The Demon Core: How a Softball-Sized Sphere of Plutonium Killed Two Scientists

On the afternoon of May 21, 1946, in a laboratory at Los Alamos, a screwdriver slipped. Around a softball-sized sphere of plutonium sat two half-shells of beryllium, and for a fraction of a second they closed. There was a blue flash. The Canadian physicist Louis Slotin yanked the top shell off with his bare hands, turned to the seven other men in the room, and said, almost calmly, “Well, that does it.”

He was right. Nine days later he was dead of acute radiation poisoning. He was 35. The plutonium sphere that killed him had already killed another physicist, Harry Daghlian, the year before. After the second accident, the core stopped being called Rufus. People started calling it the demon core.

If you have read our stories on nature’s nuclear reactor at Oklo or Starfish Prime, you already know that nuclear chain reactions do not always wait for engineers to be ready. The demon core is the opposite kind of lesson. It was not a bomb going off. It was a quiet, hand-held experiment that went supercritical for less than a second, and that was enough.

A third bomb that never left New Mexico

The demon core was built in the summer of 1945 for a war that ended before it could be used. It was a solid sphere of plutonium–gallium alloy, about 89 millimeters across and 6.2 kilograms in weight, roughly the size of a softball. Like the core that destroyed Nagasaki, it was made of two hemispheres and a small ring designed to keep neutrons from jetting out through the seam during implosion. The metallurgists at Los Alamos stabilized the plutonium in its workable delta phase with a little gallium, hot-pressed it into shape, and coated it with nickel so it would not corrode.

By early August 1945, Los Alamos already had the Trinity and Nagasaki cores behind it. Inventory notes list the next finished pieces as HS-5, HS-6 and ring R-2, with more metal still in the metallurgy shop. On August 10, Major General Leslie Groves told General George Marshall that the next implosion bomb should be ready to drop on Japan after about August 17 or 18. Marshall scribbled a presidential order on the memo: it was not to be released without express authority from Truman. Japan surrendered on August 15. The third core never left New Mexico.

Once assembled, the bare sphere sat at what the criticality experts called “minus five cents,” a unit of reactivity borrowed from reactor physics. That meant it was deliberately just short of a self-sustaining chain reaction, with only a small safety margin against anything that might bounce neutrons back into it. Compression, extra metal, or a neutron reflector could push it over the edge. The experiments that killed Daghlian and Slotin were designed to find out exactly how close that edge was.

August 21, 1945: a brick, a sphere, and a lone physicist

Harry K. Daghlian Jr. was 24, the son of Armenian immigrants, a Purdue-trained physicist who had come to Los Alamos with Marshall Holloway’s cyclotron team. He worked in Otto Frisch’s critical assemblies group at the Omega Site, where scientists stacked neutron-reflecting materials around pieces of fissile metal to see how close they could get to criticality without going over. The work was so dangerous that Richard Feynman nicknamed it “tickling the dragon’s tail.”

On the evening of August 21, 1945, six days after Japan’s surrender, Daghlian came back to Omega alone after dinner. That already broke the rules: criticality work was supposed to be done with a partner. A security guard, Private Robert J. Hemmerly, sat at a desk about ten to twelve feet away. Daghlian had a neutron monitor running as he stacked tungsten-carbide bricks around the plutonium sphere. Four layers went up without trouble. As he moved to place a brick over the center of the fifth layer, the monitor told him the assembly was about to go supercritical. He pulled back. The brick slipped from his hand and landed on the core.

The sphere went critical. Daghlian knocked the brick away and began tearing the stack apart with his bare hands, but he had already taken a lethal dose of neutron and gamma radiation. His right hand, which had been closest to the core, blistered and burned. Hemmerly, farther away, received a much smaller dose and survived. Daghlian was taken to the Los Alamos hospital. Over the next 25 days his condition followed the grim arc of acute radiation syndrome: nausea, hair loss, collapsing blood counts, infection. He died on September 15, 1945. He was the first person known to have been killed by a criticality accident.

After his death, Los Alamos tightened some of the rules. Hands-on criticality experiments were supposed to use physical spacers so that neutron reflectors could not close by accident. Not everyone followed them.

Louis Slotin, bomb assembler

Louis Alexander Slotin was born in Winnipeg in December 1910, the son of Yiddish-speaking Jewish refugees from Russia. He finished high school early, entered the University of Manitoba at 16, and took gold medals in physics and chemistry before earning a doctorate in physical chemistry at King’s College London. At the University of Chicago he helped build a cyclotron and worked with radiocarbon. In 1942 he joined the Manhattan Project. He was at Chicago when Enrico Fermi’s team brought the world’s first artificial nuclear reactor, Chicago Pile-1, to life, and he later worked on plutonium production at Oak Ridge before moving to Los Alamos in late 1944.

At Los Alamos, Slotin became known as one of the most skilled assemblers of the bomb’s nuclear heart. He helped put together the core of the Gadget, the device detonated at Trinity on July 16, 1945. He also ran criticality tests, often in blue jeans and cowboy boots, in front of a roomful of observers. Enrico Fermi reportedly warned him and others that if they kept doing the experiment that way, they would be “dead within a year.”

May 21, 1946: tickling the dragon one more time

By the spring of 1946, Slotin was preparing to leave Los Alamos. Operation Crossroads, the first postwar nuclear tests at Bikini Atoll, was scheduled for July, and the demon core was a candidate for one of the shots. On May 21, Slotin was showing his replacement, Alvin C. Graves, how to run a final criticality check.

The setup was different from Daghlian’s brick stack. Two half-spheres of beryllium, a light metal that is an excellent neutron reflector, were meant to close around the plutonium like a clamshell. The standard protocol called for shims between the halves so they could not meet. Slotin skipped the shims. He rested one edge of the top shell on the bottom shell and held the other side open with the flat blade of a screwdriver, raising and lowering it to watch the neutron count climb and fall. It was a method he had used almost a dozen times.

Then the screwdriver slipped. The top shell fell shut. Instantly the room filled with a flash of blue light, the signature of ionized air around a sudden burst of radiation. In less than a second Slotin flipped the shell off with his hand. Later estimates put the interval between closure and separation at about half a second. That was long enough. Slotin absorbed roughly 1,000 rad of neutron radiation and about 114 rad of gamma rays, many times a lethal dose. His body, bent over the apparatus, partly shielded the others. Graves, standing at his shoulder, still took a heavy dose and spent weeks in the hospital. He survived, though he died of heart failure at 55; whether radiation played a role has never been settled.

Slotin knew at once what had happened. He sketched the positions of everyone in the room so doctors could reconstruct the doses, then walked out under his own power. A blue glow, a metallic taste, and the sudden heat on his hands were the classic signs. Nine days later, on May 30, 1946, he died in the Los Alamos hospital. The Associated Press reported the accident only in vague terms: men injured by “accidental exposure to radiation” during “an experiment with fissionable material.”

What a criticality accident is, and is not

A criticality accident is not a nuclear explosion. There is no city-flattening blast. What happens is that a mass of fissile material briefly sustains a runaway chain reaction, dumping an intense pulse of neutrons and gamma rays into anything nearby, then usually blowing itself apart just enough, by heat expansion, to shut the reaction down. The danger is the radiation, not the bang. That is why Daghlian and Slotin could stand next to the core, receive a fatal dose in a second, and walk away under their own power while the sphere itself sat quietly on the table.

The blue flash that witnesses described is Cherenkov radiation and the glow of ionized air, the same family of light you see in the water around a research reactor. It lasts only for the pulse. The damage does not. Neutrons smash into soft tissue and leave a trail of secondary radiation; white-cell counts crash; the gut lining fails; infection follows. Doctors in 1945 and 1946 had almost nothing that could change the outcome once the dose was high enough. The same brutal physics shows up, in a different register, in our story of the Radium Girls, who carried a slow poison home under their fingernails. The demon core delivered its poison in a flash.

After the dragon: remote hands and a melted core

The second death ended the era of tickling the dragon by hand. Raemer Schreiber, one of the men in the room that day, helped design remote-control machines and closed-circuit television so that criticality tests could be run from a quarter-mile away. Physical spacers and interlocks replaced screwdrivers. The phrase “hands-on criticality experiment” became a warning rather than a job description.

The core itself was radioactive enough after the second accident that it could not go straight to Bikini. The Able and Baker shots of Operation Crossroads used other cores. The demon core was penciled in for a third test, tentatively called Charlie, but Charlie was canceled after Baker’s underwater blast left the target fleet too contaminated to clean. In the summer of 1946 the sphere was melted down and its plutonium recycled into other cores. Rufus, the demon, disappeared into the stockpile.

Why the story still matters

The demon core is often told as a tale of reckless genius, and there is some truth in that. Slotin ignored the shim protocol. Daghlian worked alone at night. Fermi’s warning was not metaphor. But the deeper point is how thin the margin was. A few centimeters of beryllium, a single tungsten-carbide brick, a screwdriver blade the width of a coin: those were the differences between a subcritical sphere and a lethal burst. The same plutonium that needed precision high explosives to become a weapon could kill with nothing more than a misplaced reflector.

Los Alamos still remembers both men. A building on the laboratory site carries Slotin’s name. In New Albany, Indiana, a park memorial honors Daghlian. Their deaths helped drive the safety culture that now surrounds fissile material: distance, shielding, interlocks, and the rule that no one should ever again hold criticality open with a tool in their hand. The blue flash lasted less than a second. The lesson has lasted eighty years.

The core is gone. The photographs of the brick stacks and the beryllium shells remain, along with the room sketch Slotin drew while he was already dying. They are among the clearest documents we have of how close early nuclear science ran to the edge of its own fire, and of what it cost to learn where that edge was.

For anyone who wants a tangible sense of the radiation those men walked into, a modern Geiger counter clicking next to a weakly radioactive rock or an old glow-in-the-dark watch is a distant cousin of the instruments Daghlian kept running while he stacked bricks. The numbers on a handheld meter are nothing like the pulse that filled that Los Alamos room. But they are a reminder that ionizing radiation is invisible until something, an instrument, a photographic plate, or a human body, records it.

Further reading