acute radiation poisoning and subsequent death of a scientist . After these incidents the spherical plutonium pit was referred to as the " demon core . " The demon core was used in the first atomic bomb test to be conducted after World War II , five weeks after the second fatal accident . It performed normally and with the same explosive yield as the next core used in this set of two tests . First incident . On August 21 , 1945 , the plutonium core produced a burst of neutron radiation that led to Harry Daghlian ' s death . Daghlian was a physicist who made a mistake while working alone performing neutron reflection experiments on the core . The core was placed within a stack of neutron - reflective tungsten carbide bricks , and the addition of each brick moved 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 critical , a self - sustaining chain reaction . Despite quick action in moving the brick off the assembly , Daghlian received a fatal dose of radiation . He died 25 days later from acute radiation poisoning . Another person who was in the lab at the time of the accident — Private Robert J . Hemmerly , a Special Engineer Detachment ( SED ) guard — received an exposure of approximately 31 roentgen s ( 0 . 31 Gy ) of soft X - rays ( 80 kV equivalent ) and less than 1 roentgen ( 0 . 01 Gy ) of gamma rays . Hemmerly died in 1978 ( 33 years after the accident ) from acute myelogenous leukemia at the age of 62 . Second incident . On May 21 , 1946 , physicist Louis Slotin and seven other Los Alamos personnel were in a Los Alamos laboratory conducting an experiment to verify the exact point at which a subcritical mass ( core ) of fissile material could be made critical by the positioning of neutron reflectors . The test was known as " tickling the dragon ' s tail " for its extreme risk . It required the operator to place two half - spheres of beryllium ( a neutron reflector ) around the core to be tested and manually lower the top reflector over the core via a thumb hole on the top . As the reflectors were manually moved closer and farther away from each other , scintillation counters measured the relative activity from the core . Allowing them to close completely could result in the instantaneous formation of a critical mass and a lethal power excursion . Under Slotin ' s unapproved protocol , the only thing preventing this was the blade of a standard flathead screwdriver , manipulated by the scientist ' s other hand . Slotin , who was given to bravado , became the local expert , performing the test almost a dozen separate times , often in his trademark bluejeans 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 it . While lowering the top reflector , Slotin ' s screwdriver slipped outward a fraction of an inch , allowing the top reflector to fall into place around the core . Instantly there was a flash of blue light and a wave of heat across Slotin ' s skin ; the core had become supercritical , releasing a massive burst of neutron radiation . He quickly knocked the two halves apart , stopping the chain reaction and presumably saving the lives of the other men in the laboratory , though it is now known that the heating of the core and shells stopped the criticality within milliseconds of its initiation . Slotin ' s body ' s positioning over the apparatus also shielded the others from much of the neutron radiation . He received a lethal dose of 1000 rads neutron / 114 rads gamma in under a second and died nine days later from acute radiation poisoning . The nearest person to Slotin , Alvin C . Graves , was watching over Slotin ' s shoulder and was thus partially shielded by him , received a high but non - lethal radiation dose of 166 neutron / 26 gamma rads . Graves was hospitalized for several weeks with severe radiation poisoning , developed chronic neurological and vision problems as a result of the exposure , and died of a heart attack 20 years later . Besides Graves , the others in the room were the following . Two final affected persons were Paul Long , machinist , and another unidentified machinist , these latter two in another part of the building . After Slotin ' s accident , hands - on criticality experiments were stopped , and remote - control machines were designed by Schreiber , one of the survivors , to perform such experiments with all personnel at a quarter mile distance . Demon core in use . The demon core was put to use for the Able detonation test of the Crossroads series at Bikini Atoll on July 1 , 1946 . Its yield was , the same as the next core used in the Crossroads pair of bomb tests . Cultural references . Paul Mullins wrote a play based on the last nine days of Slotin ' s life , named the Louis Slotin Sonata . It premiered in 1999 at Circle X Theatre in Los Angeles and went on to win the Los Angeles Drama Critics Circle Award for Outstanding World Premiere . Charles Stross refers to the Daghlian incident in his novel " The Atrocity Archives " , specifically in the included short novel by similar title . The 1989 film " Fat Man and Little Boy " portrays , in a character named Michael Merriman ( played by John Cusack ) , a fictional composite of Harry K . Daghlian and Louis Slotin . The character dies of radiation poisoning after two neutron - reflection hemispheres , which are separated by a screwdriver , connect accidentally . Joseph Kanon recounts events surrounding the first criticality accident , " Tickling the tail of the sleeping dragon [ . . . ] Testing how far we can go , " in his 1997 novel " Los Alamos " . The story of the second accident involving the demon core ( entitled " " Risky Radiation " " ) was featured in an episode of " " , though the first accident is mentioned .