The Demon Core

Scientists Harry Daghlian and Louis Slotin were each killed after accidentally triggering a fission reaction during separate incidents while working on the Manhattan Project. Responsible for at least two and up to six deaths, the plutonium core responsible has been nicknamed ‘the demon core’. 

Born in Waterbury, Connecticut in 1921, Harry Daghlian was accepted to the Massachusetts Institute of Technology at the age of 17. He initially intending to study mathematics, but became increasingly interested in the emerging field of particle physics. He obtained his bachelor’s degree after transferring to Purdue University to study particle physics. While a graduate student, he was invited to join the Critical Assembly Group at Los Alamos Laboratory, as part of the Manhattan Project.

Harry Daghlian, photographed in 1944. Image: Arnold S Dion

On August 21st 1945, Daghlian inadvertently fumbled a heavy tungsten carbide brick during an experiment on a plutonium-gallium alloy bomb core. As a result, he received a huge dose of neutron radiation. He collapsed with acute radiation poisoning, dying 25 days later at the age of 24. Before his death, he suffered nausea, diarrhea, blistering, loss of skin and hair, cramping and fever.

Daghlian was the first known fatality caused by a criticality incident. A guard on the project, Private Robert Hemmerly, died of leukemia 33 years later, which may have been linked to his exposure to radiation. Daghlian’s death saw safety regulations on the project scrutinised, but failed to prevent a similar fatal incident eight months later. 

Daghlian’s hand nine days after the incident. Image: Nuclear Secrecy

The son of Jewish refugees that had fled the Russian pogroms to Canada, Louis Slotin was born in Winnipeg, Manitoba in December 1910. Excelling at school, he began studying for a university degree at the age of 16. He later secured a fellowship at King’s College London, earning a Ph.D in physical chemistry in 1936.

After being turned down for a job at Canada’s National Research Council, Slotin took a research position at the University of Chicago. It was in this role that he gained his first experience with nuclear chemistry. After publishing a number of academic papers on radiobiology, he was invited to join the Manhattan Project. He began work at the Los Alamos National Laboratory in December 1944.

Louis Slotin in his Los Alamos ID picture. Image: Los Alamos National Laboratory

When the war ended in 1945, Slotin told colleagues that he intended to return to the University of Chicago, and was looking forward to taking up a teaching role. However, his expertise were still required at Los Alamos, and he remained with the project.

On 21st May 1946, Slotin carried out an experiment to create the first steps of a fission reaction by placing two half-spheres of beryllium around a plutonium core. The same core had been the one that killed Daghlian, and was later dubbed the ‘demon core’ by members of the project.

Using a screwdriver to to maintain a gap between the two half-spheres, Slotin slipped and allowed the upper hemisphere to fall, resulting in him being blasted by a dose of hard radiation. Other scientists in the room later reported the air glowing blue as it was ionized, and feeling a wave of heat pass over them. Slotin quickly lifted the beryllium half-sphere, ending the experiment, but had already received a fatal dose of radiation. He began vomiting, recognised by colleagues as a reaction to ionizing radiation. 

A recreation of the incident that killed Slotin. Image: Los Alamos National Laboratory

Slotin was taken to hospital, where he began to rapidly deteriorate after five days. After his condition began to worsen, he suffered severe diarrhea, swollen hands, reddening skin, huge blisters, stomach paralysis and gangrene. One doctor described his internal radiation burns as ‘three-dimensional sunburn’. Seven days after the incident he fell into a coma. Slotin died on 30th May 1946, with his parents at his bedside. He was 35 years old.

Alvin Graves, the colleague who had been standing closest to Slotin, survived acute radiation sickness but was left with lifelong neurological and sight damage. He died of a heart attack 20 years later. Another colleague, Marion Cieslicki, died of leukemia 19 years later, while Dwight Smith Young died of aplastic anemia and bacterial infection 27 years later. All three deaths are thought to be linked to their exposure to radiation.

Daghlian (in background) and Slotin (in sunglasses) photographed working together on the Manhattan Project. Image: Los Alamos Archive

Following Slotin’s accident, Los Alamos ended all hands-on assembly work of critical cores. Instead, remotely controlled machines were brought in. He was praised by colleagues, including Alvin Graves, for quickly removing the upper hemisphere at great risk to himself, preventing his colleagues suffering a larger dose of radiation. However, another witness argued that Slotin was using unsafe procedures that put himself and his colleagues at risk. 

Daghlian was memorialised in 2020 with a memorial stone and flagpole in Calkins Park, close to where he had grown up. Unveiled by his siblings, the plaque read ‘although not in uniform, he died in service to his country’. In 1948, colleagues from both Los Alamos and the University of Chicago established the Louis A. Slotin Memorial Fund for physics lectures. An asteroid was named 12423 Slotin in 2002 in his honour. 

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