The Radioactive Story of Louis Slotin, Harry Daghlian and the Demon Core of Los Alamos


This is a transcript of Campfire Stories: Astonishing History podcast Episode 1. You can listen on Spotify, Stitcher, Apple Podcasts, and YouTube.

A sketch of men in labcoats standing at various distances from a sphere on a table
A diagram of the Louis Slotin’s accident

Los Alamos and Rufus

Los Alamos National Laboratory was hidden in the deserts of New Mexico, a secret place where scientists from the Manhattan Project worked on the world’s deadliest weapon: the atom bomb.

The first nuclear bomb, code-name “The Gadget”, was detonated on July 16th, 1945 by the lab director Robert Oppenheimer and Army General Leslie Groves. Oppenheimer described the experience with his famous quote from a Hindu holy text, and completely not over-dramatic statement:

“Now I am become Death, the destroyer of worlds.”

The two nuclear strikes on Hiroshima and Nagasaki with the bombs nicknamed “Fat Man” and “Little Boy” killed over 200,000 people less than a month later on 9 August 1945. Emperor Hirohito broadcast the Japanese surrender to the Allies on August 14, which was the first time many of his own citizens had ever heard his voice. If he hadn’t surrendered, the United States would have detonated a third atomic bomb over Japan four days later.

But with this plan halted, the core of the unused bomb was saved for testing, all dressed up, nowhere to go. Even though it wasn’t used for the war, it would still find an opportunity to take human lives. The core was made of the radioactive, man-made material plutonium, secretly discovered and kept hidden until after the war. This particular core was code-named (and I’m 100% serious about this) Rufus. He would later earn the more fitting title of “The Demon Core” for reasons we’ll get back to soon.

During Los Alamos National Laboratory’s operations from 1943- 1946 there were twenty-four fatalities, including multiple construction accidents, an unintentional shooting, a horse-back riding incident, a drowning, an explosion, three janitors who died drinking wine laced with antifreeze… and two major accidents linked to Rufus the Demon Core.

Harry Daghlian Jr.

A black and white photo of a man smiling in a suit


Now let me introduce you to someone. His name was Harotune Krikor Daglian Jr., known as Harry to friends and colleagues, and he was born on May 4th 1921 in Waterbury, Connecticut. He was one of three children born to Armenian parents Margaret Rose Currie and Harotune Krikor Daghlian, Sr.. He had one sister Helen, and one brother, Edward. He attended Bulkeley High School and played the violin.

In 1938, when he was seventeen he started studying math at MIT. While he was there, he changed his focus to a new and exciting field: particle physics. He transferred to Purdue University, in Indiana, graduated in 1942, and began grad school. In 1944, he was recruited to join the Manhattan project, working at Los Alamos Laboratory on critical assembly with scientist Otto Frisch. Only one year later, he would suffer a horrible accident.

Harry’s Experiment

A week after V-J day, day of the famous Times Square sailor-nurse kiss, on August 21st, 1945, Daghlian was working on building a neutron reflector. God I wish I had my pal Jimmy Neutron to explain this to me, because I am not a boy genius, but Daghlin’s experiment, known as a criticality experiment, involved stacking ten-pound bricks of tungsten carbide onto the plutonium core (Rufus). The plan was to use this neutron reflector to push Rufus right up to the point of becoming super-critical, when a nuclear reaction would be triggered.

These experiments were intended to measure just how far they needed to push the core before it would reach this threshold, but the consequences of crossing the line were disastrous

Criticality experiments were incredibly dangerous, as they involved what scientists called “tickling the tail of a sleeping dragon” as they risked starting a nuclear reaction by mistake.

Which is, as you may know, very bad.

The man who created the world’s first nuclear reactor, Enrico Fermi, the radioactive dad to America’s nuclear program, had warned the Los Alamos team of the danger of what they were doing. But science pressed on without regard for safety. They probably should have listened to Fermi.

Daghlian had his dinner that night and then went back to the lab to finish some work alone. No other scientists were there, only a single security guard there, which broke safety protocol. He was about to place the final tungsten brick when neutron monitors went off to tell him that adding the last brick would cause the core to become supercritical. Or in layman’s terms, that dragon was about to wake up. He tried to pull the brick away from the core, but in a highly relatable move he dropped the brick directly onto the middle of the system, immediately triggering a criticality accident.

A wave of heat and a crackle of blue light filled the room, as he tried to knock the brick away from the core, and then had to disassemble the system by hand to stop the reaction, feeling his hands tingling as he did so.

While doing this, he received a huge dose of neutron radiation. Modern day analysis suggests that the initial radiation dose may not have killed him, however, the extra gamma radiation and beta burns from disassembling the experiment was too much for his body to handle. Daghlian was rushed to the hospital, but the radiation poisoning was too severe, and he fell into a coma. He died on September 15th, 1945, twenty-five days after the accident. He was twenty-four. The security guard survived his lower dose of radiation.

Harry’s Legacy

Daghlian was the first known person to die from a criticality accident, and unfortunately he would not be the last. The lab at Los Alamos formed a committee to update their safety guidelines. At least two people needed to conduct experiments together, at least two neutron monitors needed to be used with audio alerts, and they began researching a way to do experiments like this through remote-control (so no one needed to be sticking their hands around plutonium).

In May of 2000, the city of New London and Daghlin’s brother and sister unveiled a memorial for him. It is inscribed:

“Though not in uniform, he died in service to his country.”

Dr. Louis Slotin

Louis Slotin


Now let me introduce you to Daghlin’s friend and colleague, Louis Slotin. Louis Alexander Slotin was born on December 1st, 1910. He was the oldest of three children of Israel and Sonia Slotin, Jewish refugees who fled the pogroms of Russia and settled in an Eastern European immigrant neighborhood of Winnipeg, Canada.

Slotin was something of a childhood whiz kid. His brother Sam would recall years later that Slotin “had an extreme intensity that enabled him to study long hours”, which I personally can’t relate to but it worked for him. Sam also remembered that Slotin would make him play games with his friends so he could be left alone to study. He started college at the University of Manitoba when he was only sixteen years old, showing up our boy Daghlin the 17-year-old college freshman, and earned a University Gold Medal for physics and chemistry. He received his bachelors in geology in 1932, and his masters in science a year later in 1933.

He soon earned a chemistry fellowship at King’s College London. During this time he also took up amateur boxing and won the college championship, although he did joke with his brother that all he received for it was a black eye. (He was more than just an egg-head, guys.) He also went to Spain, which was in a period of brutal civil war, seeking excitement. It seems he may have originally claimed that he fought in the war as an anti-aircraft gunner, but later his brother Sam told reporters that Slotin had only gone on a walking tour, and had not fought, so this is unclear.

One article attempted to explain this by stating:

“[Slotin] regularly amused himself with the gullible by planting false clues to an imaginary and stylish past. Many of his friends came to believe, for example, that he had fought with the Loyalists in Spain and flown with the RAF and this seemed to please some strain of romance in him.”

Little did he know, he would earn a place in the history books without any need for exaggeration.

In 1936, Slotin received his Ph.D. in physical chemistry, becoming the newly minted Dr. Slotin. He worked for six months on Dublin’s Great Southern Railways testing rechargeable nickel-zinc batteries. He was rejected from a position with Canada’s National Research Council, prompting his move to the U.S. to research nuclear chemistry with the University of Chicago. He built the first cyclotron in the midwest, which may sound like the name of a Transformer, but is actually a type of particle accelerator that produced a material known as radio-carbon.

The position did not pay well at all, and Slotin spent this time of his life being financially supported by his father. He also researched how plant cells use CO2 for carbohydrate synthesis, so I can partially thank Slotin for one of two things I remember from 9th-grade biology. That and the mitochondria is the powerhouse of the cell.

Slotin and the Manhattan Project

Louis Slotin working on a machine
Slotin working on an unknown machine

Eventually Slotin’s publications on radio-biology earned him an invitation to the Manhattan Project, one that in hindsight, he for sure should have said no to. In 1944 he started at Los Alamos in New Mexico to work on bomb physics. He worked on criticality experiments with uranium and later plutonium, where he would encounter, that’s right, say it with me: Rufus.

Slotin also established himself as a risk-taker with a disregard for protocol in a particular incident described by his colleague Dr. K.Z. Morgan as follows:

“It was Friday afternoon and Louis wanted to shut down the reactor to make adjustments to an experiment at the bottom of the tank of water which was used to absorb radiation. We said that was impossible, and we planned to shut down the reactor that weekend. When we came back on Monday morning, I found that Louis had stripped down to his shorts, dived into the tank and made the adjustments under water. I was appalled that anyone would take such risks. It shows what kind of person he was. He was like a cowboy — but a good experimental scientist.”

Louis Slotin, the cowboy scientist, stands with the first nuclear bomb

He was not wearing a dosimetry badge, which the scientists were supposed to wear to measure their exposure to radiation.

Slotin created the core of Trinity, which was the first atomic device to be detonated, and earned him the title of “chief armorer of the United States” as several commemorative lead pins, because this man simply wasn’t touching enough deadly materials. He kept the paper receipt that he received for delivering the bomb as a treasured memento. He was unable to travel to the military base for the launch of Fat Man and Little Boy because he had not fully completed his American citizenship process yet.

After the end of World War II, Slotin began training a replacement for himself as he wanted to return to teaching and research at the University of Chicago, which is a bit of a bummer considering the way things turned out for him. It is likely that he would have left sooner, however his expertise was needed. He said, “I have become involved in the Navy tests, much to my disgust… I am one of the few people left here who are experienced, bomb putter-togetherers” which is a title I’d love to drop on a LinkedIn profile. His desire to leave was hastened by the fact the Slotin was uneasy about his participation in projects that caused such devastation. It may have also been sped along by his upset over his assistant and friend Daghlian’s death, but if that accident was a sign to run from Los Alamos, Slotin didn’t heed the warning quick enough.

Slotin’s Accident

A hand holds a screwdriver next to an open sphere with a second metal sphere inside
A recreation of Slotin’s accident

Let me take you to May 21st, 1946, six months to the day after Daghlian’s death. Slotin was demonstrating an experiment for seven of his colleagues. Slotin had Rufus the Demon Core, which radiated a constant warmth from its radioactivity, on a table in the center of the room. The plan was to create the first step of a fission reaction by placing a hemi-spheres of beryllium onto the fourteen pound plutonium core. This dome, called a tamper, acted like the bricks Daghlin had used to reflect neutrons back at Rufus, but it was vital that the tamper never completely covered the core to allow a gap for some neutrons to escape.

Slotin was holding the tamper with his left hand through a thumb hole in the top, and was keeping the hemispheres separate from each other with the blade of a screwdriver held in his right hand. Shockingly, this was not official protocol as metal shims were usually used instead but they had been removed.

But then at exactly 3:20 p.m. the screwdriver slipped, the hemisphere fell and there was an instant burst of radiation. According to eyewitness reports, room began to glow blue with ionization and there was a wave of heat just like Daghlin’s accident. A sour taste filled Slotin’s mouth and his left hand began to burn. Slotin shielded his colleagues with his body and tried to end the reaction. An eyewitness said,

“The total duration of the flash could not have been more than a few tenths of a second. Slotin reacted very quickly in flipping the tamper piece off.” But it was already too late.

According to another scientist in the room, Slotin knew the consequences of what he had done. He had been with Harry Daghlin as he suffered from radiation poisoning in the hospital, and when the blue glow of the room had faded, Slotin’s first words were, “Well… that does it.” On his way to the hospital, Slotin also told a colleague:

“You’ll be ok, but I think I’m done for.”

The Aftermanth

The security guard, Private Patrick Cleary, fled the room in terror while the room flashed and the scientists began to yell. The lab was evacuated, and the scientists gathered outside to wait for ambulances to arrive. He recalled what happened as follows:

“After the accident I ran out the East door and down the ramp. Probably took me about five seconds or so. When I got to the gate, it was still locked and Mr. Kline, Cieslicki and myself were the only ones there. Mr. Kline told the M.P. to open the gate. He had some trouble getting his whistle out of his pocket, but, when he did, he opened the gates and then blew the whistle. I ran up the hill approximately 1,000 feet with the others. Pretty soon Dr. Slotin and Mr. Young came out. Mr. Young called and told us to come down to the laboratory again. As soon as we got there we drew a diagram to figure out approximately where everyone was when it happened. The only other conversation was that everyone was wondering who had gotten most of the radiation.”

Slotin sketched out a diagram of where he and his colleagues were standing at the time of the accident. He also tried to use a radiation detector on several objects in the room with them, including a Coca-Cola bottle and a bristle brush. However, the detector itself was contaminated with radiation, so the readings were not accurate.

Luckily, a tool existed for this exact purpose. Film dosimetry badges were by Los Alamos scientists to measure people’s exposure to radiation during an accident exactly like this. But unluckily, not a single person in the room was wearing their badges which was another break of protocol. A confused and sick Slotin sent one of his colleagues back into the contaminated lab to take the badges out of the locked lead box that they were in but they were unusable. Slotin likely would have known this, but as a later report highlighted after an exposure like this, people “are in no condition for rational behavior.”

Slotin began vomiting immediately when he left the building, and continued to do so on his first day in the hospital. When he arrived, he reiterated his sense of doom to college Alvin Graves, saying:

“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.”

On the second day, his condition actually seemed to get better. But after that day, Slotin began to suffer a steep decline in his condition. The hand that had been holding the core became a waxy blue, and blisters, and doctors kept it packed in ice. He suffered intestinal distress, reduced urine output, swollen hands, erythema, blisters covering his arms, intestinal paralysis and gangrene. The radiation throughout his body was described by a medical expert as a “three-dimensional sunburn”.

Slotin made sure that while he was suffering the effects of the radiation, the doctors and scientists with him were using it as an opportunity for case study. He repeatedly discussed his tests results with the doctors and nurses.

General Leslie Groves, the same man who had been with Oppenheimer at the detonation of the Gadget, was sent to retrieve Slotin’s parents from Winnipeg and they arrived on day four. By day five, his white blood cell count dropped severely. By day seven, he was confused, his lips turned blue, and he was placed in an oxygen tent. He fell into a coma and died nine days after the accident on May 30th, 1946, at age 35.

He was buried in Winnipeg in a sealed military casket. As Slotin’s family were Orthodox Jews, the army made sure that they brought their son home at sundown on a Friday, the beginning of the Jewish sabbath. Sam Slotin met them at the airport, and the local newspaper covered the event with the headline: “Hero’s Body Home.” To further study the effects of the sickness, an autopsy was requested by the scientists. Although it was against their religious traditions, Israel Slotin agreed, as one article reported him saying that:

“Louis had been a scientist all his life, and that when it could do him no harm, it would be wrong to prevent him adding to knowledge.”

Over 3,000 mourners attended his funeral.

The Observers

Four of Slotin’s colleagues were discharged by May 25th, and the others were described to have a “satisfactory” condition. The observer who was closest to Slotin during the accident, Alvin Graves, who Slotin was training as his replacement (not sure he wanted that job anymore) stayed in the hospital for several weeks with acute radiation poisoning. He did survive, although he had vision and neurological problems the rest of his life.

Three of the scientists, Graves, Kline, and Perlman sued for compensation from the accident in 1948. Graves received $3,500. Three of the scientists later died from radiation-related illnesses. Graves died 20 years later at age 55 from a heart attack. Cieslicki died nineteen years later from acute myeloid leukemia 19 years later at age 42. Young died of a plastic anemia and a bacterial infection of his heart 27 years later at age 83. The security guard, Private Patrick Cleary died in 1950 in North Korea after being promoted to Sergeant First Class.


Strangely, Slotin and Daghlin’s accidents happened on the same day of the month, the 21st, and they died in the same hospital room, which only added to the Demon Core’s nefarious reputation.

What happened to the demon core? Slotin’s experiment was intended to be a last test of its ability to reach critical status before it would be detonated in a nuclear weapons test in the Bikini Atoll. Slotin was going to attend and then leave Los Alamos to return to teaching. After the accident, it’s detonation was rescheduled to give it time to cool down, but it was eventually melted down in 1946 and returned to the national stockpile.

It was very difficult for scientists and doctors to determine how much radiation that Slotin, Daghlin and the others received. At the time, the badges that were placed around the room only measured gamma radiation, which left out the large dose of neutron radiation that was released. Slotin and the observers were not wearing their badges, and badges were also supposed to be placed under tables, but they were not there. Instead, scientists used measurements of activation of sodium in blood and urine samples to estimate the neutron radiation. It is estimated that Daglin may have received 110 R of gamma radiation and 480 R of x-ray radiation and Slotin received 114 R of gamma and 1930 R of x-ray. 500 R is usually fatal in humans. These calculations were done with the methods of the time.


After Slotin’s accident, Los Alamos finally stopped critical assembly experiments from being done by hand. All tests after this were done by remote control to protect scientists from radiation exposure.

The narrative after the accident was one of Slotin being a hero for removing the hemisphere quickly and ending the reaction, protecting the other observers from more severe radiation exposure. A quote from the time states that:

“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.”

Alvin Graves also hailed Slotin as a hero from protecting him and the others. However another observer, Raemer Schreiber, challenged this decades later by stating that Slotin risked the lives of the team by not following proper safety procedures. As for my personal feelings, I think he is a bit of a mix, with a heavy lean towards hero. And it’s undeniable the achievements that he contributed to his fields.

Slotin’s accident was also fictionalized in the 1955 novel “The Accident”, in a 2001 off-Broadway play called the Louis Slotin Sonata, and the 1989 film “Fat Man and Little Boy” where the character Michael Merriman, played by John Cusack, was based on Slotin. Slotin also came up with the name “dollar” as a unit of measurement for criticality processes.

Thomas P. Ashlock, an associate editor of the Los Alamos Times newspaper published a poem in June of 1946 titled “Slotin- A Tribute”. It read as follows:

“May God receive you, great-souled scientist! While you were with us, even strangers knew The breadth and lofty stature of your mind Twas only in the crucible of death We saw at last your noble heart revealed.”

In 1948 the Louis A. Slotin Memorial Fund was established by his colleagues at Los Alamos and the University of Chicago to fund physics lectures by leading scientists. In 2002, an asteroid was named the 12423 Slotin.

Louis Slotin with a female toddler on his lap
Louis Slotin holds an unknown child




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