During his time there, Dr. Slotin's duties were to perform criticality tests on fissile uranium and plutonium. The tests required him to bring two half-spheres of the radioactive materials into close proximity, and measure the beginnings of the fission reaction. This way, scientists could indirectly determine the critical mass of the material without actually starting a nuclear chain reaction. Slotin's preferred method of keeping the two hemispheres separated was an ordinary screwdriver, because he had a strong distrust of automated safety systems.
Dr. Richard Feynman, a fellow Los Alamos scientist, had remarked that these criticality tests were "tickling the tail of a sleeping dragon," because they brought the fissile materials so close to a dangerous critical mass. In fact, another physicist at Los Alamos, Harry K. Daghlian, had been injured by radiation exposure during a criticality test just nine months earlier, when he accidentally dropped a brick of tungsten carbide onto a plutonium mass. Tungsten carbide is a neutron reflector, which decreases the amount of nuclear material needed to go critical. In Daghlian's case, it caused the plutonium mass to go supercritical, bathing him in radiation. He died less than a month later from radiation sickness.
Louis Slotin's experiment of May 21, 1946 didn't involve such neutron reflecting bricks, but it did utilize the same plutonium cores as Daghlian's experiment, and it was similarly ill-fated. At the secret Omega Site Laboratory, as six observers looked on, Slotin was training a colleague who was meant to replace him, one Alvin C. Graves. As Slotin demonstrated the criticality test, the screwdriver he used to separate the two sphere halves slipped, and the hemispheres came into contact.
Louis, having been closest to the event, soon complained of a painful burning sensation in his left hand, and a sour taste in his mouth. His colleagues rushed him outside and into a car bound for the hospital, but he had already begun vomiting, a sign of acute radiation poisoning. The other scientists who had been in the room didn't experience any immediate symptoms, indicating that Louis' reflex to separate the plutonium hemispheres had spared them from a similar level of exposure. All too familiar with the consequences of such a powerful dose of radiation, he said to his colleagues in the car, "You'll be OK, but I think I'm done for." After arriving at the Los Alamos hospital, Slotin said to Alvin Graves, "I'm sorry I got you into this. I'm afraid I have less than a 50 per cent chance of living. I hope you have better than that."
He had a telegram sent to his parents in Winnipeg to inform them of the accident, and a few days later, he telephoned them with the assistance of a nurse who held the receiver for him. Major-General Leslie R. Groves, the administrator of the Manhattan Project, sent a U.S. Army DC-3 to pick up his parents, and they arrived just days before he died of radiation exposure, nine days after the incident. Within the next couple of years, two of the scientists who had been observing the experiment died with symptoms of radiation sickness.
Louis Slotin is generally regarded as a hero for his quick action, and his sacrifice was indeed selfless and heroic. The true tragedy, however, is that the entire accident could have been avoided with some simple safety precautions. A better method would have been to fix the upper plutonium hemisphere in an elevated position, with a levered mechanism to lift the lower hemisphere up towards it. In the event of a problem with such an assembly, the lower hemisphere would fall harmlessly down and away. He had also neglected to use two tiny safety spacers which had been developed after his colleague had been killed by radiation nine months earlier.
As he had awaited the inevitable, the Los Alamos authorities issued a special citation, which was read to him in the hospital:
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.