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After the United States dropped atomic bombs on Hiroshima and Nagasaki in August 1945, the US Army Air Force was preparing to drop a third such bomb if the government of Japan refused to surrender. Manhattan Project scientists had a plutonium core ready to install in another “Fat Man” style atomic device. On 15 August 1945, however, Japan’s Emperor Hirohito announced his country’s unconditional surrender, and the third bomb was never built.

After the war, a few of the Manhattan Project scientists remained on duty at Los Alamos to further explore the behaviors and potential of nuclear technology. Among them was Dr. Louis Slotin, who in his own words was kept around because he was “one of the few people left here who are experienced bomb putter-togetherers.”

During his time there, one of Dr. Slotin’s duties was to use the undeployed plutonium core to perform criticality tests. The tests required him to lower a dome of beryllium, a neutron reflector, over the radioactive material, 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. It was necessary to leave a gap between the two halves of the beryllium shell, otherwise a dangerous reaction would occur. Slotin’s preferred method of maintaining this gap was an ordinary screwdriver. 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 state. 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 the plutonium sphere. 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 bathe him in neutron and gamma radiation. He soon collapsed from the acute radiation poisoning. He died less than a month later.

Louis Slotin’s experiment of 21 May 1946 didn’t involve such neutron reflecting bricks, but it did utilize the same plutonium core as Daghlian’s experiment. 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. Slotin gently lowered the beryllium dome over the sphere of plutonium, using his screwdriver to maintain the gap so neutrons could escape safely. The screwdriver slipped, and the dome fell to fully cover the core.

Immediately, all eight scientists in the room felt a wave of heat accompanied by a blue flash as the plutonium sphere vomited an invisible burst of gamma and neutron radiation into the room. As the lab’s Geiger counter clicked hysterically, Slotin used his bare hand to push the beryllium dome off and onto the floor, which terminated the prompt critical reaction moments after it began. “Well,” Slotin said gravely, “that does it.”

Slotin, 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 Slotin’s reflex to swat away the dome 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 percent 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 would die of radiation exposure. As Slotin 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.

He died nine days after the incident.

Louis Slotin is generally regarded as a hero for his quick, selfless action to end the reaction with his bare hand, but the true tragedy is that the entire accident could have been avoided with some simple safety precautions. A better method would have been to raise the beryllium dome from below the core on a levered mechanism⁠—in the event of a slip with such an assembly, the dome would fall harmlessly down and away instead of into the dangerous prompt critical position. Slotin also neglected to use two safety spacers which had been developed after his colleague was killed by the same core nine months earlier. Louis Slotin saved the immediate lives of some colleagues, but his neglect of safety protocols is what created the danger in the first place.

Within the next couple of years, two of the scientists who had been observing the experiment died with symptoms of radiation sickness.

A previous version of this article incorrectly described the demonstration as using two hemispheres of plutonium rather than two halves of a beryllium neutron shield.

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