What Caused The Demon Core Criticality In Each Incident?

Funny enough, the story of the demon core keeps popping up whenever people ask for the human side of the Manhattan Project. I got pulled into it the first time I read Richard Rhodes' 'The Making of the Atomic Bomb' — he treats the Daghlian and Slotin accidents with a kind of careful, almost literary attention that made me picture the labs and late-night experiments in a way a dry report never would. That book is probably the single best place to start if you want a well-researched narrative that places the demon core in the broader sweep of World War II and early nuclear history. From there I chased down interviews and archival material; Los Alamos' oral histories and accident reports are publicly available and make the events feel immediate and real, not just a cautionary footnote. Documentary makers have definitely been interested in the drama. If you want film, check out 'The Day After Trinity' — it’s more about Oppenheimer and the Trinity test, but it’s emblematic of how documentaries stitch personal stories into the technical history and it touches the culture at Los Alamos. The 1989 film 'Fat Man and Little Boy' dramatizes the project and includes incidents inspired by those accidents, though it takes cinematic liberties. For more focused, modern takes, historians and writers like Alex Wellerstein have detailed posts on his 'Restricted Data' blog and in journal articles that read like long-format detective work: timelines, people, and the scientific context that made those mishaps possible. Beyond books and big documentaries, the demon core shows up in lots of niche media — magazine longreads, science-channel YouTube pieces, and history podcasts that love telling one-off morality tales about technology. I’ve listened to several podcast episodes that reconstruct the moments surrounding Louis Slotin’s final hands-on test and Charles Daghlian’s earlier accident; they’re good for a compact, human-focused retelling. If you’re curious, mix Rhodes for depth, Wellerstein for technical-historical analysis, and a documentary or two to feel the period. It’s weirdly compelling and sad in equal measure — a reminder that brilliant people can still make tragic mistakes, and that history preserves those moments so we can learn from them.

There’s something almost mythic about the phrase 'demon core'—not because of supernatural forces, but because of how a few human decisions and a very unforgiving bit of physics combined into tragedies. I dug into the stories years ago while reading 'The Making of the Atomic Bomb' late one sleepless night, and what struck me most was how normal the setting felt: tired scientists, hands-on tinkering, casual confidence. Two incidents stand out: one where a tungsten-carbide reflector brick was dropped onto the core, and another where a pair of beryllium hemispheres were being nudged apart with a screwdriver. Both were trying to push a subcritical plutonium mass closer to criticality to measure behavior, and both crossed a deadly threshold. From a physics perspective, the core was dangerously close to critical mass as-built, because the design intended to be compressed into a supercritical state in a bomb. Neutron reflectors—metallic bricks or hemispheres—reduce leakage of neutrons and thus increase reactivity. In plain terms, adding or closing a reflector can turn a harmless pile into a prompt-critical event almost instantly. The accidents produced an intense burst of neutron and gamma radiation (a prompt critical excursion) that didn’t blow the core apart like a bomb, but was enough to deliver a fatal dose to whoever was nearest. People weren’t vaporized; they received overwhelming radiation that caused acute radiation syndrome over days to weeks. Why did this happen twice? There was a blend of human factors: informal experimental practices, assumptions that dexterity and care were sufficient, single-person demonstrations, and a culture that prized hands-on 'knowing' over remote, engineered safety. The first incident involved dropping a reflector brick by mistake; the second was a public demonstration with the hemisphere only held apart by a screwdriver. Both show how ad hoc methods—bricks, hands, and tools—were being used where remote apparatus or interlocks should have been. There was also secrecy and pressure: schedules, wartime urgency, and the novelty of the devices meant procedures lagged behind what the hazards really demanded. Those deaths changed things. Afterward, strict criticality safety rules, remote handling, and formalized procedures became the norm. The name 'demon core' stuck because it felt like a cursed object, but the real lesson is less mystical: when you’re working with systems that have non-linear thresholds, casual handling and human overconfidence can turn boring measurements into lethal events. I still picture those cramped lab benches and feel a chill at how close those teams walked to disaster before the safety culture finally caught up.

I've always been fascinated by the weird mix of hands-on tinkering and high-stakes physics those Los Alamos experiments had. The core people later called the 'demon core' was tested the old-fashioned way: you make a subcritical assembly and then you add neutron-reflecting material around it bit by bit while watching the neutron instruments. Practically that meant stacking heavy tungsten-carbide bricks or closing beryllium hemispheres around the plutonium mass to bounce escaping neutrons back into the fuel. Each added reflector raises the effective multiplication factor, so the experimenters could see how close they were to criticality by monitoring neutron count rates, oscilloscopes, and ionization chambers. The goal was to map reactivity — how the assembly responds as neutron economy changes — not to explode anything, but to understand at what point the chain reaction becomes self-sustaining. I was struck, reading about those days, by how tactile the work was. Instead of remote rigs and interlocks we have now, scientists literally adjusted spacer shims, stacked bricks by hand, or used a screwdriver to keep two beryllium halves slightly apart. That screwdriver trick is infamous: people later called it 'tickling the dragon's tail' because it was a delicate tease toward prompt criticality. When the separation failed, the core emitted an intense burst of neutrons and gamma rays — a bluish flash and a spike on the detectors — and those nearby got severe radiation doses in an instant. Two tragedies stand out: one incident involved a dropped brick and another involved the screwdriver slipping; both resulted in fatal exposures days later. Those events shocked the community into modernizing procedures. After those accidents the culture changed fast. Measurements that used to be eyeballed were redesigned for remote control, mechanical interlocks and automated shutters became standard, and teams moved to monitor from behind thick shielding or via instruments that could be read out at a distance. Film badges and ion chambers stayed, but shockingly banal tools like a screwdriver were swept out in favor of engineered jigs that would prevent human fingers from being the last line of defense. Reading about it now — flipping through photographs, procedural notes, and personal remembrances — you get a real sense of how a handful of mistakes reshaped experimental safety for decades. It’s a grim lesson, but one that pushed safer, smarter methods into the mainstream of experimental nuclear work, and that feels oddly reassuring.

Whenever the demon core topic comes up at a museum talk or online forum, I get that little shiver of morbid curiosity — it’s one of those stories that sits at the crossroads of human error, science, and tragedy. The straightforward truth is that you won't find a neat glass case labeled 'The Demon Core' in any public museum. After the two fatal criticality accidents in 1945–46, the 6.2-kilogram plutonium core was deemed far too hazardous and politically sensitive to display. In the years that followed, the material was melted down and recycled into other plutonium components for tests and production, so there’s no intact, original core preserved for public viewing today. That said, museums and archives do hold a lot of the story: replicas, photographs, lab notebooks, safety reports, and the critical assembly apparatus that help tell what happened. If you want to get a tangible sense of the events, the Bradbury Science Museum at Los Alamos and the National Museum of Nuclear Science & History near Albuquerque often have exhibits about the Manhattan Project and criticality experiments (they show models, diagrams, and sometimes period tools). The Department of Energy and Los Alamos National Laboratory retain classified and radioactive materials in secure facilities, and any actual plutonium remnants would be under strict DOE control — not on public display. Museums instead balance public education with safety by using models, declassified documents, and oral histories. There’s also a lot of mythos around the incidents — the famous screwdriver story from Slotin’s experiment is part of that lore — and museums try to be careful about separating dramatized retellings from the documented facts. If you’re hunting for the closest thing to a relic, archival photos, the original procedural reports, and curator-led interpretations are your best bet. I always walk away from these exhibits feeling a mix of awe and somber respect; the physical pieces might be gone or locked away, but the lessons and the human stories are very much on display.

When I dug into those grim little episodes from Los Alamos history, the hair on my neck stood up — not because the physics was mystical, but because it was brutally simple. A plutonium sphere doesn't need to be part of a bomb to produce a dangerous burst of neutrons and gamma rays; it just needs the right combination of mass, shape, density, and nearby materials that reflect or slow neutrons. In practice that means a near-critical chunk like the so-called demon core could still be pushed into a prompt supercritical state if someone surrounded it with a good reflector (think beryllium or tungsten carbide), closed gaps that let neutrons escape, or accidentally soaked it in something that moderated neutrons like water. I like to picture the old experiments: heavy hands moving reflective bricks, a gap closing millimeter by millimeter. That tactile closeness is what made the original incidents lethal. Modern nuclear materials are kept in geometries and containers designed to be subcritical under all credible disturbances — lots of boronated materials, fixed spacing, and administrative rules to prevent ad hoc tinkering. So yes, in theory, a demon-core-like object could go prompt supercritical if mishandled in the very specific ways that increase reactivity (adding reflectors, shaping into a compact sphere, compressing it). In reality, the combination of safeguards today makes accidental criticality from casual mishandling very unlikely, but the physics hasn’t changed, and that old, cold fact always gives me a chill when I read those accounts.

My curiosity about odd historical mishaps always pulls me toward the strange story of the so-called demon core. Two people died directly from handling that plutonium core during criticality experiments: Harry K. Daghlian Jr. and Louis Slotin. Daghlian’s accident happened on August 21, 1945. He was doing a late-night experiment stacking tungsten carbide bricks as a neutron reflector around the core when one slipped and fell onto it, causing a prompt critical reaction. Witnesses later described a bluish flash; Daghlian was exposed to a massive dose of radiation and developed acute radiation sickness, dying a few weeks later on September 15, 1945. That sequence of events — the nervous fumbling with the reflected core, the sudden flash, the tragic decline — has stuck with me every time I read about the era. The second fatality was Louis Slotin, on May 21, 1946. Slotin was performing the infamous ‘tickling the dragon’s tail’ demonstration, manually separating two beryllium hemispheres that acted as a reflector with a screwdriver. The screwdriver slipped, the hemispheres closed, and a critical excursion occurred. Slotin saw the flash, absorbed a lethal dose, and died on May 30, 1946. Several others in the room received significant exposures but survived. These incidents reshaped how hands-on criticality work was handled and are chilling reminders of how improvised procedures in high-risk labs can have fatal consequences. If you want a deeper dive, I’d recommend reading 'The Making of the Atomic Bomb' — it paints the human side of these technical disasters really well.

I still get a chill thinking about those cramped lab rooms at Los Alamos, the hum of equipment and the quiet confidence everyone had back then. The two technicians who actually handled the infamous 'demon core' during the fatal incidents were Harry Daghlian and Louis Slotin. Harry Daghlian, a young physicist in his twenties, was working alone in late August 1945 stacking tungsten-carbide bricks as a neutron reflector around the core when one brick slipped and fell onto the assembly. That single mistake produced a prompt increase in reactivity — he received a lethal dose of radiation and died a few weeks later. The accident was heartbreaking because it came down to one moment of bad luck and the brutal physics of a supercritical assembly. The second incident, less than a year later in May 1946, involved Louis Slotin. He was famous for performing delicate criticality experiments — the so-called 'tickling the dragon's tail' technique — where a beryllium shell was used as a reflector around the plutonium core and separated by a small gap. Slotin used a screwdriver to keep the halves apart during a demonstration; when the screwdriver slipped the shells closed, the core went prompt critical. Slotin took the brunt of the radiation and passed away about a week and a half later. Several colleagues and supervisors were present in the room during that test, and the event led to a major tightening of safety procedures and a rethinking of how these experiments should be performed. Reading about these incidents always reminds me how dangerous hands-on work with fissile material was before remote tooling and strict protocols became standard—human curiosity and skill were mixed with dreadful risk, and the losses left a deep mark on the laboratory community.

I dug into this because the story of those criticality accidents always felt like a grim little chapter of Cold War science — and the way physicians reacted at the time tells you a lot about how medicine meets the unknown. When Harry Daghlian and Louis Slotin were exposed, the immediate response was basic emergency care layered over painstaking radiation monitoring. Teams first moved the men away from the source, removed clothing that might be contaminated, and washed skin thoroughly while using Geiger counters and film badges to check for external contamination. Doctors and health physicists worked together: the physicists tried to quantify dose with badges, film, and blood tests, while the physicians focused on symptom control and preventing infection once the bone marrow began to fail. Treatments were largely supportive and reactive. Both men developed acute radiation syndrome — nausea, vomiting, severe burns in irradiated tissues, and progressive loss of white blood cells. Physicians gave fluids, pain control (morphine and other analgesics), antibiotics to chase opportunistic infections, and transfusions when blood counts dropped. Wound care was carried out for irradiated or burned skin; isolation and strict hygiene were emphasized because an irradiated patient’s immune defenses were collapsing. There weren’t targeted anti-radiation drugs then for internal contamination, and hematopoietic growth factors didn’t exist, so doctors were limited to transfusions and palliative measures when the marrow stopped producing cells. What always gets me is the human side — the medical teams were trying their best with incomplete knowledge. Slotin’s agony and Daghlian’s decline were documented with clinical notes that show careful monitoring of temperature, blood counts, and wound infections as they progressed to multi-system failure. Contemporary physicians tried novel things when possible, but many modern interventions (cytokine support, bone marrow/stem cell transplants, specific chelators for transuranics) weren’t available yet. Reading the reports over a coffee in the archives, I felt both respect for those clinicians and a chill at how much they were improvising. It’s a reminder that medicine evolves, and that those early tragedies shaped protocols that protect people today.